Overcoming the low reactivity of biobased, secondary diols in polyester synthesis

Size-Exclusion Chromatography-Electrospray-Ionization Mass Spectrometry To Characterize End Group and Chemical Distribution of Poly(lactide-co-glycolide) Copolymers

The characterization of the microstructure of in vivo degradable polyesters is gaining increased interest thanks to their high-performance applications, such as drug delivery systems. The design of such material requires a high level of understanding of the critical material attributes of the polyesters, such as molecular-weight distribution (MWD), chemical-composition distribution (CCD), and end-groups (functionality-type distribution, FTD). Size-exclusion chromatography (SEC) hyphenated with mass spectrometry (MS) is an effective method for analyzing the microstructure of polymers. While the MWD can be determined by size-exclusion chromatography hyphenated with ultraviolet spectrometry and refractive index, the CCD and FTD can be determined by SEC-MS. However, previous applications of SEC-MS have not assessed if polymer fragmentation can occur during the analysis process. In order to correctly interpret CCD and FTD, it is important to establish whether SEC-MS methods can be applied to biodegradable polymers and to recognize if fragmentation processes occur. In this study, we investigate whether SEC-MS methods can be applied to PLGA biodegradable polyesters. The research demonstrates that the choice of alkali metal salt used during ionization can influence the stability of PLGA during SEC-MS analysis. CsI was found to minimize fragmentations during ESI-MS, simplifying the MS spectra and allowing isomeric PLGA structures to be distinguished. The resulting method facilitates FTD and CCD determination. Additionally, when combined with selective degradation, the described method can provide insights into the "blockiness" of the polymer and support the development of sequence-controlled PLGA synthesis.

Designing Biobased Poly(ethylene-co-isosorbide terephthalate) Copolyesters with Tunable Properties and Degradability

Production of high-performance polyesters with tailored degradability remains a challenge. Here, a series of poly(ethylene-co-isosorbide terephthalate) (PEIT) copolyesters were synthesized by varying the isosorbide (IS) content (0-20 mol %) using tetrabutyl titanate (TBT) as the catalyst. By variation of the IS content, the thermal, mechanical, and optical properties of the copolyesters were effectively tailored. As the IS content increased, the Tg was raised from 80 to 101 °C, and the tensile strength from 58.8 to 68.7 MPa. Moreover, excellent transparency was maintained (up to 90% light transmittance). Interestingly, the susceptibility to hydrolytic degradation was significantly enhanced by the incorporation of IS, with PEIT-20 showing approximately 3.5 times higher weight loss compared to PET after 50 days of alkaline degradation in 0.1 M NaOH solution. This outlines an attractive approach for developing high-performance copolyesters with tunable properties and degradation rates, suitable for applications in transparent thermal packaging materials.

Molecular Design for Dual Circularity: Polyester with Complementary Mechanical and Chemical Recyclability under Mild Conditions

The plastic waste accumulation requires facile yet effective solutions. Currently mechanical recycling typically leads to downcycling, while the environmental footprint of chemical recycling is often unacceptable. Here, we introduce a dual circularity concept, where rational molecular design paves the way for complementary closed-loop mechanical and chemical recyclability under mild conditions. Very small changes in macromolecular structures, thermal and mechanical properties were observed, after as many as six repeated mechanical recycling cycles, showing that a wide processing window could be the key for closing the mechanical recycling loop. Facile, time and energy efficient closed-loop chemical recycling into products with identical performance to original ones was also realized, thanks to the abundant free volume and accessible ester-functionalities. With these design criteria at hand, dual circulation in recyclability can be realized; proceeding with mechanical recycling when we can, and switching to chemical recycling when we must.

