Fiber-Forming Acrylonitrile Copolymers: Synthesis and Properties

17 ISSN 18112382 Polymer Science Series C 2020 Vol 62 No 1 pp 1750 Pleiades Publishing Ltd 2020 Russian Text The Authors 2020 published in Vysokomolekulyarnye Soedineniya Seriya C 2020 Vol 62 No 1 pp 2054 FiberForming Acrylonitrile Copolymers From Synthesis to Properties of Carbon Fiber Precursors and Prospects for Industrial Production E V Chernikovaab R V Tomsc A Yu Gervaldc and N I Prokopovc aFaculty of Chemistry Moscow State University Moscow 119991 Russia bTopchiev Institute of Petrochemical Synthesis Russian Academy of Sciences Moscow 119991 Russia cLomonosov Institute of Fine Chemical Technologies Russian Technological University MIREA Moscow 119571 Russia еmail chernikovaelenamailru Received January 13 2020 revised January 20 2020 accepted February 5 2020 AbstractThe latest achievements in the field of synthesis of acrylonitrile copolymers are summarized New ecofriendly methods of synthesizing acrylonitrile copolymers in ionic liquids and supercritical media are analyzed The potential of various techniques of controlled radical and anionic polymerizations in tai loring the structure and properties of acrylonitrile copolymers is discussed Methods for the synthesis of acrylonitrile copolymers which may be used for the melt spinning of fibers are considered A patent search is conducted and information relevant to new methods and recipes for the synthesis of acrylonitrile copolymers is generalized DOI 101134S1811238220010026 INTRODUCTION The interest in reinforced multipurpose compos ite materials based on carbon fibers has increased dramatically in past decades In general in the period from 2005 to 2015 the global consumption of carbon fibers increased by more than a factor of two 1 Among them of special interest are high strength fibers with the breaking strength above 6 GPa and the elastic modulus above 300 GPa the main producer of these fibers is the Japanese com pany Toray 2 It should be noted that these charac teristics are much lower than those predicted theo retically This naturally facilitates the emergence of basic research works in the field of synthesis of both precursors and final carbon fibers Another no less important task concerns creation of more economi cally sound and environmentally friendly methods of producing carbon fibers whose mechanical charac teristics may be not so high The production of carbon fibers is a complex multi parameter task including polymer synthesis if neces sary fiber spinning precursor synthesis and subse quent thermooxidative stabilization carbonization and graphitization processes Each of these stages con tributes to the properties of the final product Acrylonitrile ANbased polymers are of the most practical importance among the known precur sors This is associated with their high specific strength and rigidity combined with small mass and low cost as well as a high yield of carbon during car bonization Acrylic textile fibers made from acrylonitrile copo lymers were patented for the first time in the mid 1940s by DuPont and their industrial production was launched in the 1950s 3 The method was based on the suspension polymerization of acrylonitrile and the spinning of films from DMF solution In the mid 1950s using the same method for the synthesis of PAN Monsanto developed the technology of film spinning from DMAA 4 and later Cyanamid pat ented the method of suspension PAN spinning from the aqueous solution of NaSCN 5 The latter version stimulated Courtaulds to develop the solution method for the synthesis and processing PAN in the aqueous solution of NaSCN 6 Subsequent producers were forced to introduce new elements to the existing tech nologies in order not to violate the rights of patent holders Eventually in the 1970s all major technolo gies of PAN precursor production were developed including solution DMSO DMF DMAA ethylene carbonate propylene carbonate ZnCl2 aqueous NaSCN aqueous HNO3 and heterophase pre dominantly suspension and precipitation polymer ization processes 18 CHERNIKOVA et al An alternative approach to the manufacture of tion make it possible to manufacture carbon fibers of PAN precursor is its spinning from melt rather solu similar quality Toray is the absolute leader in the vol tion This imposes certain limitations on the molecu ume and quality of the produced fibers lar characteristics of PAN and its synthetic proce The final properties of carbon fibers depend on the dures In addition melt spinning is more environ efficiency of previous stages including the synthesis of mentally friendly and economically sound because it copolymers 810 Just at the stage of synthesis the does not need the use of organic solvents and consid molecular structure of a chain its compositional het erably increases polymer concentration during fiber erogeneity and MWD are set these characteristics processing affect the morphology and structure of the spun fiber The use of AN copolymers for the production of and its behavior during thermal stabilization 1113 structural carbon fibers is related to the studies of Jap Because of low solubility and crystallinity which are anese researcher A Shindo who first developed and related toa strong interaction between nitrile groups of patented the carbonization of PAN according to the macromolecules the PAN homopolymer is rarely batch method without stretching to produce carbon used in producing carbon fibers 14 15 Instead of the fibers with strength less than 2 GPa 7 Later focus PAN homopolymer binary and ternary copolymers on civilian applications of carbon fibers was the seri with the weight content of comonomers not above ous reason for the success of Japanese producers 15 are applied 1619 The choice of comonomers whose joint effects contributed to advancing the tech js associated with their ability to influence the cycliza nologies of carbon fiber PAN precursor manufactur tion of PAN which leads to formation of the ladder ing using their own methods to produce acrylic fibers polymer Generally cyclization initiated by the pres In the United States considerable attention was given ence of various defects in the chain structure or resid to viscose carbon fibers therefore initially American yal initiator occurs by the radical mechanism and is producers were unengaged in the elaboration of their accompanied by an intense exothermic effect 20 own technologies but used the finished PAN precur sor manufactured by Courtaulds Exlan or Toray In aot the United Kingdom developments were mostly R CET AL eee focused on military applications rather than civilian a NEN hw S uses subsequently this was the reason for a low com N N N N NNR petitiveness of the carbon fibers produced compared with the Japanese carbon fibers In Russia the pro However in the case of monomers containing duction of acrylic fibers relying on the Courtaulds acidic or amide groups the ionic mechanism may be technology was launched in Saratov in the 1960s At realized cyclization begins at a lower temperature and present even though all main methods of producing proceeds in a wider temperature interval and its total AN copolymers by solution or suspension polymeriza exo effect is reduced 21 age eee eet O Q SN Sn O O SN SN O O SN NH H H In practice either binary copolymers with a neutral COOCH COOCH inert comonomer for example methyl acrylate or CH CH ternary copolymers with addition of a monomer accel 2 2 erating cyclization are used 2225 Every year doz ot AL OY ens of publications appear in which the authors use S Ss SS new monomers to improve the properties of the PAN 0 NH N N O N N NH precursor or seek to increase the degree of stereoregu H larity of PAN 2629 However attempts are also made to synthesize binary copolymers in which the Owing to the interactions of nitrile groups along a comonomer contains simultaneously ester and acidic chain a PAN macromolecule adopts the helical con or amide groups for example 3aminocarbonyl3 formation 31 During cyclization a sequence of con butenoic acid methyl ester 30 jugated pyridine cycles is formed and once a critical POLYMER SCIENCE SERIES C Vol 62 No 1 2020 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 19 length of this sequence is attained according to theo retical estimates it amounts to five to seven units ste ric hindrances hampering further cyclization appear in the chain 32 As a result defects appear in the cyclic ladder structure 17 33 Reduction in the amount of such defects and as a consequence improvement of the physicomechanical properties of carbon fibers are an important challenge One of the ways to solve it consists in the uniform arrangement of neutral not influencing cyclization and accelerating initiating cyclization comonomer units along the chain It is assumed that a higher degree of cyclization allows the polymers to withstand hightemperature carboniza tion and ensures higher strength parameters of the car bon fiber 3436 The first attempt to control the dis tribution of comonomers in a PAN chain was made by JS Tsai et al in 1991 37 in the conventional radical solution copolymerization of AN with methyl acrylate MA itaconic acid and 2ethylhexyl acrylate which were loaded in a reactor at a controlled rate It was shown that the slowed down introduction of comono mers causes a decrease in the degree of crystallinity and the size of crystallites neutral comonomers МА and 2ethylhexyl acrylate increase the initial tem perature of cyclization while the acidic comonomer itaconic acid decreases it Attempts to improve the properties of the PAN pre cursor contributed to the active fundamental research into solution suspension and emulsion homo and copolymerization and precipitation polymerization 3843 At present the main kinetic features of AN copolymerization in various media were ascertained for example the data on the solution polymerization of AN were summarized in 44 The reactivity ratios of AN and a large number of comonomers including vinyl acids and alkyl acrylates were determined 18 4547 The data on the relative reactivity of mono mers in copolymerization and the composition of the monomer mixture make it possible to estimate the average length of AN blocks which exerts a strong effect on the behavior of the precursor for example in thermooxidative stabilization 48 The advent of modern controlled radical polymerization tech niques provided the opportunity to control not only the MWD but also the molecular structure of poly mers 49 To date many experimental data on the methods of preparing acrylonitrile copolymers and the effect of their chemical composition on the procedure and conditions of spinning and thermooxidative stabiliza tion processes have been accumulated However in our opinion the latest achievements in the field of synthesis of AN copolymers call for generalization from the point of view of controlling the structure and properties of carbon fibers precursors and the pros pects of their industrial application The discussion of these issues is the subject of the present review GREEN CHEMISTRY IN THE SYNTHESIS OF ACRYLONITRILE COPOLYMERS The specific feature of AN polymerization is the insolubility of the resultant polymer in its own mono mer even at a conversion of about 10 PAN precipi tates from the monomer solution 50 This fact trig gered the intense development and investigation of both homogeneous in solution and heterogeneous in bulk emulsion suspension and precipitation radical polymerizations of AN However in recent years growing attention has been paid to environmen tal issues this has led to searching for alternative media for polymerization processes Supercritical CO2 and ionic liquids have gained the widest popularity among the socalled green media The former is an environmentally friendly fairly cheap and nonflam mable medium 5153 Ionic liquids in turn occur in the liquid state in a wide temperature range and fea ture high specific conductivity good dissolving ability nonvolatility and incombustibility they are nonex plosive and nontoxic and can be reused 54 Let us consider their application in the synthesis of PAN and its copolymers Polymerization in Supercritical CO2 For carbon dioxide the critical conditions are tem perature Тcr 31С and pressure Рcr 74 MPa 55 In the supercritical state its density is typical of liquids and diffusion and viscosity are characteristic of gases For example the viscosity of supercritical CO2 is almost an order of magnitude lower than those of organic solvents and its selfdiffusion coefficient is of the same order of magnitude as that of gaseous СО2 and one to two orders of magnitude higher than those of organic solvents 56 Owing to these features super critical СО2 readily dissolves monomers and plasticizes polymers At the fixed pressure corresponding to the critical Рcr the phase state of the system supercritical СО2monomer may change depending on the critical temperature of the latter Therefore polymerization may be accomplished under supercritical conditions or in the twophase or singlephase liquid or gaseous state of the system 56 57 The main advantage of using supercritical СО2 is the absence of finished product drying and solvent removal stages The key issue for the wide use of CO2 as a solvent in the synthesis of polymers is that in the supercritical state it is a poor solvent for many poly mers including PAN 58 Therefore the polymeriza tion of AN in CO2 proceeds in the heterophase mode In supercritical CO2 PAN is primarily synthesized by dispersion or precipitation polymerization in these systems the monomer concentration is usually in the range of 1015 wt 5961 20 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al The first information on the polymerization of AN in supercritical CO2 was published in 2000 59 The authors conducted the precipitation and dispersion polymerization of AN in the latter case the stabilizer was polyfluoroalkyl acrylate soluble in supercritical СО2 or its block copolymer with styrene The precipitation and dispersion polymerization reactions with the use of polyfluoroalkyl acrylate prevented the production of particles of the desired morphology The formation of spherical PAN parti cles with the numberaverage diameter in the range of 0205 μm and a narrow particle size distribution PSD at a monomer conversion of about 70 was provided only by the block copolymer Unfortunately the authors presented no data on the kinetics of the process and the MWD of the reaction product These studies were further developed by XR Teng et al 62 63 who investigated the precip itation polymerization of AN in supercritical CO2 These authors varied the concentration of the mono mer and RAFT agent time temperature and pressure and revealed that the polymerization of AN in a monomer concentration range of 614 wt occurs at a low rate and yields PAN with a viscosityaverage MW of 14110 103 Mn 515 103 and dis persion Ð 3032 The monomer conversion is not above 50 The PAN precursor obtained by spinning from DMF solution features a defective structure and low strength characteristics The effect of conditions of AN precipitation polymerization on the morphology of particles was studied in more detail by М Okubo et al 64 The authors managed to look for conditions under which the monomer conversion close to 100 was attained and the polymer with a high degree of crystallinity and a viscosityaverage MW of about 50 103 was isolated The produced micronsized PAN particles were uni form and spherical in shape which was facilitated by the intense mixing of the reaction medium The ideas concerning the precipitation and disper sion polymerization of AN in scCO2 were developed in 61 Polymerization was carried out in a wide pres sure range from 77 to 30 MPa and a temperature of 65С in the presence of a radical initiator AIBN The resulting particles were stabilized using the block copolymer formed from PAN and polyfluoroalkyl acrylate which provided the formation of submicron spherical particles with a narrow PSD With an increase in pressure higher conversions of the monomer and MW of the polymer and the narrow PSD were reached in dispersion polymerization rather than in precipitation polymerization As the pressure was decreased especially in the vicinity of Рcr 77 78 MPa this difference leveled off and in both cases a high molecular weight crystalline PAN with a monomer conversion above 90 Mn 130190 103 and a close PSD was produced It should be expected that in supercritical media the radical polymerization of AN will yield an atactic polymer This was confirmed by the authors of 65 who compared the chain microstructure of PAN formed by precipitation and suspension polymeriza tion reactions in supercritical СО2 An analysis of the triad and pentad composition of the polymers showed that PAN synthesized in supercritical СО2 is an atactic polymer and features the Bernoulli distribution of tri ads and pentads The content of isotactic sequences in it is lower than that in the samples obtained by precip itation and suspension polymerizations The copolymerization of AN in supercritical СО2 was described in 6668 The data on the AIBNinitiated precipitation copolymerization of AN with methyl methacrylate MMA and 2chlorostyrene conducted at 70С and 20 kPa in supercritical СО2 were discussed in 66 Particular attention was paid to the compositions of the ANcomonomer monomer mixture in which the comonomer content was less than 10 mol Under increased pressure the reaction medium remained homogeneous during polymerization for 3 h after wards the polymer precipitated as a separate phase The final product of polymerization was a powder soft solid mass or gel depending on the comonomer nature and copolymer composition With an increase in the fraction of AN in the mixture a powdery prod CH block CH2 n C CH CH2 m O OCH2C7F15 CH co CH2 C C CH2 m O OCH2CH2OC9F17 CH2 block CH CN C CH3 CH3 C O C2H5O CH3 nBr FIBERFORMING ACRYLONITRILE COPOLYMERS 21 uct was formed and the yield of the copolymer was observed for binary and ternary copolymers The increased Unusual is the fact that ifthe molar con advantage of this method