Thermal reactions of methyl and acetyl manganese pentacarbonyls. Part 1.—Initiation of free-radical polymerization and formation of methyl(2-methyl 4-oxopentanoate)

Abstract

The manganese pentacarbonyl derivatives CH₃Mn(CO)₅ and CH₃COMn(CO)₅ are low-molecular-weight analogues of macroinitiators which carry terminal Mn(CO)₅ groups and their thermal reactions in methyl methacrylate at 100°C have been studied from this aspect. Under these conditions isomerization reactions are rapid and the two compounds behave similarly.

The derivatives initiate free-radical polymerization above 60°C but give rise to marked retardation except at very low concentrations. The rate of polymerization ω is increased by the presence of “halide” or “non-halide” additives and at the same time retardation is greatly reduced. The dependence of ω upon additive concentration is of the familiar form showing a plateau value at high concentrations. The rate of polymerization is depressed by the presence of carbon monoxide.

However, initiation of polymerization is only a minor component of the overall reaction at 100°C, the major process (∼90%) being formation of methyl(2-methyl 4-oxopentanoate), CH₃COCH₂CH (CH₃)COOCH₃. This reaction follows a non-radical route. A mechanism is suggested involving intramolecular interaction between an acetyl anion and the coordinated monomer (M) in CH₃COMn(CO)₄M; the process is completed by proton transfer from water present in the system in trace quantities. This mechanism is strongly supported by investigations with D₂O.

Spectral (u.v.-visible) changes accompanying these transformations are recorded. Thermal decomposition of CH₃Mn(CO)₅ and CH₃COMn(CO)₅ in benzene yields Mn₂(CO)₁₀; in methyl methacrylate an additional peak develops at 385 nm which we attribute to Mn(CO)₅OH. This peak is not much affected by the presence of CCl₄ or CO.