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Model catalysts having specially designed active species on support surfaces were employed to understand relationships between the structure/state of active sites and their catalytic performances for two classes of industrial olefin polymerization catalysts: Ziegler‐Natta and Phillips catalysts. Propylene polymerization using a TiCl3/MgCl2 model Ziegler‐Natta catalyst clarified an important role of the Ti dispersion state for the polymer stereo & chemical structures. Model Phillips catalysts having monochromate and dichromate structures on silica support imparted knowledge that dinuclear species is more responsible for the production of α‐olefin comonomer in ethylene polymerization, thus giving more branched polyethylene.

The Phillips CrO x /SiO 2 catalyst is an important industrial catalyst for ethylene polymerization. However, understanding of the state of active sites and chain propagation mechanisms concerning the Phillips catalyst is still waiting for conclusive evidence. In this work, the Phillips CrO x /SiO 2 catalyst, having been calcined, was used for investigating the copolymerization of ethylene and cyclopentene in the presence of triethylaluminum as a cocatalyst for the first time. The microstructures of the polymers were investigated with 13 C-NMR and gel permeation chromatography methods. Because of the absence of internal double bond (C¼ ¼C) in the copolymer main chain, the ring-opening metathesis polymerization of cyclopentene was excluded during the copolymerization stage of ethylene and cyclopentene. Also, the 1,2-insertion and 1,3-insertion of cyclopentene into the polyethylene main chain were confirmed. This evidence strongly implies that Cr¼ ¼C species may not be the active sites for chain propagation; instead, the CrAC active site model under the Cossee-Arlman chain propagation mechanism may be responsible for the chain propagation during the normal polymerization period.

Control factors of branching in ethylene polymerization on CrOx/SiO2 catalysts were comprehensively investigated by varying catalyst structure and polymerization conditions. Variation of the polymerization conditions such as polymerization time, catalyst concentration, polymerization temperature, and alkylaluminum concentration indicated that the branch frequencies in produced polyethylene were highly relevant to the polymerization yield, plausibly through concentration of in‐situ‐produced α‐olefins. A method to fairly compare the branching abilities of different catalysts was proposed. The application of the method to model catalysts with controlled chromium nuclearity proved an important role of dinuclear species in methyl branching through olefin metathesis.

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