was a more effective use of tent of the comonomer is less than 10 the number the reaction volume by 23 times monomer loads average MW of the copolymer is almost independent were up to 50 vol compared with solution or con of the monomer mixture composition and amounts to ventional heterophase polymerization Under these 5060 x 10 and D 35 while at the equimolar conditions finely dispersed spherical particles were composition the MW increases abruptly while the synthesized this simplified purification and subse MWD narrows DP 1520 An increase in the frac quent processing of the polymer However copoly tion of the comonomer Causes a decrease in the degree merization suffers from a substantial drawbackthe of crystallinity of the copolymer and its swelling in copolymer produced exhibits a high compositional CO solution The asplasticized copolymer has a heterogeneity This result is well illustrated by DSC reduced glass transition temperature which also studies The data on the thermal behavior of the binary affects the thermal behavior of the copolymer and ternary copolymers suggest that introduction of a Another example is the precipitation homopoly small amount of an inert monomer MA into a chain merization and binary and ternary copolymerizations causes an unexpectedly sharp reduction in the activa of acrylonitrile with methyl acrylate and itaconic acid tion energy of cyclization compared with PAN while or its derivatives monomethyl and monoethyl itacon addition of the comonomer accelerating cyclization ates and monoamide and mononoctylamide of itaconic acid or its derivatives does not lead to addi itaconic acid in supercritical CO at 6580C and tional reduction in the activation energy of cyclization 40 kPa 67 In homopolymerization an increase in the concentration of AIBN entails an increase in the The synthesis of the block copolymer of P AN with conversion of the monomer and a reduction in the polyvinyl acetate was investigated for the first time MW of the polymer the synthesized PAN exhibits a using cobaltII acetylacetonate CoAcAc as an wide MWD PD 35 An analogous result was example 68 0 Zon Tt ene RCoAcAc supercritical CO 45C R n os AN pn LAN Honan supercritical CO 45C n m O CN MeO where R XY NC os At the initial stage the polymerization of vinyl ace oligomeric polyvinyl acetate As a result the block tate was carried out and it was proved that the process copolymer with M 50 x 10 and D 20 was occurs according to the living mechanism ie the obtained at an AN conversion of 80 MW of the polymer linearly increases with conversion In general the copolymerization of AN in and the dispersion D is below 11 With an increase in supercritical CO may be regarded as a promising the MW of polyvinyl acetate above 10 its solubility technology for the synthesis of AN copolymers in the reaction medium worsens The synthesized The developed methods provide the opportunity to polymer can function as a macroinitiator in the control the MW and composition of the copolymers polymerization of its own monomer and AN In the in a wide range and to synthesize random and block latter case the controlled synthesis requires the use of copolymers Easy removal of the solvent via the POLYMER SCIENCE SERIES C Vol62 No1 2020 22 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al transformation of carbon dioxide into the gaseous state a high conversion of the monomer which enables one to avoid the stage of purification of the final product and the possibility to isolate the final product as a fine powder are the advantages of this synthetic procedure Polymerization in Ionic Liquids The use of ionic liquids in radical polymerization was described for the first time at the beginning of the 2000s 69 At present the data on the effect of ionic liquids on the kinetics and mechanism of polymeriza tion and polycondensation processes are summarized in reviews 7073 The polymerization of AN in ionic liquids may proceed in the homogeneous or heterophase regime 54 73 When salts of 13dialkylsubstituted imidazole where R1 and R2 CnH2n 1 n 14 R1 and n 25 R2 and Y Cl Br BF PF and CF3SO22N are used as ionic liquids the polymerization of 20 and 50 AN solutions occurs under homogeneous condi tions only if R1 CH3 R2 C3H7 and Y Cl or Br 54 In comparison with polymerization in DMF the rate of polymerization and the MW of the polymer are much higher It is believed that this phenomenon is explained by a decrease in the activation energy of chain propagation and termination reactions and a reduction in the probability of occurrence of side reac tions The glass transition temperatures of PAN syn thesized in the ionic liquid and DMF almost coincide while the temperature of the onset of decomposition is much higher and depends on the nature of the ionic liquid At R1 R2 C4H9 and Y PF this tempera ture is 340С and at R1 CH3 R2 C3H7 and Y CF3SO22N it is 280С while for PAN synthesized in DMF this value is 220С It could be assumed that improvement of the thermal stability of the polymers is related to the presence of the residual ionic liquid However the experiments described in 73 are incon sistent with these considerations the presence of even a small amount of ionic liquid leads to a decrease in the glass transition temperature that is it functions as a plasticizer Regardless of the presence or absence of the ionic liquid in the polymerization product the thermal stability of the polymer synthesized in DMF is worse Unfortunately no data are available on the chain microstructure and degree of crystallinity of the polymer Changes in these characteristics could be responsible for these differences Analogous data were reported for the copolymer ization of AN with MMA The rate of copolymeriza tion and the MW of the copolymers decrease with an increase in the fraction of AN in the monomer mix ture An important point is that the compositions of the copolymers synthesized in DMF and the ionic liq uid are different at different conversions This result indicates that the activities of the monomers in copo lymerization in these media are different The same tendency was observed for the homopo lymerization of AN in the solution of 1butyl3 methylimidazolium tetrafluoroborate R1 CH3 R2 C4H9 and Y BF 73 PAN synthesized under these conditions was characterized by Mn 3050 103 and a wide MWD Ð 3 Thus the higher rate and MW in the polymerization of AN are the general phenomenon independent of the structure of the imidazolium salt The most remarkable achievement in this field is associated with controlled atomtransfer radical polymerization 7478 which is stipulated by the use of transitionmetal compounds and the ability of ionic liquids to function as a ligand and to stabilize the cat alyst complex In what follows this issue will be dis cussed in more detail CONTROLLED SYNTHESIS OF ACRYLONITRILE POLYMERS In the past decades along with the development of new technologies for the synthesis of AN copoly mers which are based on the known polymerization mechanisms and polymerization techniques efforts of a large number of research teams have been directed at seeking ways to control the molecular characteristics of PAN This should include studies in the field of anionic polymerization and reversible deactivation radical polymerization Anionic Polymerization Anionic polymerization is of interest to researchers because it can be implemented in the living chain regime that is in the absence of chain termination and chain transfer reactions This technique became known thanks to М Szwarc in the mid1950s 79 At present the living anionic polymerization allows for the synthesis of polymers of complex architecture with the desired MW narrow MWD high stereoregularity and compositional homogeneity 80 The first data on the anionic polymerization of AN were published in the 1950s1960s when it was shown that alkali metals their amides alcoholates and hydroxides organometallic compounds quaternary ammonium bases and metals in liquid ammonia can N N R1 R2 Y 4 6 6 4 FIBERFORMING ACRYLONITRILE COPOLYMERS 23 initiate the polymerization of AN 8189 In our during anionic polymerization at temperatures above country the intense studies of the anionic homo and 0C a colored product is frequently formed this is copolymerization of AN evolved in the 1960s1970s feasible when both conjugated bonds CN and 9098 It was demonstrated for the first time that naphthirydinetype rings are formed CH CH CH CH CH CH or cH CRE OH CN CN CN CN CH CH ap CHa FH CHa CH CH ce CH CN C Cc C HO SN Sy N The main kinetic parameters of anionic polymeriza The search for new initiators is under way In the tion in the presence of initiators of different types were cited papers various issues are being solved namely determined the conditions to suppress chain termina the synthesis of high molecular weight PAN andor tion and chain transfer side reactions were assessed the the search for metalfree polymerization initiators solidphase Pp olymerization of AN was described and a For example the authors of 108 synthesized PAN high molecular weight P AN with M 2x 10 Was syN with M 10 and D 1922 by polymerizing AN in thesized It was shown that in anionic copolymerization DMF in th f lithium amides obtained b acrylonitrile is more active than vinyl monomers for the j mn the of butvlli hi ith ami dii y example MMA e interaction of butyllithium with amines iisopro pylamine diethylamine hexamethyldisilazane dicy When industrial technologies relying on radical pro jghexylamine and 2266tetramethylpiperidine cesses were used for the synthesis of PAN and ANbased copolymers the interest in the anionic polymerization of Bul AN declined However these studies persist albeit less RNH aE RNLi intensely than the study of radical processes One of the longstanding issues in the anionic The authors managed to find conditions under polymerization of AN which researchers tried to which side reactions of active center isomerization were resolve is the synthesis of stereoregular PAN These almost suppressed Subsequently these authors attempts were made many times but all of them extended this work and conducted the anionic polym resulted in the synthesis of atactic and frequently zation of AN in a flow microreactor using the same branched rather than linear polymers 99103 It was initiators Asa result a polymer with a narrower MWD assumed that the reason behind this failure was the D 15 albeit a lower MW M 400800 x 10 ketenimine structure of the growing anion 104 The was prepared 109 polymerization of AN catalyzed by magnesium dial The singlecomponent bimetallic initiator kyls in a nonpolar solvent toluene or xylene atatem nOCHCHNMeOH Nag Ln Yb Nd perature above 100C may be considered a good solu ang Sm was proposed in 110 This initiator was effi tion to this problem 105 Using several dozen initia Gjent ina wide temperature interval from 78 to 50C tors these authors performed the systematic research polymerization was completed within 10 min and into effects of the nature of the RAFT agent the tem yielded atactic PAN with a viscosityaverage MW of A rane of synthe SIs ane the re and P olny or 2040 x 10 The rate of polymerization and the yield the solvent and showed that only magnesium dialky s of the polymer increased with an increase in the polarity can improve the stereoregularity isotacticity of PAN of the solvent DMF toluene THE hexane decrease in the temperature of synthesis leads to an increase in the fraction of heterotriads that is causes Along with standard approaches anionic polymer the loss of regularity of the chain structure The follow ization was initiated for example by the electrochem up of this work is the study of the synthesis of highly iso ical generation of anion radicals using benzophenone tactic PAN by dibutylmagnesiumcatalyzed precipita and 22bipyridyl 111 The authors proved the tion polymerization in xylene 106 Recently the stere anionic mechanism of reaction and focused on study oregular copolymers of AN acrylamide and itaconic ing the chain propagation reaction determined its rate acid were prepared by templateassisted solid phase constant and estimated effect of the type and concen polymerization using nickel or magnesium chloride tration of the supporting electrolyte nC4H94NCIO anhydrous salts as templates 107 or NaClO and initiator on its value POLYMER SCIENCE SERIES C Vol62 No1 2020 24 CHERNIKOVA et al The first papers describing the use of metalfree 40C the monomer conversion close to 100 was initiators were published in the early 1990s 112 It attained over a period of 24 h Unfortunately under turned et that ney AN H were unable to es the chosen conditions the MW of the resulting poly this nility when com lexe d wi thepox de acquire mer did not exceed 104 and its MWD was fairly wide y P P The wide MWD could be attributed both to slow initi R3N R CH CH Rp CH CH NR ation and to formation of the branched polymer O O Recently this direction has been further elaborated by Russian researchers from the Institute of Problems of This effect was demonstrated using 14diazabicy Chemical Physics Russian Academy of Sciences 113 clo222octane and various cyclic oxides If glycidyl U4 There th h ded ini ine thi methacrylate or allyl glycidyl ether was used as an There the authors succeeded in improving this Pro epoxide then a macromonomer capable of polymer SS It was shown that the products of ethylene oxide ization was obtained It is significant that polymeriza interaction with bicyclic amines containing tertiary nitro tion occurred under mild conditions in DMSO at gen atoms at the vertex of the bicyclic structure CH N Hc 2 CH CH H2C Ho cH N CH NT CH 7 an Hac HC HyC2 7 CH SO CH Sy CH Ni HC ay NO HC NO 2 are efficient initiators of the anionic polymerization of Another example of a metalfree initiator is tetraal acrylonitrile The polymerization of acrylonitrile initi kylammonium salts for example tetrabutylammonium ated by the system ethylene oxidebicyclic amine in carbazolide 115 However in their presence a colored the polar solvent DMSO at room temperature occurs polymer that is PAN containing conjugated bonds is under homogeneous conditions within several min formed The authors of 116 used samarium diodide utes while in a weakly polar solvent THF it proceeds for the polymerization of AN asa result PAN with M at a lower rate under heterogeneous conditions over 35 x 103 was synthesized The assumed mechanism of several hours Furthermore polymerization success yolymerization includes formation of the AN anion fully proceeds in a mixture of these solvents in a wide jadical which undergoes recombination to forma dian temperature range from room temperature to below jon on which subsequently chain propagation occurs zero values The proposed initiating systems made it possible to considerably increase the MW of the polymer Later trivalent phosphorus compounds which are M 25480 x 10 while preserving the typical val mild bases and are able to add to the CC bond of AN ues of dispersion D 155340 related to intra and without abstracting a monomer proton were proposed intermolecular chain transfer reactions The synthesis of as initiators In contrast to tertiary amines phosphines AN copolymers with ethylene oxide MA ethylacrylate are capable of initiating the anionic polymerization of and dimethyl itaconate was less effective the yield and AN For example triphenylphosphine was used in the MW were smaller than those of the homopolymer It polymerization of AN However the MW of the poly should be noted that the exploration of these initiating mer was fairly low 117 The authors advanced the zwit systems is still at the initial stage because there are no terionic mechanism of polymerization and assumed data on the kinetic features of polymerization the topol that the active center capable of initiating further ogy of macromolecules and their stereoregularity polymerization is formed on a monomer NC CN te PPh CH CH Ph3P OCH CIDE CHTCN ph3P JCH ON CH5 CHCH CN nerechcN Ph3P CH cut CH CHCH CH CN CN CN POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 25 These ideas were further developed in 118121 of 30 is suitable for fiber formation Tetraethylam For example it was shown that the high molecular monium bromide accelerates polymerization probably weight PAN is formed in the presence of triethylphos owing to the rapid exchange by cations and the phite at room temperature in DMF solution The replacement of the phosphoruscontaining cation resultant spinning solution with a PAN concentration with the ammonium one Br EtsNBr EtO3P CHCH CH CH CH CH EtO3P CHCH CH CH CH CHNEty CN CN CN CN CN CN Initiators based on tricyclohexylphosphine were the additionfragmentation mechanism RAFT 124 also efficient in polymerization The main features of these processes and their potential Thus the metalfree initiators of anionic polymer 2 discussed in reviews 125128 In what follows we ization are of potential interest for use in practice will consider their application to the controlled synthe Progress was achieved in the synthesis of not only a sis of PAN and ANbased copolymers high molecular weight homopolymer of AN but also its copolymers A significant limitation of this area of Polymerization in the presence of stable and low research is the sensitivity of such initiators tothe func activity radicals Reversible inhibition is based on tional groups of comonomers which could hardly be the reversible termination reaction of macroradicals overcome without resorting to complicating tricks to protect the required group P with low molecular weight lowactivity or sta ble radicals Xstable radicals iniferters or organocobalt compounds 129 Reversible Deactivation Radical Polymerization Radical polymerization may be living that is pro ceed in the terminationless regime only in excep tional cases 122 However the features of living PX PxX anionic polymerization can be imparted to radical polymerization The key idea of such radical polymer ization which is generally referred to as reversible deactivation radical polymerization is that special Reversible inhibitors 130 and iniferters 131 are compounds able to reversibly interact