Anionic polymerization of phenyl-substituted isoprene derivatives: polymerization behaviour and cyclization-enabled fluorescence

Abstract

1,3-Dienes are important monomers for living anionic polymerization. However, phenyl-substituted diene monomer structures have been rarely investigated. Based on DFT calculations and ¹³C NMR spectroscopy, a discrepancy in the reactivity of the two monomers 1-phenyl isoprene (1PhI) and 4-phenyl isoprene (4PhI) in anionic polymerization is expected. Starting from a Wittig reaction including an optimized extraction procedure, disubstituted 1,3-dienes were obtained that resulted in polymers with different degrees of 1,3-incorporation. The polymers have been characterized by ¹H NMR spectroscopy and using different SEC conditions. Molecular weights of up to 48.8 kg mol⁻¹ with narrow dispersities (Đ ≤ 1.13) were achieved. The addition of the modifier THF led to an initial increase of vinyl units as well as a loss of control over the polymerization of 4PhI. Increasing the THF concentration further resulted in a rather unusual decrease of the vinyl units and ended with more than 80% 1,4-units in pure THF. Copolymerizations with styrene (S) and isoprene (I), respectively, were tracked via in situ¹H NMR kinetics. The observed ideally random copolymerizations of I and 1PhI as well as the gradient copolymers with S were further investigated via the synthesis of copolymers with a targeted M_n of 40 kg mol⁻¹. In a subsequent reaction step, the homopolymers were cyclized using trifluoromethyl sulfonic acid inducing fluorescence properties. The different microstructures and substitution patterns of the original polymers differ in both emission maxima and quantum yields.