A renewably sourced, circular photopolymer resin for additive manufacturing.

The additive manufacturing of photopolymer resins by means of vat photopolymerization enables the rapid fabrication of bespoke 3D-printed parts. Advances in methodology have continually improved resolution and manufacturing speed, yet both the process design and resin technology have remained largely consistent since its inception in the 1980s1. Liquid resin formulations, which are composed of reactive monomers and/or oligomers containing (meth)acrylates and epoxides, rapidly photopolymerize to create crosslinked polymer networks on exposure to a light stimulus in the presence of a photoinitiator2. These resin components are mostly obtained from petroleum feedstocks, although recent progress has been made through the derivatization of renewable biomass3-6 and the introduction of hydrolytically degradable bonds7-9. However, the resulting materials are still akin to conventional crosslinked rubbers and thermosets, thus limiting the recyclability of printed parts. At present, no existing photopolymer resin can be depolymerized and directly re-used in a circular, closed-loop pathway. Here we describe a photopolymer resin platform derived entirely from renewable lipoates that can be 3D-printed into high-resolution parts, efficiently deconstructed and subsequently reprinted in a circular manner. Previous inefficiencies with methods using internal dynamic covalent bonds10-17 to recycle and reprint 3D-printed photopolymers are resolved by exchanging conventional (meth)acrylates for dynamic cyclic disulfide species in lipoates. The lipoate resin platform is highly modular, whereby the composition and network architecture can be tuned to access printed materials with varied thermal and mechanical properties that are comparable to several commercial acrylic resins.

Improving the Performance of Photoactive Terpene-Based Resin Formulations for Light-Based Additive Manufacturing.

Photocurable liquid formulations have been a key research focus for the preparation of mechanically robust and thermally stable networks. However, the development of renewable resins to replace petroleum-based commodities presents a great challenge in the field. From this perspective, we disclose the design of photoactive resins based on terpenes and itaconic acid, both potentially naturally sourced, to prepare photosets with adjustable thermomechanical properties. Biobased perillyl itaconate (PerIt) was synthesized from renewable perillyl alcohol and itaconic anhydride via a scalable solvent-free method. Photoirradiation of PerIt in the presence of a multiarm thiol and photoinitiator led to the formation of networks over a range of compositions. Addition of nonmodified terpenes (perillyl alcohol, linalool, or limonene) as reactive diluents allowed for more facile preparation of photocured networks. Photosets within a wide range of properties were accessed, and these could be adjusted by varying diluent type and thiol stoichiometry. The resins showed rapid photocuring kinetics and the ability to form either brittle or elastic materials, with Young's modulus and strain at break ranging from 3.6 to 358 MPa and 15 to 367%, respectively, depending on the chemical composition of the resin. Glass transition temperatures (Tg) were influenced by thioether content, with temperatures ranging from 5 to 43 °C, and all photosets displayed good thermal resistance with Td,5% > 190 °C. Selected formulations containing PerIt and limonene demonstrated suitability for additive manufacturing technologies and high-resolution objects were printed via digital light processing (DLP). Overall, this work presents a simple and straightforward route to prepare renewable resins for rapid prototyping applications.

Stereocomplexed Functional and Statistical Poly(lactide-carbonate)s via a Simple Organocatalytic System.

The stereocomplexation of polylactide (PLA) has been widely relied upon to develop degradable, sustainable materials with increased strength and improved material properties in comparison to stereopure PLA. However, forming functionalized copolymers of PLA while retaining high crystallinity remains elusive. Herein, the controlled ring-opening copolymerization (ROCOP) of lactide (LA) and functionalized cyclic carbonate monomers is undertaken. The produced polymers are shown to remain crystalline up to 25 mol % carbonate content and are efficiently stereocomplexed with homopolymer PLA and copolymers of opposite chirality. Polymers with alkene and alkyne pendent handles are shown to undergo efficient derivatization with thiol-ene click chemistry, which would allow both the covalent conjugation of therapeutic moieties and tuning of material properties.

