| Abstract: | The effect of diphenyl thiourea (DPTU) on the radical polymerization of methyl methacrylate (MMA) has been studied in benzene solution at 50°C. with the use of cumene hydroperoxide (CHP), p-menthane hydroperoxide (PMHP), tert-butyl perbenzoate (tBPBz), di-tert-butyl peroxide (DBP), and dicumyl peroxide (DCP) as initiators. In the CHP-initiated polymerization, the rate of polymerization increased appreciably on addition of DPTU with a linear dependence on the square root of DPTU concentration up to a maximum which was observed when the ratio of the concentration of CHP to DPTU was 2.5. Then the rate decreased gradually with increasing DPTU concentration in the range greater than the above ratio. It was found from kinetic studied that the overall polymerization rate Rp was expressed by the equation: Rp = Kperoxide]1/2 DPTU]1/2MMA], where K is the rate constant, α = 1.2 for CHP and α = 1.0 for tBPBz. It was thought that the acceleration effect observed was due to a redox reaction caused by the interaction of a peroxide–monomer and/or a peroxide–solvent complex with DPTU, and the decrease in the polymerization rate which was observed over a certain concentration of DPTU was due to the action of the oxidized product of DPTU as a transfer agent. The effect of substituents was studied by using para and meta-substituted DPTU. It was found that the polymerization rate increased as electron-donating substituents are added to the benzene ring of DPTU with considerable dependence on Hammett's equation (p = ?0.36). The acceleration effect is also observed for PMPH-and tBPBz-initiated polymerizations, whereas the DCP- and DBP-initiated systems show no effects on the polymerization rate. |
