Molecular weight effects in the hydrogenation of model polystyrenes using platinum supported on wide-pore silica

A kinetic study of the heterogeneous catalytic hydrogenation of model polystyrenes (PS) using a catalyst comprised of Pt on a wide-pore silica support was performed. The effect of molecular weight on the initial rate of hydrogenation, r₀, was investigated under conditions where mass transfer limitations were minimized. The r₀ values for a series of model polystyrenes ranging in molecular weight from 1.5 to 276 kg/mol were measured by monitoring hydrogen uptake with a digital mass flow controller. The initial rate of hydrogenation was found to be inversely proportional to PS molecular weight. For molecular weights up to 102 kg/mol, r₀ scaled with the number-average degree of polymerization, X̄_n, to the -0.15 power. The two highest molecular weight samples, 190 and 276 kg/mol, exhibited significantly slower initial rates of hydrogenation and did not follow this trend. These results are explained in terms of equilibrium structure and conformational dynamics of a polymer on a metal surface and the relative dimensions of the polymer coils and Pt crystallites.