Selective and Sequential Catalytic Chemical Depolymerization and Upcycling of Mixed Plastics.

Chemical recycling to monomer (CRM) provides a useful technique to allow for polymer-to-monomer-to-polymer circular economies. A significant challenge remains, however, in the treatment of mixed plastics by CRM in which unselective depolymerization requires either presorting of plastics or purification processes postdepolymerization, both of which add cost to waste plastic processing. We report a simple, yet selective, chemical depolymerization of three commonly used polymers, poly(lactic acid) (PLA), bisphenol A polycarbonate (BPA-PC), and polyethylene terephthalate (PET), using inexpensive and readily available common metal salt/organobase dual catalysts. By a judicious choice of catalyst and conditions, selective and sequential depolymerization of mixtures of the polymers was demonstrated. Furthermore, the potential for upcycling of polymers to value-added monomers was explored through the application of alternative nucleophiles within the depolymerization.

2D Hierarchical Microbarcodes with Expanded Storage Capacity for Optical Multiplex and Information Encryption.

The design of nanosegregated fluorescent tags/barcodes by geometrical patterning with precise dimensions and hierarchies could integrate multilevel optical information within one carrier and enhance microsized barcoding techniques for ultrahigh-density optical data storage and encryption. However, precise control of the spatial distribution in micro/nanosized matrices intrinsically limits the accessible barcoding applications in terms of material design and construction. Here, crystallization forces are leveraged to enable a rapid, programmable molecular packing and rapid epitaxial growth of fluorescent units in 2D via crystallization-driven self-assembly. The fluorescence encoding density, scalability, information storage capacity, and decoding techniques of the robust 2D polymeric barcoding platform are explored systematically. These results provide both a theoretical and an experimental foundation for expanding the fluorescence storage capacity, which is a longstanding challenge in state-of-the-art microbarcoding techniques and establish a generalized and adaptable coding platform for high-throughput analysis and optical multiplexing.

Tuning the Functionality of Self-Assembled 2D Platelets in the Third Dimension.

The decoration of 2D nanostructures using heteroepitaxial growth is of great importance to achieve functional assemblies employed in biomedical, electrical, and mechanical applications. Although the functionalization of polymers before self-assembly has been investigated, the exploration of direct surface modification in the third dimension from 2D nanostructures has, to date, been unexplored. Here, we used living crystallization-driven self-assembly to fabricate poly(ε-caprolactone)-based 2D platelets with controlled size. Importantly, surface modification of the platelets in the third dimension was achieved by using functional monomers and light-induced polymerization. This method allows us to selectively regulate the height and fluorescence properties of the nanostructures. Using this approach, we gained unprecedented spatial control over the surface functionality in the specific region of complex 2D platelets.

Stereocomplex-Driven Morphological Transition of Coil-Rod-Coil Poly(lactic acid)-Based Cylindrical Nanoparticles.

The stereocomplexation of poly(lactic acid) (PLA) enantiomers opens up an avenue for the formation of new materials with enhanced performance, specifically regarding their mechanical and thermal resistance and resistance to hydrolysis. Despite these useful features, the study of the stereocomplexation between block copolymers based on PLA in solution is limited, and a comprehensive understanding of this phenomenon is urgently needed. Herein, triblock copolymers of poly(N-hydroxyethyl acrylamide) and PL(or D)LA in which PLA was midblock (PHEAAmy-b-PL(D)LAx-b-PHEAAmy) were synthesized and assembled into cylindrical micelles via crystallization-driven self-assembly . The stereocomplexation between enantiomeric micelles facilitates the morphological transition, and the transformation process was investigated in detail by varying the aging temperature, block composition, and solvent. It was found that the solubility of the copolymers played a vital role in determining the occurrence and the speed of the chain exchange between the micelles and the unimers, which thereafter has a significant impact on the shape transition. These results lead to a deeper understanding of the stereocomplex-driven morphological transition process and provide valuable guidance for further optimization of the transition under physiological conditions as a new category of stimuli-responsive systems for biomedical applications.

