Termination in anionic polymerization of octamethylcyclotetrasiloxane. formation of stable complexes on active sites

Abstract

AbstractThe ionic polymerization of octamethylcyclotetrasiloxane (the tetramer) proceeds through successive additions of tetramer on active centers. The active centers consist of the ion‐pair magnified image (where M stands for Na, K, Rb, Cs, Me4N−, Bu4P−, etc.) formed through the reaction of the hydroxide (oxide) of the respective alkali metal (or the respective ammonium or phosphonium base) with the siloxane bond. Measurement of the rate of polymerization of tetramer indicated changes in the efficiency of magnified image active centers caused by additions of small amounts of substances capable of changing the character of donor‐acceptor reactions taking place during polymerization. These acids such as H2SO4 to very strong bases, such as hydroxides of alkaline metals. The substances were added in the order of their decreasing acidity ranging from very strong influence of substances being added, as judged by their influence on the overall rate of reaction, can be seen to fall into several classes. The most acidic substances (ranging from very strong mineral acids to weak organic acids, e.g., phenol) neutralize the active centers; weak acids (H2O, ROH, etc.) act (as judged by changes of the overall rate of reaction) only through physicochemical effects in the polymerizing system (for instance through changes in the quantitative composition of the solvate shell of ions and ion‐pairs and through changes of the dielectric constant). Very strong bases, as strong as, or stronger than, KOH increase the number of active centers. The most interesting effects are displayed on addition of relatively strong bases of the type of LiOH and NaOH. They cause a strong reduction of the efficiency of active centers, a deceleration of the polymerization and even its complete inhibition, in the whole range of dielectric constante of the polymerizing medium investigated by the authors (viz., in the range of dielectric constants from 3.4 to 8.7). This phenomenon of deceleration and even stopping of polymerization on addition of relatively strong bases to a basic active center has been called the “isobasic termination.” Similar effects were observed also in the case of the cationic polymerization of the tetramer, viz. a slowing‐down or even stopping of the H2SO4−induced polymerization on addition of weaker acids. The mechanism of “isobasic termination” is discussed.