The photochemical polymerization of methyl acrylate vapour

Abstract
The investigation of the polymerization reactions of ethylene derivatives has defined fairly clearly the nature of the substituents most likely to favour such a reaction. For instance, it is absolutely essential to have a = CH 2 group at one end of the molecule, while the substituent attached to the other carbon atom at the end of the double bond may consists of a large variety of groups such as CH 3 , COOCH 3 , CN, halogen, CH = CH 2 , etc. In the methyl methacrylate molecule, CH 2 = C(CH 3 )COOCH 3 , both groups attached to the α -carbon atom are separately capable of favouring the tendency to form polymers, the methyl group probably making a contribution small compared with that of the carboxyl group since propylene is rather difficult t o polymerize. Consequently, if the methyl group is replaced by a hydrogen atom the ability of the molecule to polymerize should not be greatly altered. The experiments on the photo-polymerization of methyl methacrylate vapour (Melville 1937) have shown, however, that this methyl group may play an important part in determining what type of reaction stops the growth of the polymer and thereby limits its molecular weight. Indeed, it was suggested that the long life of the growing methacrylate polymer is entirely due to the immobility of the methyl group. This has the further consequence that the cross-linking of polymers, if branching could occur, should for the same reason be practically impossible-a suggestion compatible with the straight-chain characteristics of the methacrylate polymer. The question then arises as to what will happen when the methyl group is replaced by a relatively more mobile hydrogen atom. This paper is therefore concerned with the photo-polymerization of methyl acrylate vapour which has been carried out under similar conditions to those obtaining with the methacrylate.

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