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 degrees C for 5 min using sec-butyllithium (sec-BuLi) or oligo(alpha-methylstyryl)lithium (alpha MSLi) as the initiator. Poly(1)s having broad molecular weight distribution (MWD, D-M, M-w/M-n similar to 2) were quantitatively obtained using sec-BuLi and alpha MSLi, and the molecular weights could be controlled by the molar ratios between 1 and the initiators. Complete consumption of 2 was also realized using alpha MSLi 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 alpha MSLi in the presence of 10-fold cesium phenoxide (PhOCs) as an additive. The polymerization using alpha MSLi/PhOCs was completed within few minutes, and the resulting poly(1)s and poly(2) possessed narrow MWD (M-w/M-n <1.1) and predictable molecular weights. In the case of 3, the alpha MSLi/PhOCs initiator quantitatively produced poly(3) having predictable molecular weight and relatively narrow MWD (M-w/M-n = 1.2-1.3). The stability of the propagating carbanions was demonstrated at -78 degrees 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.

Automated summary

Anionic polymerizations of 4-chlorostyrene, 4-bromostyrene, and 4-iodostyrene were successfully carried out at low temperatures using sec-butyllithium or oligo(alpha-methylstyryl)lithium as initiators. The choice of initiators and conditions allowed for control of the molecular weight distribution and molecular weights of the resulting polymers. The addition of cesium phenoxide as an additive led to the production of polymers with narrow molecular weight distributions and predictable molecular weights.