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.

Automated summary

In the olefin polymerization process using MgCl2-supported Ziegler-Natta (Z-N) catalysts, the apparent propagation rate constant (kp) decreases over time due to the monomer diffusion limitation in the polymer/catalyst particles. A simplified multi-grain particle model was proposed to establish a correlation between (kp) and experimentally determined kinetic parameters. The decline in (kp) during polymerization was accurately fitted using a linear correlation between lg(kp) and [(?(cat)m(p))/(?(p)m(cat)) + 1](1/3) based on the particle model. The real propagation rate constant (kp) was estimated by extrapolating the fitting line to the starting point of polymerization, where no concentration gradient exists. The degree of monomer diffusion limitation was represented by the slope of the lg(kp) versus [(?(cat)m(p))/(?(p)m(cat)) + 1](1/3) line (lgd), which varied in different reaction systems based on the knowledge of olefin diffusion in the polymer phase.