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

Abstract In olefin polymerization with MgCl 2 ‐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 MgCl 2 ‐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 [(ρ cat m p )/(ρ 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 [(ρ cat m p )/(ρ p m cat ) + 1] 1/3 line (lg d ) 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.