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-Me(2)Si(2-Me-Ind)(2)ZrCl(2) (3) and methylaluminoxane (MAO) was investigated and compared with that by rac-Et(Ind)(2)ZrCl(2) (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 (13)C 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-P(12)N and Mt-P(13)N 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.