P olymers with a lower critical solution temperature (LCST) undergo an entropy-driven phase transition from a soluble coiled state at temperatures below the LCST to an insoluble aggregated globular state at temperatures above the LCST. 1 Owing to thermoresponsiveness, polymers exhibiting LCST behavior have found a diverse range of applications spanning advanced "smart" materials, drug delivery, and catalysis. 2 Poly(Nisopropylacrylamide) is a typical substance which displays LCST behavior. Its derivatives have been widely studied due to their good biocompatibility and easy adjustment of the phase transition temperature to near body temperature. 3 These polymers have a flexible main-chain conformation and respond to temperature changes in a water medium by varying their hydrogen bonds. In recent years, few examples of new LCST systems utilizing organic media have been reported, in which those LCST behaviors were induced by host−guest, 4 chargetransfer, 5 and hydrogen-bonding 6 interactions with additives. Although these demonstrations broaden the scope of the design of LCST polymers, they use ternary systems with various additives and are still illustrated with the polymers having a flexible main-chain conformation.Aromatic polyimides (PIs) are typical rigid polymers and high-performance polymeric materials with excellent thermal, mechanical, and electrical properties. 7 However, most PIs are insoluble and infusible as a result of their rod-like chain conformation and strong interchain interactions. Therefore, much research effort on PIs has been devoted to synthesizing soluble polymers in a fully imidized form without a deterioration of their thermal and mechanical properties. 8 Among the many approaches attempted thus far, the incorporation of trifluoromethyl (CF 3 ) groups onto polymer chains is considered as an effective means of realizing soluble PIs without deteriorating their useful properties. 9 Recently, we reported new soluble PIs which were prepared by introducing two CF 3 groups onto the repeating units of the chain. 10 These polymers showed good solubility while retaining their good thermal and optical properties due to the existence of CF 3 groups as substituents. Against all expectations, we noted that the PIs showed LCST behavior in organic media. This phenomenon was very interesting because LCST behavior in rigid polymers such as PIs is unprecedented to date.In this context, we describe herein the observation of novel LCST behavior in rigid polymers. The polymers were prepared from 4-(4′-aminophenoxy)-3,5-bis(trifluoromethyl)aniline and two aromatic dianhydrides, 4,4′-oxidiphthalic anhydride (PI1) and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (PI2), via a one-pot solution imidization method (Scheme 1).These polymers showed good solubility in various organic solvents such as tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, N,N-dimethylacetamide, m-cresol, anisole, ethyl acetate, and acetone. Among these solvents, ethyl acetate and acetone solutions exhibit...