Stimuli Responsive Gradient Polymers

Stimuli-responsive polymers have attracted much attention in recent years due to their unique properties. These polymers can respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals, and cause the change in the physical or chemical properties of the system. Gradient polymers form a new class of materials that come into existence with the advent of living polymerization techniques. Unlike random and block polymers, gradient polymers vary their monomer compositions gradually along molecular chains. Among them, amphiphilic gradient polymers which consist of hydrophilic units and hydrophobic units, can self-assemble to form various aggregates in selective solvents. Amphiphilic gradient polymers reveal many potential applications in different fields, such as supramolecular assemblies, smart coatings, networks or some combination of these possibilities. This article summarizes recent research progress on the synthesis of gradient polymers, such as living anionic polymerizations, controlled/living radical polymerizations (including nitroxide-mediated polymerization, atom transfer radical polymerization and reversible addition-fragmentation chain transfer polymerization) and ring opening polymerizations. The second part discusses their self-assembly and stimuli-responsive behaviors, especially for distinctive thermo-sensitivity and pH-sensitivity. In particular, the divergent stimuli responsive behaviors of aggregates self-assembled from amphiphilic block polymers and amphiphilic gradient polymers are discussed and compared in detail. The development prospect of this research field and its potential applications are discussed as well in the last part.