Polymerization of vinyl monomers initiated by chromium(II) acetate + organic peroxides

Abstract

The redox initiator systems consisting of chromium(II) acetate (Cr²⁺) and organic peroxides are studied in dimethylformamide (DMF) at low temperatures. The second-order rate constants of reduction of benzoyl peroxide (BPO) with Cr²⁺ are 0.65, 0.24 and 0.075 dm³ mol^–1 min^–1 at –10, –20 and –28°C, respectively.

The activity of organic peroxides for the redox polymerization of methyl methacrylate (MMA) at –28°C decreases in the following order; hydroperoxide > diacylperoxide > perester = dialkyl peroxide.

The kinetics of polymerization of MMA are different from that of a conventional free-radical polymerization. The dependence of the initial rate of polymerization (R_p) on the concentrations of initiator components, Cr²⁺ and BPO, is very small; the kinetic orders are 0.1 and 0.2, respectively. However, R_p is proportional to the square of the concentration of MMA. This result is explained in terms of a primary radical termination.

The polymerization of MMA with the Cr²⁺+ BPO system is found to occur continuously without a dead end polymerization after red Cr²⁺ changed to blue Cr³⁺, i.e., after Cr²⁺ was consumed by the redox reaction with BPO. The degree of polymerization of PMMA obtained by this initiator system below 30°C increases with an increase in monomer conversion.