Retarded Anionic Polymerization of Styrene, 6. Influence of Zinc and Boron Derivatives on the Reactivity of Polystyryllithium

Zinc and boron derivatives were used as additives for the retardation of styrene polymerization initiated by sec ‐butyllithium. The formation of mixed complexes between polystyryllithium (PSLi) and alkyl zinc or boron derivatives as well as a transmetallation reaction between PSLi and alkoxyborane was studied by UV‐vis spectroscopy. Kinetic studies performed in the presence of these additives show that the rate of polymerization is divided only by two with diethylzinc at [Zn]/[Li] = 8 suggesting the contribution of remaining non‐complexed PSLi. On the contrary, the rate was found to decrease noticeably with triethylborane upon increasing the [B]/[PSLi] ratio, in agreement with the formation, at 20°C, of “ate” complexes more stable than with diethylzinc. However, the dissociation constant of the complexation equilibrium between R 3 B and PSLi is highly dependent on the temperature, and the retardation is not so effective at T > 100°C. The same retardation effect was observed with borane alkoxides. Transmetallation reaction involving PSLi and B—OR bonds is first observed yielding B(PS) x derivatives. Retardation is governed by the concentration of the remaining PSLi species.