Diimine Chelates of Late Transition Metals

Nickel and palladium a-diimine chelates in the presence of MAO polymerize ethylene to polymers with weight-average molecular weights of 104-106. Palladium initiators yield higher molecular weights than do Ni initiators. Unlike Ni and Pd, platinum shows no activity. 

Both Ni and Pd initiators polymerize propene to give a combination of different microstructures, including 1,2- and 3,1-placements as well as methyl branches (via 2,3-placement) and long-chain branching. Room-temperature polymerizations with both Ni and Pd initiators yield atactic polypropene. Low temperature polymerizations proceed by chain end control to yield moderate syndioselectivity, (rr) as high as 0.8, but usually less [Busico and Cipullo, 2001 McCord et al, 2001 Pappalardo et al., 2000 Zambelli et al., 2001]. 

Ni and Pd initiators polymerize cycloalkenes through the double bond with cis 1,3-place-ment (Sec. 8-6a). The only reported polymerization of an acyclic 2-alkene, specifically trans-2-butene, involves the use of an a-diimine initiator [Leatherman and Brookhart, 2001]. 

and Ru initiators are much less studied. They are not as active as the Ni and Pd initiators, but this can be altered in some cases by appropriate ligands. 

Many other chelates of Ni and Pd have been studied, including the phosphorus and alkoxide chelate XLVII [Gibson and Spitzmesser, 2003 Mecking, 2001]. 

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