Alkenylsilane structure effects on mononuclear and binuclear organotitanium-mediated ethylene polymerization: Scope and mechanism of simultaneous polyolefin branch and functional group introduction

Smruti B. Amin, Tobin J. Marks

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Abstract

Alkenylsilanes of varying chain lengths are investigated as simultaneous chain-transfer agents and comonomers in organotitanium-mediated olefin polymerization processes. Ethylene polymerizations were carried out with activated CGCTiMe2 and EBICGCTi2Me4 (CGC = Me2Si(Me4C5)(NtBu); EBICGC = (μ-CH2CH2-3,3′){(η5-indenyl)[1- Me2Si(tBuN)]}2) precatalysts in the presence of allylsilane, 3-butenylsilane, 5-hexenylsilane, and 7-octenylsilane. In the presence of these alkenylsilanes, high polymerization activities (up to 10 7 g of polymer/(mol of Ti-atm ethylene·h)), narrow product copolymer polydispersities, and substantial amounts of long-chain branching are observed. Regardless of Ti nuclearity, alkenylsilane incorporation levels follow the trend C8H15SiH3 < C6H 11SiH3 ≈ C4H7SiH3 < C3H5SiH3. Alkenylsilane comonomer incorporation levels are consistently higher for CGCTiMe2-mediated copolymerizations (up to 54%) in comparison with EBICGCTi2Me 4-mediated copolymerizations (up to 32%). The long-chain branching levels as compared to the total branch content follow the trend C 3H5SiH3 < C4H7SiH 3 ≈ C6H11SiH3 ≈ C 8H15SiH3, with gel permeation chromatography-multi-angle laser light scattering-derived branching ratios (gM) approaching 1.0 for C8H15SiH3. Time-dependent experiments indicate a linear increase of copolymer Mw with increasing polymerization reaction time. This process for producing long-chain branched polyolefins by coupling of an α-olefin with a chain-transfer agent in one comonomer is unprecedented. Under the conditions investigated, alkenylsilanes ranging from C3 to C8 are all efficient chain-transfer agents. Ti nuclearity significantly influences silanolytic chain-transfer processes, with the binuclear system exhibiting a sublinear relationship between Mn and [alkenylsilane]-1 for allylsilane and 3-butenylsilane, and a superlinear relationship between Mn and [alkenylsilane]-1 for 5-hexenylsilane and 7-octenylsilane. For the mononuclear Ti system, alkenylsilanes up to C 6 exhibit a linear relationship between Mn and [alkenylsilane]-1, consistent with a simple silanolytic chain termination mechanism.

Original languageEnglish
Pages (from-to)2938-2953
Number of pages16
JournalJournal of the American Chemical Society
Volume129
Issue number10
DOIs
Publication statusPublished - Mar 14 2007

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ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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