Aqueous polymerization of methyl methacrylate initiated by ceric ion reducing agent systems in sulfuric acid medium

Abstract

Polymerization of methyl methacrylate (MMA) was carried out in aqueous sulfuric acid medium at 30°C using ammonium ceric sulfate (ACS)/methyl ethyl ketone (MEK) and ammonium ceric sulfate/acetone as redox initiator systems. A short induction period was observed with both the initiator systems, as well as the attainment of limiting conversion for polymerization reactions. The rate of ceric ion consumption, R Ce , was first order with respect to Ce(IV) concentration in the concentration range (0.5-5.5) X 10 -3 m, and 0.5 order with respect to reducing agent concentration in the concentration ranges (0.0480-0.2967M) and (0.05-0.3912M) for Ce(IV)-MEK and Ce(IV)-acetone initiator systems, respectively. A fall in R Ce was observed at higher reducing agent concentrations. The plots of R Ce versus reducing agent concentrations raised to the half power yielded straight lines passing through the origin, indicating the absence of complex formation between reducing agents and Ce(IV). The addition of sodium sulfate to maintain constant sulfate ion concentration in the reaction medium could bring down the R Ce values in the present reaction systems. The rate of polymerization of MMA, R p , increased with increase in Ce(IV), reducing agent, and monomer concentrations for the Ce(IV)-MEK initiator. The rate of polymerization of MMA is independent of Ce(IV) concentration and increased with an increase in reducing agent and monomer concentrations for the Ce(IV)-acetone initiator. At higher concentrations of reducing agent (0.4-0.5M), a steep fall in R p values was observed with both the initiator systems. The orders with respect to Ce(IV), MEK, and MMA using the Ce(IV)-MEK initiator were found to be 0.23, 0.2, and 1.29, respectively. The orders with respect to Ce(IV), acetone, and MMA using the Ce(IV)-acetone initiator were found to be zero, 0.42, and 1.64, respectively. Maintaining constant [SO 4 2- ] in the reaction medium could bring down R p values for the Ce(IV)-MEK initiator system. On the other hand, a rise in R p values with an increase in [Na 2 SO 4 ] could be observed when constant [SO 4 2- ] was maintained in the reaction medium for the Ce(IV)-acetone initiator system. A kinetic scheme involving direct attack of Ce(IV) on reducing agent, production of radicals, initiation, propagation, and termination of the polymeric radicals by bimolecular interaction is proposed. An oxidative termination of primary radicals by Ce(IV) is also included.