Living Anionic Polymerization of 2-Isopropenylthiophene Derivatives

The anionic polymerization of 2-isopropenylthiophene (1), 2-(1-adamantyl)-5-isopropenylthiophene (2), 5-phenyl-2-isopropenylthiophene (3), and 2-cyano-5-isopropenylthiophene (4) was performed in tetrahydrofuran (THF) with various initiators including sec-BuLi, oligo(α-methylstyryl)lithium, potassium naphthalenide, and diphenylmethylpotassium at −78 °C. The anionic polymerization of 2–4 proceeded quantitatively to provide novel polymers with the predicted molecular weights and narrow molecular weight distributions (M_w/M_n < 1.2), whereas the polymerization of 1 often suffered from side reactions, probably due to proton abstraction on the thiophene ring. After the complete polymerization at −78 °C, the propagating carbanion of the resulting polymers of 2–4 can be depolymerized to give the starting monomer by elevating the temperature to 0 °C. In particular, 3 showed a reversible equilibrium polymerizability similar to that of α-methylstyrene by varying the temperature of the polymerization system. From the plot of logarithm of equilibrium monomer concentration, ln[M]_e, against reciprocal temperature, the thermodynamic parameters, ΔH and ΔS, and the ceiling temperature (T_c) of the anionic polymerization of 3 in THF were estimated to be −8.09 ± 0.22 kcal mol^–1, −27.3 ± 0.9 cal mol^–1 K^–1, and 24 °C, respectively.