4D Printable Salicylic Acid Photopolymers for Sustained Drug Releasing, Shape Memory, Soft Tissue Scaffolds.

3D printing of pharmaceuticals offers a unique opportunity for long-term, sustained drug release profiles for an array of treatment options. Unfortunately, this approach is often limited by physical compounding or processing limitations. Modification of the active drug into a prodrug compound allows for seamless incorporation with advanced manufacturing methods that open the door to production of complex tissue scaffold drug depots. Here we demonstrate this concept using salicylic acids with varied prodrug structures for control of physical and chemical properties. The role of different salicylic acid derivatives (salicylic acid, bromosalicylic allyl ester, iodosalicylic allyl ester) and linker species (allyl salicylate, allyl 2-(allyloxy)benzoate, allyl 2-(((allyloxy)carbonyl)oxy)benzoate) were investigated using thiol-ene cross-linking in digital light processing (DLP) 3D printing to produce porous prodrug tissue scaffolds containing more than 50% salicylic acid by mass. Salicylic acid photopolymer resins were all found to be highly reactive (solidification within 5 s of irradiation at λ = 405 nm), while the cross-linked solids display tunable thermomechanical behaviors with low glass transition temperatures (Tgs) and elastomeric behaviors, with the carbonate species displaying an elastic modulus matching that of adipose tissue (approximately 65 kPa). Drug release profiles were found to be zero order, sustained release based upon hydrolytic degradation of multilayered scaffolds incorporating fluorescent modeling compounds, with release rates tuned through selection of the linker species. Cytocompatibility in 2D and 3D was further demonstrated for all species compared to polycarbonate controls, as well as salicylic acid-containing composites (physical incorporation), over a 2-week period using murine fibroblasts. The use of drugs as the matrix material for solid prodrug tissue scaffolds opens the door to novel therapeutic strategies, longer sustained release profiles, and even reduced complications for advanced medicine.

Correction to Crystal Structure of Poly(7-heptalactone).

[This corrects the article DOI: 10.1021/acs.macromol.3c00710.].

The Utilisation of Hydrogels for iPSC-Cardiomyocyte Research.

Cardiac fibroblasts' (FBs) and cardiomyocytes' (CMs) behaviour and morphology are influenced by their environment such as remodelling of the myocardium, thus highlighting the importance of biomaterial substrates in cell culture. Biomaterials have emerged as important tools for the development of physiological models, due to the range of adaptable properties of these materials, such as degradability and biocompatibility. Biomaterial hydrogels can act as alternative substrates for cellular studies, which have been particularly key to the progression of the cardiovascular field. This review will focus on the role of hydrogels in cardiac research, specifically the use of natural and synthetic biomaterials such as hyaluronic acid, polydimethylsiloxane and polyethylene glycol for culturing induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). The ability to fine-tune mechanical properties such as stiffness and the versatility of biomaterials is assessed, alongside applications of hydrogels with iPSC-CMs. Natural hydrogels often display higher biocompatibility with iPSC-CMs but often degrade quicker, whereas synthetic hydrogels can be modified to facilitate cell attachment and decrease degradation rates. iPSC-CM structure and electrophysiology can be assessed on natural and synthetic hydrogels, often resolving issues such as immaturity of iPSC-CMs. Biomaterial hydrogels can thus provide a more physiological model of the cardiac extracellular matrix compared to traditional 2D models, with the cardiac field expansively utilising hydrogels to recapitulate disease conditions such as stiffness, encourage alignment of iPSC-CMs and facilitate further model development such as engineered heart tissues (EHTs).

Synthesis, Morphology, and Crystallization Kinetics of Polyheptalactone (PHL).

