# Influence of Monomer Sequence on the Cyclization Behavior of Poly(acrylonitrile-_co_-acrylamide)
###### Abstract
In the latter model, the amide group initiates the cyclization reaction if the neighboring nitrile group has isotactic placement, and the conjugation spreads over the backbone in the case of isotactic configuration of AN units (intramolecular cyclization) and transfers to another chain in the case of sindiotactic configuration of AN units (intermolecular cyclization). In air atmosphere, new reactions take place along with cyclization in similar temperature range. This conclusion is confirmed by FTIR analysis of the copolymers subjected to thermo-oxidative stabilization, which revealed the slow formation of pyridine structure and slow consumption of NH\({}_{2}\) groups [25]. The authors proposed that two processes compete with each other: intramolecular and intermolecular cyclization followed by oxidative reactions, according to Scheme 2. It was supposed that thermal treatment in air leads to the acceleration of dehydration reaction rather than cyclization. Besides, during the short induction period, amide groups react with neighboring nitrile groups forming cyclic structures. After reaching the critical concentration of these structures, the conjugation starts, first through dehydration reaction, followed by cyclization reaction. Cylization behavior of various AN and AAM copolymers is described [22; 23; 24; 25; 26; 27; 28]. However, they were synthesized in different conditions (solvent, temperature, etc.). Meanwhile, the change of the radical copolymerization method (copolymerization in solution, precipitative copolymerization, etc.) affects the monomer reactivity and changes the monomer sequence distribution. For example, in DMF or DMF-water mixtures (where water is the non-solvent for the AN and AAM copolymer), the monomer reactivity ratios are r\({}_{\text{AN}}\) = 0.86 and r\({}_{\text{AAM}}\) = 0.81 [29]. The polarity of non-solvent gives rise to a large variety of the monomer apparent reactivity ratios for AN and AAM [30]. Copolymerization in DMSO results in r\({}_{\text{AN}}\) = 0.65 and r\({}_{\text{AAM}}\) = 1.68 [31]. As a result, the comparison of the kinetics of cyclization of nitrile groups for the copolymers that differ simultaneously in their composition, monomer sequence distribution, and compositional heterogeneity becomes impossible. So, the question about the optimal molecular structure of AN and AAM copolymer that is suitable for production of carbon fibers remains unsolved. Previously, we have shown that RAFT copolymerization in dimethylsulfoxide (DMSO) solution may be applied to the AN and AAM and results in the formation of copolymers with M\({}_{\text{n}}\) = 10-30 kg\(\cdot\)mol\({}^{-1}\) and dispersity D = 1.5-1.6 [32]. We have also demonstrated that rheological and cyclization behavior of RAFT-based copolymers and copolymers containing 2 and 10 mol. % of AAM prepared by conventional free radical copolymerization is different. However, the detailed study of the cyclization behavior of the copolymers was beyond our interest at that time. Recently, we have also studied the influence of alkyl acrylate nature on the rheological and cyclization behavior of ternary copolymers of AN, AAM, and alkyl acrylate of given composition synthesized by RAFT copolymerization [33; 34]. We have shown that the cyclization rate and formation of ladder structure increases in the following range of acrylate monomer: methyl acrylate \(<\) butyl acrylate \(<\) 2-ethylhexyl acrylate \(<\) lauryl acrylate. In the present work, we continued working with the RAFT-based copolymers of AN and AAM, aiming to study in detail cyclization behavior of RAFT-based copolymers. The combination of RAFT mechanism and the controlled addition of AAM in the reaction should provide the copolymer with a tunable composition profile and allows the study of the influence of the molecular structure of the copolymers on their cyclization behavior.