Kinetics of the Polymerization of Styrene by Friedel–Crafts Catalysts
ALTHOUGH the mechanisms and kinetics of the free-radical catalysed polymerization of styrene have been fairly extensively investigated1,2, the polymerization employing Friedel Crafts catalysts has received much less attention. Williams3 has investigated the stannic chloride catalysed polymerization of styrene in carbon tetrachloride solution and shown that the overall rate of monomer consumption can be represented by the expression: where m is initial styrene concentration, x is styrene consumed in time t, b is initial catalyst concentration. He has also shown that induction periods are present. We are studying the aluminium chloride catalysed polymerization of styrene in carbon tetrachloride solution from a kinetic point of view, and it is proposed to summarize here the principal conclusions reached to date.
SOME ASPECTS OF THE MECHANISM OF ADDITION POLYMERIZATION
It has long been recognized and is now generally accepted that, aa a rule, the process of addition polymerization differs fundamentally from condensation polymerization. The latter involves only types of reactions with which the chemist is quite familiar, with the important characteristic feature that the monomer molecules are so constructed that the condensation process is capable of indefinite repetition to build up a polymeric. product. The reaction proceeds in a regular, stepwise manner, yielding material of steadily increasing degree of polymerization throughout the course of the reaction. This mode of formation contrasts sharply with that involved in typical addition polymerizations, which are characterized by the fact that the polymer molecules first formed are essentially the same size aa the rest. In other words, the polymer molecules do not grow gradually, but each polymer molecule grows rapidly to a certain size and is then stabilized. This behavior, coupled with the uusceptibility of these reactions to catalysis and inhibition, supports the suggestion that these reactions are chain reactions involving activation of the monomer (initiation) followed by rapid successive additions of monomer molecules to the active intermediate (propagation) until the activated polymer becomes stabilized (cessation).
The photochemical polymerization of methyl acrylate vapour
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 = CH2 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 CH3, COOCH3, CN, halogen, CH = CH2, etc. In the methyl methacrylate molecule, CH2 = C(CH3)COOCH3, 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.