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Living polymerization techniques1 allow the synthesis of polymers with predictable molecular weights and narrowmolecular weight distributions, end-functionalized polymers andwell-defined block copolymers. While intense efforts haverecently focused on development of homogeneous single-site,early metal d0 and d0fn catalysts for polymerization of ethyleneand R-olefins (particularly propylene),2 rapid chain transferprocesses in these systems have limited their use as iving polymerization catalysts. The living polymerization of R-olefins has been achieved only in rare instances and at very low temperatures.3,4 We describe here the development of a procedurefor living polymerization of R-olefins based on recently reported NiII-R-diimine catalysts5 and application of this procedure to the synthesis of diblock and triblock poly(R-olefins). Access to well-defined transition metal olefin polymerizationcatalysts has allowed detailed mechanistic studies of this importantclass of catalysts. While much attention has centered on earlytransition metal d0 and lanthanide d0fn systems,1-7 we have focusedon developing single-site late metal catalysts. Our initial report8 that Ni(II) and Pd(II) aryl-substituted R-diimine complexes bearing bulky ortho substituents were capable of polymerizing ethyleneand R-olefins has stimulated extensive experimental9-20 andtheoretical21-27 work on these catalysts. The Pd(II) systems yield highly branched polyethylene under typical polymerization conditions,while the microstructure of polyethylene produced by theNi(II) systems is dependent on monomer concentration, reactiontemperature, and ligand structure. |
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zhangzws
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2楼2010-09-16 21:12:47
zhangzws
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3楼2010-09-16 21:38:01
zhangzws
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4楼2010-09-16 21:40:03
