Cobalt complexes - chain transfer agents

Enikolopyan et al.til found that certain Co11 porphyrin complexes (eg. 87) function as catalytic chain transfer agents. Later work has established that various square planar cobalt complexes (e.g. the cobaloximes 88-92) are effective transfer agents.Ij2 m The scope and utility of the process has been reviewed several times,1 lt>JM ns most recently by Hcuts et al,137 Gridnev,1 3X and Gridnev and Ittel."0 The latter two references1provide a historical perspective of the development of the technique. 

Cobalt tetraphenylporphyrin complex promotes a chain-transfer reaction in the radical polymerization of MMA to give an MMA oligomer with vinylidene unsaturation at the chain end.124 An alternative method of introducing the terminal unsaturation was disclosed by Meijs et al,125 Substituted allylic sulphides are used as chain transfer agents in which sulphide groups act as leaving group as follows ... 

Thus these cobalt complexes function as extremely efficient catalytic chain transfer agents, with each polymer chain having an olefinic end group. 

Abstract Macromonomers of structure 4. prepared by the free radical polymerization of MMA using cobalt complexes as chain transfer agents, become incorporated into polymer chains by copolymerization but also act as efficient chain transfer agents by an addition-fragmentation reaction. Macromonomers of general structure 1 have been prepared by utilizing appropriately substituted allylic sulfides as chain transfer agents in free radical polymerizations. 

Olefin-terminated poly(methyl methacrylate) (PMMA) 4 can be prepared by free radical polymerization of MMA using cobalt complexes as catalytic chain transfer agents (3). 

Most catalytic systems used in industrial production yield polyalkenes with very long chains which are unsuitable for current processing procedures and applications. For regulating molecular mass, H2 is preferred to organometal-lics. Hydrogen is not a suitable transfer agent in diene polymerizations on cobalt complexes [67] because it reduces the Co (II) zr-allylic centre to inactive Co (I) particles. 

In terms of the influence of the olefin structure, CCT activity is especially high for methacrylates and styrene, while acrylates tend to yield more efficient OMRP trapping by cobalt complexes. A study of H transfer from CpCr(CO)3H to a variety of olefins yields the rate constants and relative rates are shown in Figure 30, but no equivalent information is apparently available on the catalytically more relevant H transfer from the chain-carrying radical to the transfer agent, which is usually the rate-determining step in CCT. 

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