Penultimate Effects and Chain Epimerization in Propene-Norbornene Copolymers by rac-Me2Si(2-Me-Ind)2ZrCl2 C2-Symmetric Metallocene

The copolymerization of propene (P) and norbornene (N) by the catalytic system composed of rac-Me2Si(2-Me-Ind)(2)ZrCl2 (3) and methylaluminoxane (MAO) was investigated and compared with that by rac-Et(Ind)(2)ZrCl2 (1) and MAO. Methyl 2 substitution on the indenyl ligand in 3 has an unexpected and strong influence on the catalyst behavior in P-N copolymerization, causing a strong decrease in catalytic activity, molar fraction f(N), T-g, and M-n values. P-N copolymers with maximum of 16 mol % of norbornene were obtained by 3-MAO in contrast with those highly alternating obtained by 1-MAO. The microstructural analysis by C-13 NMR of the copolymers at triad level and reactivity ratios r(i) and r(ij) obtained from diads and triads, respectively, give evidence of the tendency to alternate or not to alternate P and N comonomers and information on the probability of insertions of N and of the possible forms of P insertions (P-12, P-13, and P-21). Penultimate effects were demonstrated as 3-MAO has difficulty in accommodating a norbornene into Mt-P12N and Mt-P13N bonds. A great decrease in the tacticity oldie PP blocks in copolymers prepared with 3-MAO when increasing norbornene content in the feed revealed the high probability of R S isomerization of the last inserted propene. Chain end groups analysis showed a greater amount of 2-butenyl end groups arising from termination at a Mt-P-21 than of vinylidene arising from termination at a Mt-P-12. This is an evidence that the limiting step in P N copolymerization is the difficulty to insert a propene after N, which causes 2,1 insertions with subsequent isomerization to 1,3 propene insertions. This effect also originates chain epimerization under starved propene conditions with catalyst 3-MAO.