Chain length dependence of radical-radical termination in free radical polymerization: A pulsed laser photolysis investigation

A new method for analyzing the chain length dependence of termination rate coefficients (k(t)(i,j) where i and j are the chain lengths of the reacting radicals) in low conversion free radical polymerizations is described. The method involves comparing experimental molecular weight distributions of polymers formed in pulsed laser photolysis experiments with those predicted by kinetic simulation. The method is enabled by direct measurements of the concentration of radicals generated per laser pulse. Knowledge of the radical concentrations should mean that the only unknowns in the simulations are the termination rate coefficients. The analysis demonstrates the need for chain length dependent termination rate constants in describing polymerization kinetics. Polymerization kinetics were analyzed in terms of two models which purport to describe the functional form of k(t)(i,j). Use of a Smoluchowski model for k(t)(i,j), in conjunction with independently measured monomer diffusion coefficients, provides molecular weight distributions that closely match those obtained experimentally. However, the finding that reasonable fits can also be obtained using a geometric mean model to describe k(t)(i,j) indicates molecular weight distributions alone do not contain sufficient information for unambiguous model discrimination.