Isotactic 1-butene polymerization promoted by C2- symmetric metallocene catalysts

1-Butene polyinsertion promoted by two typical C-2-symmetric zirconocene catalysts (i.e. rac-ethylenebis(1-indenyl)ZrCl2 (1) and rac-dimethylsilylbis(1-indenyl)ZrCl2) (2) was investigated under both hydrooligomerization and polymerization conditions and compared with that of propene. It was found that, in spite of a similar regiospecificity for the two monomers, the fraction of ''dormant'' sites is higher in 1-butene polymerization, as a result of a (much) lower reactivity of a 2,1 last-inserted unit. Accordingly, in the investigated cases, 2,1 units of 1-butene were not incorporated in the growing chains but either isomerized to 4,1 units or remained at the chain end until they underwent chain transfer via beta-hydrogen elimination. Experimental evidence is also presented of an epimerization reaction of the growing poly(1-butene) chain with a 1,2 last-inserted unit, leading to a decrease of polymer stereoregularity with decreasing monomer concentration.