Studies in polymerization. XIII. The activated initiation of vinyl polymerization by metal carbonyls

It has been found that the hexaearbonyls of group VI metals in the presence of some organic halogen compounds are active initiators of vinyl polymerization. Molybdenum hexacarbonyl is the most active, and halogen compounds containing —CC1 3 , —CBr 3 are generally more effective than those with fewer halogen atoms joined to a single carbon. Fluorine and iodine compounds are inactive. A study has been made of the initiation of the polymerization of methyl methacrylate by the Cr(CO) 6 + CCl 4 system. The polymerization is markedly retarded by carbon monoxide. It is shown that the reaction proceeds through free-radical intermediates, and a kinetic mechanism for initiation is derived (equation (4)) leading to the expression (5) for the rate of chain starting. This quantity may be calculated from (5) with the aid of the numerical values in table 3. Kinetic observations are in satisfactory agreement with this mechanism. The nature of the participating reactions is discussed.