Toughening Poly(lactic acid) without Compromise - Statistical Copolymerization with a Bioderived Bicyclic Lactone

Poly(lactic acid) (PLA) offers a renewable and degradable alternative to petroleum-based plastic, but its mechanical properties are not ideal for many applications. Herein, we describe the synthesis and polymerization of 2-oxo-3,8-dioxabicyclo[3.2.1]octane (ODO), a bioderived bicyclic lactone, and show that copolymers of l-lactide (LA) with small amounts of ODO have improved mechanical properties over PLA. Homopolymerization of ODO to poly(oxo-3,8-dioxabicyclo[3.2.1]octane) (PODO) is optimized for both solution-phase, organocatalytic and melt-phase, metal-catalyzed conditions. In comparison to the monocyclic analog, ε-caprolactone (CL), ODO has a lower enthalpy of polymerization and faster rate of polymerization. PODO is an amorphous, elastomeric polyester that has a Tg 90 °C higher than poly(ε-caprolactone) (PCL). Statistical copolymerization of LA with small fractions of ODO yields tough and transparent thermoplastics that have over 12× elongation at break compared to native PLA, while maintaining Tg, Young's modulus (E), and yield strength. Together, these results describe how the incorporation of the tetrahydrofuran ring alters lactone polymerizability and the thermomechanical properties of the homopolymer and copolymer materials.

Catalyst free PET and PEF polyesters using a new traceless oxalate chain extender

Plastic material performance is strongly correlated to the polymer's molecular weight. Obtaining a sufficiently high molecular weight is therefore a key goal of polymerization processes. The most important polyester polyethylene terephthalate (PET) and the new polyethylene furanoate (PEF) require metal catalysts and time-consuming production processes to reach sufficiently high molecular weights. Metal catalysts, which are typically antimony or tin for polyesters, end up in the plastic products which may result in sustainability and ecological challenges. When the less reactive comonomer isosorbide is introduced to produce (partly) biobased materials with enhanced thermal properties, such as polyethylene-co-isosorbide furanoate (PEIF), reaching high enough molecular weight becomes even more challenging. This study presents an easily implementable approach to produce high molecular weight PET and PEF polyesters and their isosorbide copolyesters PEIT and PEIF by coupling lower molecular weight polymer chains by the reactive diguaiacyl oxalate (DGO) chain extender. DGO is so reactive, that the use of metal catalysts can be completely avoided and it helps avoiding an extra solid-state polymerization step. In addition, DGO distinguishes itself from typical chain extenders by its ability to be completely removed from the resulting polymer, thereby avoiding the inherent drawbacks associated with typical chain extenders.

Biorenewable and circular polyolefin thermoplastic elastomers

Polymers capable of depolymerizing back to their own monomers offer a promising solution to address the challenges in polymer sustainability. Despite significant progress has been achieved in plastics circularity, chemical recycling of thermoplastic elastomers is relatively less concerned, largely because of their intrinsic complex multicomponents. This work creates a homopolymer-based platform towards chemically recyclable but tough thermoplastic elastomers. It is enabled by a semicrystalline polymer with high molecular weight but low crystallinity, which is prepared through ring-opening metathesis polymerization of a fully biobased cyclic olefin. By shifting the ring-chain equilibrium, quantitative conversions were achieved for both forward polymerization and reverse depolymerization. This simple circular, high-performance thermoplastic elastomer platform based on biomass highlights the importance of monomer design in addressing three challenges in sustainable polymers: the feedstock renewability, depolymerization selectivity, and performance trade-offs.

The search for rigid, tough polyesters with high T g - renewable aromatic polyesters with high isosorbide content

