4D Printed Protein-AuNR Nanocomposites with Photothermal Shape Recovery.

4D printing is the 3D printing of objects that change chemically or physically in response to an external stimulus over time. Photothermally responsive shape memory materials are attractive for their ability to undergo remote activation. While photothermal methods using gold nanorods (AuNRs) have been used for shape recovery, 3D patterning of these materials into objects with complex geometries using degradable materials has not been addressed. Here, we report on the fabrication of 3D printed shape memory bioplastics with photo-activated shape recovery. Protein-based nanocomposites based on bovine serum albumin (BSA), poly (ethylene glycol) diacrylate and gold nanorods were developed for vat photopolymerization. These 3D printed bioplastics were mechanically deformed under high loads, and the proteins served as mechanoactive elements that unfolded in an energy-dissipating mechanism that prevented fracture of the thermoset. The bioplastic object maintained its metastable shape-programmed state under ambient conditions. Subsequently, up to 99% shape recovery was achieved within 1 min of irradiation with near-infrared light. Mechanical characterization and small angle X-ray scattering (SAXS) analysis suggest that the proteins mechanically unfold during the shape programming step and may refold during shape recovery. These composites are promising materials for the fabrication of biodegradable shape-morphing devices for robotics and medicine.

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.

Boosting the Reactivity of Bis-Lactones to Enable Step-Growth Polymerization at Room Temperature.

The development of new sustainable polymeric materials endowed with improved performances but minimal environmental impact is a major concern, with polyesters as primary targets. Lactones are key monomers thanks to ring-opening polymerization, but their use in step-growth polymerization has remained scarce and challenging. Herein, we report a powerful bis(γ-lactone) (γSL) that was efficiently prepared on a gram scale from malonic acid by Pd-catalyzed cycloisomerization. The γ-exomethylene moieties and the spiro structure greatly enhance its reactivity toward ring-opening and enable step-growth polymerization under mild conditions. Using diols, dithiols, or diamines as comonomers, a variety of regioregular (AB)n copolymers with diverse linkages and functional groups (from oxo-ester to ß-thioether lactone and ß-hydroxy-lactame) have been readily prepared. Reaction modeling and monitoring revealed the occurrence of an original trans-lactonization process following the first ring-opening of γSL. This peculiar reactivity opens the way to regioregular (ABAC)n terpolymers, as illustrated by the successive step-growth polymerization of γSL with a diol and a diamine.

Overcoming the Limitations of Organocatalyzed Glycolysis of Poly(ethylene terephthalate) to Facilitate the Recycling of Complex Waste Under Mild Conditions.

Although multiple methods have been reported in the literature for the chemical recycling of poly(ethylene terephthalate) (PET), large-scale depolymerization is not yet widely employed. The main reasons for the limited adoption of chemical recycling of PET are the harsh conditions required and the lack of selectivity. In this study, the organocatalytic glycolysis of PET mediated by organic bases at low temperatures is studied, and routes to avoid the deactivation of the catalyst are explored. It is shown that the formation of terephthalic acid by uncontrolled hydrolysis leads to issues which can be resolved using potassium tert-butoxide as a cocatalyst. Finally, complex PET waste obtained from a mechanical recycling plant was depolymerized under optimized conditions, obtaining bis(2-hydroxyethyl) terephthalate yields >90% in less than 15 min at only 100 °C. These results open the way to efficient recycling of PET-enriched waste streams under milder conditions.

A Single Amino Acid Able to Promote High-Temperature Ring-Opening Polymerization by Dual Activation.

Amino acids are indispensable compounds in the body, performing several biological processes that enable proper functioning. In this work, it is demonstrated that a single amino acid, taurine, is also able to promote the ring-opening polymerization (ROP) of several cyclic monomers under industrially relevant conditions. It is shown that the unique zwitterionic structure of taurine, where the negatively charged sulfonic acid group and the protonated amine group are separated by two methylene groups, not only provides high thermal stability but also leads to a dual activation mechanism, which is corroborated by quantum mechanical calculations. This unique mechanism allows for the synthesis of polylactide of up to 50 kDa in bulk at 180 °C with good end-group fidelity using a highly abundant catalyst. Furthermore, cytotoxicity tests confirm that PLLA synthesized with taurine is non-toxic. Moreover, it is demonstrated that the presence of taurine does not have any detrimental effect on the thermal stability of polylactide, and therefore polymers can be used directly without any post-polymerization purification. It is believed that the demonstration that a simple structure composed of a single amino acid can promote polymerization can bring a paradigm shift in the preparation of polymers.

Oxime metathesis: tuneable and versatile chemistry for dynamic networks.

Oxime chemistry has emerged as a versatile tool for use in a wide range of applications. In particular, the combination of oximes with esters and urethanes has enabled the realisation of Covalent Adaptable Networks (CANs) with improved and tunable dynamic properties. Nevertheless, an exclusively oxime-based chemistry has not yet been explored in the fabrication of CANs. In this work, we investigate the mechanism of the acid-catalysed dynamic exchange of oximes. We propose a metathesis mechanism that is well supported by both experimental and computational studies, which highlight the importance of the substituent effect on the exchange reaction kinetics. Then, as a proof of concept, we incorporate oxime groups into a cross-linked polymeric material and demonstrate the ability of oxime-based polymers to be reprocessed under acid catalysis while maintaining their structural integrity.

Degradable Alternating Copolymers by Radical Copolymerization of 2-Methylen-1,3-dioxepane and Crotonate Esters.

Producing backbone degradable copolymers via free-radical copolymerization is a promising, yet challenging method to develop more sustainable materials for many applications. In this work, we present the copolymerization of 2-methylen-1,3-dioxepane (MDO) with crotonic acid derivative esters. MDO can copolymerize by radical ring-opening polymerization incorporating degradable ester moieties in the polymer backbone, although this can often be difficult due to the very unfavorable reactivity ratios. Crotonic acid derivatives, on the other hand, can be easily produced completely from biomass but are typically very difficult to (co)polymerize due to low propagation rates and very unfavorable reactivity ratios. Herein, we present the surprisingly easy copolymerization between MDO and butyl crotonate (BCr), which shows the ability to form alternating copolymers. The alternating nature of the copolymer was characterized by MALDI-TOF and supported by the reactivity ratios calculated experimentally (rMDO = 0.105 and rBCr = 0.017). The alternating nature of the copolymers favored the degradability that could be achieved under basic conditions (in 2 h, all chains have molar masses smaller than 2 kg/mol). Last, the work was expanded to other crotonate monomers to expand the portfolio and show the potential of this copolymer family.

Tuning Reprocessing Temperature of Aliphatic Polyurethane Networks by Alkoxyamine Selection.

Recent studies have shown that the largest employed thermoset family, polyurethanes (PUs), has great potential to be reprocessed due to the dynamic behavior of carbamate linkage. However, it requires high temperatures, especially in the case of aliphatic PUs, which causes side reactions besides the desired exchange reaction. To facilitate the reprocessing of aliphatic PUs, in this work, we have explored the dynamic potential of alkoxyamine bonds in PU networks to facilitate the reprocessing under mild conditions considering their fast recombination ability. Taking advantage of the structural effect of the nitroxide and alkyl radicals on the dissociation energy, two different alkoxyamine-based diols have been designed and synthesized to generate PU networks. Our study shows that replacing 50 mol % of a nondynamic diol chain extender with these dynamic blocks boosts the relaxation times of the networks, enabling reprocessing at temperatures as low as 80 °C.