Factors Affecting the Apparent Chain Transfer Rate Constants of Chromium Metalloradicals: Mechanistic Implications

Abstract

When a high concentration of (C5Ph5)Cr(CO)3• (1) is present during the initiator-induced oligomerization of MMA, UV−vis spectra show that the concentration of the metalloradical decreases sharply at the beginning of the reaction (an “induction period”) and remains constant subsequently (a “stable period”). With dimethyl 2,2‘-azobis(isobutyrate) (AIBMe) as the initiator the decrease is due to the transfer of H• from the methyl isobutyryl radical and the chain-carrying radicals to 1, forming the corresponding hydride 2. When the catalysis of chain transfer during a polymerization reaction is studied by the Mayo method, it is assumed that all the added 1 remains active. In truth the chain transfer constant (CS) and chain transfer rate constant (ktr) from the Mayo method are only “apparent” values; the true [1] during the polymerization is lower, and the true values of CS and ktr for this Cr catalyst are larger than those obtained from a Mayo plot. Simulations based on a simplified kinetic model reproduce our observations satisfactorily.