Mayo plots have been used to determine the rate constant k(tr) for chain transfer catalysis by (C5Ph5)Cr(CO)(3)(.) of the AIBN-initiated polymerization of MMA from 60 to 80 degreesC. The value of ktr decreases with temperature, implying that abstraction of H-. from the chain-carrying radical by (C5Ph5)Cr(CO)(3)(.) does not occur in a single step and suggesting the associative formation of a caged radical pair. No CIDNP is observed for the reaction of (C5Ph5)Cr(CO)(3)H with MMA.