Cobalt complexes chain transfer constants

As with cobaloximes, substituents on the equatorial ligand have only a moderate effect on the value of Cc for the complexes in Table 3. The same is true for substituents on cobalt porphyrins, 1 and 45—51 (Table 4). For tetrakis(pentafluoroethylphenyl)-porphyrin—Co11 the substituent effect is not clear. The fluorinated porphyrin works moderately for the polymerization of MMA in supercritical C02 with chain-transfer constant Cc = 550 at 60 °C.126 Unfortunately, no data on the chain-transfer constant in bulk polymerization are available, so that it is not clear whether this reduced value of Cc is the result of solvent or the presence of a strong EWG such as pentafluorophenyl in the porphyrin macrocycle. Similar experiments with 9c (Table 2) led to Cc = 378 000, which is 20 times higher than in bulk MMA or in organic solvents.30 We may conclude at this point that additional experiments are required with different catalysts to allow us to make reliable conclusions. 

There is little support for the mechanism expressed by eqs 12 and 13. MMA is able to form a -complex with cobalt porphyrins,160 but the chain-transfer constant for its formation (1.8 L/mol s) is not high and is much smaller than the observed CCT chain-transfer constants. If the mechanism of eqs 12 and 13 is correct, then reduced concentrations of monomer should disfavor formation of LCoM, resulting in a decrease in the rate of CCT. The chain-transfer constant of the chain transfer is independent of the concentration of monomer.14,52 The mechanism expressed by eqs 12 and 13 will not be considered further. 

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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