The bulk ring-opening copolymerization mechanism of octamethylcyclotetrasiloxane (D4) with N-β-aminoethyl-γ-aminopropylmethyldimethyoxysilane (APAEDMS) and its key controlling elementary reactions were firstly investigated and demonstrated by means of the Monte Carlo method in this paper.With consideration of the cost of computation as well as the precision of simulated results

a new computational model was developed to simulate the reaction system based on the free volume theory and the chemical reaction kinetic theory.The intrinsic reaction rate constants were obtained by simulating the main elementary reactions of copolymerization. This paper indicates from microscopic level: (1) The bulk copolymerization mechanism of D4 with APAEDMS is an anion chain-reaction mechanism with the characteristics of the step polymerization.(2) The chain-condensation reaction and its reversible reaction are the key controlling elementary reactions of the copolymerization mechanism for the studied system

while the chain initiation elementary reactions are not the key controlling elementary reactions of the copolymerization mechanism for the studied system.