Aliphatic polyesters are widely studied due to their excellent properties and low-cost production and also because, in many cases, they are biodegradable and/or recyclable. Therefore, expanding the range of available aliphatic polyesters is highly desirable. This paper reports the synthesis, morphology, and crystallization kinetics of a scarcely studied polyester, polyheptalactone (PHL). First, we synthesized the η-heptalactone monomer by the Baeyer-Villiger oxidation of cycloheptanone before several polyheptalactones of different molecular weights (in the range between 2 and 12 kDa), and low dispersities were prepared by ring-opening polymerization (ROP). The influence of molecular weight on primary nucleation rate, spherulitic growth rate, and overall crystallization rate was studied for the first time. All of these rates increased with PHL molecular weight, and they approached a plateau for the highest molecular weight samples employed here. Single crystals of PHLs were prepared for the first time, and hexagonal-shaped flat single crystals were obtained. The study of the crystallization and morphology of PHL revealed strong similarities with PCL, making PHLs very promising materials, considering their potential biodegradable character.

Uniform segmented platelet micelles with compositionally distinct and selectively degradable cores.

The creation of nanoparticles with controlled and uniform dimensions and spatially defined functionality is a key challenge. The recently developed living crystallization-driven self-assembly (CDSA) method has emerged as a promising route to one-dimensional (1D) and 2D core-shell micellar assemblies by seeded growth of polymeric and molecular amphiphiles. However, the general limitation of the epitaxial growth process to a single core-forming chemistry is an important obstacle to the creation of complex nanoparticles with segmented cores of spatially varied composition that can be subsequently exploited in selective transformations or responses to external stimuli. Here we report the successful use of a seeded growth approach that operates for a variety of different crystallizable polylactone homopolymer/block copolymer blend combinations to access 2D platelet micelles with compositionally distinct segmented cores. To illustrate the utility of controlling internal core chemistry, we demonstrate spatially selective hydrolytic degradation of the 2D platelets-a result that may be of interest for the design of complex stimuli-responsive particles for programmed-release and cargo-delivery applications.

Enhanced drug delivery to cancer cells through a pH-sensitive polycarbonate platform.

Polymer-drug conjugates are widely investigated to enhance the selectivity of therapeutic drugs to cancer cells, as well as increase circulation lifetime and solubility of poorly soluble drugs. In order to direct these structures selectively to cancer cells, targeting agents are often conjugated to the nanoparticle surface as a strategy to limit drug accumulation in non-cancerous cells and therefore reduce systemic toxicity. Here, we report a simple procedure to generate biodegradable polycarbonate graft copolymer nanoparticles that allows for highly efficient conjugation and intracellular release of S-(+)-camptothecin, a topoisomerase I inhibitor widely used in cancer therapy. The drug-polymer conjugate showed strong efficacy in inhibiting cell proliferation across a range of cancer cell lines over non-cancerous phenotypes, as a consequence of the increased intracellular accumulation and subsequent drug release specifically in cancer cells. The enhanced drug delivery towards cancer cells in vitro demonstrates the potential of this platform for selective treatments without the addition of targeting ligands.

Click Step-Growth Polymerization and E/Z Stereochemistry Using Nucleophilic Thiol-yne/-ene Reactions: Applying Old Concepts for Practical Sustainable (Bio)Materials.

