The mechanism of ethylene polymerization by the widely used Phillips catalyst remains controversial. In this work, we compare initiation, propagation, and termination pathways computationally using small chromasiloxane cluster models for several previously proposed and new mechanisms. Prohibitively high activation barriers for propagation rule out previously proposed chromacycle ring expansion and alternating alkylidene/chromacycle mechanisms. A new oxachromacycle ring expansion mechanism has a plausible propagation barrier, but initiation is prohibitively slow. On sites with adjacent bridging hydroxyls, either Si(OH)CrII-alkyl or Si(OH)CrIII-alkyl, initiated by proton transfer from ethylene, chain growth by a Cossee-Arlman-type mechanism is fast. However, the initiation step is uphill and extremely slow, so essentially all sites remain trapped in a dormant state. In addition, these sites only make oligomers, because when all pathways are considered, termination is faster than propagation. A monoalkylchromium(III) site without an adjacent proton, (≡SiO)2Cr-alkyl, is viable as an active site for polymerization. Initiation may occur with oxidative addition of ethylene with spin-crossing, followed by Cr-C homolysis.

Activation of the Phillips polymerization catalyst by ethylene oxidative addition followed by homolysis: A computational assessment

251st National Meeting (2016)

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