Studies on the Thermal Polymerization of Methyl Linoleate. VI.

Abstract

The mechanism of thermal dimerization of methyl linoleate has been studied by a kinetic method. Based on the results of analysis of kinetic data obtained, the following reaction mechanism is proposed : This involves three steps. The first is the draw off of an isolated α-methylenic hydrogen atom from the methyl linoleate molecule by a second molecule, where the linoleate reacts in both as a hydrogen donor and as a hydrogen accepter. This will give rise to two radicals, octadecadienoate radicals R·1 and octadecenoate radicals (R·2). The second step is the formation of cis, trans-conjugated isomers of methyl linoleate and the isomerization of the cis, trans-isomers into the trans, trans-form. The cis, trans-isomers are formed by a chain reaction in which R·1 of the conjugated form draws off a hydrogen atom from the linoleate molecule to form the conjugated isomer and a new R·1 radical. The final step is the dimer-formation by (1) addition reactions between R·1 and R·2 and by (2) the Diels-Alder reactions of the conjugated isomers and the linoleate, where the trans, trans-isomer acts as diene and the trans, trans-, the cis, trans-isomer or the linoleate act as dienophile. Besides these reactions, the formation of octadecenoate by the interaction of R·2 with the linoleate also occurs.