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

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 (Rp) on the concentrations of initiator components, Cr2+ and BPO, is very small; the kinetic orders are 0.1 and 0.2, respectively. However, Rp 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.