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)(NtBu); EBICGC = (mu-CH2CH2-3,3'){(eta5-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 107 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 approximately C4H7SiH3 < C3H5SiH3. Alkenylsilane comonomer incorporation levels are consistently higher for CGCTiMe2-mediated copolymerizations (up to 54%) in comparison with EBICGCTi2Me4-mediated copolymerizations (up to 32%). The long-chain branching levels as compared to the total branch content follow the trend C3H5SiH3 < C4H7SiH3 approximately C6H11SiH3 approximately C8H15SiH3, 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 alpha-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 C6 exhibit a linear relationship between Mn and [alkenylsilane](-1), consistent with a simple silanolytic chain termination mechanism.
Amin
Smruti B
SB
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
Marks
Tobin J
TJ