Mechanism of Propylene Polymerization with MgCl₂-Supported Ziegler–Natta Catalysts Based on Counting of Active Centers: The Role of External Electron Donor

Abstract

Four R1R2Si(OMe)2 type compounds were added as an external electron donor (De) in propylene polymerization with TiCl4/Di/MgCl2 type supported Ziegler–Natta catalysts (Di = internal donor). Each polypropylene (PP) sample was fractionated into three parts (atactic, medium-isotactic and isotactic PP), and the number of active centers ([C*]/[Ti]) in each PP fraction was counted using 2-thiophenecarbonyl chloride as the quenching and tagging agent. The gradual decrease of [C*]/[Ti] with De/Ti ratio is ascribed to competitive and reversible coordination of De on either central Ti of the active center or Mg adjacent to the central Ti. The former coordination leads to deactivation of C*, and the latter one leads to still living C*. The chain propagation rate constant (kp) of the active centers producing atactic, medium-isotactic and isotactic PP change with De/Ti in different ways. Only the kp of active centers producing isotactic PP was evidently increased by De. Enhancement in isotacticity of PP product is found to be a combined result of both deactivation of active centers by De and selective activation of the active centers that produce isotactic PP. Changing the alkyl groups of R1R2Si(OMe)2 leads to an altered balance between the deactivation and activation effects of De.