Metallocene-catalyzed ethene polymerization : long-chain branched polyethene

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52, [98]
Acta polytechnica Scandinavica. Ch, Chemical technology series, 290
Long-chain branches and narrow molecular weight distribution is a novel structure combination in polyethene, which has only been possible to achieve with single-center catalysis. Long-chain branches, even at very low concentrations, have a strong effect on the polymer melt behavior and, thereby, the processing properties. This work deals with ethene polymerization using group IV metallocene catalysts and the examination of long-chain branching in polyethene. Long-chain branching in metallocene catalysis is believed to take place via a copolymerization route, in which a vinyl terminated polyethene chain is incorporated into a growing polymer chain. Understanding the chain transfer mechanisms (vinyl end-group formation) and copolymerization abilities of metallocene catalysts have been important issues in this work. The examination of the polymerization behavior of several metallocene compounds revealed that chain transfer mechanisms were catalyst specific. Depending on the catalyst structure, the termination of chain growth occurred via β-H elimination, chain transfer to the monomer, or chain transfer to the cocatalyst. The vinyl selectivities were between 20 and 100%. Comonomer response in ethene and 1-olefin copolymerization also depended on the catalyst structure. 10-fold differences in comonomer reactivity ratios were observed. Long-chain branching analysis of homopolyethenes produced with different metallocene catalysts indicated that the catalysts with high vinyl selectivity and good copolymerization ability were the most prominent in producing a polymer with modified rheological properties. In addition to rheological measurements, 13C NMR spectroscopy also showed the presence of long-chain branches. Besides the choice of catalyst, the polymerization conditions had a major impact on long-chain branch contents. Adjusting the ethene, hydrogen, or comonomer (1-olefin or nonconjugated α,ω-diene) concentration changed the rheological properties of the polymers.
metallocene catalysts, polyethylene, copolymerization, characterization, long-chain branching, rheology
Other note
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