Alkynyl Ether Labeling: A Selective and Efficient Approach to Count Active Sites of Olefin Polymerization Catalysts

Abstract

Accurately measuring molecular kinetics of catalytic olefin polymerization has been a challenging objective. Many methods have been proposed in the literature, but all of them have drawback(s). In this paper, we introduce a labeling method to count active sites employing methyl propargyl ether (MPE) as the quench-labeling agent. It is commercially available, does not react with Al-alkyl species, and has a labeling efficiency close to 100%. The labeling reaction was evidenced by a mechanistic study on the reaction between the model system Cp2ZrMe2/MAO (Cp = cyclopentadienyl) and MPE that it may occur through a coordination–insertion mechanism without noticeable multiple insertions. The method was benchmarked by studying a MgCl2-supported Ziegler–Natta catalyst in 1-hexene polymerization. The fraction of the active transition metal χ* is found to be <1%. It rises in the very first few seconds and reaches a plateau within 10 s in the absence of precontact. With precontact, χ* remains constant over polymer yield from 0.3 up to 5.3 g(PH) g(cat)−1, demonstrating that χ* does not necessarily grow during polymerization. Longer precontact resulted in decreased χ* and decreased average rate constant of propagation <kp> which is in the order of 102 M–1 s–1.