with propagat rarely used in the polymerization of AN compared ing radicals are introduced into the polymerization with other controlled radical polymerization proce system 123 As a result a macroradical becomes dures because the dissociation of bond PX needed inactive dormant but under the reaction condi for the activation of macroradicals is hardly possible tions it activates revives and continues to grow until for most of the known stable and lowactivity radicals it again reacts with the additive Thusa macromolecule An exception is the reaction of copolymerization with grows in the stepbystep manner the more often it 4 more active monomer that with a high probability Tevives and falls asleep the closer the features of occurs at the end of a propagating radical and forms a radical polymerization to the living anionic One jabile bond with radical X Depending on the procedure used to activate macro molecules there are reversible inhibition reversible Nevertheless in the past 1020 years certain atom transfer and reversible chain transfer according to progress has been achieved in this area For example To a polyurethane macroiniferter based on tetrapheny In the domestic literature the terms pseudoliving quasiliv lethane was used for the polymerization of AN in ing living and controlled polymerization have been used to date DMF in the temperature range of 6080C 132 POLYMER SCIENCE SERIES C Vol62 No1 2020 26 CHERNIKOVA et al CH 0c0NH nee CH 0CHCHCHCHC0CNH nee I O CN CN O O 0 The rate of polymerization shows the first order More fascinating results were obtained in the copo with respect to the macroiniferter concentration and lymerization of AN with vinyl monomers the oneandahalf order with respect to the monomer a concentration Taking into account that the reaction The TEMPOmediated copolymerization of sty orders from 0 to 05 are typical of iniferters 133 the rene and AN was described in 138 The azeotropic above data indicate that the mechanism of the process copolymerization 25 wt AN proceeded according is more complex than the reversible dissociation of to the living mechanism up to high conversions in the bond ANCPh as supposed by the authors The narrow temperature range 110130C and was charac process is characterized by features of the living pro terized by the linear growth of M with increase in cess the M of the polymer linearly grows with an jonomer conversion and a narrow MWD D 15 increase in monomer conversion and the MWD ofthe 1 6 This outcome was explained by a high content of reel wh aut of p on ee PAN 1S fairly dos tow D styrene in the comonomer mixture and its higher reac 16 ent synt osize Y 1S Teused as a Macro tivity in copolymerization ran 008 and Fevrene iniferter its efficiency is low it is slowly consumed y during polymerization as a result even at high con 043 Asa consequence styrene which occurred at versions the MWD of the polymer is bimodal the end of the growing chain in a wide conversion A d to inif ble nitroxide radical range provided realization of the reversible inhibition S opposed to Iniferters stable nitroxide radicals 1 echanism These data are confirmed by independent and nitrones are more efficient in the controlled syn y P thesis of polymers 134 135 Among nitroxide radi quantumchemical calculations of dissociation energy cals 22 b te trame thyl1piperi dinvloxy TEMPO for alkoxyamine adducts with styrene and AN in terms and its derivatives are used most frequently However of the terminal and penultimate models as well as no remarkable success was achieved in the homopoly Par steric and resonance effects 139 merization of ed By nitroxide radicals ine Owing to the progress in the chemistry of nitroxide presence 6 TEMP O oon at high temperatures above radicals and the appearance of new radicals and their 120C polymerization decayed in the course of time adducts the content of AN in the copolymer was and the ultimate conversion was not above 1520 increased For example the authors of 140 used Nvert 136 The replacement of TEMPO with alkoxyamines butylcisopropylnitrone for the synthesis of random functionalized by a succinimide ester group did not and block copolymers of styrene and AN The copoly enable one to increase the MW M 14 103andD mercontaining 40 mol AN with M 355 x 10 and 114126 and yield of the polymer 137 D 13 was synthesized using an unusual trick POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 27 Uncontrolled X yr polymerization 85C 4h p SN C o O Generation of nitroxide radicals CN i Controlled polymerization Initially nitroxide radicals were generated at alower ternary copolymerization of AN styrene and MMA temperature in the presence of styrene and benzoyl per mediated by two nitroxides TEMPO and SG1 142 oxide Once nitroxides were accumulated in the reac 143 It was shown that for comonomers with differ tion medium AN was added and temperature was ent activities the equilibrium constant indicating the increased This trick allowed the authors to control the possibility to implement the living mechanism is MWand MWD of the polymer Moreover the obtained determined by the equilibrium constant of the mono copolymer functioned asa macroinitiator and provided mer more active in copolymerization Among the formation of the block copolymer with styrene three chosen monomers the activity changes in the Owing to a stronger bond in the PANnitroxide following sequence styrene MMA AN For exam adduct an interesting application of small additives of ple in azeotropic terpolymerization 52 mol sty AN in the nitroxidemediated polymerization of MMA rene 18 mol MMA and 30 mol AN the revers was found 141 The main problem encountered in real ible inhibition mechanism is realized owing to styrene ization of the living mechanism of nitroxidemediated Given this SG1 ensures a better control over MWD methacrylate polymerization is related to the reaction of than TE MP Ob fa hich val f th lib disproportionation between a propagating radical and van cease Of a Men value oF le equrl nitroxide However when nitroxide SG1 Nertbutyl U constant Asa followup of this concept gr adient N1diethylphosphono22dimethylpropyl nitrox copolymers were synthesized from the monomer mix ide or its adduct with methacrylic acid BlocBuilder ture consisting of 30 mol styrene 10 mol MMA was introduced into the reaction mixture containing and 60 mol AN with participation of the same MMA the use of even a small amount of AN enabled nitroxides Using the adduct polystyreneSG1 M the authors to almost fully suppress this reaction 52 x 10 gradient copolymers with M up to 40 x 10 and D 13 were synthesized YY On the whole it should be recognized that the YY reversible inhibition method is poorly suitable for the NO N0 synthesis of AN copolymers designed to produce car o 0 COOH bon fiber precursors This method requires the use of a 7 P Ya high temperatures and is characterized by a consider Eto SOFt EtO OEt able duration of the process and a low yield of the polymer moreover it does not make it possible to pre SG1 BlocBuilder pare copolymers with the AN content above 80 mol and a high MW This result can be explained by the fact that the Atom transfer radical polymerization ATRP This constant of dissociation of the adduct decreases by process is based on the reversible interaction of several orders of magnitude on transition from organometallic compounds primarily transition PMMAnitroxide to PANnitroxide metal halides with alkyl halides to generate a radical Later ways to control the living regime of polymer able to interact with a monomer and to provide chain ization were elaborated by MYu Zaremskii for the propagation 144 POLYMER SCIENCE SERIES C Vol62 No1 2020 28 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al where Мt is the transition metal with valence n Hal is the halogen L is the organic ligand R is the alkyl or aryl M is the monomer and P is the macroradical The evolution of approaches to the implementation of atomtransfer polymerization namely normal and reverse atomtransfer polymerizations initiators for continuous activator regeneration atom transfer radi cal polymerization ICAR ATRP 145 activators regenerated by electron transfer for atomtransfer rad ical polymerization AGET ATRP 146 single elec tron transfer living radical polymerization SETLRP 147 electrochemically mediated atomtransfer radi cal polymerization eATRP 148 photoinduced metalfree atomtransfer radical polymerization metalfree ATRP 149 and photoinduced electron transfer atomtransfer radical polymerization without using organometallic compounds PETATRP 150 is accompanied not only by seeking conditions for the controlled synthesis of polymers but also by widening the scope of monomers catalysts and reaction media The main advances in the synthesis of acrylonitrile homopolymers and copolymers by atomtransfer polymerization were summarized in two review papers 151 152 Unfortunately the structure and properties of the copolymers were not discussed in these reviews This is probably caused by the absence of cor responding information in the publications These reviews mainly concern effects of nature of the cata lyst initiator ligand activator and solvent on the rate of polymerization and molecular weight characteris tics of the synthesized polymers It should be emphasized that first attempts to apply this method to the synthesis of PAN were made at the fall of the 1990s 153 154 The initial achievements did not go beyond the synthesis of the polymers with a low MW less than 104 and a narrow MWD However at the beginning of the 2000s conditions for the syn thesis of a higher molecular weight PAN were ascer tained 155160 To date a wide variety of catalysts mostly based on copper and iron salts and appropriate components initiators ligands reducers has been proposed for the synthesis of PAN and AN copolymers with a wide range of MW and narrow MWD this infor mation was summarized in the cited reviews 151 152 At present the main efforts in the synthesis of PAN by ATRP are reduced to seeking conditions for the synthesis of high molecular weight PAN with a narrow MWD macromolecular design and creation of new architectures metalfree catalysts and new reaction media The replacement of metal compounds with photoin duced ATRP metalfree ATRP is a novel direction 161 162 For example the photoinduced polymeriza tion of AN was accomplished using aniline and pheno thiazines for example 10phenylphenothiazine 1 10 4methoxyphenylphenothiazine 2 or 101naph thalenylphenothiazine 3 The key idea of this mechanism consists in the photoactivation of phenothiazine and its subsequent interaction with alkyl halide to form an alkyl radical and a phenothiazinyl radical cation However as regards acrylonitrile this method still gives rise to oligomers albeit with a narrow MWD It should be pointed out that the existing standard approaches such as reverse atomtransfer polymer R Hal R Mtn 1LxHal Pn Mtn 1LxHal Pn Hal MtnLx MtnLx M n S N S N OCH3 S N 1 2 3 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 29 ization allow for the synthesis of a higher molecular weight PAN for example with Mn 2030 103 and Ð 12 163 As for the synthesis of ultrahigh molecular weight PAN mention should be made of paper 164 in which PAN with the viscosityaverage molecular weight above 106 was synthesized by ICAR ATRP The synthesis of starshaped polymers from PAN or ANМА copolymers containing a small amount of acrylate is of practical interest 165 PAN of the star shaped architecture was obtained by the normal atom transfer polymerization in the presence of polyfunc tional initiators The authors succeeded in the synthe sis of a set of homopolymers and copolymers with dif ferent numbers of arms It was shown that the star shaped PAN Mn 1070 103 did not feature unusual properties in DMSO solution typical of non linear polymers the viscosity of its solution was only slightly smaller than the viscosity of solution of the lin ear PAN In contrast the addition of a small amount of methyl methacrylate considerably increased poly mer flexibility and reduced viscosity For the star shaped copolymer as opposed to its linear counter part the glass transition temperature was registered more distinctly These copolymers may show promise as additives to linear copolymers affecting the rheol ogy of PAN solutions New opportunities of the method in the controlled synthesis of PAN may be illustrated by the ICAR ATRP synthesis of block copolymer PEOblock PAN particles by dispersion polymerization inducing selfassembly 166 This study is of purely academic interest However it extends our knowledge of the ways to implement atomtransfer polymerization A much more promising direction of ATRP involves the use of ionic liquids 167 168 The prog ress of this direction is related to seeking novel green solvents and simplification of the reaction system The use of ionic liquids with a high dissolving ability including inorganic compounds in atomtransfer polymerization makes it possible to reduce the num ber of necessary components of the catalytic system and to facilitate the stage of separation of metal com pounds from the reaction product Another advantage of the mentioned systems is that the ionic liquid and the catalyst may be regenerated and reused The first reports on the application of ionic liquids in the atomtransfer polymerization of AN date back to 2008 169 170 Both the normal and reverse reactions were employed In the first case 1butyl3methylimidaz olium tetrafluoroborate AIBN FeCl3 and succinic acid were used The rate of polymerization in the ionic liquid was higher than that in the bulk In the second case the authors used 1methylimidazolium acetate valerate or caproate FeBr2 catalyst and 2bro moisobutyrate initiator Note that the rates of reac tion in the ionic liquid and DMF solution coincided However despite a good control over MW in both cases only the oligomeric PAN Mn 104 Ð 12 was produced Its MW was increased owing to postpo lymerization that is introduction of a new monomer portion when polymerization was completed Mn 3040 103 and Ð 1213 A variant of ionic liquids used as reaction media is a microemulsion based on ionic liquid it is prepared using ionic surfactants for example cetylmethylam monium bromide Reverse ATRP of the monomer pair ANstyrene was carried out in the microemul sion ionic liquid 1butyl3methylimidazolium hexa fluorophosphate in the presence of a complex of FeCl3 hexahydrate and succinic acid and benzoyl peroxide as an initiator 171 The authors provided evidence that the living mechanism is realized under these condi tions and demonstrated that the copolymer with Mn 36 103 and Ð 13 can be synthesized by the post polymerization of a macroinitiator formed in micro emulsion polymerization The most curious result of this work is that the activities of the comonomers in copolymerization change with the composition of the monomer mixture with an increase in the content of AN in the mixture its reactivity decreases A similar change in the activity of the comonomers was observed in conventional radical copolymerization conducted in the ionic liquid microemulsion 172 In another version of ATRP AGET ATRP in addition to the catalytic complex based on FeBr3 2bromoisobutyrate and ascorbic acid ionic liquids such as 1methylimidazolium acetate propionate and butyrate were used 173 The authors managed to carry out the living polymerization of AN in the absence and presence of oxygen and to synthesize PAN with Mn 20 103 and Ð 12 When PAN was repeatedly used in synthesis the product with Mn 70 103 and Ð 13 was obtained In the case of SET LRP Fe0 and analogous ini tiator and ionic liquids that is 2bromoisobutyrate and 1methylimidazolium acetate propionate and valerate were used and the results were close to those described above 174 In general the combination of advantages offered by ionic liquids over common organic solvents and aqueoussaline media with the advantages of atom transfer polymerization enables one to forecast further growth of interest in the synthesis of ANbased copo lymers by this method to obtain PAN precursors To sum up it can be stated that atomtransfer polymerization albeit more effective in terms of implementation for the synthesis of AN copolymers with the predetermined MW and narrow MWD than reversible inhibition processes also suffers from a number of drawbacks as regards the synthesis of PAN precursors First as in anionic polymerization it requires a high purity of reactants because of a high 30 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al sensitivity of catalytic systems to impurities and sensi tivity of the monomers themselves to catalysts Sec ond the indicated method is inapplicable directly to the synthesis of copolymers containing acidic and amide groups Moreover the purification of polymers from organometallic catalysts that adversely influence subsequent stages of carbon fiber CF production is required However the use of ionic liquids andor the transition to metalfree catalysts can help to solve these problems in the future Reversible additionfragmentation chain transfer RAFT polymerization This process relies on the use of sulfurcontaining compounds of the general for mula RSCSZ Along with chain initiation propagation and termination elementary reactions common for radical polymerization it includes reversible additionfragmentation chain transfer reactions as a result of which the majority of the formed macromolecules contain terminal R and SCSZ groups of the initial RAFT agent 175 In contrast to the reversible inhibition and ATRP processes discussed above RAFT polymerization is tolerant to the functional groups of monomers and solvents and can be performed in a wide temperature range under substantial initiation and UV or γ irradi ation 176 177 At present this is the only controlled polymerization procedure which makes it possible to follow the entire chain from the synthesis of copolymers with the predetermined MW and nar row MWD to the production of precursor and carbon fiber 178 The first report on RAFT polymerization as applied to the synthesis of PAN was published in 2003 