Polymerization of sterically hindered a-olefins with single-site group 4 metal catalyst precursors

A variety of group 4 metal catalytic systems (C2-symmetric {EBTHI}-, {SBI}-type zirconocene complexes (C2-1–4); C1-symmetric (C1-5–8) and Cs-symmetric (Cs-9) {Cp/Flu}-type zirconocene complexes; Cp*2ZrCl2 (Cp* 2-10)), half-metallocene complexes (CpTiCl3, HM-11), constrained-geometry (CGC-12) titanium catalysts) and post-metallocene catalysts (Dow’s ortho-metallated amido-pyridino hafnium complex (PM-13)) have been screened in the polymerization of the sterically demanding 3-methylbut-1-ene (3MB1) and vinylcyclohexane (VCH). All systems proved to be sluggishly active under regular conditions (toluene, 20°C; MAO as cocatalyst) towards 3MB1, with productivities in the range 0–15 kg.mol–1.h–1. Higher productivities (up to 75 kg.mol–1.h–1) were obtained in the polymerization of VCH with C1-symmetric metallocene catalysts under the same conditions, while Cs-symmetric systems were found to be completely inactive. For both 3MB1 and VCH, under all conditions tested, the most productive catalyst appeared to be Dow’s post-metallocene system PM-13/MAO. Optimization of the polymerization conditions led to a significant enhancement of the productivities of this catalyst system towards both 3MB1 and VCH up to 390 and 760 kg.mol–1.h–1, respectively (Tpolym = 70°C). 13C NMR spectroscopy studies revealed that all isolated P(3MB1) and P(VCH) polymers were isotactic, regardless the nature/symmetry of the (pre)catalyst used. The nature of the chain-end groups in P(3MB1) is consistent with two different chaintermination mechanisms, namely b-H elimination/transfer-to-monomer for C2-1/MAO and chain-transfer to Me3Al for PM-13/MAO systems, respectively. For polymerization of VCH with PM-13/MAO at 70°C, b-H elimination / transfer-to-monomer appeared to be the main chain termination reaction.