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 CGCTiMe(2) and EBICGCTi(2)Me(4) (CGC = Me2Si(Me4C5)((NBu)-Bu-t); EBICGC = (mu-CH2CH2-3,3'){(eta(5)-indenyl)[1-Me2Si((BuN)-Bu-t)]}(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 < C6H11SiH3 approximate to C4H7SiH3 < C3H5SiH3. Alkenylsilane comonomer incorporation levels are consistently higher for CGCTiMe(2)-mediated copolymerizations (up to 54%) in comparison with EBICGCTi(2)Me(4)-mediated copolymerizations (up to 32%). The long-chain branching levels as compared to the total branch content follow the trend C3H5SiH3 < C4H7SiH3 approximate to C6H11SiH3 approximate to C8H15SiH3, 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 alpha-olefin with a chain-transfer agent in one comonomer is unprecedented. Under the conditions investigated, alkenylsilanes ranging from C-3 to C-8 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.