Abstract:
In propylene polymerization catalyzed by transition metal complexes, chain transfer reactions exert crucial influence on the structures of terminal groups as well as molecular weights of the resultant polymers. Among the known chain transfer reactions,
β-methyl elimination (
β-Me elimination) could lead to the production of ally-terminated polymer chain selectively, thus attracts great attention. Up to data, it is still a great challenge to achieve high
β-Me elimination selectivity systematically during propylene polymerization process. In this work, a series of ethylene-bridged indenyl-fluorenyl complexes, C
2H
4(5,6-cyclopenta-Ind)(Flu)ZrCl
2(
C1), C
2H
4 (5,6-cyclopenta-Ind)(2,7-
tBu
2-Flu)ZrCl
2(
C2), C
2H
4(3-Bn-5,6-cyclopenta-Ind) (2,7-
tBu
2-Flu)ZrCl
2(
C3), C
2H
4 (3-Bn-5,6-cyclopenta-Ind)(2,7-
tBu
2-Flu)HfCl
2(
C4), were synthesized and characterized. Upon activation with methylaluminoxane(MAO), these complexes could catalyze the polymerization of propylene with moderate to high activities to produce propene oligomers or polymers with vinylidene or allyl-dominant chain ends. Complex
C3 bearing a benzyl group at the 3- position of the indenyl ring showed significantly higher
β-Me elimination selectivities (80%—83%) at 40—100 °C than the other zirconocene complexes
C1 and
C2, and meanwhile led to the formation of propene oligomers with significantly lower molecular weights (
Mn = 800~13 600) than those obtained by the other zirconocene complexes, suggesting that the dominant
β-Me elimination chain transfermode led to the decrease of molecular weights of resultant polymeric products. The hafnocene complex
C4 also showed the highest
β-Me elimination selectivity among these complexes (87%), however its activity was very low.