Dual site ethene/1-hexene copolymerization with MAO activated (1,2,4-Me3Cp)(2)ZrCl2 and (Me5Cp)(2)ZrCl2 catalysts. Possible transfer of polymer chains between the sites

摘要

Ethene homopolymerization and copolymerization in toluene solution with varying amounts of 1-hexene were catalyzed by the methylaluminoxane (MAO) activated zirconocene catalysts (1,2,4-Me-3-Cp)(2)ZrCl2 and (Me5Cp)(2)ZrCl2 and a mixture of these two in a 1:1 molar ratio. The dual site catalyst (1:1 mixture) has a homo- and copolymerization activity intermediate between the individual catalysts. Deviations from the expected average behavior taking into account the ca. 6-fold higher 1 h yield of (1,2,4-Me3Cp)(2)ZrCl2 are observed. The dual site system results in a copolymer with ca. 30% too low comonomer incorporation. The melting endotherm attributed to (Me5Cp)(2)ZrCl2 is found to be ca. 30-70% too high, and the observed molecular weights for the copolymers are higher than expected and even higher than for the two individual catalysts. Chain transfer to trimethylaluminum (TMA) is found to become less important at high comonomer concentrations concurrent with an increase of the concentration of vinyl and trans-vinylene unsaturations. Further, it is noteworthy that with the dual site system it is possible to produce ethene/1-hexene copolymers with narrow molecular weight distribution as a mixture of chains containing low and high comonomer contents. The observed discrepancies in the mixed system from the average of the (1,2,4-Me3Cp)(2)ZrCl2 and (Me5Cp)(2)ZrCl2 catalysts are provisionally explained by chain transfer between the sites coupled with an activity increase of the latter of these sites. [References: 30]