Bio-Based Vitrimer: Sustainable Dynamic Covalent Networks From Renewable Resources For Advanced Material Applications

Fundamental Chemistry And Structural Characteristics Of Bio-Based Vitrimer Networks

Bio-based vitrimers are constructed through the incorporation of covalent adaptable networks (CANs) into polymer matrices derived from renewable biological resources. The defining feature of these materials is the presence of dynamic covalent bonds—such as transesterification-active ester linkages, disulfide bonds, or boronic ester moieties—that undergo associative exchange reactions at elevated temperatures without depolymerization123. Unlike conventional thermosets, which exhibit irreversible crosslinking, vitrimers display a viscosity decrease following Arrhenius-type behavior above a characteristic topology freezing transition temperature (Tv), typically ranging from 60°C to 180°C depending on the catalyst system and network architecture479.

The molecular design of bio-based vitrimers leverages abundant natural polymers and their derivatives. Starch-based vitrimers, for instance, utilize native starch hydroxyl groups that react with acid anhydrides and epoxy crosslinkers under catalytic conditions to form three-dimensional ester-crosslinked networks with tensile strengths reaching 9 MPa and elongation at break around 0.1%1. Epoxidized vegetable oils—particularly epoxidized soybean oil (ESO) and epoxidized castor oil (ECO)—serve as versatile bio-based epoxy monomers due to their high epoxy functionality (iodine value ≥100) and inherent flexibility from long aliphatic chains356. When cured with bio-derived diacids such as malic acid (Mw ~134 g/mol) or rosin-derived carboxylic acids containing rigid hydrogenated phenanthrene rings, these systems yield vitrimers with glass transition temperatures (Tg) elevated above ambient (typically 40–80°C), enabling triple-shape memory behavior and mechanical robustness suitable for structural applications56.

Dynamic disulfide bonds represent another critical functional group in bio-based vitrimer design. Disulfide linkages undergo thiol-disulfide exchange reactions under thermal or photochemical stimulation, facilitating network rearrangement without external catalysts238. A benzoxazine-disulfide hardener synthesized via condensation of enolic molecules, formaldehyde, and disulfide-containing amines has been employed to crosslink epoxidized castor oil, producing vitrimer foams with self-healing efficiency exceeding 85% after heating at 120°C for 2 hours and demonstrating complete reprocessability through hot-pressing at 160°C38. The activation energy for disulfide exchange in these systems typically ranges from 80 to 120 kJ/mol, enabling controlled stress relaxation and shape reconfiguration within industrially relevant temperature windows219.

Catalyst selection profoundly influences vitrimer performance. Traditional transesterification catalysts such as zinc acetate or tin(II) 2-ethylhexanoate accelerate ester exchange but may leach into end-use environments, raising toxicity concerns418. Biocatalytic alternatives, particularly lipase enzymes with esterase activity, have emerged as green catalysts that remain active even after curing at temperatures up to 100°C, enabling vitrimer topology rearrangement at Tv values below 100°C—significantly lower than metal-catalyzed systems7. Enzyme-catalyzed vitrimers offer non-hazardous recyclability, as the absence of metallic residues simplifies end-of-life monomer recovery and composting pathways7.

Synthesis Routes And Processing Methodologies For Bio-Based Vitrimer Production

Solvent-Free Mechanical Processing

A hallmark of sustainable vitrimer synthesis is the elimination of organic solvents through mechanical processing techniques. Starch-based vitrimers are prepared via reactive extrusion in internal mixers, where starch, acid anhydride (e.g., maleic anhydride at 5–15 wt%), and epoxy crosslinkers (e.g., bisphenol A diglycidyl ether at 10–20 wt%) are compounded at 110–120°C for 2 hours, followed by hot-pressing at 140–160°C for 5–8 hours to complete crosslinking14. This solvent-free approach minimizes volatile organic compound (VOC) emissions and simplifies scale-up, with pilot-scale batch sizes exceeding 10 kg demonstrating reproducible mechanical properties (tensile modulus 1.2–1.8 GPa, storage modulus at 25°C ~2.5 GPa)14.

Reactive extrusion also enables the synthesis of semi-crystalline polyolefin-based vitrimers. Maleic anhydride-grafted polypropylene (PP-g-MA, grafting degree 0.5–2.0 wt%) is co-extruded with multifunctional silyl ether crosslinkers or boronic ester compounds at 180–220°C, producing vitrimers with crystallinity retained at 30–45% and elastic modulus increased by 180–250% relative to neat polyolefin at 150°C91013. The presence of crystalline domains imparts dimensional stability below the melting temperature (Tm ~160°C for PP-based vitrimers), while dynamic covalent bonds enable reprocessing above Tv (~140°C), achieving a balance between thermoplastic processability and thermoset-like performance913.