179 Polymerization was carried out in ethylene car bonate at 60С with the use of 2cyanoethyl dithio benzoate and cumyl dithiobenzoate as RAFT agents Under common conditions dithiobenzoates retarded the polymerization of many vinyl monomers 180 as result the authors managed to synthesize only the oligomeric PAN with Mn 5 103 and Ð 13 Attempts to synthesize a higher molecular weight PAN by the RAFT technique were made many times 181185 However for a long time researchers failed to exceed the threshold 2030 103 while preserving the narrow MWD furthermore the highest conver sion was not above 5060 The synthesis of PAN with Mn 105 and Ð 15 was described for the first time in 186 To achieve this goal the authors consid erably decreased the concentration of the RAFT agent 42carbazole9carbodithioate2methylpropionic acid phenyl ester and as a consequence the concentration of the initi ator AN RAFT AIBN was reduced from 2500 25 05 to 30000 25 05 Polymerization was carried out in DMSO at AN DMSO 1 2 volvol Surprisingly at such a low concentration of the initia tor AN AIBN 30000 05 within 48 h the conversion of AN attained 60 the viscosityaverage MW was 400 103 and the dispersion was Ð 13 Pn S C S Z R C Z S S R Pn C Z S S Pn R R M Pm Pm S C S Z Pn C Z S S Pm Pn C Z S S Pm Pn O C O C CH3 CH3 S C S N O C O C H3C CH3 S C S N POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 31 Owing to the additional introduction of the Lewis acid AlCl3 001 molar equivalent with respect to the monomer the fraction of isotactic triads in PAN was increased from 25 to 34 Nanofibers with diameters in the range of 2501100 nm depending on the molec ular weight of PAN were obtained by electrospinning from PAN solution in DMSO An alternative way to synthesize high molecular weight PAN is based on photoinduced RAFT polym erization 187 188 The process rests on the ability of a sulfurcontaining compound RSCSZ to undergo homolysis via the CS bond accompanied by generation of an initiating radical R and a less active radical SCSZ which reversibly interacts with active radicals present in the system In fact this is the mechanism of iniferter polymerization advanced by Т Otsu in 1981 it is analogous to the photoactivated polymerization of vinyl monomers mediated by benzyl dithiocarbamate and tetramethylthiuram disulfide 189 190 Obviously in this case there is no need to use the radical initiator and polymerization may be conducted in a wide temperature range because the rate of adduct decomposition is determined solely by the irradiation dose The interest in this direction is probably associated with the fact that appropriate compounds capable of reversible homolysis under mild conditions were revealed Using common mono functional trithiocarbonate 2cyano2propyldo decyl trithiocarbonate PAN with Mn 220 103 and Ð 12 was synthesized at AN RAFT 8000 in ethylene carbonate under irradiation with a wave length of 460470 nm at room temperature 187 The rate of the process was acceptable a 60 conversion was achieved during polymerization over 10 h In another version of this process 188 the authors used 4cyanopentanoic acid dithiobenzoate which participated in the redox reaction in the presence of TiO2 and generated radicals able to initiate photoin duced RAFT polymerization the conditional mecha nism of this reaction is presented in the scheme This process occurred in a miniemulsion and yielded the oligomer with a fairly narrow MWD Unfortunately the authors used a much lower by 40 times concen tration ratio of the monomer and RAFT agent which did not allow the fullest potential of this process to be disclosed PC is photocatalyst If a common photoinitiator is present in the sys tem the conventional RAFT mechanism of polymer ization is realized instead of the iniferter one This procedure was applied in 189 using 1235 tetrakiscarbazol9yl46dicyanobenzene as a pho toinitiator and 2cyanoprop2yl1dithionaphthalate as a RAFT agent When AN was polymerized in DMSO at AN RAFT photoinitiator 1500 1 0075 and at a wavelength of 458 nm for 4 h the mono mer conversion was 60 and the Mn of PAN was 75 103 Similar results were obtained in radiationinduced polymerization mediated by the same RAFT agent in ethylene carbonate at room temperature 190 How ever in this case the dose rate should be controlled in order to avoid the random destruction of the polymer chain This factor was taken into account therefore the rate of polymerization was lower and the conver sion of the monomer was smaller as a consequence the MW of PAN was lower According to the data described above a new strat egy was elaborated for the synthesis of polymers macroinitiators carrying the photosensitive terminal group 191 The authors synthesized the RAFT agent SbenzylO2oxo12diphenylethyl carbonodith ioate which was an efficient RAFT agent and ensured the formation of PAN with a narrow MWD at 70С in DMF solution However under irradiation at 350 nm the terminal group of PAN underwent dissociation and in the presence of 1ethoxy2methylpyridinium hexafluorophosphate the polymeric radical con verted to a macrocation able to initiate polymeriza tion for example of butyl vinyl ether S S S Z Pn S S S Z Pm S S S Z Pn S S S Z Pn Pm S S S Z Pn PC hν PC M M 32 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al According to this approach functional polymers may be synthesized by combining radical RAFT polymerization and cationic polymerization Another fascinating RAFT polymerization proce dure was described in 192 The RAFT mechanism was applied to polymerization in aqueoussaline media NaSCN and ZnCl2 that are used in the com mercial production of PAN In the presence of 4cyanopentanoic acid dithiobenzoate which is fairly stable in alkaline media and AIBN at AN RAFT initiator 2000 1 04 PAN with Mn 60 103 and Ð 14 was synthesized However in a zinc chloride solution the RAFT mechanism was violated probably because of complexation of the Lewis acid with a growing macroradical 193 Thus the techniques for the synthesis of AN homopolymer with a broad range of MW and func tionality of terminal groups and a fairly narrow MWD were developed However from the viewpoint of using PAN as a precursor of carbon fibers it is more import ant to ensure the controlled synthesis of its copolymers with a low content of comonomers in a chain Initial advances in the synthesis of highmolecular weight copolymers of AN are related to studies dealing with the synthesis of ANbutadiene copolymers by the RAFT method in which the fraction of butadiene was above 50 mol 194198 In 194 the nitrile butadiene rubber was obtained by the solution polym erization of AN and 13butadiene in DMAA at 100С in the presence of mono and bifunctional trithiocar bonates and dithioacetate This was the first considerable step forward in the RAFT process because previously these copolymers were synthesized solely by emulsion copolymeriza tion The equimolar ratio initiator RAFT agent or a twofold excess of the former was used under these conditions the soluble product with Mn up to 60 103 was isolated The dispersity of the copolymer increased in the course of polymerization from 12 to 20 This process was investigated in more detail in 195 for azeotropic copolymerization carried out in the presence of the monofunctional trithiocarbonate depicted above three azo initiators and a wide range of solvents such as dimethylacetamide chloroben zene 14dioxane tertbutanol isobutyronitrile tolu ene trimethylacetonitrile dimethyl carbonate aceto nitrile methyl acetate acetone and tertbutyl methyl ether to optimize polymerization conditions while preserving its living mechanism It was found that the solvent nature influences only the rate of initiator decomposition which in turn defines the overall rate of polymerization However when an attempt was made to increase the conversion of monomers the control of MWD worsened and on the contrary a high molecular weight copolymer with a narrow MWD was synthesized under conditions of a low rate of polymerization A solution to this problem was reported in 196 but it is doubtful that it can be real ized under manufacturing conditions The copoly mers were synthesized in the presence of monofunc tional trithiocarbonate containing the alkynyl group S S O O S S O O CH2 CH CN S S O O CH2 CH CN n n hν CH2CHCN S S S S O OH S 11S S S POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 33 The copolymer carrying the terminal alkynyl group was involved in the cycloaddition reaction with 14 bisazidomethylbenzene which resulted in doubling the MW of the copolymer Thus the linear copoly mers with Mn 70 103 and Ð 16 were obtained The same trick that is the combination of RAFT polymerization and click chemistry was applied to the synthesis of branched and network nitrilebutadiene rubbers For this purpose the terpolymer of AN 35 mol 13butadiene 56 mol and propargyl methacrylate 9 mol with Mn 39 103 and Ð 13 was synthesized by the RAFT method and involved in the cycloaddition reaction with 14bisazidometh ylbenzene Later this trick was elaborated and used for the synthesis of linear and starshaped block copo lymers from nitrilebutadiene and nitrilestyrene rub bers 197 Unfortunately in the cited papers there are no data on the properties of the given copolymers However this gap was filled in 198 where the nitrile butadiene copolymers and their block and graft copo lymers with the nitrilestyrene rubber were synthe sized by azeotropic copolymerization mediated by monofunctional trithiocarbonate The copolymers of butadiene and AN exhibited two glass transition tem peratures Тg 24С which is typical of the AN butadiene random copolymer containing 38 mol AN units and Тg 66С which is close to that of polybutadiene obtained by the radical polymerization and containing cis14 trans14 and 12 units Tak ing into account a fairly narrow MWD of the copoly mers this result provides evidence for their block gra dient structure According to the DMA data the block and graft copolymers AN butadiene and styrene feature three relaxation modes in the range of 70 to 80С and 2 to 6С corresponding to the phase of the soft block the ANbutadiene copolymer and in the range of 90105С corresponding to the phase of the rigid block the ANstyrene copolymer In the synthesis of AN copolymers styrene methacrylates vinylpyridine and other copolymers are often used as comonomers For example the cumyl dithiobenzoatemediated RAFT copolymer ization of styrene and AN in DMF yielded random and diblock copolymers with a controlled MW and narrow MWD Ð 1315 199 In a later paper 200 the activity of styrene and AN was studied in the bulk RAFT copolymerization mediated by 2cyano2 propyl dithiobenzoate and it was demonstrated that the apparent activities of the monomers change This fact may be explained by the selective solvation of the monomers by the active center For example at a molar fraction of AN in the monomer mixture of 09 the reаctivity ratios of the monomers are rAN 081 S O O S S 11 and rSt 009 while at molar fraction of AN of 01 rAN 02 and rSt 035 This circumstance should be taken into account when synthesizing copolymers with the desired distribution of units in a chain The synthesis of compositionally homogeneous binary copolymers of AN with styrene МА and n and tert butyl acrylate through their bulk copolymerization mediated by bifunctional trithiocarbonate dibenzyl trithiocarbonate as well as of random block copoly mers from the same monomers was described in 201 According to the authors copolymerization and block polymerization in bulk occur under homogenous con ditions at a comonomer content of 20 mol or above and the resultant copolymers are characterized by Mn 2040 103 and Ð 1213 The targetoriented synthesis of PS and its block copolymer with AN having a bimodal MWD was accomplished in 202 The authors used a mixture of monofunctional and bifunctional trithiocarbonates It should be emphasized that this result is very nontriv ial because functionality of the RAFT agent should exert no effect on the MW of the polymer Unusual synthesis of ANbased block copolymers was described in 203 Initially poly4vinylpyridine containing the terminal trithiocarbonate group was prepared then it was used as a polymeric RAFT agent in the copolymerization of the azeotropic mixture of styrene and AN in solution and emulsion In both cases the block copolymers with a narrow MWD were formed in the latter case a stable dispersion of block copolymer particles was obtained A similar approach was used in 204 the block copolymer was prepared by dispersion polymerization in water using dithiobenzoateterminated polyeth ylene oxide as a RAFT agent As a result the disper sion of block copolymer particles with a diameter of 5080 nm was produced in which the polyethylene oxide block functioned as a stabilizer and the PAN block formed the core of particles Other monomer pairs also attracted the attention of researchers For example the alternating copoly merization of βpinene rβpinene 0 and AN rAN 066 mediated by 2cyanopropyl2yl dithiocarbon ate at 60С in dichloroethane was carried out 205 The living mechanism was observed only at the β pinene content not above 25 mol and low conver sions subsequently the RAFT mechanism was vio lated because of the degradative chain transfer to the monomer Upon addition of the Lewis acid Et2AlCl to the mixture the tendency to the alternation of units became more distinct Using the same RAFT agent in DMF the authors of 206 synthesized the copolymers of AN and 236methyl4oxo1234tetrahydropyrimidin2 ylureido ethyl methacrylate no more than 10 mol which were able to efficiently bind Hg2 ions from aqueous solutions A satisfactory control of MW was 34 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al achieved and a set of copolymers with Mn in the range of 2040 103 and Ð 1216 was synthesized Linear and combshaped thermoresponsive block copolymers of AN and Nisopropylacrylamide were synthesized by the combination of RAFT polymeriza tion and atomtransfer polymerization 207 Linear block copolymers were obtained by RAFT polymer ization and starshaped polymers were prepared by atomtransfer polymerization from the brominated copolymer of AN and 2hydroxyethyl methacrylate synthesized by RAFT polymerization In both cases block copolymers featuring a narrow MWD and pos sessing thermal sensitivity and surface wettability were obtained Starting in 2011 researchers not only developed methods for the controlled synthesis of PAN and its copolymers but also began to examine its properties including the behavior of copolymer solutions and the cyclization and thermooxidative stabilization processes These studies enabled one to gain insight into effects of the narrow MWD compositional homogeneity of the copolymers and control over dis tribution of units along the chain on the properties of the PAN precursor The effects of MW MWD width comonomer nature and copolymer composition on the thermal behavior of copolymers synthesized by RAFT polym erization were investigated in detail in 208215 It is well known that synthesis conditions ie sol vent nature polymerization temperature and initiator concentration influence the processes of the thermo oxidative stabilization of PAN 216 The reason behind this phenomenon may be the plasticizing effect of the residual solvent different MW of the polymer and impurities of the unreacted RAFT agent 217 218 The first experiments showed that PAN with a narrow MWD but with Mn 20 103 is involved in cyclization at a temperature 4050С lower than PAN with a wide MWD and a higher MW synthesized by conventional radical polymerization in the same sol vent 219 The behavior of the oligomeric PAN of the same MW depends on the nature of terminal groups R of the initial RAFT agent in the case of the benzyl ter minal group cyclization begins at a higher tempera ture compared with the ester group However as the MW of the polymer is increased this dependence van ishes On the contrary for PAN with a high MW and narrow MWD RAFT cyclization begins at a higher temperature and proceeds in a narrower temperature interval compared with PAN with a wide MWD classics 213 Systematic research into the effect of MW and MWD width unveiled an interesting ten dency the activation energy of the cyclization reaction for PAN with a narrow MWD is 4090 kJmol lower than that for PAN with a wide MWD and the value of the preexponent which makes it possible to indi rectly evaluate molecularity of the reaction differs by five orders of magnitude or more ie the mecha nisms of cyclization reaction for these polymers are mostly likely different In addition the exo effect of the cyclization reaction for PAN with a narrow MWD in higher by a factor of 25 It appears that reduction in the synthesis temperature due to the use of redox or radiation initiation additionally contributes to a decrease in the amount of defects in the polymer chain such as branchings and shifts the exo effect to high temperatures and its intensity grows 218 Thus it can be argued that the controlled radical polymeriza tion decreases the amount of defects in the structure of macromolecules this entails a more avalanche occur rence of cyclization in homopolymers When analyzing the thermal behavior of copoly mers the task is complicated by the fact that in con ventional radical copolymerization the conversion compositional heterogeneity is added to the wide MWD Therefore RAFT polymerization which is able to eliminate both undesirable factors is a useful tool for studying the cyclization of AN copolymers A comparison of the thermal behavior of binary copolymers of AN with inert monomers such as МА nbutyl acrylate and MMA with a narrow MWD Ð 1314 suggests that as in conventional copo lymers when the content of the comonomer is increased from 2 to 10 mol cyclization begins at higher temperatures and the intensity of the exo effect decreases 210 220 Another situation is typical of the AN copolymers with tertbutyl acrylate which are able to split isobuty lene at elevated temperatures As a result acrylic acid units which initiate cyclization appear in a macro molecule the higher