Reorganization of polymer structures based on dynamic covalent chemistry: polymer reactions by dynamic covalent exchanges of alkoxyamine units

The recent progress of research on polymer reactions utilizing dynamic covalent exchanges of alkoxyamine units-adducts of styryl and stable nitroxide radicals-is reviewed. The alkoxyamine derivatives derived from 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO, a stable free radical) are frequently utilized as unimolecular initiators for the nitroxide-mediated radical polymerizations. In the absence of monomers, however, the alkoxyamine derivatives can undergo intermolecular crossover reactions via a radical process upon heating. The central C-ON bonds in the alkoxyamine derivatives reversibly cleave and reform upon heating, and a mixture of alkoxyamine derivatives is able to equilibrate thermally, meaning that the covalent bonds in the alkoxyamine derivatives are considered as 'dynamic covalent bonds.' Unlike conventional polymers, the structures and constitutions of polymers with dynamic covalent bonds, dynamic covalent polymers, can be reorganized under appropriate conditions even after polymerization. In the present review, various types of macromolecular design, polymer reactions based on dynamic covalent exchanges of alkoxyamine units and their related research are described. Various examples of polymer reactions of main-chain-type, side-chain-type, crosslinked and star-shaped poly(alkoxyamine)s are systematically shown. Furthermore, the progress of the polymer reactions can be confirmed by diverse characterization techniques, such as spectroscopic, chromatographic, microscopic and scattering methods.