Coordination polymerization constrained geometry

The Dow corporation has recently developed constrained geometry addition polymerization catalysts (CGCT), typically Me2Si(C5Me4)(NBut)MCl2 (M = Ti, Zr, Hf) (141) activated with MAO. The homo-polymerization of a-olefins by CGCT afford atactic or somewhat syndiotactic (polypropylene rr 69%) polymers. The metal center of the catalyst opens the coordination sphere and enables the co-polymerization of ethylene to take place, not only with common monomers such as propylene, butene, hexene, and octene, but also with sterically hindered a-olefins such as styrene and 4-vinylcyclohexene [202]. 

Examination of Figure 3.27 shows that the constrained geometry complex is a coordinatively unsaturated chromium(III) alkyl that polymerizes ethylene, however, it does not react with 1-hexene to provide an ethylene/1-hexene copolymer. In reactions with propylene, a head-to-tail dimerization reaction takes place without any homopolymerization of higher 1-olefins. 

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