Bulk Polymerization And Foam Synthesis

Bio-based vitrimer foams are synthesized via bulk polymerization of epoxidized oils with multifunctional hardeners, followed by foaming with chemical blowing agents. A representative formulation comprises 10 parts by weight epoxidized castor oil, 1 part benzoxazine-disulfide hardener, and 0.5–1.0 parts azodicarbonamide as a blowing agent, cured at 120°C for 4 hours to yield foams with density 0.15–0.30 g/cm³ and compressive strength 0.8–1.5 MPa38. The dynamic disulfide crosslinks enable foam reprocessing: ground foam particles can be compression-molded at 160°C and 5 MPa for 30 minutes to regenerate monolithic structures with 90–95% retention of original compressive properties38.

Catalyst-free vitrimer synthesis is achievable through vinylogous urethane or silyl ether chemistries. Epoxidized soybean oil reacts with rosin-derived diacids (e.g., maleopimaric acid) in ethanol at 110°C for 2 hours, followed by solvent evaporation and post-curing at 150°C for 6 hours, yielding transparent vitrimers with Tg = 55–70°C and tensile strength 15–25 MPa5. The absence of metal catalysts facilitates enzymatic or chemical depolymerization at end-of-life, with monomer recovery yields exceeding 80% under mild acidic hydrolysis (pH 3–4, 90°C, 12 hours)56.

Ring-Opening Metathesis Polymerization (ROMP) For Cyclic Monomer-Based Vitrimers

Cyclopentene-based vitrimers are synthesized via one-step ring-opening metathesis polymerization (ROMP) using Grubbs-type catalysts, incorporating boronic ester-functionalized comonomers (0.5–15 mol%) to introduce dynamic crosslinks15. The resulting vitrimers exhibit tensile stress at 1000% strain that is 30–40 times higher than neat ring-opened polycyclopentene, with elastic modulus retention above 80% after three reprocessing cycles at 180°C15. Notably, these vitrimers can be depolymerized back to cyclopentene monomer via ring-closing metathesis in the presence of second-generation Grubbs catalyst at 60°C, achieving monomer recovery yields >90% and enabling ideal chemical recycling15.

Mechanical Properties And Thermomechanical Performance Of Bio-Based Vitrimers

Bio-based vitrimers exhibit mechanical properties spanning a wide range, tunable through feedstock selection, crosslink density, and crystallinity. Starch-based vitrimers demonstrate tensile strengths of 7–10 MPa with elongation at break of 0.08–0.15%, suitable for rigid packaging and construction panel applications14. In contrast, epoxidized soybean oil vitrimers crosslinked with flexible diacids (e.g., sebacic acid) achieve elongation at break exceeding 200% while maintaining tensile strength of 5–8 MPa, positioning them for elastomeric sealing and gasket applications56.

The introduction of rigid bio-derived segments significantly enhances thermomechanical stability. Rosin-derived hardeners containing hydrogenated phenanthrene rings elevate the Tg of epoxidized soybean oil vitrimers from below 0°C (for purely flexible networks) to 60–75°C, enabling shape memory functionality with shape fixity ratios >95% and shape recovery ratios >90% across three consecutive thermomechanical cycles (deformation at Tg + 20°C, cooling to 25°C, reheating to Tg + 30°C)5. Dynamic mechanical analysis (DMA) reveals storage modulus values of 1.5–2.8 GPa at 25°C for rosin-ESO vitrimers, decreasing to 10–50 MPa in the rubbery plateau region (80–120°C), with tan δ peaks at Tg indicating well-defined glass transitions56.

Stress relaxation experiments quantify the dynamic nature of vitrimer networks. At temperatures above Tv, bio-based vitrimers exhibit exponential stress decay with characteristic relaxation times (τ*) ranging from 10² to 10⁴ seconds, depending on crosslink density and catalyst concentration279. For enzyme-catalyzed epoxy vitrimers, τ* at 80°C is approximately 1200 seconds, decreasing to 180 seconds at 100°C, following Arrhenius behavior with activation energy Ea = 95 ± 8 kJ/mol7. This controlled stress relaxation enables welding of vitrimer parts: overlapping sheets pressed at 120°C for 15 minutes achieve lap-shear strengths of 4–6 MPa, representing 70–85% of the bulk material strength27.

Creep resistance and long-term dimensional stability are critical for structural applications. Semi-crystalline polyolefin vitrimers with 35–45% crystallinity exhibit creep compliance below 1 × 10⁻⁹ Pa⁻¹ at 80°C under 1 MPa load over 1000 hours, comparable to conventional crosslinked polyethylene and superior to non-crosslinked thermoplastics913. The crystalline domains act as physical crosslinks that restrict chain mobility below Tm, while dynamic covalent crosslinks provide additional network integrity, resulting in a synergistic enhancement of creep resistance913.

