Modified natural rubber latex by graft copolymerization with methyl methacrylate or styrene and hydrogenation using OsHCI(CO)(O₂)(PCy₃)₂ as a catalyst

Abstract

Graft copolymerization of methyl methacrylate or styrene onto natural rubber in latex form has been carried out using cumene hydroperoxide redox initiator. The synthesized graft copolymers were purified and then characterized by proton nuclear magnetic resonance. Grafting efficiency is discussed with respect to the influence of initiator concentration, monomer to rubber ratio and reaction temperature. The purified graft copolymer was then hydrogenated by using a homogeneous osmium complex catalyst, OsHCl(CO)(O₂)(PCy₃)₂. A detailed kinetic investigation of the hydrogenation was carried out by monitoring the amount of hydrogen consumption as a function of reaction time using a gas-uptake apparatus. The kinetic results for the hydrogenation of graft natural rubber indicated that the rate of hydrogenation exhibited a first -order dependence on the catalyst concentration and a first -order dependence on [C=C] for methyl methacrylate grafted natural rubber and a second-order dependence on [C=C] for styrene grafted natural rubber. Due to the presence of impurities within the natural rubber the rate of hydrogenation was found to decrease with increasing polymer concentration. The apparent activation energy for hydrogenation of natural rubber grafted with methyl methacrylate and natural rubber grafted with styrene was found to be 70.3 and 106.5 kJ/mol, respectively. The addition of a small amount of acid and PMMA or PS provided a beneficial effect on the hydrogenation of grafted natural rubber. The hydrogenated grafted natural rubber could be used as a compatibilizer for PMMA/EPDM or PS/EPDM blends. Thermal properties, mechanical properties and morphology of the blends were also investigated.