the content of tertbutyl acrylate in the copolymer the more pronounced the shift of the exo effect to low temperatures 220 When comparing the thermal behavior of the copo lymers containing units of monomers accelerating cyclization acrylamide or acrylic acid which were synthesized by the RAFT technique and conventional radical polymerization the same tendency was detected as that for the homopolymers the tempera ture interval of cyclization narrowed and shifted to higher temperatures 212 218 221 In such copoly mers both cyclization mechanismsionic lowtem perature and radical hightemperatureare opera tive 1625 A higher compositional homogeneity of the copolymers synthesized by the RAFT technique and their narrow MWD at certain copolymer compo sitions lead to a change in the contributions of these mechanisms and affect the conjugation chain sequence during cyclization 212 In addition to the average composition of the copo lymer its properties are influenced by the character of distribution of units along the chain For the AN copolymers this parameter determines the length of conjugation chain segment that is the defectiveness and properties of the thermally stabilized PAN precur sor Studies in this direction with the use of both the POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 35 conventional radical polymerization 222 and the RAFT process 223230 are being conducted In the first case the attempt to gain insight into the effect of the length of the sequence of AN units in the ANMA copolymer on cyclization processes was made for the synthesis of the copolymers by conven tional radical copolymerization at early monomer conversions 222 This made it possible to prepare a set of copolymers with the average length of the sequence of AN units varying from 13 to 5 An increase in the content of methyl acrylate that is a decrease in the length of the segment of AN units caused reduction in the average length of the conju gated structure increased the activation energy of oxi dation and decreased the content of oxidized struc tures in the copolymer subjected to stabilization The degree of cyclization changed nonmonotonically and passed through a maximum at a content of methyl acrylate units in the copolymer of 8 wt With an increase in the degree of cyclization the thermal sta bility and the amount of coke residue in the polymer increased As far as the RAFT process is concerned these studies were preceded by the works of Chinese researchers who advanced the theoretical model of the semibatch RAFT copolymerization to provide the same distribution of monomer units along the entire length of the polymer chain 223 This model was verified by the synthesis of copolymers of styrene and butyl acrylate 224226 As applied to the AN copolymers this trick was used in 221 227 In the RAFT copolymerization of AN r1 039 and Nisopropylacrylamide accelerating cyclization r2 072 carried out in ethylene carbonate at 30С in the presence of a lowtemperature azo initiator and 2cyano2propyldodecyl trithiocarbonate the sec ond monomer was dosed at a chosen rate during the reaction 227 In the polymerization product the fraction of the second monomer was 2564 mol According to 227 a slow introduction of Nisopro pylacrylamide on one hand and its higher concentra tion on the other hand contribute to reduction in heat release during cyclization and widening of the temperature interval of cyclization due to the shift of its onset to the lowtemperature region compared with the simultaneous introduction of the monomers in synthesis In general a more uniform distribution of a comonomer accelerating cyclization in a chain increases the rate of cyclization and improves the ther mal stability of the polymer during heating to 700С under an inert atmosphere The copolymerization of AN rAN 049 with another monomer accelerating cyclization acrylic acid rAA 25 was conducted at 55С in DMSO at different rates of introduction of acrylic acid into polymerization using potassium persulfate as an initi ator and 2dodecylthiocarbonothioylthio2meth ylpropionic acid as a RAFT agent 221 The copoly mers having similar average composition and MW but different distribution of units in a chain demonstrated different thermal behavior For example an increase in the time of introduction of acrylic acid into copoly merization caused a rise in the intensity of the high temperature peak corresponding to the radical mech anism of cyclization which was related to the length ening of the sequence of AN units If acrylic acid was introduced in the reaction at a higher rate the contri bution of the ionic mechanism of cyclization in the copolymer increased and the total exo effect decreased The chain microstructure also influenced the activation energies of ionic and radical cyclization reactions Thus at present the methods to tune the thermal behavior of AN copolymers by controlling their MW MWD composition and distribution of units in a chain using RAFT polymerization have been devel oped The abovementioned factors also affect the rheol ogy of polymer solutions Knowledge of these data provides basis for choosing the regimes of PAN spin ning The flow curves of PAN and its binary with sty rene or acrylamide and ternary copolymers with methyl acrylate and itaconic acid in such solvents as DMSO DMF or DMAA which are often used in solution spinning are typical of polymers 212 228 230 These are Newtonian fluids in a wide concentra tion range at high shear stresses determined by the MW and MWD of the polymer transition to an unsta ble flow regime occurs However a narrower MWD of the copolymers synthesized by RAFT polymerization is the cause of their lower viscosity at the same poly mer concentration in solution compared with the con ventional polymer Moreover on the flow curves the region of the Newtonian behavior the highest Newto nian viscosity is observed in a wider shear rate range An examination of the dynamic properties of solutions of RAFT polymers indicates that gelation occurs later compared with conventional polymers Thus the main advantage of the RAFT method in terms of spin ning is that polymers with a narrower MWD can be synthesized which makes it possible to use more vis cous polymer solutions for fiber spinning There is no doubt that the studies which cover the entire chain extending from the RAFT synthesis of the polymer to the production of the carbon fiber on its basis are of greatest interest Only two examples of such studies are available in the literature 210 211 214 However they illustrate wide prospects offered by the RAFT method The ANMMA copolymer with a MMA content of 4 mol Mn 150 103 and Ð 17 was synthe sized in ethylene carbonate using 2cyano2propyl dodecyl trithiocarbonate 210 The fibers were wet spun from a 24 copolymer solution in DMF PAN precursor fibers with a dense structure without voids and circular cross section were spun for these fibers 36 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al the breaking strength was 4050 cNtex the elastic modulus was 9001000 cNtex the density was 1178 gcm3 the degree of crystallinity was 80 and the degree of orientation was 87 The authors chose the conditions for the thermooxidative stabilization of the PAN precursor and carbonization and produced the carbon fiber with a breaking strength of 25 GPa The terpolymer of AN МА and itaconic acid with Mn 300 103 and Ð 12 was synthesized by 2cyano2isopropyldithiobenzoatemediated RAFT polymerization in DMSO the PAN precursor was obtained from this terpolymer and the carbon fiber was spun 214 For comparison analogous operations were performed for the terpolymer synthesized by conventional radical polymerization PAN was pre pared according to a nonstandard approach Polymer ization was accomplished by setting a certain tempera ture profile of the reaction It included a short initial period less than 30 min during which polymeriza tion was carried out at an elevated temperature above 65С and a long period 1520 h when tempera ture was maintained below 60С As a result a mono mer conversion of 60 atypical of dithiobenzoate mediated polymerizations was attained The fibers were wet spun from 15 DMAA solu tion Owing to a narrower MWD of the RAFT poly mer the viscosity of its solution was much lower and the structure of the fiber was denser The mechanical characteristics of the PAN precursor obtained by the RAFT method were much better than those of the classical precursor For example for the former PAN precursor the breaking strength was higher by 50 and the elastic modulus was higher by 40 Thermooxidative stabilization was conducted in the temperature gradient from 208 to 255С and carbon ization was performed in an inert atmosphere in the temperature range of 4001350С Upon carboniza tion the mechanical characteristics of the carbon fiber produced from the RAFT polymer remained higher than those of the reference sample Thus control over the molecular structure of AN copolymers under conditions of the RAFT process may be considered a promising direction for the pro duction of highmodulus and highstrength carbon fibers ACRYLONITRILE COPOLYMERS CAPABLE OF MELT SPINNING A factor limiting expansion of the sector of civilian applications of carbon fibers is their fairly high cost Therefore alternative more economically sound methods for the synthesis of PAN precursors have been extensively developed in past decades Among them certainly the leading position belongs to the melt spinning rather than solution spinning of fibers This method does not require the use of organic sol vents and allows for a considerable increase in the con centration of the polymer during spinning Moreover acrylonitrile copolymers intended for obtaining pre cursors by solution spinning are unsuitable for the method This is due to the fact that these polymers are distinguished by a fairly high MW and a low content of the comonomers as a result the processes of cycliza tion in them begin earlier than transition to the viscous flow state There are two main strategies to reduce the melting temperature of PAN the synthesis of copolymers with the predetermined molecular characteristics and the physical modification of the copolymerplasti cization 231 232 In the first case primarily certain comonоmers should be introduced into the polymer in an amount sufficient for distortion of the crystalline structure of PAN Furthermore the MW of the poly mer should be reduced to critical values usually Mn 50 103 In the second case organic compounds such as Nacetylmorpholine ethylene carbonate alcohol and water are used as plasticizers The meth ods of synthesizing of PAN capable of melt spinning will be considered below These copolymers are synthesized by conventional radical polymerization in solution suspension and emulsion and precipitation polymerization in water 233236 The latter process is more promising from a manufacturing point of view because it affords an easytoisolate product requiring no additional purifi cation The MW of the polymer is primarily controlled by using chaintransfer agents and varying the tem perature of synthesis and the ratio of the molar con centrations of the monomer and initiator Mercaptans for example dodecyl mercaptan taken in an amount of 005070 mol 234 and less frequently isopropa nol 233 are most often employed as chaintransfer agents The first patent disclosing the synthesis of the ANmethacrylonitrile copolymer which passes to melt without any change in chemical structure cyclization was published in 1996 237 four decades since these copolymers were mentioned for the first time as PAN precursors 238 Active studies in the field of synthesis and characterization of similar copo lymers began only in the 2000s All known AN copo lymers capable of melt spinning may be divided into four groups the binary and ternary copolymers of AN and MA the terpolymers of AN МА and a third comonomer containing a photosensitive functional group the copolymers of AN with vinylimidazole and the copolymers of AN with other vinyl monomers The detailed studies of the copolymerization of AN with МА to yield melt spun precursors were reported in 233 234 239241 It was shown that at the MA content less than 10 mol even at Mn 20 103 the copolymer cannot be melt processed The lifetime of the melt in this case is fairly short because the temperature of the onset of cyclization and formation of the ladder structure is POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 37 close to the flow temperature and already several minutes after melting the viscosity begins to grow as a result of cyclization Above this threshold value 10 mol МА the properties of the copolymer such as dynamic viscosity and storage modulus change substantially at elevated temperatures 234 For example at 220С the dynamic viscosity of melt and the elastic modulus of the copolymer containing 10 and 15 mol МА decrease by almost four orders of magnitude compared with the copolymers containing 27 mol МА An increase in the content of МА widens the temperature interval in which the copoly mer can be melt processed and decreases the activa tion energy of viscous flow and the temperature of transition to the viscous flow state The dynamic vis cosity of melt is much more sensitive to the MW of copolymers than to their composition at 220С the viscosity drops by three orders of magnitude as the MW of the copolymer decreases by an order of magni tude from 115 103 to 9 103 An analogous decrease in viscosity can be achieved by adding a low molecular weight copolymer to the high molecular weight one The stability of melt is determined not only by the temperature but also by the composition and MW of the copolymer the smaller the MW the later the gain in viscosity due to the cyclization and formation of the threedimensional structure occurs These studies allowed the authors to propose copolymer character istics optimum from their point of view for obtaining PAN precursor by melt spinning the MA fraction in the copolymer is 15 mol and Mn 20 103 The viscosity of the ANМА copolymer melts may be additionally reduced with the use of СО2 as a plas ticizer for this purpose the copolymer is preliminarily saturated with СО2 for a long time under fairly severe conditions 242 or through its adsorption immedi ately during extrusion 243 Another way of preparing ANМА copolymers capable of melt processing is the microencapsulation of a phase change material2 into it 244 To this end the precipitation copolymerization of AN and МА 15 mol was carried out at 30С in water in the presence of mercaptan and after polymerization for 2 h a new portion of the RAFT agent and from 5 to 25 wt nоctadecane the phase change material were introduced into the reaction system As a result the copolymer with Mn 50 103 and a wide MWD Ð 25 was produced An increase in the concentra tion of the microencapsulated octadecane decreased the melt flow index and simultaneously increased the glass transition temperature of the copolymer from 88 in the absence of octadecane to 92С in the pres ence of 25 wt octadecane The octadecanecon taining fiber melt spun at 200С had a microporous 2 These materials referred to phase change materials can store and release latent heat during phase transitions in a certain tempera ture interval structure As a consequence the breaking strength decreased from 32 to 10 cNdtex and the elonga tion at break decreased from 26 to 10 with an increase in the content of octadecane In the subse quent paper the content of the microencapsulated octadecane was increased to 40 it was shown that the copolymer with М 30 103 and an MA content of 15 mol was able to be melted at 206С 245 Various procedures of copolymer synthesis were used in the above studies A comparison of viscosities of melts of copolymers with close MW and composi tion but synthesized by different methods revealed that these values are different This result may be explained by the fact that the activity of the monomers in radical copolymerization changes appreciably upon transition from the homogeneous to heterophase synthesis 129 Eventually the degree of disorder of copolymer mac romolecules influencing the properties of the polymer melt changes depending on the extent of uniform insertion of MA into a polymer chain The effect of synthesis conditions on the behavior of ANМА copolymer melts was investigated in 233 Unfortu nately different parameters the MW and MWD of the copolymer were changed at the same time this did not allow one to follow the effect of chain microstruc ture on the properties of the melt This issue still remains open According to the data reported in 233 it may be stated that along with the copolymer com position the MWD of the copolymer is the main fac tor affecting the viscosity of the melt An increase in the proportion of the high molecular weight fraction leads to a considerable rise in viscosity and causes the interval of stability of the melt to narrow The studies described above made it possible to use the melt spinning of the ANMA copolymers 15 mol МА to produce hollow fibers with satisfac tory physicomechanical characteristics Тg 96С tensile strength of 16 cNdtex elongation at break of 187 and elastic modulus of 3 GPa 246 Evidently during the thermal stabilization of fibers based on the ANМА copolymers initially their melting and then cyclization will proceed Hence a special approach should be developed in order to use them as carbon fiber precursors The first trick involved the introduction of comonomers accelerating cyclization It was antici pated that conditions would be found at which the copolymer could be melt processed and then at a higher temperature could rapidly be subjected to thermooxidative stabilization For example a small amount of the third comonomer acrylic acid meth acrylic acid or acrylamide at a molar ratio of AN МА comonomer from 85 11 4 to 85 14 1 was introduced into the ANМА copolymer 241 The effect of the content of the third monomer was inves tigated in length using itaconic acid as an example All the copolymers were synthesized by copolymerization in DMF with the use of dodecyl mercaptan