# Influence of Monomer Sequence on the Cyclization Behavior of Poly(acrylonitrile-_co_-acrylamide)
###### Abstract  
In this research, we have developed the approach to controlled synthesis of acrylonitrile-acrylamide copolymers with narrow molecular weight distribution and various monomer sequence distributions. By using diphenylxl trifluoracorbate and batch/semibatch polymerization, we have first synthesized random, gradient, and block-gradient copolymers containing 3.4-10.2 mol. % of acrylamide and revealed the influence of the monomer sequence on the cyclization behavior of poly(acrylonitrile-_co_-acrylamide) by combination of differential scanning calorimetry and Fourier transform infrared spectroscopy. This allowed us to find differences in cyclization behavior of the copolymers in argon and air atmosphere. Intramolecular cyclization was the main process proceeding in argon atmosphere. The radical mechanism of cyclization was suppressed already at the molar part of acrylamide units in copolymer exceeding \(\sim\)3 mol. % for random copolymer and \(\sim\)6 mol. % for block-gradient copolymer. The activation energy of ionic cyclization was equal to 89 \(\pm\) 3 kJ\(\cdot\)mol\({}^{-1}\) and was not influenced by both copolymer composition and chain microstructure in contrast to the rate of cyclization. The latter was increased with the rise of acrylamide content, the content of hetero-triads and in the range block-gradient < gradient < random structure. In air atmosphere, the oxidation reactions dominated over cyclization. The oxidation reactions were found to be less sensitive to copolymer composition and chain microstructure. polyacrylonitrile; precursor of carbon fibers; cyclization; thermo-oxidative stabilization +
Footnote †: journal: Applied Sciences  
average composition in conditions of free radical copolymerization, seems unlikely. At the same time, it becomes possible in conditions of reversible deactivation radical polymerization (RDRP) [10; 11; 12]. RDRP differs from conventional free radical polymerization due to appearance of new reversible termination and/or transfer reactions of the propagating radicals with special compounds, which suppress the irreversible termination reaction of the propagating radicals and result in the continuous conversion of deactivated macromolecules into propagating radicals. Thus, RDRP has similar features as a living anionic polymerization, i.e., the number-average molecular weight (M\({}_{\text{n}}\)) increases linearly with progress in monomer conversion, molecular weight distribution becomes narrow, and the random or gradient copolymers with high compositional homogeneity can form from the monomers with different reactivity [10]. The formation of gradient copolymers with different monomer sequence distribution in RDRP is possible even if the difference between the reactivity ratios of the comonomers is not significant, when monomers are dosed with different rates. So, semibatch reactors can be used to produce gradient copolymers by using different comonomer feed policies. The model was used to predict the following characteristics of copolymers of styrene-methyl methacrylate and acrylonitrile-methyl methacrylate in atom transfer radical polymerization (i.e., one of the RDRP techniques) using semibatch reactors: average MW, dispersity, copolymer composition, and comonomer sequence length at any polymerization time [13]. This simulation has demonstrated the possibility of varying the monomer unit distribution from random to gradient under conditions mentioned above. The combination of RDRP and the use of semibatch reactor was implemented for reversible addition-fragmentation chain transfer (RAFT) copolymerization of AN and N-isopropyl acrylamide (NIPA) for the synthesis of the copolymer with uniform distribution of NIPA units along the chain rather than its gradient distribution [14]. The batch copolymerization of AN and NIPA resulted in the formation of the gradient copolymer with the compositional drift of NIPA along the backbone.