Intrinsic Branching Effects in Syndiotactic Copolymers of Propylene and Higher α-Olefins

Copolymers of propylene with higher α-olefins (4-methyl-1-pentene, 1-hexene, 1-octene, and 1-decene) have been synthesized at 0 °C using the syndioselective catalysts Me₂C(η⁵-C₅H₄)(η⁵-C₁₃H₈)ZrCl₂ (1), Me₂C(η⁵-C₅H₄)(η⁵-C₂₉H₃₆)ZrCl₂ (2), and Me₂Si(η¹-C₂₉H₃₆)(η¹-N-tBu)ZrCl₂·OEt₂ (3) activated with methylaluminoxane (MAO). For each catalyst, the observed melting temperature (T_m) depression of the syndiotactic copolymers was found to be linearly dependent on the molar fraction of comonomer incorporated, but little dependence on the identity of the comonomer was detected. The observed rmrr stereochemical pentad fraction present in the copolymers was proportional to the mol % of comonomer incorporation, suggesting that each comonomer insertion induces an average of 1.2 site epimerization stereoerrors for the three catalysts operating at 0 °C. The catalyst 3/MAO produced the fewest enantiofacial misinsertions and the copolymers with the highest melting temperatures for any given incorporation of comonomer. The unequaled syndioselectivity of 3/MAO has made it possible to more accurately measure the intrinsic effect of branches on the melting temperature of syndiotactic copoly[propylene/higher α-olefin]. The best fit linear relationship was found to be T_m = 161.7 °C − 7.6 °C(mol % comonomer).