The physicochemical characteristics of many polymers are strongly affected by external stimuli. For example, the solubility of polymers may depend strongly on the temperature, and not just in the way we know it for small molecules. Some polymers exhibit a so-called lower critical solution temperature (LCST), losing their solubility upon heating. Alternatively, some polymers exhibit the opposite type of behavior, an upper critical solution temperature (UCST), where the polymer dissolves upon heating and even combinations of both behavior can be realized.
These thermoresponsive properties are influenced by several factors, such as polymer concentration, molar mass and architecture and the presence of salts. Besides thermoresponsiveness, numerous polymers exist which respond to other external stimuli such as light, pH, redox and so on. The development of modern polymerization, modification and functionalization methods has enabled also the preparation of systems, which respond to multiple changes in the environment. The applications of responsive polymers include drug-delivery, smart coatings, hydrogels, sensors, bioconjugates and degradable materials.