Renewable polyesters with a good balance between impact strength and elastic modulus (stiffness) are not very common, especially when combined with high glass transition temperature (T g). Achieving such high performance properties would enable the substitution of high performance polymers like ABS and polycarbonate with chemically recyclable polyesters from bio-based or recycled sources. One of the challenges in developing these materials is to select the right composition of the right monomers/comonomer ratios and making these materials with high molecular weight, which can be challenging since some of the most promising rigid diols, such as isosorbide, are unreactive. This study comprises aromatic polyesters from (potentially) renewable monomers, using bio-based isosorbide as a means to increase their T g and to inhibit their crystallization, while using flexible co-diols to improve impact strength. To incorporate a high amount of isosorbide into the targeted polyesters, we used the synthesis method with reactive phenolic solvents previously developed in our group. The selected compositions display high T g's (>90 °C) and high tensile modulus (>1850 MPa). We show that more polar monomers such as the stiffer 2,5-furandicarboxylic acid (FDCA) and diethylene glycol cause high stiffness but decreased impact strength (<5 kJ m-2). Combining terephthalic acid and isosorbide with more flexible diols like 1,4-butanediol, 1,4-cyclohexanedimethanol (CHDM) and 1,3-propanediol provides a better balance, including the combination of high tensile modulus (>1850 MPa) and high impact strength (>10 kJ m-2).

Terephthalate Copolyesters Based on 2,3-Butanediol and Ethylene Glycol and Their Properties

This study explores the synthesis and performance of novel copolyesters containing 2,3-butanediol (2,3-BDO) as a biobased secondary diol. This presents an opportunity for improving their thermal properties and reducing crystallinity, while also being more sustainable. It is, however, a challenge to synthesize copolyesters of sufficient molecular weight that also have high 2,3-BDO content, due to the reduced reactivity of secondary diols compared to primary diols. Terephthalate-based polyesters were synthesized in combination with different ratios of 2,3-BDO and ethylene glycol (EG). With a 2,3-BDO to EG ratio of 28:72, an Mn of 31.5 kDa was reached with a Tg of 88 °C. The Mn dropped with increasing 2,3-BDO content to 18.1 kDa for a 2,3-BDO to EG ratio of 78:22 (Tg = 104 °C) and further to 9.8 kDa (Tg = 104 °C) for the homopolyester of 2,3-BDO and terephthalate. The water and oxygen permeability both increased significantly with increasing 2,3-BDO content and even the lowest content of 2,3-BDO (28% of total diol) performed significantly worse than PET. The incorporation of 2,3-BDO had little effect on the tensile properties of the polyesters, which were similar to PET. The results suggest that 2,3-BDO can be potentially applied for polyesters requiring higher Tg and lower crystallinity than existing materials (mainly PET).

Relationship between Composition and Environmental Degradation of Poly(isosorbide-co-diol oxalate) (PISOX) Copolyesters

To reduce the global CO2 footprint of plastics, bio- and CO2-based feedstock are considered the most important design features for plastics. Oxalic acid from CO2 and isosorbide from biomass are interesting rigid building blocks for high Tg polyesters. The biodegradability of a family of novel fully renewable (bio- and CO2-based) poly(isosorbide-co-diol) oxalate (PISOX-diol) copolyesters was studied. We systematically investigated the effects of the composition on biodegradation at ambient temperature in soil for PISOX (co)polyesters. Results show that the lag phase of PISOX (co)polyester biodegradation varies from 0 to 7 weeks. All (co)polyesters undergo over 80% mineralization within 180 days (faster than the cellulose reference) except one composition with the cyclic codiol 1,4-cyclohexanedimethanol (CHDM). Their relatively fast degradability is independent of the type of noncyclic codiol and results from facile nonenzymatic hydrolysis of oxalate ester bonds (especially oxalate isosorbide bonds), which mostly hydrolyzed completely within 180 days. On the other hand, partially replacing oxalate with terephthalate units enhances the polymer's resistance to hydrolysis and its biodegradability in soil. Our study demonstrates the potential for tuning PISOX copolyester structures to design biodegradable plastics with improved thermal, mechanical, and barrier properties.