Polymer sustainability is synonymous with "bioderived polymers" and the zeitgeist of "using renewable feedstocks". However, this sentiment does not adequately encompass the requirements of sustainability in polymers. In addition to recycling considerations and mechanical performance, following green chemistry principles also needs to be maximized to improve the sustainability of polymer synthesis. The synthetic cost (i.e., maximizing atom economy, reducing chemical hazards, and lowering energy requirements) of producing polymers should be viewed as equally important to the monomer source (biomass vs petrol platform chemicals). Therefore, combining the use of renewable feedstocks with efficient syntheses and green chemistry principles is imperative to delivering truly sustainable polymers. The high efficiency, atom economy, and single reaction trajectories that define click chemistry reactions position them as ideal chemical approaches to synthesize polymers in a sustainable manner while simultaneously expanding the structural scope of accessible polymers from sustainably sourced chemicals.Click step-growth polymerization using the thiol-yne Michael addition, a reaction first reported over a century ago, has emerged as an extremely mild and atom-efficient pathway to yield high-performance polymers with controllable E/Z stereochemistry along the polymer backbone. Building on studies of aromatic thiol-yne polymers, around 10 years ago our group began investigating the thiol-yne reaction for the stereocontrolled synthesis of alkene-containing aliphatic polyesters. Our early studies established a convenient path to high-molecular-weight (>100 kDa) E-rich or Z-rich step-growth polymers by judiciously changing the catalyst and/or reaction solvent. This method has since been adapted to synthesize fast-degrading polyesters, high-performance polyamides, and resilient hydrogel biomaterials. Across several systems, we have observed dramatic differences in material properties among polymers with different alkene stereochemistry.We have also explored the analogous thiol-ene Michael reaction to create high-performance poly(ester-urethanes) with precise E/Z stereochemistry. In contrast to the stereoselective thiol-yne polymerization, here the use of monomers with predefined E/Z (geometric) isomerism (arising from either alkenes or the planar rigidity of ring units) affords polymers with total control over stereochemistry. This advancement has enabled the synthesis of tough, degradable materials that are derived from sustainable monomer feedstocks. Employing isomers of sugar-derived isohexides, bicyclic rigid-rings possessing geometric isomerism, led to degradable polymers with fundamentally opposing mechanical behavior (i.e., plastic vs elastic) simply by adjusting the stereochemistry of the isohexide.In this Account, we feature our investigation of thiol-yne/-ene click step-growth polymers and efforts to establish structure-property relationships toward degradable materials with practical mechanical performance in the context of sustainable polymers and/or biomaterials. We have paid attention to installing and controlling geometric isomerism by using these click reactions, an overarching objective of our work in this research area. The exquisite control of geometric isomerism that is possible within polymer backbones, as enabled by convenient click chemistry reactions, showcases a powerful approach to creating multipurpose degradable polymers.

Stimuli-responsive and core cross-linked micelles developed by NiCCo-PISA of helical poly(aryl isocyanide)s.

We report the synthesis of redox- and pH-sensitive block copolymer micelles that contain chiral cores composed of helical poly(aryl isocyanide)s. Pentafluorophenyl (PFP) ester-containing micelles synthesised via nickel-catalysed coordination polymerisation-induced self-assembly (NiCCo-PISA) of helical poly(aryl isocyanide) amphiphilic diblock copolymers are modified post-polymerisation with various diamines to introduce cross-links and/or achieve stimulus-sensitive nanostructures. The successful introduction of the diamines is confirmed by Fourier-transform infrared spectroscopy (FT-IR), while the stabilisation effect of the cross-linking is explored by dynamic light scattering (DLS). The retention of the helicity of the core-forming polymer block is verified by circular dichroism (CD) spectroscopy and the stimuli-responsiveness of the nanoparticles towards a reducing agent (l-glutathione, GSH) and pH is evaluated by following the change in the size of the nanoparticles by DLS. These stimuli-responsive nanoparticles could find use in applications such as drug delivery, nanosensors or biological imaging.

Intrinsically Re-curable Photopolymers Containing Dynamic Thiol-Michael Bonds.

