Special issue: CO₂: capture of, utilization of, and degradation into

Carbon dioxide (CO₂) capture and utilization have been challenging issues in the field of polymer science. Due to the steady increase in CO₂ concentration in the atmosphere and its potential threat as a Greenhouse gas, numerous efforts have been devoted to the development of polymer membranes for CO₂ capture in the past few decades. CO₂ is generally recognized as an inert gas, but can react with other reagents and catalysts. Indeed, a variety of polymerization processes with CO₂ as a raw material have been developed since the pioneering work on epoxide/CO₂ copolymerization by Prof. Inoue et al. Recent studies have also demonstrated that biosynthesis is an attractive and promising process to prepare functional polymer materials from CO₂. These CO₂-derived polymer materials store the carbon from CO₂ in a certain period and then release it as CO₂ by combustion or biodegradation. Accordingly, the coupling of CO₂ capture, CO₂ utilization, and degradation into CO₂ can close the carbon cycle, contributing to the next-generation polymer engineering for a circular economy. Considering the environmental pollution issues, including microplastics in the ocean, degradation of polymeric materials into CO₂ is also an essential science to realize the sustainable society. In this context, we organized a diverse team consisting of the Editor-in-Chief, an associate editor, and guest editors to publish this special issue on CO₂-related polymer science.

In this special issue, the chemical synthesis of CO₂-based polymers is one of the hot topics as a typical utilization of CO₂. Meng et al. reviewed the syntheses of CO₂-based copolymers with cyclic anhydrides and cyclic esters [1]. Darensbourg et al. reported copolymerization to synthesize completely alternating regioregular ¹³C-labelled poly(propylene carbonate) [2]. Kerton et al. reviewed iron-catalyzed reactions of CO₂ and epoxides to synthesize cyclic and polycarbonates [3]. Sugimoto et al. reported terpolymerization of CO₂, propylene oxide, and various epoxides with bulky side groups to tune the resultant copolymers’ thermal properties [4]. Nakano et al. reported the synthesis of polycarbonate-block-polycycloalkenes via epoxide/CO₂ copolymerization and ring-opening metathesis polymerization [5]. Tominaga et al. reported ion-conductive and dielectric behavior of poly(ethylene carbonate)-based Li electrolytes [6]. For the CO₂ capture, several membranes for CO₂ separation are introduced in this issue. Taniguchi et al. reported piperazine-immobilized polymeric membranes for CO₂ capture [7]. Matsuyama et al. reported inorganic/organic double-network ion gel membrane for CO₂ separation [8]. Yamada et al. overviewed recent progress of the development of carbon capture process and amine-based CO₂ capture technologies [9]. Hoshino et al. reported thermo-responsive CO₂ absorbent for high and low concentration CO₂, which functions by tuning pK_a of ammonium ions [10]. Fujikawa et al. reviewed the general understanding of the polymeric membranes for CO₂ capture and separation [11]. Biodegradable polymers are also essential and target topics in this special issue. Sudesh et al., Tsuge et al., Kasuya et al., Taguchi et al., and Mota et al. reported biosynthesis and biodegradation of polyhydroxyalkanoates [12,13,14,15,16]. Another bacterial polymer, cyanobacterial polysaccharides, is also reviewed by Kaneko et al. [17]. As other biodegradable and biological polymers, Fujisawa et al. reported cellulose and its composites [18]. Numata et al. reported silk materials as biological and biodegradable materials [19].

We believe that this special issue will be valuable for readers who have scientific and practical interests in CO₂-related polymer science and contribute to significant advances in this area in the future. Finally, we sincerely appreciate all authors and referees for their contribution to this special issue.