Topology-Accelerated and Selective Cascade Depolymerization of Architecturally Complex Polyesters.

Despite considerable recent advances already made in developing chemically circular polymers (CPs), the current framework predominantly focuses on CPs with linear-chain structures of different monomer types. As polymer properties are determined by not only composition but also topology, manipulating the topology of the single-monomer-based CP systems from linear-chain structures to architecturally complex polymers could potentially modulate the resulting polymer properties without changing the chemical composition, thereby advancing the concept of monomaterial product design. To that end, here, we introduce a chemically circular hyperbranched polyester (HBPE), synthesized by a mixed chain-growth and step-growth polymerization of a rationally designed bicyclic lactone with a pendent hydroxyl group (BiLOH). This HBPE exhibits full chemical recyclability despite its architectural complexity, showing quantitative selectivity for regeneration of BiLOH, via a unique cascade depolymerization mechanism. Moreover, distinct differences in materials properties and performance arising from topological variations between HBPE, hb-PBiLOH, and its linear analogue, l-PBiLOH, have been revealed where generally the branched structure led to more favorable interchain interactions, and topology-amplified optical activity has also been observed for chiral (1S, 4S, 5S)-hb-PBiLOH. More intriguingly, depolymerization of l-PBiLOH proceeds through an unexpected, initial topological transformation to the HBPE polymer, followed by the faster cascade depolymerization pathway adopted by hb-PBiLOH. Overall, these results demonstrate that CP design can go beyond typical linear polymers, and rationally redesigned, architecturally complex polymers for their unique properties may synergistically impart advantages in topology-augmented depolymerization acceleration and selectivity for exclusive monomer regeneration.

Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers.

Development of photocatalysts (PCs) with diverse properties has been essential in the advancement of organocatalyzed atom transfer radical polymerization (O-ATRP). Dimethyl dihydroacridines are presented here as a new family of organic PCs, for the first time enabling controlled polymerization of challenging acrylate monomers by O-ATRP. Structure-property relationships for seven PCs are established, demonstrating tunable photochemical and electrochemical properties, and accessing a strongly oxidizing 2 PC.+ intermediate for efficient deactivation. In O-ATRP, the combination of PC, implementation of continuous-flow reactors, and promotion of deactivation through addition of LiBr are critical to producing well-defined acrylate polymers with dispersities as low as 1.12. The utility of this approach is established through demonstration of the oxygen-tolerance of the system and application to diverse acrylate monomers, including the synthesis of well-defined di- and triblock copolymers.

Comparative Performance of PETase as a Function of Reaction Conditions, Substrate Properties, and Product Accumulation.

There is keen interest to develop new technologies to recycle the plastic poly(ethylene terephthalate) (PET). To this end, the use of PET-hydrolyzing enzymes has shown promise for PET deconstruction to its monomers, terephthalate (TPA) and ethylene glycol (EG). Here, the Ideonella sakaiensis PETase wild-type enzyme was compared to a previously reported improved variant (W159H/S238F). The thermostability of each enzyme was compared and a 1.45 Šresolution structure of the mutant was described, highlighting changes in the substrate binding cleft compared to the wild-type enzyme. Subsequently, the performance of the wild-type and variant enzyme was compared as a function of temperature, substrate morphology, and reaction mixture composition. These studies showed that reaction temperature had the strongest influence on performance between the two enzymes. It was also shown that both enzymes achieved higher levels of PET conversion for substrates with moderate crystallinity relative to amorphous substrates. Finally, the impact of product accumulation on reaction progress was assessed for the hydrolysis of both PET and bis(2-hydroxyethyl) terephthalate (BHET). Each enzyme displayed different inhibition profiles to mono(2-hydroxyethyl) terephthalate (MHET) and TPA, while both were sensitive to inhibition by EG. Overall, this study highlights the importance of reaction conditions, substrate selection, and product accumulation for catalytic performance of PET-hydrolyzing enzymes, which have implications for enzyme screening in the development of enzyme-based polyester recycling.

Comparative Performance of PETase as a Function of Reaction Conditions, Substrate Properties, and Product Accumulation.