The development of photopolymers that can be depolymerized and subsequently re-cured using the same light stimulus presents a significant technical challenge. A bio-sourced terpenoid structure, l-carvone, inspired the creation of a re-curable photopolymer in which the orthogonal reactivity of an irreversible thioether and a dynamic thiol-Michael bond enables both photopolymerization and thermally driven depolymerization of mechanically robust polymer networks. The di-alkene containing l-carvone was partially reacted with a multi-arm thiol to generate a non-crosslinked telechelic photopolymer. Upon further UV exposure, the photopolymer crosslinked into a mechanically robust network featuring reversible Michael bonds at junction points that could be activated to revert, or depolymerize, the network into a viscous telechelic photopolymer. The regenerated photopolymer displayed intrinsic re-curability over two recycles while maintaining the desirable thermomechanical properties of a conventional network: insolubility, resistance to stress relaxation, and structural integrity up to 170 °C. Our findings present an on-demand, re-curable photopolymer platform based on a sustainable feedstock.

Methanolysis of Poly(lactic Acid) Using Catalyst Mixtures and the Kinetics of Methyl Lactate Production.

Polylactic acid (PLA) is a leading bioplastic of which the market share is predicted to increase in the future; its growing production capacity means its end-of-life treatment is becoming increasingly important. One beneficial disposal route for PLA is its chemical recycling via alcoholysis. The alcoholysis of PLA leads to the generation of value-added products alkyl lactates; this route also has potential for a circular economy. In this work, PLA was chemically recycled via methanolysis to generate methyl lactate (MeLa). Four commercially available catalysts were investigated: zinc acetate dihydrate (Zn(OAc)2), magnesium acetate tetrahydrate (Mg(OAc)2), 4-(dimethylamino)pyridine (DMAP), and triazabicyclodecene (TBD). Dual catalyst experiments displayed an increase in reactivity when Zn(OAc)2 was paired with TBD or DMAP, or when Mg(OAc)2 was paired with TBD. Zn(OAc)2 coupled with TBD displayed the greatest reactivity. Out of the single catalyst reactions, Zn(OAc)2 exhibited the highest activity: a higher mol% was found to increase reaction rate but plateaued at 4 mol%, and a higher equivalent of methanol was found to increase the reaction rate, but plateaued at 17 equivalents. PLA methanolysis was modelled as a two-step reversible reaction; the activation energies were estimated at: Ea1 = 25.23 kJ∙mol-1, Ea2 = 34.16 kJ∙mol-1 and Ea-2 = 47.93 kJ∙mol-1.

Critical advances and future opportunities in upcycling commodity polymers.

The vast majority of commodity plastics do not degrade and therefore they permanently pollute the environment. At present, less than 20% of post-consumer plastic waste in developed countries is recycled, predominately for energy recovery or repurposing as lower-value materials by mechanical recycling. Chemical recycling offers an opportunity to revert plastics back to monomers for repolymerization to virgin materials without altering the properties of the material or the economic value of the polymer. For plastic waste that is either cost prohibitive or infeasible to mechanically or chemically recycle, the nascent field of chemical upcycling promises to use chemical or engineering approaches to place plastic waste at the beginning of a new value chain. Here state-of-the-art methods are highlighted for upcycling plastic waste into value-added performance materials, fine chemicals and specialty polymers. By identifying common conceptual approaches, we critically discuss how the advantages and challenges of each approach contribute to the goal of realizing a sustainable plastics economy.

Ultra-Tough Elastomers from Stereochemistry-Directed Hydrogen Bonding in Isosorbide-Based Polymers.

The remarkable elasticity and tensile strength found in natural elastomers are challenging to mimic. Synthetic elastomers typically feature covalently cross-linked networks (rubbers), but this hinders their reprocessability. Physical cross-linking via hydrogen bonding or ordered crystallite domains can afford reprocessable elastomers, but often at the cost of performance. Herein, we report the synthesis of ultra-tough, reprocessable elastomers based on linear alternating polymers. The incorporation of a rigid isohexide adjacent to urethane moieties affords elastomers with exceptional strain hardening, strain rate dependent behavior, and high optical clarity. Distinct differences were observed between isomannide and isosorbide-based elastomers where the latter displays superior tensile strength and strain recovery. These phenomena are attributed to the regiochemical irregularities in the polymers arising from their distinct stereochemistry and respective inter-chain hydrogen bonding.