Recent progress on the synthesis of cyclic polymers via ring‐expansion strategies

ABSTRACT Cyclic polymers are the simplest topological isomers of linear macromolecules, but exhibit properties that differ from linear chains in ways that remain imperfectly understood. The difficulty of synthesizing appropriately pure and high molecular weight cyclic samples has hindered experimental studies. Ring‐closure methods, while versatile, are inherently limited in the range of molecular weights that can be achieved. Ring‐expansion methods are a much more promising strategy toward obtaining high molecular weight cyclic polymers. The current review focuses on recent developments in ring‐expansion polymerization strategies toward the synthesis of high molecular weight cyclic polymers. Significant progress in the last decade has made the synthesis of cyclic polymers possible by a variety of methods, such as ruthenium‐ and tungsten‐catalyzed ring‐expansion metathesis polymerization, organocatalytic and Lewis acid‐catalyzed zwitterionic polymerization, RAFT and nitroxide‐mediated radical polymerization, among many others. While the study of cyclic polymers has long been hampered by synthetic challenges, the recent resurgence of interest in this field presents an exciting opportunity for chemists. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 2892–2902