Steric hindrance effect on the thermo- and photo-responsive properties of pyrene-based polymers

Abstract

In this work, the influence of the steric hindrance effect between a polymeric backbone and the side groups on the lower critical solution temperature (LCST)-type behavior and photodegradation behavior, respectively, has been carefully studied. A series of pyrene-based polymers namely poly(pyren-1-yl-o(m,p)-vinylbenzoate) (P(Py-o(m,p)-VB)) were successfully prepared by free-radical polymerization. All the polymers exhibited an LCST-type phase separation in THF, and the mechanism was investigated by variable temperature ¹H NMR analysis. In addition, the polymers showed a “structural self-quenching effect” (SSQE) and presented a photo-cleavage reaction under UV irradiation. The photo-cleavage mechanism was confirmed by gel permeation chromatography (GPC) measurements. Both the cloud point temperature (T_c) value and the photo-cleavage rate of the polymers increased with the enhancement of the steric hindrance effect which was analyzed by two-dimensional ¹H nuclear Overhauser enhancement spectroscopy (¹H NOESY) experiments. For thermo-responsive behavior, the T_c value increased from 37.6 °C to 45.1 °C and then above 65 °C (boiling point of THF) with the position of pyrene chromophores changing from a para to meta and then to ortho position. Meanwhile, the increasing steric hindrance effect reduced the stability of the polymer, thus dominating the photo-cleavage responsivity. In this work, we propose a novel and convenient strategy to regulate the thermo- and photo-responsive properties via simply tailoring the position of the side groups. This novel LCST-type polymer with photo-degradation behavior may find applications in sensors, dispersion stabilizers, fluorescent probes and so on.