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Copolymerization of Isoprene with p‑Alkylstyrene Monomers: Disparate Reactivity Ratios and the Shape of the Gradient
journal contribution
posted on 2019-01-11, 17:52 authored by Philipp von Tiedemann, Jan Blankenburg, Kamil Maciol, Tobias Johann, Axel H. E. Müller, Holger FreyThe 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 Pn and the comonomer mole fraction, X. In situ 1H NMR kinetics characterization
revealed a strong gradient structure for all three copolymer systems
(rI = 21.9, rp‑ES = 0.022; rI = 19.7, rp‑iPS = 0.027; rI = 19.8, rp‑tBS = 0.022). The rate of crossover
from a polyisoprenyllithium chain end (I) to a p-alkylstyrene (S) unit relative to the alkylstyrene homopolymerization, kIS/kSS (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.