Copolymerization of Isoprene with p‑Alkylstyrene Monomers: Disparate Reactivity Ratios and the Shape of the Gradient

The statistical copolymerization of isoprene with p-ethyl- (p-ES), p-isopropyl- (p-iPS), and p-tert-butylstyrene (p-tBS) initiated by sec-butyllithium in cyclohexane was investigated with respect to kinetics, reactivity ratios, and formation of tapered block copolymers with pronounced monomer gradient. An efficient synthetic route to the monomers was developed on a multigram scale, relying on the precipitation of the side-product triphenylphosphine oxide at low temperature. The copolymerization kinetics and resulting molecular weight distributions were analyzed. The dispersity, Đ, of the copolymers depends on the p-alkyl substituent, the the degree of polymerization P_n and the comonomer mole fraction, X. In situ ¹H NMR kinetics characterization revealed a strong gradient structure for all three copolymer systems (r_I = 21.9, r_p‑ES = 0.022; r_I = 19.7, r_p‑iPS = 0.027; r_I = 19.8, r_p‑tBS = 0.022). The rate of crossover from a polyisoprenyl­lithium chain end (I) to a p-alkylstyrene (S) unit relative to the alkylstyrene homopolymerization, k_IS/k_SS (in 10^–3 (L mol^–1)^−1/4), decreases in the order p-MS (19.1) > p-ES (11.3) > p-iPS (5.71) ≈ p-tBS (5.63), supporting the observed, increasingly bimodal character of the molecular weight distributions and the higher dispersity. Thermogravimetric analysis revealed that all poly­(p-alkylstyrene) homopolymers are stable up to 300 °C.