It is evi 38 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al dent that the addition of a cyclization accelerator should decrease the temperature interval of stability of the melt compared with the binary copolymer ANМА containing the same amount of AN The dynamic viscosity of the melt in the temperature range of 200220С is determined by the amount of the comonomer and its nature At 200С the melts are stable for more than 30 min and at 220С the viscos ity begins to grow more rapidly because of the cycliza tion reaction and the formation of conjugated and threedimensional structures The rate of its growth varies in the sequence itaconic acid acrylamide methacrylic acid acrylic acid The kinetic analysis of a change in the viscosity of the melt allowed one to estimate the activation energy of cyclization and the reaction rate constant and to determine the optimum compositions of the terpolymers For example the introduction of 3 mol itaconic acid was acceptable for the synthesis of the terpolymer preserving melt processability at 200С and undergoing cyclization at a high rate at 220С In a recent paper 247 dimethyl itaconate was used for this purpose The ANМА and AN dimethyl itaconate copolymers containing the same amount of the comonomer 5 10 and 15 mol were compared The chosen synthesis conditions are not typical of the problems posed because the ANMA copolymers are characterized by Mn 105 and Ð 20 and the ANdimethyl itaconate copolymers feature Mn 4060 103 and Ð 1117 According to the authors of 247 the copolymer containing 15 mol dimethyl itaconate is more preferable than a higher MW copolymer containing 15 mol МА because it melts without cyclization at a lower temperature 190С However for this copolymer the time of iso thermal cyclization at any temperature is higher than that of the MAbased copolymer It is probable that a more suitable way to stabilize the PAN precursor obtained by melt spinning consists in its electronbeam irradiation The authors of 248 obtained the fiber by melt spinning of the ANМА copolymer 15 mol МА at 190С followed by its electronbeam irradiation with a dose of 1500 kGy and stepwise stabilization to a temperature of 250С and carbonization at 1200С The resultant CF was char acterized by a tensile strength of 14 GPa an elastic modulus of 110 GPa and an elongation at break of 13 Cyclization induced by various irradiation proce dures and leading to formation of the ladder PAN was also described in 249252 For example the same effect may be reached by UV irradiation of the ANМА copolymer 249 As in the abovedescribed case this leads to the generation of radicals and causes a partial crosslinking of the polymer For comparison the authors used the commercial ANМА copoly mers obtained by solution spinning 6 mol МА and melt spinning 12 mol МА Both samples were subjected to UV radiation followed by thermooxida tive stabilization and carbonization and the physico mechanical characteristics of the fibers were investi gated at each stage The carbonized fiber obtained from the PAN melt was characterized by a tensile strength of 03 GPa an elastic modulus of 60 GPa and an elongation at break of 06 These parameters were lower than those for the copolymerbased carbon fiber obtained by solution spinning Another trick involving the introduction of a pho tosensitive monomer into the ANMA copolymer may be more effective under irradiation at a certain wavelength this monomer can pass to the excited state and generate radicals on a polymer chain Among these monomers are 4benzophenylvinyl carbonate 4benzophenylvinyl carbamate 2vinyloxycarbonyloxy methylanthraquinone and benzoin vinyloxycarbonyl ether The authors of 235 described the synthesis of the terpolymer of AN MA and acryloylbenzophenone by emulsion polymerization conducted in the presence of mercaptan the copolymer contained 14 mol MA and 1 mol acryloylbenzophenone The properties of the binary ANMA copolymer Mn 26 103 with the same content of AN and the terpolymers with different MW values Mn 16 103 and 22 103 were compared An important result was evidence that acryloylbenzophenone incorporated in a chain was fairly thermally stable and did not experi ence any changes when heated to 200260С The chemorheological study of the copolymer and terpoly mers revealed that the presence of the photosensitive monomer facilitates reduction in the rate of viscosity growth owing to formation of the network polymer C O O C O O CH2 CH C O N C O O CH2 CH H C C O O CH2 C O O CH2 CH O C O C H O C O O CH CH2 253 C O C O O CH2 CH POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 39 and exerts no effect on the stability of the melt Thus it can be anticipated that similar terpolymers may be of interest as precursors for the manufacture of carbon fibers The melt spun PAN precursors obtained with the use of analogous copolymers and terpolymers were subjected to UV irradiation and a change in their chemical structure was studied 254 The binary copolymers preserved their properties upon irradiation In contrast the chemical structure of the terpolymers changed upon irradiation which manifested itself in particular as a change in color the higher the irradia tion dose the more intense this change The mecha nism of this reaction may be depicted as follows The authors of 254 systematically studied this process by UV and IR spectroscopy However this study was not followed by the investigation of stabili zation and carbonization and the mechanical proper ties of fibers This theme was further developed by producing the carbon fiber from the AN МА and acryloylbenzo phenone terpolymer with a molar ratio of 85 14 1 which was synthesized by emulsion polymerization and processed from the melt subjected to UV radia tion thermal stabilization and carbonization 255 The carbon fiber had a tensile strength of 06 GPa an elastic modulus of 130 GPa and an elongation at break of 04 However the synthesis of the analogous ter polymer by suspension polymerization and the same manipulations produced the carbon fiber with a tensile strength of 06 GPa an elastic modulus of 73 GPa and an elongation at break of 04 256 It should be noted that in the latter case no mercaptan was used to control MW and possibly this was the reason behind lower strength characteristics of the carbon fiber The next attempt to synthesize mats from carbon fibers was made by the authors of 257 The precursor of ter polymer AN МА acryloylbenzophenone 85 14 1 emulsion polymerization was used to manufacture nonwoven mats that were exposed to UV treatment thermal stabilization and carbonization at 1500С The tensile strength was 1 cNtex the elastic modulus was 110 cNtex and the elongation at break was 13 The results discussed above make it possible to consider this approachthe synthesis of a terpolymer with a photosensitive monomera promising method of obtaining the PAN precursor and carbon fiber on its basis However it requires optimization of both the method of synthesis and terpolymer composition and the choice of conditions for producing carbon fiber Another potential option of carbon fiber precursor is the copolymers of AN and 1vinylimidazole VIM 258 Their copolymerization in solution gives rise to copolymers having an alternating structure or one close to it rAN 012 and rVIM 024 259 Such copolymers have a fairly low MW which is achieved via the use of mercaptan as a chaintransfer agent they can pass to the viscous flow state and then can undergo cyclization in air to produce the ladder structure The optimum composition of the copolymer was deter mined through the systematic research into the effect of the fraction of vinylimidazole on the glass transition temperature coke residue viscoelastic properties of the melt at various temperatures and stability of the melt 258 The best results were obtained for the copolymer containing 18 mol vinylimidazole The R R R R R R R R H H R H R CH2 CH C O O C O CH2 CH C O O C O hν CH2 CH C O O HC OH 40 CHERNIKOVA et al melt of this copolymer was stable at 192C and above The PAN precursor obtained by solution spinning 210C it underwent cyclization at a high rate of the ANvinylimidazole copolymer has found an Further studies were aimed at seeking optimum unusual application It is known that nitrogen oxide conditions for melt processing of the mentioned copo NO is capable of reversible interaction with acceptors lymers for example reduction in the spinning tem to form diazeniumdiolate In the case of PAN the rate perature or spinning time For this purpose common of this reaction is low However the introduction of plasticizers ethylene carbonate propylene carbonate vinylimidazole units into the copolymer enhances the or ethylene glycol polyhedral oligomeric silsesquiox bility of PAN to react with NO 262 ane octaphenyl and octaisobutylsilsesquioxane Oo and a new oligomer based on acrylonitrile and methyl ret N 21Himidazol1ylacrylate 20 mol with M H NZ 25 x 103 were used 260 HY 2NQ HNN Ht x yen x Yin Hs N CN me CN qQy LX m These fibers initially accumulating and then releas CN N ing NO are of interest in surgery for the healing of y wounds The released NO hampers their infection It turned out that the fiber melt spun from the N ANvinylimidazole copolymer stores a larger amount The latter plasticizer had the most pronounced of NO than the fiber obtained by the electrospinning effect on the properties of the melt For example the of the same copolymer or AN homopolymer Note addition of 8 wt oligomer leads to reduction in the that the additional modification of the fiber with poly glass transition temperature of the ANvinylimidaz caprolactone allowed for the controlled release of NO ole copolymer M 50 x 10 its synthesis is similar for several days to that described in 258 7 115C by 40C and Among other monomers which were used for the causes a twofold reduction in the shear stress A synthesis of AN copolymers capable of transition to change in the temperature at which cyclization begins the viscous flow state one should mention N Ndi2 is as low as 5 10C relative to the initial value 280C propylacryloamidines N Ndi2propylacryloami Therefore the temperature interval of melt processing dine and N2propylNfertbutylacryloamidines of the copolymer can be widened substantially The 263 fiber was spun at 180192C and its thermooxidative stabilization was conducted under heating from 100 to H NO H 300C at a rate of 1Cmin It was shown that the oligomer is incorporated into the ladder structure of NJ ZN NN the cyclizing polymer Carbonization at 1000C me y yielded carbon fibers with a tensile strength of 19 GPa ZA ZA and an elastic modulus of 190 GPa Acryloamidines do not exert any marked effect on These studies resulted in application for a patent the glass transition temperature but change the ther 261 which disclosed the method of producing car mal behavior of the ANbased copolymers In an inert bon fiber from the PAN precursorthe copolymer of atmosphere the temperature of the beginning of AN and 1vinyl 4vinyl 2vinyl or lmethyl2 cyclization decreases from 198C PAN to 178C vinylimidazole 7080 mol ANwith the addition upon addition of 1 mol NNdi2propylacrylo of 510 wt acrylonitrilecoNimidazole acrylate amidine to the polymer and to 138C when its content oligomer with M 12 x 10 the strength charac is 19 mol In air the reaction is more exothermic teristics of the fiber were similar to those described in and for the copolymers with 20 mol acryloamidines 258 and M 25 x 10 it begins at temperatures ranging R from 140 to 170C depending on the nature of substit uents at nitrogen In the absence of additives the 0 O copolymers pass to the viscous flow state at tempera tures above 150C while upon adding 10 wt DMF f m the temperature at which melt processing is feasible CN N decreases to 130C According to these data the pre 2 cursor was synthesized and its thermooxidative stabi lization and carbonization were performed in an inert N atmosphere by heating the fibers from room tempera where R H CH CH 4 ture to 1400 or 1800C at a rate of 10Cmin The POLYMER SCIENCE SERIES C Vol 62 No 1 2020 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 41 resultant carbon fiber was characterized by a tensile strength of 09 GPa an elastic modulus of 85 100 GPa and an elongation at break of 12 Another example is the use of methacrylonitrile MAN 237 In radical copolymerization this monomer is more active than AN 264 therefore it is consumed in copolymerization at a higher rate This factor has the decisive effect on the properties of the PAN precursor therefore one of the ways to control the properties of the precursor is to maintain a con stant monomer mixture composition during copoly mer synthesis which is ensured by different rates of introducing comonomers into copolymerization The MW is set by mercaptan or alcohol additives The ANMAN copolymers with a content of the latter monomer of 2550 wt and a fairly low MW which are synthesized by emulsion polymerization can pass to the viscousflow state before the onset of cyclization and may be used for melt processing On the whole the analysis of the published data suggests that the technologies of PAN precursor melt processing are still imperfect The main requirements for such copolymers include a low MW and the pres ence of no less than 10 mol comonomer However the optimum chemical nature of such a comonomer and requirements for the molecular structure of the chain the character of distribution of units in a chain and the length of AN sequence segments have not been ascertained up to now and it appears that this field will progress rapidly in the coming years ANALYSIS OF THE PATENT LITERATURE The review of domestic and foreign patents disclos ing the synthesis of acrylonitrile homopolymer and copolymers used in the manufacture of PANbased carbon fiber precursors which have been issued during the past three decades shows that researchers concentrated their efforts on modifying the conditions of the known processes new initiators sol ventsmedia monomers and elucidation of new mechanisms of polymerization Works in the field of synthesis of AN copolymers used in other areas eg as thermoplastics are generally based on the same synthetic approaches 265295 but they are beyond the scope of this review Radical Polymerization Domestic and foreign researchers developed new methods for the сopolymerization of acrylonitrile in bulk 296 solution 297299 and emulsion 300 and the precipitation polymerization in water 301 305 and supercritical CO2 306308 For example the patent 309 describes the appli cation of new germinal bishydroperoxides 310 311 of the general formula where R1 is lower alkyl R2 is Н or СН3 R3 is Н or lower alkyl R4 is Н or lower alkyl or R1 and R4 are branched alkyls In this case the noncrosslinked PAN soluble in DMF and DMAA is synthesized by bulk polymerization in the presence of germinal bishydroperoxides at a moderate temperature 50 60С The use of these initiators is economically sound because they are obtained from commercial raw material such as ketones and hydrogen peroxide aqueous solutions Moreover the implementation of polymerization in the monomer bulk rather than in solution makes the technology of synthesis and isola tion of polyacrylonitrile cheaper and simpler and decreases the ecological load on the environment Solution polymerization was used for the synthesis of acrylonitrile copolymers with sodium acrylate a mixture of sodium acrylate and methyl acrylate or a mixture of sodium acrylate with methyl methacrylate and itaconic acid in 4552 aqueous solution of sodium rhodanide at 4090С 297 These authors replaced a more expensive monomer vinyl acetate with a cheaper onesodium acrylateand obtained an easily colorable PAN fiber The patent 298 describes the method of synthe sizing acrylonitrile copolymers with the comonomer content no greater than 10 and Mw 80120 103 by AIBN or azobisisovalericacidinitiated radical polymerization in DMSO or DMF in much the same way as in patent 299 or precipitation polymerization in water in the presence of sodium thiocyanate zinc chloride or sodium perchlorate This method is dis tinguished by the technique of synthesizing the mono mer acrylonitrile from glycerol The fiber is produced by wet spinning using an appropriate organic solvent Once the optimum diameter of the fiber is attained it is subjected to thermooxidative stabilization and afterwards carbonization Emulsion polymerization is extremely rarely used for the synthesis of AN copolymers Nevertheless the fiberforming copolymers of acrylonitrile with esters of acrylic and methacrylic acids were synthesized by radiationinduced emulsion polymerization in the presence of a cationic or anionic surfactant 300 This makes it possible to implement polymerization in the range of 560С and to synthesize copolymers with MW up to 15 106 and a conversion of 8095 One of the advantages of this method is a high rate of polymerization and the opportunity to control the MW of the polymer A more common version of heterophase polymer ization is precipitation polymerization that is carried R2 HOO OOH R1 R4 R3 42 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al out more frequently in water 301304 For example the precipitation polymerization of acrylonitrile with methyl acrylate and a comonomer containing an acidic group acrylic methacrylic and itaconic acids was conducted using the common redox system based on potassium persulfate and sodium metabisulfite in the temperature range from 40 to 60С at pH 2 to 3 301 Particular attention was paid to the purity of the monomers and water under the assumption that the mentioned factors contribute to an increase in the MW and chemical purity of AN copolymers used for the production of carbon fiber precursors In another version of the precipitation polymeriza tion of AN the redox system consisting of potassium persulfate and sodium bisulfite was combined with ironII sulfate 302 This combined system enabled one to achieve a high rate of the process at moderate temperatures the reaction duration was 560 min at 4560C This method provided the synthesis of PAN with Mn 