Kazuki Fukushima

Sustainable polymers are often designed to be degradable or recyclable via low-energy, low-CO₂-emission pathways while demonstrating high physical properties comparable to those of conventional plastics. Aliphatic polycarbonates (APCs), studied as functionalized degradable polymers, are promising candidates for use as sustainable materials. However, enhancing their physical properties remains essential. Polymer properties depend on various factors, including chemical structure, molecular weight and its distribution, branching, higher-order structures, and thermal history, and are therefore not uniquely determined. Machine learning models are powerful tools for predicting polymer properties with associated uncertainty, particularly for polymers absent from open datasets. Herein, we investigate uncertainty-aware predictions of the glass transition temperature ( T _g ) of APCs using ensembles of multiple molecular descriptors and machine learning methods. The T _g data for more than 50 APCs were collected from previous experimental studies. Prediction models for T _g were constructed using more than 100 combinations of molecular descriptors and machine learning algorithms based on datasets of common polymers. The top five models, selected by coefficients of determination, exhibited moderate predictive performance for APCs. The mean values and standard deviations of the predictions obtained from the top five models provide T _g predictions with quantified uncertainty. This ensemble-based framework will be extended to uncertainty-aware prediction of various polymer properties.

Polymers and plastics pose environmental challenges, including marine pollution from waste and CO₂ emissions from incineration. Recycling and upcycling are crucial strategies for conserving petroleum resources and reducing waste discharge. Additionally, developing sustainable polymers is essential for achieving a circular economy. Polymer degradation is a key process in both recycling and sustainable polymer development. This review examines the degradation of condensation polymers, such as polyesters and polycarbonates, when organic catalysts are used to enhance transesterification. Organic bases exhibit high catalytic efficiency in polymer degradation, whereas others facilitate the controlled polymerization of substituted cyclic esters and carbonates. Notably, 1,5,7-triazabicyclo[4.4.0]dec-7-ene has exceptional efficiency in degrading various condensation polymers, including aliphatic polycarbonates and liquid-crystalline wholly aromatic polyesters, via a dual hydrogen-bonding activation mechanism. The functionalization of aliphatic polycarbonates via side-chain modifications is a promising approach for producing functionalized degradable polymers, supported by efficient monomer synthesis and established ring-opening polymerization (ROP) techniques using organic catalysts. Precise polymer synthesis enhances mechanical and thermal properties by incorporating rigid moieties while enabling degradation control. These advancements contribute to the development of sustainable materials within a future circular economy.

Plastic circular economy needs to be established to solve environmental issues related to plastic waste. Superengineering plastics such as liquid-crystalline (LC) polymers exhibit excellent thermal and mechanical properties, resulting in poor degradability in natural environment. Herein, we report the degradation of a wholly aromatic thermotropic LC polyester, poly(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid) (Vectra) mediated by superbases. Methanolysis and hydrolysis of Vectra yield its monomeric compounds, 4-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, and their methyl esters. Among several transesterification catalysts explored, 1,5,7-triazabicylco[4.4.0]dec-5-ene (TBD) is the most suitable for the methanolysis of Vectra. The complete degradation of Vectra is achieved under reflux. The degradation proceeds heterogeneously via a surface erosion mechanism, preferentially starting from less chain-packed regions. Model reactions using aryl arylates reveal that monomeric compound-superbase complexes could mediate the cleavage of the ester bonds in both homogeneous and heterogeneous systems. The ester bonds of Vectra have inherent poor reactivity and are protected by oriented robust structures of the polymer. Nevertheless, the superbases enable the degradation of Vectra via the cleavage of the ester bonds by methanol. These outcomes open the way for recycling high-performance plastics as well as demonstrate the feasibility of recovering precious aromatic compounds from plastic waste as aromatic feedstock.

