Efficient synthesis of photoresponsive azobenzene-containing side-chain liquid crystalline polymers with high molecular weights by click chemistry

Abstract

A new and highly efficient approach to obtaining high molecular weight (up to 303000) azobenzene (azo)-containing side-chain liquid crystalline polymers by copper(I)-catalyzed polymer analogous click chemistry is described. A series of azo compounds with an azido end group and different flexible spacers were successfully attached onto a polymer bearing pendant acetylene groups (i.e., poly(propargyl methacrylate)) in rather high functionalization efficiency (≥97%). The chemical structures, phase transition behaviors, and photoresponsivity of the obtained polymers were characterized, and they were also compared with those of the corresponding low molecular weight azo polymers (prepared via conventional free radical polymerization) to study the effects of the largely different molecular weights on the properties of the polymers. Both the high and low molecular weight azo polymers with a flexible spacer = (CH₂)₁₀ showed smectic C liquid crystallinity and an increase in the molecular weight led to a broader range of the liquid crystalline phase, which is positive for potential applications. Furthermore, the highly reversible photoisomerization of the polymer solutions was proven to be hardly affected by the increase of the molecular weights.