Alternating copolymerization cyclocopolymerization

In this section wc consider systems where the radical formed by propagation can eyclizc to yield a new propagating radical. Certain 1,4-dicncs undergo cyclocopolymerization with suitable olefins. For example, divinyl ether and MAH are proposed to undergo alternating copolymerization as illustrated in Scheme 4.19.167 These cyclo-copolymerizations can he quantitative only for the case of a strictly alternating copolymer. This can be achieved with certain electron donor-electron acceptor pairs, for example divinyl ether-maleic anhydride. 

Choo, T. N. Waymouth, R. M. Cyclocopolymerization a mechanistic probe for dual-site alternating copolymerization of ethylene and a-olefins. J. Am. Chem. Soc. 2002,124, 4188-4189. 

Recently, Choo and Way mouth performed the copolymerization of ethylene with 1,5-HD using various metallocene catalysts (12, 13, 14, Figure 19.2). 1,5-HD cyclopolymerized exclusively to give MCP units in the copolymers, with only traces of uncyclized 1,2-inserted 1,5-HD. The diaste-reoselectivity of the cyclocopolymerization favored the formation of 1,3-cyclopentane rings for metallocenes (74% trans for 12, 81% trans for 13, and 66% trans for 14). For metallocenes 12 and 14, the ethylene/1,5-HD copolymerization yielded copolymers with similar comonomer compositions and sequence distributions to those observed for ethylene/1-hexene copolymerization with these catalysts. On the other hand, the copolymers derived from metallocene 13 showed very different compositions and sequence distributions. At comparable comonomer feed ratios, the poly(ethylene-c -l,5-HD)s were enriched in the 1,5-HD comonomer and deficient in ethylene as compared to the analogous polymers prepared from ethylene and 1-hexene. The copolymerization behavior of 13 provided support for a dual-site alternating mechanism for 1,5-HD incorporation, wherein one coordination site of the active catalyst center is highly selective for the initial 1,2-inserion of 1,5-HD and the other site is selective for cyclization. 

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