Investigating polymer degradation mechanisms enables the establishment of controlled degradation techniques for the development of sustainable and recyclable materials. Hydration can play a crucial role in controlling the hydrolysis of polymers. Here, ether-functionalized aliphatic polycarbonates (APCs) susceptible to nonenzymatic hydrolysis were developed for application as biocompatible biomaterials. Among these polymers, those grafted with 2-methoxyethyl and 3-methoxypropyl side chains via an amide group were highly wettable, strongly interacted with water, and experienced almost complete hydrolysis in phosphate-buffered saline over 30 days, which was attributed to the hydrogen bonding between water and the amide/methoxy groups. In an alkaline medium, all amide-linked APCs were completely hydrolyzed within 30 days, regardless of the side-chain structure. In contrast, the nonamide-linked APCs and a representative aliphatic polycarbonate, poly(trimethylene carbonate), were minimally degraded in the buffer and experienced <31% degradation under alkaline conditions. The APC with the 3-methoxypropyl side chain exhibited platelet adhesion properties comparable to those of ether-functionalized APCs previously reported as blood-compatible polymers. Thus, our results demonstrate the effects of an amide linker on the hydration and hydrolytic properties of APCs and can help establish new design concepts for degradable polymers.

Side-chain-functionalized aliphatic polyesters are promising as functional biodegradable polymers. We have investigated ring-opening reactions of γ-carbonyl-substituted ε-caprolactones (gCCLs) to obtain poly(ε-caprolactone) (PCL) analogues. Organic catalysts and Sn(Oct)₂ often used for the ring-opening polymerization (ROP) of ε-caprolactone (CL) have been explored to find the conditions for the formation of polymeric products of gCCLs. We confirmed the consumption of gCCLs in all catalyzed reactions. However, chain propagation hardly occurs, as the propagating species are preferentially transformed to α-substituted five-membered lactones when the substituents are linked by ester or not sterically hindered. Intramolecular cyclization to form thermodynamically stable five-membered lactones releases alcohols and amines, serving as nucleophiles for the subsequent ring opening of other gCCLs. Thus, apparent chain reactions are realized for continuous consumption of gCCLs. The reaction preference remains unchanged independent of the catalysts, although the reactions of the amide-linked gCCLs by acidic catalysts are slightly mitigated. Finally, copolymerization of CL and a gCCL catalyzed by diphenyl phosphate has been investigated, which enables the chain propagation reaction to yield the linear oligomers of PCL analogues containing up to 16 mol% of gCCL units. This study contributes to understanding the chemistry of ring-opening reactions of substituted lactones for designing functional degradable polymers.

Polylactide (PLA) crystallizes to form extendedchain crystals in a Langmuir monolayer because crystallization is accelerated on the water surface. This is a unique situation where chain packing can be analyzed by simply measuring the lamellar thickness. Herein, star-shaped poly(L-lactide)s (PLLAs) with 2-12 arms were synthesized through the polymerization of L-lactide with various polyols as initiators, and their crystallization behavior in a monolayer was studied via atomic force microscopy. The PLLAs comprising 2-4 arms crystallized with all arms aligned in the same direction and being folded at the central polyol unit. Meanwhile, the PLLAs comprising 6 and 12 arms crystallized with both halves of the arms extended from the center to the opposite directions, most likely due to the steric hindrance of the crowded arms. Considering that the PLLAs crystallized from a once-formed condensed amorphous state during compression, they have a strong tendency to crystallize with the arms aligned in the same direction. The crystallization rate of star-shaped PLAs is known to reduce compared with that of a linear PLA even if the number of arms is as few as 2. This should be closely related to the unique crystallization behavior of the star-shaped PLLAs with the arms aligned in the same direction.

Aliphatic polycarbonates have attracted attention as degradable and sustainable materials contributing to the circular plastic economy. Their chemical recycling has not been sufficiently studied. In this study, the efficacy of organocatalysts for depolymerization of poly(trimethylene carbonate) (PTMC), a representative aliphatic polycarbonate, is investigated using several organic acids and bases. The hydrolysis of PTMC produces the water‐soluble degradates propane‐1,3‐diol (PD) and CO₂. A phosphazene base P₂‐t‐Bu shows high activity for the hydrolysis, yielding up to 31% and 89% of PD in the homogeneous reaction at around 27°C and the inhomogeneous reaction under the reflux condition, respectively. By contrast, 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD) exhibits exceptionally high catalytic activity for the methanolysis of PTMC, producing PD and dimethyl carbonate. This is because of dual hydrogen‐bonding activation, which completes the inhomogeneous reaction in a few hours at around 27°C while yielding more than 90% of PD. The reaction rate of the TBD‐catalyzed methanolysis depends on the concentration of the nucleophile and catalyst, and the ratio of the nucleophile to the substrate affects the PD yield. These results provide a highly promising standard for chemical recycling of functionalized aliphatic polycarbonates that could potentially be applied to sustainable materials in the future.