110125 103 and dispersion Ð 23 In the precipitation copolymerization of AN with vinyl acetate and vinyl carbonic acid or its deriva tives a mixture of ammonium persulfate sodium bisulfite and ironII sulfate is applied as an initiator 303 Polymerization is conducted at 5070C and pH from 2 to 5 The content of acrylonitrile in the sys tem is 2030 wt per water weight The advantage of this method is a relatively low cost of the feedstock A similar procedure was disclosed in the patent 304 in which the precipitation copolymerization of AN with acrylic or methacrylic acid their esters or amides and itaconic acid was initiated by ammonium persulfate and sodium bisulfite or sulfite in the temperature range of 3080C for 110 h The concentrations of the components were chosen in order to obtain the copolymers with Mn 17526 103 and dispersion Ð 2729 The precipitation copolymerization of AN with methacrylic acid and acrylamide was also initiated by a mixture of ammonium persulfate ammonium hydrosulfite and ironII sulfate at 50С and рН 3 305 The specific feature of this method is the multi stage isolation of the polymer which can be justified only by the high strength characteristics of the resul tant carbon fiber The unusual precipitation polymerization proce dure was proposed in the patent 312 in which the copolymerization of polar monomers specifically acrylonitrile and methacrylic acid was carried out in a mixture of appropriate solvents The temperature of polymerization was chosen to be above the lower crit ical solution temperature of the polymer In this way the authors managed to finely tune the relative activi ties of the monomers in copolymerization and to syn thesize the copolymers with different microstructure with М 103105 and a narrow MWD The precipitation copolymerization of AN in supercritical carbon dioxide was described in patents 306308 According to the authors the advantages of this synthetic procedure are environmental friendli ness cost effectiveness and energy efficiency The copolymerization of acrylonitrile with itaconic acid or its derivatives is initiated by common initiators for example AIBN at 6580C The abovementioned systems differ from precipitation polymerization in water in an extremely broad MWD In order to solve this problem the authors of 307 recommended using thiols ethanethiol which decrease the dispersity of the copolymer by 4045 ReversibleDeactivation Radical Polymerization The ascertainment of general features of reversible deactivation radical polymerization made it possible to develop and patent a number of fascinating methods for the synthesis of acrylonitrile copolymers with the controlled MW and a fairly narrow MWD The radical polymerization mediated by nitroxide radicals was used by the authors of patent 265 in which a wide range of comonomers including acrylo nitrile was enumerated It should be emphasized that this invention covers copolymers of different struc ture homopolymer diblock and triblock copolymers mutiblocks and gradient copolymers The major monomer providing the controlled synthesis of acry lonitrilebased copolymers of varying structure is sty rene and its derivatives polymerization is carried out at temperatures above 100С ATRP was used for the synthesis of AN copolymers in patents 313315 The distinctive feature of this method concerns the use of transitionmetal com pounds which cannot be directly used for the polym erization of vinyl acids and amides It should be noted that the mentioned patents lack information on puri fication of the polymers from the catalyst and proper ties of the PAN precursor or the carbon fiber obtained on its basis For example the copolymerization of acrylonitrile methyl acrylate and dimethyl itaconate in DMSO at 6070С with the use of the RAFT agent carbon tetrachloride and a complex catalytic system was described in 313 314 The catalytic system con sisted of a catalyst copperI bromide two ligands tris2pyridylmethylamine and tris2dimethyl aminoethylamine and an activating agent glu cose The copolymer produced by this method had Mn 60 103 and a narrow MWD Ð 15 The atomtransfer copolymerization of acryloni trile with styrene or its derivatives at 6090С in eth ylene carbonate propylene carbonate DMSO DMF or their mixture was developed 315 The polymeriza tion reaction was catalyzed by a copper salt with an organic ligand 22bipyridine or its derivative At the initial stage a macroinitiator was synthesized from the copolymer of acrylonitrile and vinyl comonomer for 0120 h The content of acrylonitrile in the macroini tiator should be from 10 to 80 mol and its Mw should POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 43 be 0550 103 If the weight content of acryloni trile and its comonomers is taken as 100 phr then the content of the macroinitiator in the system should be 0510 phr and that of the solvent should be 500 2000 phr As declared by the authors this method pro vides a way to obtain the copolymers of acrylonitrile and styrene their solutions possess a low viscosity because structurization gelation is suppressed In addition RAFT polymerization was in demand for producing PAN precursors 307 316 317 For example the authors of 316 described the trithiocar bonatemediated polymerization of AN in DMSO DMF or ethylene carbonate under radiation irradia tion at room temperature 316 Among the given sol vents ethylene carbonate is the most ecofriendly and at the same time it ensures a good control over polym erization The process consists of two stages At the first step lasting from 1 to 10 h the oligomeric RAFT agent with Mw 1339 103 is synthesized At the second step which also lasts from 1 to 10 h the polymerization of AN mediated by this agent is con ducted The conditions of synthesis are chosen so that the final PAN is characterized by Mw 55 103 and Ð 15 A wider scope of solvents was used in the patent 317 The RAFT copolymerization of acrylonitrile with vinyl acids vinyl esters vinyl amides and imid azoles mediated by trithiocarbonates was conducted not only in an organic solvent but also in aqueous solutions of zinc chloride or sodium thiocyanate The polymerization was initiated by organic peroxide or azo compound at 4085C The preferred content of the solvent in the system was 7295 wt the fraction of AN in the monomer mixture was above 90 wt and the content of the RAFT agent was less than 1 wt per monomer The copolymers synthesized according to this procedure feature Mw 60500 103 and Ð 2 The method of producing AN copolymers in the presence of dithiobenzoates and trithiocarbonates is described in the patent 318 in which the concentra tion conditions for the synthesis of polymers of the desired MW are disclosed For example the molar ratio of the comonomers and RAFT agent concentra tions should be at least 1 1 the molar ratio of AN and RAFT agent concentrations should be at least 400 1 and the molar ratio of the RAFT agent and initiator concentrations should be from 5 1 to 10 1 These conditions provide a high MW no less than 200 103 and narrow MWD of the copolymers Ð 13 In the authors opinion the assynthesized PAN precursors make it possible to substantially improve the physico mechanical properties the elastic modulus and break ing strength of carbon fibers obtained on their basis compared with carbon fibers obtained from PAN pre cursors with a lower molecular weight andor wider MWD A method of synthesizing AN copolymer in super critical CO2 in the presence of dibenzyl trithiocarbon ate was developed 307 The heterophase character of polymerization exerts no effect on dispersion of the product when polymerization is carried out in carbon dioxide that is it does not ensure control over MWD Anionic Polymerization and GroupTransfer Polymerization The methods of synthesizing acrylonitrile copoly mers whose macromolecules are formed according to the anionic mechanism are presented in patents 319 321 A new initiating anionictype system is designed which according to the authors does not introduce hardtoremove admixtures to the polymer 319 320 The copolymerization of AN with oxygencontaining monomers was carried out in DMSO THF or their mixtures at temperatures from 20 to 80C The ini tiating system was free of metal atoms and consisted of compounds containing only С Н N and O atoms for example ethylene oxide22diazabicyclooctane system or a mixture of propylene oxide and 22diaz abicyclooctane The proposed method allows for the synthesis of oligomers and branched polymers in a wide MW range Mn 6400 103 and Ð 1235 Common metalcontaining anionic initiators were used for the synthesis of an ultrahigh molecular weight fiberforming PAN 321 Acrylonitrile was polymerized in DMF in the presence of a low concen tration of lithium 12bisdiethylamino2oxoethano late The researchers demonstrated that the polymer ization may be accomplished at a high concentration of the monomer up to 28 molL at moderately low temperatures to 20С In this case the authors managed to maximally limit the occurrence of side reactions and to avoid cyclization and gelation pro cesses The viscosityaverage molecular weight of the acrylonitrile homopolymers was 570840 103 Finally in 322 grouptransfer polymerization was used for the synthesis of PAN Polymerization was carried out in DMF DMAA or DMSO at a tempera ture of 20 to 60C using tetrabutylammonium flu oride as a catalyst and trimethylcyanosilane or 1methoxy1trimethylsiloxy2methyl1propene as an initiator This method enabled one to synthesize stereoregular acrylonitrile polymers with Mw 50 300 103 and Ð 1215 CONCLUSIONS The modern approaches to the synthesis of AN copolymers intended for use as carbon fiber precur sors which are considered in this review make it pos sible to make some considerations regarding prospects for their use in industry 44 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al The first question arises as to what the require ments are for AN copolymers An analysis of the pub lished data suggests that the properties of the PAN precursor are determined by its chemical composition MW MWD and the average length of the sequence of AN units From the point of view of synthesizing AN copolymers with the predetermined molecular struc ture the RAFT polymerization is the most promising technique It is tolerant to the functional groups of monomers may be used in homogeneous and het erophase polymerizations and solves the problem of control over the MWD and compositional heteroge neity of the copolymers Owing to a wide range of the MW of the synthesized copolymers from tens of thou sands to hundreds of thousands the RAFT process may be proposed for producing PAN precursors spun from both solution and melt The former is confirmed by the analysis of the patent literature and the described production of carbon fibers from RAFT copolymers The second direction has not yet been realized in practice To date the application of other versions of controlled radical or anionic polymeriza tion has not gone beyond the synthesis of copolymers and study of their thermal behavior or the rheology of their solutions In our opinion the use of green media may be of interest for designing melt processable PAN precursors For example when the copolymerization of AN is conducted in supercritical СО2 the copolymer can adsorb a certain amount of СО2 which can plasticize polymers Accordingly the glass transition tempera ture will decrease and the transition of the copolymer to the viscous flow state will be facilitated Moreover once the reaction is completed СО2 can readily be removed and the final polymeric product does not require purification Copolymers with a high MW and wide MWD are commonly synthesized in supercritical СО2 but the use of mercaptans as molecular weight regulators could solve this problem The combination of atomtransfer polymerization with the use of ionic liquids may be a promising approach In this case the formation of copolymers with a relatively low MW which is common for these systems and the opportunity to more easily separate a catalyst from a polymer owing to a high solubility of the catalyst in ionic liquids will be beneficial Urgent studies are focused on seeking new inex pensive comonomers which enable one to govern the rheology of solutionsmelts of copolymers and their cyclization and thermooxidative stabilization Finally research into control over the distribution of comonomers in the PAN chain which along with other factors determines the defectiveness of stabi lized and carbonized fibers is of interest Solution to this problem may be copolymerization performed at a constant or controlled rate of comonomer introduc tion into copolymerization in combination with the reversible deactivation mechanism In general the carbon fibers manufactured from the PAN precursor using methods known from the open sources refer to the segment of mediumstrength and mediummodulus fibers It is hoped that new syn thetic approaches elaborated during the past decades which make it possible to tailor the molecular struc ture of polymers and to minimize defects in their structure will open new horizons for designing carbon fibers with improved strength characteristics FUNDING This work was supported by the Russian Foundation for Basic Research project no 182917004mk REFERENCES 1 T Roberts in The Carbon Fiber Industry Global Stra tegic Market Evaluation 20062010 Mater Technol Publ Watford UK 2006 pp 10 93177 2 TORAY Innovation b y Chemistry httpcs2toraycojpnewstorayennewsrrs02nsf0 B126D9D8433675EE49257D11001770C5 Cited 2020 3 R C Houtz US Patent No 2404727A 1944 4 J P Knudsen and P A Ucci US Patent No 3088793A 1956 5 H D Mackenzie and F Reeder UK Patent No 944 217 1963 6 W G Schmidt US Patent No 2923694A 1955 7 A Shindo JPN Patent No 28287 1959 8 Carbon Fibers and Their Composites Ed by P Morgan Taylor and Francis New York 2005 9 S Chand J Mater Sci 5 1303 2000 10 Carbon Fiber Composites Ed by D L Chung Butter worthHeinemann Newton 1994 11 J Liu L He S Ma J Liang Y Zhao and H Fong Polymer 61 20 2015 12 S Dalton F Heatley and P M Budd Polymer 40 5531 1999 13 A Gupta and I R Harrison Carbon 35 809 1997 14 S Dalton F Heatley and P M Budd Polymer 40 5531 1999 15 A Ju S Guang and H Xu Carbon 54 323 2013 16 X Huang Materials 2 2369 2009 17 D D Edie Carbon 36 345 1998 18 P Bajaj D K Paliwal and A K Gupta J Appl Polym Sci 49 823 1993 19 R C Bansal and J B Donnet in Comprehensive Poly mer Science Ed by G Allen and J C Bevington Per gamon Press Oxford 1989 Vol 6 p 501 20 Q Ouyang L Cheng H Wang and K Li Polym Degrad Stab 93 1415 2008 21 L A Beltz and R R Gustafson Carbon 34 561 1996 22 V Bhanu P Rangarajan K Wiles M Bortner M Sankarpandian D Godshall and G Wilkes Polymer 43 4841 2002 23 J S Tsai and C H Lin J Appl Polym Sci 43 679 1991 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 45 24 P Bajaj and A K Roopanwal J Macromol Sci Polym Rev 37 97 1997 25 S P Rwei T F Way and Y S Hsu Polym Degrad Stab 98 2072 2013 26 N A M J Siti D Rusli and A Ishak Int J Chem Eng Appl 3 416 2012 27 N A M J Siti D Rusli and A Ishak Materials 7 6207 2014 28 B G Min T W Son B C Kim and W H Jo Polym J 24 841 1992 29 G S Krishnan A Burkanudeen N Murali and H Phadnis eXPRESS Polym Lett 6 729 2012 30 AQ Ju MJ Li M Luo and MQ Ge Chin Chem Lett 25 1275 2014 31 G HenriciOlive and S Olive Adv Polym Sci 32 123 1980 32 J S Tsai and C H Lin J Mater Sci 26 3996 1991 33 E Fitzer W Frohs and M Heine Carbon 24 387 1986 34 Z Wangxi L Jie and W Gang Carbon 41 2805 2003 35 R C Bansal and J B Donnet in Comprehensive Poly mer Science Ed by G Allen and J C Bevington Per gamon Press Oxford 1989 Vol 6 p 501 36 B Saha and G C Schatz J Phys Chem B 116 4684 2012 37 J S Tsai and C H Lin J Mater Sci 26 3996 1991 38 W M Thomas Adv Polym Sci 2 401 1961 39 L H GarciaRubio and A E Hamielec J Appl Polym Sci 23 1397 1979 40 D N Bort G F Zvereva and S I Kuchanov Vyso komol Soedin Ser A 20 1827 1978 41 N T Srinivasan and M Santappa Makromol Chem 26 80 1958 42 Z Izumi J Polym Sci Part A Polym Chem 5 469 1967 43 D Edwin Moore and A Gustav Parts Makromol Chem 37 108 1960 44 M D Goldfein Vysokomol Soedin Ser A 32 2243 1990 45 I Mathakiya V Vangani and A K Rakshit J Appl Polym Sci 69 217 1998 46 K B Wiles V A Bhanu A J Pasquale T E Long and J E McGrath J Polym Sci Part A Polym Chem 42 2994 2004 47 C Hou L Ying and C Wang J Mater Sci 40 609 2005 48 C Hou C Sun L Ying and C Wang J Appl Polym Sci 96 483 2005 49 H S Bisht and A K Chatterjee J Macromol Sci Part C Polym Rev 41 139 2001 50 N Shavit M Konigsbuch and A Oplatka US Patent No 3345350A 1963 51 K A Shaffer Trends Polym Sci 3 146 1995 52 D A Canelas Adv Polym Sci 133 103 1997 53 J L Kendall Chem Rev 99 543 1999 54 Ya S Vygodskii O A Melnik E I Lozinskaya and A S Shaplov Polym Sci Ser A 47 122 2005 55 R Span and W Wagner J Phys Chem 25 1509 1996 56 Supercritical Fluid Extraction Ed by M McHugh ButterworthHeinemann Newton 1994 57 D E Knox Pure Appl Chem 77 513 2005 58 P B Zetterlund F Aldabbagh and M Okubo J Polym Sci Part A Polym Chem 47 3711 2009 59 H Shiho Macromolecules 33 1565 2000 60 Y Yang Q Dong and Z Wang J Appl Polym Sci 102 5640 2006 61 Z Wang Y J Yang and Q Dong Polymer 47 7670 2006 62 XR Teng J Appl Polym Sci 87 1393 2003 63 XR Teng HL Shao and XC Hu J Appl Polym Sci 86 2338 2002 64 M Okubo S Fujii H Maenaka and H Minami Colloid Polym Sci 281 964 2003 65 XR Teng XC Hu and HL Shao Polym J 34 534 2002 66 S Yeo and E Kiran Macromolecules 37 8239 2004 67 A V Shlyahtin I E Nifantev V V Bagrov D A Lemenovskii A N Tavtorkin and P S Tima shev Green Chem 16 1344 2014 68 A Kermagoret N D Q Chau B Grignard D Cor della A Debuigne C Jérôme and C Detrembleur Macromol Rapid Commun 37 539 2016 69 M Harrison S MacKenzie and D M Haddleton Chem Commun 2002 2850 2002 70 P Kubisa Prog Polym Sci 34 1333 2009 71 Ionic Liquids Applications and Perspectives Ed by A Kokorin InTech Rijeka 2011 72 A S Shaplov Candidates Dissertation in Chemistry INEOS RAN Moscow 2005 73 L Cheng Y Zhang and T Zhao Macromol Symp 216 9 2004 74 C Hou R Qu C Ji C Sun and C Wang J Polym Sci Part A Polym Chem 46 2701 2008 75 C Hou R Qu C Ji C Sun and C Wang Polymer 49 3424 2008 76 H Chen Y Meng Y