Understanding structure and composition of thermally rearranged polymers based on small‐molecule chemistry: a perspective

Abstract

We provide a critical perspective of the burgeoning literature on microporous polymers prepared using thermal rearrangement (TR) processes based on the learning derived from analogous chemistry involving small‐molecular‐weight compounds. TR polymers have shown interesting permeability–selectivity relationships in gas separation and, thus, have generated wide interest as potential membrane materials for industrial applications. The intractable nature of the products obtained by TR processes has precluded rigorous structural elucidation of the polymers. Based on small‐molecule chemistry, we conclude that structures are likely to be more complex than generally depicted in the published literature. Interestingly, a simpler chemistry, namely thermal dehydrocyclization (TCD), leads to products identical to those derived from TR, but at significantly lower temperatures. However, TCD chemistry does not involve a skeletal rearrangement of the kind purported in TR during the conversion of imide to oxazole ring resulting in spatially confined heterocyclic ring polymers. Yet, they show similar fractional free‐volume elements as exhibited by TR polymers. This is intriguing and points to a need for more careful examination of the factors responsible for microporosity in such materials. TR chemistry as currently practiced appears limited to only benzoxazole‐type structures. The ability to precisely control and reproducibly produce materials with well‐defined structure and properties will be a key to large‐scale manufacture and industrial applications of such materials. Seen from this perspective, TR processes leave much to be desired and further improvements are clearly warranted. © 2019 Society of Chemical Industry