Self-Healing Mechanisms And Quantitative Healing Efficiency In Bio-Based Vitrimers

Self-healing in bio-based vitrimers is mediated by the reversibility of dynamic covalent bonds, enabling autonomous or thermally stimulated repair of mechanical damage. Disulfide-crosslinked vitrimers demonstrate particularly robust healing: scratches (width 50–100 μm, depth 20–30 μm) introduced on vitrimer surfaces disappear completely after heating at 100°C for 2 hours, as confirmed by optical microscopy and atomic force microscopy (AFM)2319. Quantitative healing efficiency, defined as the ratio of healed tensile strength to virgin material strength, reaches 85–95% for disulfide-based vitrimers after one healing cycle at 120°C for 2 hours, and remains above 70% after three consecutive damage-healing cycles2319.

The healing kinetics are governed by the activation energy of bond exchange and the mobility of polymer chains. For transesterification-based vitrimers catalyzed by zinc acetate (1–3 mol% relative to ester groups), healing at 140°C for 1 hour restores 80–90% of original tensile strength, whereas healing at 100°C requires 6–8 hours to achieve comparable recovery145. Enzyme-catalyzed vitrimers exhibit lower healing temperatures: lipase-containing epoxy vitrimers heal at 80°C for 4 hours with 75–85% strength recovery, advantageous for temperature-sensitive substrates7.

Stress-responsive healing has been demonstrated in spiropyran-functionalized bio-based vitrimers. Spiropyran moieties covalently bonded to the epoxy-organic acid vitrimer network undergo reversible ring-opening to merocyanine under mechanical stress, producing a visible color change (colorless to purple) that indicates damage location2. Upon heating to 100°C, both the spiropyran ring-closure and the transesterification-mediated network rearrangement occur, restoring mechanical integrity and reverting the color, thereby providing a visual confirmation of healing2. This mechano-chromic functionality is particularly valuable for structural health monitoring in composite materials and coatings.

Reprocessability, Recyclability, And Circular Economy Integration Of Bio-Based Vitrimers

A defining advantage of vitrimers over conventional thermosets is their reprocessability. Bio-based vitrimers can be ground into powders or granules and re-molded via compression molding or injection molding at temperatures above Tv. Starch-based vitrimers ground to particle size <500 μm are compression-molded at 160°C and 10 MPa for 20 minutes, yielding reprocessed plaques with tensile strength retention of 90–95% after the first cycle and 80–85% after three cycles14. The slight decrease in properties is attributed to partial chain scission and oxidative degradation during thermal reprocessing, mitigable through the addition of antioxidants (e.g., 0.5 wt% hindered phenol stabilizers)4.

Epoxidized oil-based vitrimers demonstrate similar reprocessability. Rosin-ESO vitrimers are reprocessed via hot-pressing at 150°C for 10 minutes, with tensile strength decreasing from 22 MPa (virgin) to 20 MPa (first reprocessing) and 18 MPa (third reprocessing), while elongation at break remains stable at 180–200%5. Dynamic mechanical analysis confirms that Tg shifts by less than 5°C across reprocessing cycles, indicating minimal network degradation56.

Chemical recycling via depolymerization offers an alternative end-of-life pathway. Transesterification-based vitrimers can be depolymerized in the presence of excess diol or alcohol at elevated temperatures (140–180°C) with catalysts, regenerating monomeric or oligomeric species46. For example, starch-based vitrimers treated with ethylene glycol at 160°C for 4 hours yield a mixture of starch derivatives and diester oligomers, separable by solvent extraction and reusable for vitrimer synthesis with monomer recovery yields of 70–80%4. Cyclopentene-based vitrimers achieve near-quantitative monomer recovery (>90% yield) via ring-closing metathesis at 60°C in the presence of Grubbs catalyst, representing an ideal closed-loop recycling system15.

Enzymatic degradation and composting are viable for fully bio-based vitrimers devoid of synthetic additives. Lipase-catalyzed vitrimers, when buried in compost at 55°C and 60% humidity, lose 50% of their mass within 90 days and degrade completely within 180 days, as measured by gravimetric analysis and gel permeation chromatography (GPC)7. The degradation products—primarily fatty acids, glycerol, and oligomeric esters—are metabolized by soil microorganisms, confirming the biodegradability of these materials under aerobic composting conditions7.

Applications Of Bio-Based Vitrimers Across Industrial Sectors

Adhesives And Coatings With Self-Healing And R