Cyclic Polymers with Pendent Carbazole Units: Enhanced Fluorescence and Redox Behavior

Abstract

Cyclic polymers, which are also referred to as polymer rings or macrocycles, have specific properties that because of the absence of chain ends are in stark contrast to linear polymers, such as smaller hydrodynamic volume, reduced viscosity, larger refractive index, and higher glass transition temperature. These unique properties of cyclic polymers have attracted extensive attention in macromolecular science in the last decades. To date, cyclic polymers that are based on homopolymers and copolymers and have various shapes, such as sun-shaped, tadpole-shaped, eight-shaped, and qshaped structures, have been extensively investigated. Moreover, cyclic polymers with functional monomers have also been designed and prepared. Among the strategies for preparation of cyclic polymers, a-alkyne–w-azide click chemistry combined with living free radical polymerization (LFRP) was shown to be highly efficient and popular. Liu et al. and Winnik et al. have reported the syntheses of cyclic poly(N-isopropylacrylamide) by LFRP and click chemistry, respectively. Very recently, Liu et al. reported the synthesis of cyclic block copolymers by selective click cyclization at a relatively high concentration. However, cyclic topological polymers that bear functional mesogens have been underexplored to date because of the limited availability of well-defined linear precursors. The sole example was reported by Zhao et al. in which a liquid crystalline side chain with azobenzene as mesogen unit was introduced to a cyclic polymer. These cyclic polymers with functional moieties have exhibited improved or unique properties compared with their linear precursors. Therefore, it is of great interest to design and fabricate cyclic topologic polymers with multifunctional moieties, and especially to investigate their topological effects on characteristic properties by comparing with linear counterparts. Poly(4-vinylbenzyl-carbazole) (PVBCZ) is a versatile polymer with strong fluorescence and specific electrochemical properties that render it particularly attractive for use in a variety of optoelectronic applications. Inspired by this, cyclic PVBCZ is believed to significantly improve optoelectronic properties. Herein, we present the first example of a cyclic PVBCZ synthesized by the combination of atom transfer radical polymerization (ATRP) and a click reaction. The synthetic route to cyclic PVBCZ is depicted in Scheme 1. The thermal properties, fluorescence, and redox behaviors of cyclic PVBCZ were investigated and compared with those of its linear precursor.