On the mechanism of ionic polymerization

Abstract

AbstractThis lecture is devoted mainly to individual elementary reactions, especially those of chain growth taking place on a pair of ions, and termination in ionic polymerization. Two mechanisms of chain growth are possible and are known to occur: first, a gradual chain growth taking place at the ion pair (as is known in the case of “living” polymers), second, chain growth taking place at free ions. The velocity of chain growth is controlled by a number of factors amongst which may be cited the following (a) The type of the chemical bond in the monomer which undergoes cleavage during polymerization. In this respect the author correlates results of kinetic measurements (partly his own and partly other authors' data) relating to the bonds >CC< (in isobutylene, styrene), >CO (in formaldehyde), and (in octamethylcyclotetrasiloxane, OTS). (b) The size and type of ions forming the ion‐pair. Here different anions are compared for their effects on the cationic polymerization of isobutylene and, similarly, different cations are compared for their effects on the anionic polymerization of OTS. (c) The ability of monomer to form complexes with the ion‐pair at which growth occurs. During the anionic polymerization of OTS the free orbital of the Si atom of the monomer is directly engaged in the dissociation of the terminal ion‐pair . The role of the aromatic nucleus in the polymerization of styrene falls within the same category. More detailed data on the dissociation of ion pairs have been obtained by the study of electrical conductivities of catalyst‐cocatalyst systems on the one hand and of systems involving monomer on the other. The study of electrical conductivities in the presence of a monomer was carried out in the anionic polymerization of OTS which is characterized by relatively low rates of polymerization. Here it was possible to prove the active part played by the monomer in the dissociation of the ion‐pair at which chain growth takes place. The termination reaction on the other hand is also influenced by a number of factors. Among the most important factors is the acidity (or basicity) of the growing ion which decides whether the actual step is one of transfer or termination. By way of an example one may draw the attention to the difference in behaviour of water on the polymerization of the CC bond on one hand, that of the CO bond on the other. The reactivity of the very strong cations (of anions ) is sufficient for even relatively strong bases (or acids) to add irreversibly. As an example one may show the action of various bases in the cationic polymerization of isobutylene and that of acids in the anionic polymerization of OTS. In conclusion, absolute rates of the individual elementary reactions and the new possibilities opened by the study of electrical conductivities of monomer‐containing systems are discussed.