Influence of the catalyst and polymerization conditions on the long-chain branching of metallocene-catalyzed polyethenes

A study was made on the effects of polymerization conditions on the long-chain branching, molecular weight, and end-group types of polyethene produced with the metallocene-catalyst systems Et[Ind](2)ZrCl2/MAO, Et[IndH(4)](2)ZrCl2/MAO, and (n-BuCp)(2)ZrCl2/MAO. Long-chain branching in the polyethenes, as measured by dynamic rheometry, depended heavily on the catalyst and polymerization conditions. In a semibatch flow reactor, the level of branching in the polyethenes produced with Et[Ind](2)ZrCl2/MAO increased as the ethene concentration decreased or the polymerization time increased. The introduction of hydrogen or comonomer suppressed branching. Under similar polymerization conditions, the two other catalyst systems, (n-BuCp)(2)ZrCl2ZrCl2/MAO and Et[IndH(4)](2)ZrCl2/MAO, produced linear or only slightly branched polyethene. On the basis of an end-group analysis by FTIR and molecular weight analysis by GPC, we concluded that a chain transfer to ethene was the prevailing termination mechanism with Et[Ind](2)ZrCl2/MAO at 80 degrees C in toluene. For the other catalyst systems, beta-H elimination dominated at low ethene concentrations. (C) 2000 John Wiley & Sons, Inc.