Liang Z Lu and P Lu J Ma ter Res 24 1880 2009 77 H Chen Y Liang D Liu and Z Tan Mater Sci Eng C 30 605 2010 78 GX Wang M Lu ZH Hou and H Wu J Polym Res 20 80 2013 79 M Szwarc Nature 178 4543 1168 1956 80 A Hirao R Goseki and T Ishizone Macromole cules 47 1883 2014 81 W C Higginson and N S Wooding J Chem Soc 1952 760 1952 82 F C Foster J Am Chem Soc 74 2299 1952 83 M Imoto and M Kinoshita J Chem Soc Jpn Ind Chem Sect 61 452 1958 84 M Frankel A Ottolenghi M Albeck and A Zilkha J Chem Soc 1959 3858 1959 85 A Zilkha B A Feit and M Frankel J Chem Soc 1959 928 1959 46 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al 86 A Zilkha B A Feit and M Frankel J Polym Sci 49 231 1961 87 A Zahn and P Schäfer Makromol Chem 30 225 1959 88 C G Overberger H Juki and N Urakawa J Polym Sci 45 127 1960 89 R B Cundall D D Eley and J Worrall J Polym Sci 58 869 1962 90 M A Askarov and S N Trubitsyna Vysokomol Soe din 5 1235 1963 91 B L Erusalimskii and I V Kulevskaya Vysokomol Soedin 7 184 1965 92 A A Berlin E V Kochetov and N S Enikolopyan Vysokomol Soedin 7 187 1965 93 I M Panaiotov and A T Obreshkov Vysokomol Soedin 7 366 1965 94 E V Kochetov A A Berlin and N S Enikolopyan Vysokomol Soedin 8 1018 1966 95 V N Krasulina A V Novoselova and G A Orlova Vysokomol Soedin Ser A 12 1029 1970 96 E V Kochetov A A Berlin E M Masalskaya and N S Enikolopyan Vysokomol Soedin Ser A 12 1118 1970 97 N A Kubasova D S Din M A Geiderikh and M V Shishkina Vysokomol Soedin Ser A 13 162 1971 98 A A Korotkov V N Krasulina and A V Novoselo va Vysokomol Soedin Ser A 13 2661 1971 99 K Matsuzaki T Uryu M Okada and H Shiroki J Polym Sci Part A1 Polym Chem 6 1475 1968 100 Y Inoue and A Nishioka Polym J 3 149 1972 101 C Brandy D Gervaris and J P Vairon Eur Polym J 20 619 1984 102 S Raynal Eur Polym J 22 559 1986 103 O Hirofumi H Kunio and K Kenji Polym J 25 245 1993 104 H C Bach and R S Knorr in Encyclopedia of Poly mer Science and Engineering Wiley New York 1985 Vol 1 p 339 105 K Kamide H Ono and K Hisatani Polym J 24 917 1992 106 F Zhang ZX Xu and RG Jin Acta Polym Sin 1 1102 2008 107 G S Krishnan A Burkanudeen N Murali and H Phadnis eXPRESS Polym Lett 6 729 2012 108 X Shi and J Jiang Chin Chem Lett 30 473 2019 109 X Shi and J Jiang Polym Sci Ser B 61 511 2019 110 Y Cao J M Li Y R Wang Y M Yao Q Shen Chin Sci Bull 56 2780 2011 111 S Miertuš O Kysel and P Májek Chem Zvesti 19471984 33 167 1979 112 I Ikeda S Mitsumoto Y Yamada and K Suzuki Polym Int 26 115 1991 113 Y I Estrin A E Tarasov A A Grishchuk A V Chernyak and E R Badamshina RSC Adv 6 106064 2016 114 Y I Estrin A A Grishchuk A E Tarasov E O Perepelitsina and E R Badamshina Polym Sci Ser B 58 19 2016 115 M T Reetz and T Knauf US Patent No 5 494 983 1996 116 T Zhang L Jiang W Sun Z Shen and S Ma Eur Polym J 38 2329 2002 117 V Jaaks C D Eisenbach and W Kern Makromol Chem 161 139 1972 118 T Ogawa and P Quintana J Polym Sci Part A Polym Chem 13 2517 1975 119 T Ogawa and J Romero Eur Polym J 13 419 1975 120 F Schaper S R Foley and R F Jordan J Am Chem Soc 126 2114 2004 121 N Terán M R Estrada and T Ogawa J Macromol Sci Part A Pure Appl Chem 42 1679 2005 122 A M Kaplan I L Stoyachenko V B Golubev and V I Goldanskii Vysokomol Soedin Ser B 17 259 1975 123 J Qiu B Charleux and K Matyjaszewski Polimery 46 453 2001 124 Fundamental of ControlledLiving Radical Polymeriza tion Polymer Chemistry Series Ed N V Tsarevsky R Soc Chem Oxford 2013 125 H S Bisht and A K Chatterjee J Macromol Sci Polym Rev 41 139 2001 126 W A Braunecker and K Matyjaszewski Prog Polym Sci 32 93 2007 127 K Matyjaszewski and J Xia Chem Rev 101 2921 2001 128 G Moad E Rizzardo and S H Thang Aust J Chem 62 1402 2009 129 Handbook of Radical Polymerization Ed by K Maty jaszewski and T P Davis WileyIntersci Hoboken 2002 130 T Fukuda T Terauchi A Goto and K Ohno Mac romolecules 29 6393 1996 131 K Tharanikkarasu and G J Radharishnan J Polym Sci Part A Polym Chem 34 1723 1996 132 K Tharanikkarasu S Sundar C V Thankam and G Radhakrishnan J Polym Mater 15 243 1998 133 T Otsu J Polym Sci Part A Polym Chem 38 2121 2000 134 T Fukuda T Terauchi A Goto K Ohno Y Tsujii and T Miyamoto Macromolecules 29 6393 1996 135 Nitroxide Mediated Polymerization From Fundamen tals to Applications in Materials Science Ed by D Gig mes R Soc Chem Oxford 2015 136 D F Grishin Polym Sci Ser A 50 221 2008 137 D Benoit V Chaplinski R Braslau and C J Hawk er J Am Chem Soc 121 3904 1999 138 S BrinkmannRengel and N Niessner ACS Symp Ser 768 394 2000 139 E Megiel and A Kaim J Polym Sci Part A Polym Chem 46 1165 2008 140 C Detrembleur V Sciannamea C Koulic M Claes M Hoebeke R Jérôme Macromolecules 35 7214 2002 141 J Nicolas S Brusseau and B Charleux J Polym Sci Part A Polym Chem 48 34 2010 142 M Yu Zaremski A V Plutalova and I Eremeev Macromol Theory Simul 25 413 2016 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 47 143 M Zaremski I Eremeev E Garina O Borisova and B Korolev J Polym Res 24 151 2017 144 JS Wang and K Matyjaszewski J Am Chem Soc 117 5614 1995 145 W Jakubowski and K Matyjaszewski Macromole cules 38 4139 2005 146 W Jakubowski K Min and K Matyjaszewski Mac romolecules 39 39 2006 147 V Percec T Guliashvili J S Ladislaw A Wistrand A Stjerndahl M J Sienkowska and S Sahoo J Am Chem Soc 128 14156 2006 148 W Tang and K Matyjaszewski Macromolecules 39 4953 2006 149 N J Treat H Sprafke J W Kramer P G Clark B E Barton J Read De Alaniz and C J Hawker J Am Chem Soc 136 16096 2014 150 D Konkolewicz K Schroder J Buback S Bern hard and K Matyjaszewski ACS Macro Lett 1 1219 2012 151 D F Grishin and I D Grishin Russ Chem Rev 84 712 2015 152 D F Grishin and I D Grishin Fibre Chem 50 514 2019 153 K Matyjaszewski S M Jo H Paik and S G Gay nor Macromolecules 30 6398 1997 154 K Matyjaszewski S M Jo H Paik and D A Shipp Macromolecules 32 6431 1999 155 M Lazzari O Chiantore R Mendichi and M A LópezQuintela Macromol Chem Phys 206 1382 2005 156 H Dong W Tang C Hou J S Liu and C Wang Polym Int 55 171 2006 157 C Ho R Qu L Ying and C W Wang J Appl Polym Sci 99 32 2006 158 C Hou R J Qu C N Ji and C W Wang J Polym Sci Part A Polym Chem 44 219 2006 159 K Matyjaszewski Macromolecules 40 2974 2007 160 J Jiang X Lu and Y J Lu J Appl Polym Sci 116 2610 2010 161 X Pan M Lamson J Yan and K Matyjaszewski ACS Macro Lett 4 192 2015 162 EX Liang M Zhong ZH Hou Y Huang BH He GX Wang LC Liu and H Wu J Macromol Sci Part A Pure Appl Chem 53 210 2016 163 S Li Y Wang L Ma X Zhang S Dong L Liu X Zhou C Wang and Z Shi Des Monomers Polym 22 180 2019 164 Z Huang J Chen L Zhang Z Cheng and X Zhu Polymers 8 59 2016 165 V Pitto B I Voit T J A Loontjens and R A T M van Benthem Macromol Chem Phys 205 2346 2004 166 G Wang Z Wang B Lee R Yuan Z Lu J Yan X Pan Y Song M R Bockstaller and K Matyjasze wski Polymer 129 57 2017 167 N V Tsarevsky and K Matyjaszewski J Polym Sci Part A Polym Chem 44 5098 2006 168 N V Tsarevsky and K Matyjaszewski Chem Rev 107 2270 2007 169 C Hou R Qu C Ji C Sun and C Wang J Polym Sci Part A Polym Chem 46 2701 2008 170 C Hou R Qu C Sun C Ji C Wang L Ying N Ji ang F Xiu and L Chen Polymer 49 3424 2008 171 GX Wang M Lu ZH Hou and H Wu J Polym Res 20 80 2013 172 K C Lee L M Gan C H Chew and S C Ng Polymer 36 3719 1995 173 H Chen Y Liang D Liu Z Tan S Zhang M Zheng and R Qu Mater Sci Eng C 30 605 2016 174 J Ma H Chen M Zhang and M Yu J Polym Sci Part A Polym Chem 50 609 2012 175 J Chiefari Y K Chong F Ercole J Krstina J Jef fery T P T Le R T A Mayadunne G F Meijs C L Moad G Moad E Rizzardo and S H Thang Macromolecules 31 5559 1998 176 Y Z You C Y Hong R K Bai C Y Pan and J Wang Macromol Chem Phys 203 477 2002 177 PE Millard L Barner M H Stenzel T P Davis C BarnerKowollik and A H E Müller Macromol Rapid Commun 27 821 2006 178 J Y Cai J McDonnell C Brackley L O Brien J S Church K Millington and N PhairSorensen Mater Today Commun 9 22 2016 179 C Tang T Kowalewski and K Matyjaszewski Mac romolecules 36 8587 2003 180 G Moad E Rizzardo and S H Thang Aust J Chem 65 985 2012 181 Q An J Qian L Yu Y Luo and X Liu J Polym Sci Part A Polym Chem 43 1973 2005 182 XH Liu GB Zhang XF Lu JY Liu D Pan and YS Li J Polym Sci Part A Polym Chem 44 490 2006 183 XH Liu YG Li Y Lin and YS Li J Polym Sci Part A Polym Chem 45 1272 2007 184 XH Liu GB Zhang BX Li and YG Bai Eur Polym J 44 1200 2008 185 A Aqil C Detrembleur B Gilbert and R Jerome Chem Mater 19 2150 2007 186 S Niu L Zhang J Zhu W Zhang Z Cheng and X Zhu J Polym Sci Part A Polym Chem 51 1197 2013 187 J Li C Ding Z Zhang J Zhu and X Zhu React Funct Polym 113 1 2017 188 JY He and M Lu J Macromol Sci Part A Pure Appl Chem 56 443 2019 189 Z Huang L Zhang Z Cheng and X Zhu Polymers 9 4 2017 190 S Zhang L Yin J Wang W Zhang L Zhang and X Zhu Polymers 9 26 2017 191 Z Uyar F Turgut U Arslan M Durgun and M Degirmenci Eur Polym J 119 102 2019 192 M Kopeć P Krys R Yuan and K Matyjaszewski Macromolecules 49 5877 2016 193 V A Kabanov V P Zubov and Yu D Semchikov ComplexRadical Polymerization Bukinist Moscow 1987 in Russian 48 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al 194 A Kaiser S Brandau M Klimpel and C Barner Kowollik Macromol Rapid Commun 31 1616 2010 195 C J Dürr S G J Emmerling A Kaiser S Brandau A K T Habicht M Klimpel and C BarnerKowol lik J Polym Sci Part A Polym Chem 50 174 2012 196 C J Dürr S G J Emmerling P Lederhose A Kai ser S Brandau M Klimpel and C BarnerKowol lik Polym Chem 3 1048 2012 197 C J Dürr L Hlalele A Kaiser S Brandau and C BarnerKowollik Macromolecules 46 49 2013 198 F J EnríquezMedrano F SorianoCorral P AcuñaVázquez E N CabreraÁlvarez H Saade Caballero A CastañedaFacio L V López and R D de LeónGómez J Mex Chem Soc 58 194 2014 199 D Fan J He J Xu W Tang Y Liu and Y Yang J Polym Sci Part A Polym Chem 44 2260 2006 200 M M Alam H Peng K S Jack D J T Hill and A K Whittaker J Polym Sci Part A Polym Chem 55 919 2017 201 E V Chernikova Z A Poteryaeva and A V Plutalo va Polym Sci Ser B 56 109 2014 202 H Jiang L Zhang J Qin W Zhang Z Cheng and X Zhu J Polym Sci Part A Polym Chem 50 4103 2012 203 J BožovićVukić H T Mañon J Meuldijk C Kon ing and B Klumperman Macromolecules 40 7132 2007 204 J S Kim H J Jeon K M Lee J N Im and J H Youk Fibers Polym 11 153 2010 205 AL Li Y Wang H Liang and J Lu J Polym Sci Part A Polym Chem 44 2376 2006 206 W Wang L Bai H Chen H Xu and Q Tao Polym Bull 75 4327 2018 207 LS Wan H Lei Y Ding L Fu J Li and ZK Xu J Polym Sci Part A Polym Chem 47 92 2009 208 E V Chernikova Z A Poteryaeva S S Belyaev I E Nifantev A V Shlyakhtin Yu V Kostina A S Cherevan M N Efimov G N Bondarenko and E V Sivtsov Polym Sci Ser B 53 391 2011 209 E V Chernikova Z A Poteryaeva S S Belyaev and E V Sivtsov Russ J Appl Chem 84 1031 2011 210 J M Spörl A Ota R Beyer T Lehr A Müller F Hermanutz and M R Buchmeiser J Polym Sci Part A Polym Chem 52 1322 2014 211 J Spörl F Hermanutz and M Buchmeiser Int Fi ber J 28 24 2014 212 E V Chernikova S M Kishilov A V Plutalova Yu V Kostina G N Bondarenko A A Baskakkov S O Ilyin and A Yu Nikolaev Polym Sci Ser B 56 553 2014 213 J D Moskowitz B A Abel C L McCormick and J S Wiggins J Polym Sci Part A Polym Chem 54 553 2016 214 J Cai J Y McDonnell C Brackley and L O Brien Mater Today Commun 9 22 2016 215 E V Chernikova R V Toms N I Prokopov V R Duflot A V Plutalova S A Legkov and V I Gomzyak Polym Sci Ser B 59 28 2017 216 H Khayyam R N Jazar S Nunna G Gol karnarenji K Badii S Mousa Fakhrhoseini and M Naebe Prog Mater Sci 107 100575 2020 217 E V Chernikova Yu V Kostina M N Efimov N I Prokopov A Yu Gervald R V Toms A Yu Nikolaev and M D Shkirev Polym Sci Ser B 59 116 2015 218 R V Toms N I Prokopov A Yu Gervald and E V Chernikova Plast Massy Nos 56 21 2016 219 E V Chernikova Z A Poteryaeva S S Belyaev and E V Sivtsov Russ J Appl Chem 84 1031 2011 220 E V Chernikova Z A Poteryaeva A V Plutalova and A A Baskakov Plast Massy Nos 78 33 2014 221 R V Toms M S Balashov A A Shaova A Yu Ger vald N I Prokopov A V Plutalova N A Greben kina and E V Chernikova Polym Sci Ser B 62 2020 in press 222 J Hao Y Liu and C Lu Polym Degrad Stab 147 89 2018 223 R Wang Y Luo B Li X Sun and S Zhu Macro mol Theory Simul 15 356 2006 224 X Sun Y Luo R Wang BG Li B Liu and S Zhu Macromolecules 40 849 2007 225 X Sun Y Luo R Wang BG Li and S Zhu AIChE J 54 1073 2008 226 X Li S Liang WJ Wang BG Li Y Luo and S Zhu Macromol React Eng 9 409 2015 227 J D Moskowitz and J S Wiggins Polymer 84 311 2016 228 J Kaur K Millington and J Y Cai J Appl Polym Sci 133 48 44273 2016 229 S O Ilyin A A Baskakov E V Chernikova and V G Kulichikhin Russ Chem Bull Int Ed 66 711 2017 230 I Yu Skvortsov R V Toms N I Prokopov E V Chernikova and V G Kulichikhin Polym Sci Ser A 60 793 2018 231 A K Gupta D K Paliwal and P Bajaj J Appl Polym Sci 70 2703 1998 232 S Jamil R Daik and I Ahmad Materials 7 6207 2014 233 V A Bhanu P Rangarajan K Wiles M Bortner M Sankarpandian D Godshall T E Glass A K Banthia J Yang G Wilkes D Baird and J E McGrath Polymer 43 4841 2002 234 P Rangarajan J Yang V Bhanu D Godshall J McGrath G Wilkes and D Baird J Appl Polym Sci 85 69 2002 235 M J Bortner V Bhanu J E McGrath and D G Baird J Appl Polym Sci 93 2856 2004 236 XY Gao N Han XX Zhang and WY Yu J Mater Sci 44 5877 2009 237 B S Curatolo and G S Li EU Patent No 0492909B1 1990 238 H H Weinstock Jr and G A Nesty US Patent No 2 692875A 1949 239 J Yang A K Banthia D Godshell P Rangarajan T E Glass G L Wilkes D G Baird and J E Mc POLYMER SCIENCE SERIES C Vol 62 No 1 2020 FIBERFORMING ACRYLONITRILE COPOLYMERS 49 Grath Polym Prepr Am Chem Soc Div Polym Chem 41 59 2000 240 V A Bhanu K B Wiles A K Banthia A Mansuri M Sankarpandian P Rangarajan T E Glass D G Baird G L Wilkes J E McGrath Polym Prepr Am Chem Soc Div Polym Chem 42 663 2001 241 P Rangarajan V Bhanu D Godshall G Wilkes J McGrath and D Baird Polymer 43 2699 2002 242 M J Bortner and D G Baird Polymer 45 3399 2004 243 M D Wilding and D G Baird Polym Eng Sci 49 1990 2009 244 XY Gao N Han XX Zhang and WY Yu J Mater Sci 44 5877 2009 245 N Han XX Zhang and XC Wang J Appl Polym Sci 103 2776 2007 246 W Peng N Han X Tang H Liu and X Zhang Adv Mater Res 332334 339 2011 247 SP Rwei TF Way WY Chiang and JC Tseng Text Res J 88 1479 2018 248 J H Lee JU Jin S Park D Choi NH You Y Chung BC Ku and H Yeo J Ind Eng Chem 71 112 2018 249 M C Paiva P Kotasthane D D Edie and A A Ogale Carbon 41 1399 2003 250 R E Bullock Fiber Sci Technol 7 157 1974 251 J Dietrich P Hirt and H Herlinger Eur Polym J 32 617 1996 252 S M Badawy and A M Dessouki J Phys Chem B 41 11273 2003 253 Z Czech Int J Adhes Adhes 27 195 2007 254 A K Naskar R A Walker S Proulx D D Edie and A A Ogale Carbon 43 1065 2005 255 T Mukundan V A Bhanu K B Wiles H Johnson M Bortner D G Baird A K Naskar A A Ogale D D Edie and J E McGrath Polymer 47 4163 2006 256 YZ Wang SG Wang and JL Liu Key Eng Mater 575576 151 2014 257 R A Walker A K Naskar and A A Ogale Plast Rubber Compos 35 242 2006 258 W Deng A Lobovsky S T Iacono T Wu N Tomar S M Budy T Long W P Hoffman and D W Smith Polymer 52 622 2011 259 N Pekel Z M O Rzaev and O Güven Macromol Chem Phys 205 1088 2004 260 B L Batchelor S F Mahmood M Jung H Shin O V Kulikov W Voit B M Novak and D J Yang Carbon 98 681 2016 261 D J Yang B Batchelor S F Mahmood D W Smith W Deng H Shin and M Jung US Patent No 10435821 2015 262 A Lowe W Deng D W Smith and K J Balkus Macromolecules 45 5894 2012 263 S König M M Clauss E Giebel and M R Buch meiser Polym Chem 10 4469 2019 264 K Kamide and K Hisatani Polym J 24 1377 1992 265 L A Engelbrecht and A Noordam US Patent No 7858694 B2 2005 266 D K Parker F J Feher and V Mahadevan US Pat ent No 7592409 B2 2003 267 R Tsuji and T Hiiro US Patent No 7094833 B2 2002 268 D Preti A G Rossi R Nocci and N Vecchini US Patent No 5807928 1996 269 K W Pober and C M Starks US Patent No 5405916 1991 270 T Tazaki and M Kuramoto US Patent No 5391671 1990 271 T Tagami Y Imai M Kakimoto O Kiyohara and H Narushima US Patent No 5342895 1990 272 R J Bertsch US Patent No 5312956 1991 273 S Arrighetti A Brancaccio S Cesca and G Gi uliani US Patent No 4145378 1976 274 G F Fanta E I Stout and W M Doane US Patent No 4134863 1976 275 J Hradil J Stamberg and J Coupek US Patent No 4067825 1975 276 H Alberts H Bartl and R Prinz US Patent No 4056671 1975 277 K L Howe US Patent No 3963807 1972 278 CN Patent No 101608005A 2008 279 RF Patent No 2325733C1 2007 280 EU Patent No EP1935915A1 2006 281 CN Patent No 1911974B 2006 282 E Wenz V Buchholz H Eichenauer U Wolf RJ Born U Jansen W Dietz US Patent No 20030130433A1 2002 283 USSR Patent No 1562852A1 1987 284 Z Kaaruhansu JPN Patent No 55050100 1979 285 F Wenzel HD Bruemmer and M Krieg US Pat ent No 4241203 1978 286 W H Jo T Kim I C Hwang and N H Kwon US Patent No 20110201756A1 2010 287 D R Parikh R S Rikhoff L D Harris and V V Vanis FR Patent No 2011085202A1 2011 288 CN Patent No 1986635B 2006 289 RF Patent No 2391356C1 2008 290 RF Patent No 2008111361A 2006 291 RF Patent No 2009108986A 2009 292 RF Patent No 98101121A 1995 293 RF Patent No 2113444C1 1990 294 RF Patent No 2091403C1 1994 295 RF Patent No 2032695C1 1992 296 RF Patent No 2393173C2 2008 297 RF Patent No 2017865C1 1992 298 D Plee US Patent No 20100047153A1 2008 299 M Cerf D Colombie and T NZudie US Patent No 20040068069A1 2000 300 RF Patent No 2422467C2 2009 301 RF Patent No 2084463C1 1993 302 CN Patent No 106589191B 2015 303 CN Patent No 104231158B 2013 304 CN Patent No 101161694A 2007 305 N Hirota Y Sinmen N Matsuyama T Nii and H Shibatani MEX Patent No 2010013014A 2009 50 POLYMER SCIENCE SERIES C Vol 62 No 1 2020 CHERNIKOVA et al 306 RF Patent No 2560173C2 2013 307 RF Patent No 2528395C2 2012 308 RF Patent No 2550873C2 2013 309 RF Patent No 2393173C2 2008 310 A O Terentev A V Kutkin M M Platonov Y N Ogibin and G I Nikishin Tetrahedron Lett 44 7359 2003 311 A O Terentev M M Platonov Yu N Ogibin and G I Nikishin Synth Commun 37 1281 2007 312 G Caneba and Y Dar US Patent No 20050250919A1 2002 313 RF Patent No 2627264C1 2016 314 RF Patent No 2697882C1 2018 315 J Aoki H Okuda and M Ise JPN Patent No 2014141608A 2002 316 CN Patent No 106749807B 2016 317 RF Patent No 2016129965A 2014 318 J Yun US Patent No 9777081B2 2014 319 RF Patent No 2014108588A 2014 320 RF Patent No 2565767C2 2014 321 RF Patent No 2376319C2 2007 322 CN Patent No 106589193B 2016 Translated by T Soboleva