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 chain-termination 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.