High-Molecular-Weight and Light-Colored Disulfide-Bond-Embedded Polyesters: Accelerated Hydrolysis Triggered by Redox Responsiveness

Disulfide bonds have attracted considerable attention due to their reduction responsiveness, but it is crucial and challenging to prepare disulfide-bond-based polyesters by melt polycondensation. Herein, the inherently poor thermal stability of the S-S bond in melting polycondensation was overcome. Moreover, poly(butylene succinate-co-dithiodipropionate) (PBSDi) with a light color and high molecular weights (Mn values up to 84.7 kg/mol) was obtained. These polyesters can be applied via melt processing with Td,5% > 318 °C. PBSDi10-PBSDi40 shows good crystallizability (crystallinity 56-38%) and compact lamellar thickness (2.9-3.2 nm). Compared with commercial poly(butylene adipate-co-terephthalate) (PBAT), the elevated mechanical and barrier performances of PBSDi make them better packaging materials. For the degradation behavior, the disulfide monomer obviously accelerates the enzyme degradation but has a weaker effect on hydrolysis. In 0.1 mol/L or higher concentrations of H2O2 solutions, the oxidation of disulfide bonds to sulfoxide and sulfone groups can be realized. This process results in a stronger nucleophilic attack, as confirmed by the Fukui function and DFT calculations. Additionally, the greater polarity and hydrophilicity of oxidation products, proved by noncovalent interaction analysis, accelerate the hydrolysis of polyesters. Moreover, glutathione-responsive breakage, from polymers to oligomers, is confirmed by an accelerated decline in molecular weight. Our research offers fresh perspectives on the effective synthesis of the disulfide polyester and lays a solid basis for the creation of high-performance biodegradable polyesters that degrade on demand.

Alpha-ketoglutaric acid based polymeric particles for cutaneous wound healing

Metabolites are not only involved in energy pathways but can also act as signaling molecules. Herein, we demonstrate that polyesters of alpha-ketoglutararte (paKG) can be generated by reacting aKG with aliphatic diols of different lengths, which release aKG in a sustained manner. paKG polymer-based microparticles generated via emulsion-evaporation technique lead to faster keratinocyte wound closures in a scratch assay test. Moreover, paKG microparticles also led to faster wound healing responses in an excisional wound model in live mice. Overall, this study shows that paKG MPs that release aKG in a sustained manner can be used to develop regenerative therapeutic responses.

Reactive phenolic solvents applied to the synthesis of renewable aromatic polyesters with high isosorbide content

High boiling point phenolic reactive solvents like p-cresol could play a key role in improving the synthesis of aromatic polyesters with a high content of secondary diols such as isosorbide. Previously, our group showed that this method significantly improves the synthesis of poly(isosorbide succinate). In this work, terephthalic acid and 2,5-furandicarboxylic acid were used as building blocks for the synthesis of high T_g polyesters with high isosorbide content (>30 mol% of diols) and high molecular weight (M_n > 24 kg mol^-1). A number of reactive and non-reactive solvents were tested in this work, and the results clearly point to a significant improvement when using reactive solvents, in terms of molecular weight and polycondensation time, especially for the case of p-cresol. The synthesis method was successfully scaled to 1 kg, showing promise for production at industrial scale. A method to remove these solvents (including end groups) from the polymers, which uses small excesses of isosorbide (1.5-3.0%) in the feed, is also presented.

Helical Crystals in Aliphatic Copolyesters: From Chiral Amplification to Mechanical Property Enhancement

We demonstrate herein a bottom-up strategy for achieving helical crystals via chiral amplification in copolyesters by incorporating a small amount of (d)-isosorbide into semicrystalline polyester, poly(ethylene brassylate) (PEB). During bulk crystallization of poly(ethylene-co-isosorbide brassylate)s, the molecular chirality of isosorbide in the amorphous region is transferred to PEB crystal chirality and amplified by the formation of right-handed helical crystals. Increasing isosorbide content or reducing crystallization temperature leads to thinner PEB lamellae crystals, strengthening chiral amplification by forming superhelices with a smaller helical pitch. Moreover, the superhelices with smaller helical pitch (larger chiral amplification) endow aliphatic copolyesters with enhanced modulus, strength, and toughness without sacrificing elongation-at-break. The principle outlined here could apply to the design of strong and tough materials.