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

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)(N^tBu); 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 (g_M) approaching 1.0 for C₈H₁₅SiH₃. Time-dependent experiments indicate a linear increase of copolymer M_w 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 M_n and [alkenylsilane]^-1 for allylsilane and 3-butenylsilane, and a superlinear relationship between M_n and [alkenylsilane]^-1 for 5-hexenylsilane and 7-octenylsilane. For the mononuclear Ti system, alkenylsilanes up to C ₆ exhibit a linear relationship between M_n and [alkenylsilane]^-1, consistent with a simple silanolytic chain termination mechanism.