Catalytic chain transfer cobalt porphyrins

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. 

Many catalysts have been screened for activity in catalytic chain transfer. A comprehensive survey is provided in Gridnev and Ittel s review."0 The best known, and to date the most effective, are the cobalt porphyrins (Section 6.2.5.2.1) and cobaloximes (Sections 6.2.5.2.2 and 6.2.5.2.3). There is considerable discrepancy in reported values of transfer constants. This in part reflects the sensitivity of the catalysts to air and reaction conditions (Section 6.2.5.3). 

The catalytic chain-transfer (CCT) process displays all of the features characteristic of typical, uncatalyzed chain transfer other than taking place at a rate competitive with chain propagation. Thus, the rate of polymerization at low conversions is independent of the concentration of the cobalt porphyrin (Figure 1) while the molecular weight, Mn, decreases linearly by over 2 orders of magnitude with increasing concentration of cobalt catalyst (Figure 2). As expected for a typical polymerization, the rate of polymerization increases linearly with the square root of the concentration of the azo initiator and no polymerization occurs in the absence of the initiator. 

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... 

CCTP has its origins in biochemistry where coenzyme B12 is used to conduct many free-radical reactions. Enikolopyan et al. were the first who used analogues of B12 for polymerization [257,258]. Methacrylate was polymerized by a catalyzed chain transfer using a cobalt porphyrine. AIBN was used as initiator. Two possible reaction sequences for the catalytic aspect of CCT are described in the following scheme ... 

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