Difunctional initiator based on 1,3-diisopropenylbenzene. 2. Kinetics and mechanism of the sec-butyllithium/1,3-diisopropenylbenzne reaction

Kinetics of the addition of s-butyllithium (s-BuLi) to 1,3-diisopropenylbenzene (1,3-DIB) has been studied, and the activation energy has been found to be 17.5 kcal/mol. Addition of s-BuLi to 1,3-DIB in a 2/1 molar ratio results in an oligomer rather than the desired diadduct. This observation has to be compared to the anionic polymerization of 1,3-DIB (hydrocarbon solvent at 50 °C) which leads to a polymer bearing ca. one double bond per monomeric unit. Li-NMR analysis of the addition of s-BuLi to 1,3-DIB (2/1 molar ratio) confirms the nonequivalence of the lithium cations and shows that ca. 30% s-BuLi remains unreacted when 1,3-DIB has been completely consumed. In the early stage of the addition, the propagation rate is faster than depropagation and the average molecular weight increases. When a depropagation−propagation quasi-equilibrium is reached, the oligomer molecular weight remains constant. It finally decreases when no DIB is left, as a result of depropagation. When the s-BuLi/1,3-DIB adduct is used as an initiator for styrene polymerization in the presence of a polar solvent, such as THF, initiation by monofuntional, difunctional, trifunctional, and tetrafunctional species is observed. In an apolar solvent, however, polystyrene of a narrow molecular weight distribution and a functionality of 2 is formed. This experimental observation more likely indicates that depolymerization of the 1,3-DIB oligomers occurs upon the addition of styrene. A mechanism which accounts for this propagation−depropagation process has been proposed and experimentally supported.