Living Anionic Polymerization of 4-Halostyrenes

Anionic polymerizations of 4-chlorostyrene (1), 4-bromostyrene (2), and 4-iodostyrene (3) were carried out in tetrahydrofuran (THF) at −78 °C for 5 min using sec-butyllithium (sec-BuLi) or oligo(α-methylstyryl)lithium (αMSLi) as the initiator. Poly(1)s having broad molecular weight distribution (MWD, ĐM, Mw/Mn ∼2) were quantitatively obtained using sec-BuLi and αMSLi, and the molecular weights could be controlled by the molar ratios between 1 and the initiators. Complete consumption of 2 was also realized using αMSLi under similar conditions. The conversion of 2 was only 8.8% when initiated with sec-BuLi, and the resulting poly(2) possessed broad multimodal MWD. No polymeric product of 3 was produced using sec-BuLi, indicating the presence of major side reactions involving the electrophilic C–Br and C–I bonds. On the other hand, well-defined poly(1) and poly(2) were quantitatively obtained using αMSLi in the presence of 10-fold cesium phenoxide (PhOCs) as an additive. The polymerization using αMSLi/PhOCs was completed within few minutes, and the resulting poly(1)s and poly(2) possessed narrow MWD (Mw/Mn <1.1) and predictable molecular weights. In the case of 3, the αMSLi/PhOCs initiator quantitatively produced poly(3) having predictable molecular weight and relatively narrow MWD (Mw/Mn = 1.2–1.3). The stability of the propagating carbanions was demonstrated at −78 °C by the quantitative efficiency of the sequential copolymerization of 1 or 2 and styrene as well as that of 1 and 2 in any addition order. The observed difference in polymerization behaviors of 1–3 can be explained by the relative reactivity of the carbon–halogen bonds in the monomer structures.