In the past decade, polymers of intrinsic microporosity (PIMs) have attracted much interest as versatile platform materials for applications such as gas separation membrane, 1−4 gas storage, 5,6 solvent nanofiltration, 7,8 and fuel cell catalyst support. 9 The well-studied polymer of its class is the original PIM-1, first synthesized by Budd and McKeown in 2004. 10 Ever since, in parallel to synthesis progress, postsynthesis modification has also been explored extensively. Specifically, the nitrile group in PIM-1 has been the most convenient reaction site and has been converted to various functional groups. 5,6,11−15 Among those, the carboxylic acid is a highly desirable functionality to be installed into the PIM backbone due to its ability to undergo decarboxylation, 16 polymer alloying, 17−19 and ionomer formation. 20,21 For example, the "carboxylated" PIM-1 (C-PIMs) were cross-linked via thermally induced decarboxylation. 16 The proposed reaction mechanism proceeded through similar fashion to Koros' carboxylic acid-containing polyimide work. 22 Furthermore, C-PIMs have also been modified by blending with Torlon, 17 Matrimid, 18 and P84 19 in attempts to improve the overall gas separation performance. In these studies, the significantly improved properties of the blended membranes were connected to the hydrogen bonding between C-PIMs and the other polymers. Most recently, the ionomers of C-PIMs with various metal ions Ag + , Ca 2+ , Mg 2+ , Zn 2+ , Al 3+ , and Fe 3+ have been reported. 20,21 Notably, in comparison to the parent PIM-1 membrane, the metal ion treated membranes successfully enhanced the CO 2 /CH 4 and olefin/paraffin separation performance, this being above the upper bound lines under both pure and mixed gas conditions. 20 Lastly, the "assumed" C-PIMs exhibited the selective removal of various heavy metal ions and organic dyes for wastewater treatment. 23 Originally in 2009, the preparation of C-PIMs through the conversion from nitrile to carboxylic acid was reported via a rapid 5 h base hydrolysis process. 11 The hydrolyzed products with various levels of carboxylation were obtained by subjecting PIM-1 to a basic medium between 1 and 5 h duration. Since then, this method has become the "standard" procedure to obtain C-PIMs as it is considered to be a "simple and efficient" route. To date, it has collected more than 100 citations. The original paper proposed that the resulting product contained the combination of pure PIM-1 (nitrile group only), a mixture of PIM-1 and C-PIMs (nitrile and carboxylic acid groups), and pure C-PIMs (carboxylic acid group only), as demonstrated in Scheme 1a. 11 There was no mention of the primary amide as portion of hydrolysis products.
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