Photolysis of methyl- and acetyl-manganese pentacarbonyls in methyl methacrylate. Initiation of polymerization and formation of methyl(2-methyl-4-oxopentanoate)

Abstract

Irradiation (λ= 365 nm) of solutions of CH₃Mn(CO)₅ and CH₃COMn(CO)₅ in methyl methacrylate brings about free-radical polymerization of the monomer. Rates of polymerization are enhanced by the presence of CCl₄ as coinitiator, achieving a plateau value as [CCl₄] increases. Carbon monoxide has a strong inhibiting effect on the reactions, a presence of 700 Torr reducing the rate of initiation by a factor 73 for CH₃Mn(CO)₅. Analysis of polymers prepared in the presence of CH₃Mn(CO)₅ and CCl₄(0.103 mol dm^–3) shows that 80 % of the initiation arises from CCl₃ radicals, the remaining 20 % probably being attributable to CH₃.

During irradiation of the carbonyls in methyl methacrylate (M) an increase in optical density occurs in the region below 450 nm; reasons are given for believing that this is due to CH₃COMn(CO)₄M formation. Carbon tetrachloride reduces this change considerably, particularly at longer reaction times. Light absorption by CH₃COMn(CO)₄M does not appear to initiate polymerization significantly.

Methyl(2-methyl-4-oxopentanoate) is a major product of the photochemical reaction in methyl methacrylate. The yield of this ketone is much less sensitive than the rate of initiation of polymerization to the presence of carbon monoxide.

Under our conditions the excited species CH₃Mn(CO)^*₅ decomposes by two routes: (i) by scission of CO, to give a fragment CH₃Mn(CO)₄ which yields ĊH₃ or, in the presence of CCl₄, ĊCl₃+ĊH₃: (ii) by reaction with monomer to give CH₃COMn(CO)₄M, from which methyl(2-methyl-4-oxo-pentanoate) is produced by further photolysis.

We have not been able to devise a mechanism based on primary photodissociation into ĊH₃+ Mn(CO)₅ which is compatible with our results.