Mechanism of “Autoacceleration” in the Thermal Oxidative Polymerization of α-Methylstyrene

Thermal oxidative polymerization of α-methylstyrene (AMS) has been studied at various temperatures (45−70 °C) and pressures (50−400 psi). Due to its high electron dense double bond, it undergoes thermal oxidative polymerization even at low temperatures fairly easily. The major products are poly(α-methylstyrene peroxide) (PMSP), and its decomposition products are acetophenone and formaldehyde. Above 45 °C the rate of polymerization increases sharply at a particular instant showing an “autoacceleration” with the formation of a knee point. The “autoacceleration” is supported from the fact that the plot of R p vs T shows a rapid rise, and the plot of ln R p vs 1/ T is non-Arrhenius. The occurrence of autoacceleration is explained on the basis of acetophenone-induced cleavage of PMSP during polymerization, generating more initiating alkoxy radicals, which subsequently leads to the rapid rise in the rate of polymerization. The mechanism of autoacceleration is supported by the change in order, activation energy, and activation volume before and after the knee point.