Mechanism of free radical formation by Mo(CO)6+CCl4

Abstract

The mechanism of free-radical formation in the system molybdenum carbonyl+carbon tetra-chloride in the presence of methyl methacrylate, ethyl acetate and dioxan has been studied in greater detail. For each carbonyl molecule decomposed under conditions of “high”[CCl₄] in methyl methacrylate solution one polymer molecule is formed and one chlorine atom is incorporated into the resulting molybdenum derivatives. In the above solvents the rate of carbon monoxide evolution for [CCl₄]= 0 is equal to the rate of radical formation at high [CCl₄], in agreement with earlier suggestions that the primary process is the displacement of a molecule of carbon monoxide by a solvent molecule. A second molecule of carbon monoxide is liberated on reaction with CCl₄. A mechanism involving the interaction of two intermediates is proposed to account for the inhibition of polymerization observed at high carbonyl concentrations. With a very active halide such as ethyl trichloracetate the concentration of one intermediate is very small and inhibition is negligible.

In systems containing monomer the ˙CCl₃ radical generated from CCl₄ is believed to add to the double bond of the monomer molecule already co-ordinated to the metal. With ethyl acetate and dioxan as solvents this cannot occur and radical attack on the solvents leads to a complicated mixture of products, some of which act as ligands and displace further molecules of carbon monoxide. A total of five molecules of CO are then evolved as CO from each Mo(CO)₆, the remaining one appearing as phosgene.