Understanding structure and composition of thermally rearranged polymers based on small molecule chemistry: A perspective A. Murugesan,* 1 and Swaminathan Sivaram 2 1 SSN College of Engineering (An autonomous institution), Kalavakkam, Kanchipuram, India. 2 Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pune, India. Abstract: We provide herein 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 relationship 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 have precluded rigorous structural elucidation of the polymers. Based on the small molecule chemistry, we conclude that structures are likely more complex than generally depicted in the published literature. Interestingly, a simpler chemistry, namely, thermal dehydrocyclization (TCD), leads to products identical to that 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 the imide to an 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 out 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, the TR processes leaves much to be desired and further improvements are clearly warranted. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/pi.5869 This article is protected by copyright. All rights reserved.