Influence of Monomer Sequence on the Cyclization Behavior of Poly(acrylonitrile-<i>co</i>-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 dibenzyl trithiocarbonate 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 ~3 mol. % for random copolymer and ~6 mol. % for block-gradient copolymer. The activation energy of ionic cyclization was equal to 89 ± 3 kJ·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.

Keywords

• polyacrylonitrile
• precursor of carbon fibers
• cyclization
• thermo-oxidative stabilization