The direct and autocatalytic ionic cyclization mechanism of copolymers of acrylonitrile-itaconic acid P (AN-IA) have been studied using density functional theory (DFT) at B3LYP/6-31+G (d, p) level, respectively. By comparing the direct mechanism with autocatalytic mechanism, it was found that the free energy barriers of the cyclization reaction were hugely lowered by including a nearby P(AN-IA) copolymer serving as a catalyst. The calculated free energy barrier of cyclization is about 22.74 kcal mol(-1), which is in good agreement with the experimental activation barrier of 26.1 kcal mol(-1). The cyclization of P(AN-IA) would proceed by rather an autocatalytic mechanism than a direct mechanism. (C) 2017 Elsevier B.V. All rights reserved.