Invited for this month's cover is the BOTTLE Consortium, featuring Gregg Beckham's laboratory from NREL and John McGeehan's laboratory from the University of Portsmouth. The cover image shows the application of poly(ethylene terephthalate) (PET) hydrolase enzymes on post-consumer waste plastic, towards the development of an enzymatic PET recycling strategy. The Full Paper itself is available at 10.1002/cssc.202101932.

Mixed plastics waste valorization through tandem chemical oxidation and biological funneling.

Mixed plastics waste represents an abundant and largely untapped feedstock for the production of valuable products. The chemical diversity and complexity of these materials, however, present major barriers to realizing this opportunity. In this work, we show that metal-catalyzed autoxidation depolymerizes comingled polymers into a mixture of oxygenated small molecules that are advantaged substrates for biological conversion. We engineer a robust soil bacterium, Pseudomonas putida, to funnel these oxygenated compounds into a single exemplary chemical product, either ß-ketoadipate or polyhydroxyalkanoates. This hybrid process establishes a strategy for the selective conversion of mixed plastics waste into useful chemical products.

Radical Addition to N,N-Diaryl Dihydrophenazine Photoredox Catalysts and Implications in Photoinduced Organocatalyzed Atom Transfer Radical Polymerization.

Photoinduced organocatalyzed atom transfer radical polymerization (O-ATRP) is a controlled radical polymerization methodology catalyzed by organic photoredox catalysts (PCs). In an efficient O-ATRP system, good control over molecular weight with an initiator efficiency (I* = M n,theo/M n,exp × 100%) near unity is achieved, and the synthesized polymers possess a low dispersity (D). N,N-Diaryl dihydrophenazine catalysts typically produce polymers with low dispersity (D < 1.3) but with less than unity molecular weight control (I* ~ 60-80%). This work explores the termination reactions that lead to decreased control over polymer molecular weight and identifies a reaction leading to radical addition to the phenazine core. This reaction can occur with radicals generated through reduction of the ATRP initiator or the polymer chain end. In addition to causing a decrease in I*, this reactivity modifies the properties of the PC, ultimately impacting polymerization control in O-ATRP. With this insight in mind, a new family of core-substituted N,N-diaryl dihydrophenazines is synthesized from commercially available ATRP initiators and employed in O-ATRP. These new core-substituted PCs improve both I* and D in the O-ATRP of MMA, while minimizing undesired side reactions during the polymerization. Further, the ability of one core-substituted PC to operate at low catalyst loadings is demonstrated, with minimal loss of polymerization control down to 100 ppm (weight average molecular weight [M w] = 10.8 kDa, D = 1.17, I* = 104% vs M w = 8.26, D = 1.10, I* = 107% at 1000 ppm) and signs of a controlled polymerization down to 10 ppm of the catalyst (M w = 12.1 kDa, D = 1.36, I* = 107%).

Photoinduced Controlled Radical Polymerizations Performed in Flow: Methods, Products, and Opportunities.

Photoinduced controlled radical polymerizations (CRPs) have provided a variety of approaches for the synthesis of polymers possessing targeted structures, compositions, and functionalities with the added capability for spatial and temporal control, presenting the potential for new materials development. However, the scalability and reliability of these systems can be limited as a consequence of dependence on uniform irradiation of the reaction to produce well-defined products. In this perspective, we highlight the utility and promise of photo-CRP approaches through an overview of the adaptation of these methodologies to photo-flow reactor systems. Special emphasis is placed on the current state-of-the-art in polymerization scalability, reactor design, and polymer scope.

Synthesis of Star Polymers using Organocatalyzed Atom Transfer Radical Polymerization Through a Core-first Approach.

Synthetic routes to higher ordered polymeric architectures are important tools for advanced materials design and realization. In this study, organocatalyzed atom transfer radical polymerization is employed for the synthesis of star polymers through a core-first approach using a visible-light absorbing photocatalyst, 3,7-di(4-biphenyl)-1-naphthalene-10-phenoxazine. Structurally similar multifunctional initiators possessing 2, 3, 4, 6, or 8 initiating sites were used in this study for the synthesis of linear telechelic polymers and star polymers typically possessing dispersities lower than 1.5 while achieving high initiator efficiencies. Furthermore, no evidence of undesirable star-star coupling reactions was observed, even at high monomer conversions and high degrees of polymerization. The utility of this system is further exemplified through the synthesis of well-defined diblock star polymers.