Estimation of the chain propagation rate constants of propylene polymerization and ethylene-1-hexene copolymerization catalyzed with MgCl2-supported Ziegler-Natta catalysts

In olefin polymerization with MgCl2-supported Ziegler-Natta (Z-N) catalysts, the apparent propagation rate constant (k(p))(a) calculated by R-p = (k(p))(a) [C*] C-Me (C-Me is equilibrium monomer concentration in the reaction system) declines with reaction time for gradually developed monomer diffusion limitation in the polymer/catalyst particles. In this work, a simplified multi-grain particle model was proposed to build correlation between (k(p))(a) and other kinetic parameters that can be determined experimentally. Rate profiles of propylene polymerization and ethylene-1-hexene copolymerization by three MgCl2-supported Z-N catalysts were determined, and the (k(p))(a) data was calculated using [C*] determined by quench-labelling the propagation chains with acyl chloride. Decline of (k(p))(a) in each polymerization process was precisely fitted by the linear correlation between lg(k(p))(a) and [(?(cat)m(p))/(?(p)m(cat)) + 1](1/3) developed on the particle model. Real propagation rate constant (k(p)) was estimated by extrapolating the fitting line to the starting point of polymerization, where no concentration gradient exists. According to the particle model, the slope of the lg(k(p))(a) versus [(?(cat)m(p))/(?(p)m(cat)) + 1](1/3) line (lgd) represents the degree of monomer diffusion limitation. Variations of parameter d found in the studied reaction systems can be reasonably explained based on the knowledge of olefin diffusion in the polymer phase.