Chemically recyclable polyvinyl chloride-like plastics

The study addresses the urgent challenge of plastic waste and low recycling rates, particularly for PVC, which is widely produced and valued for properties such as durability, low cost, processability, and inherent flame retardancy due to high chlorine content. Conventional recycling of PVC is problematic because depolymerization releases chlorine (deactivating catalysts) and HCl, making melt-processing or pyrolysis unsuitable and resulting in the lowest recycling rate among commodity polyolefins. Chemically recyclable polymers that depolymerize efficiently to monomers (CRM) are a promising strategy for circularity, but suitable PVC-like materials that combine recyclability with practical performance have been lacking. The research question is whether PVC-like plastics can be created from readily available monomers via efficient, reversible polymerization to deliver mechanical and flame-retardant properties comparable to PVC while enabling high-yield chemical recycling back to monomers. The authors propose reversible copolymerization of chloral (trichloroacetaldehyde) with cyclic anhydrides to access halogen-rich polyesters that can be depolymerized to monomers under thermal conditions, thus contributing to a circular plastic economy.

The paper situates the work within the broader push for chemically recyclable polymers (CRM) and circular plastics. Ring-opening polymerization (ROP) of lactones has enabled recyclable polyesters, but suitable lactones are limited for large-scale use and many reported recyclable polyesters have modest mechanical performance (few exceed tensile strength >40 MPa). The authors' prior work demonstrated alternating copolymerization of cyclic anhydrides with aldehydes to form polyesters, but those materials had low molecular weights (<20 kDa), inadequate mechanical properties, and only proceeded via uncontrolled cationic mechanisms due to instability of aldehyde oxonium species. The literature also underscores PVC's challenges in chemical upcycling (chlorine release, catalyst deactivation) and poor recyclability relative to other polyolefins. This context motivates developing new CRM-capable, PVC-like plastics from abundant monomers with improved synthesis control, performance, and practical recycling processes.

• Monomers and catalysts: Chloral (commercial, prepared industrially by Cl2 reaction with ethanol or acetaldehyde) and various cyclic anhydrides (e.g., glutaric anhydride, GA; 3-methylglutaric anhydride, 3-MGA; diglycolic anhydride, DGA; 2,2-dimethylglutaric anhydride, 2,2-DMGA; cyclopentanediacetic anhydride, CPDA; cyclohexanediacetic anhydride, CHDA; diphenic anhydride, DPA) were used. Bromal (tribromoacetaldehyde) was also tested. Catalysts for anionic copolymerization included Et3N (triethylamine), DBU, and MTBD. Cationic copolymerization used InBr3 and Brønsted acids (NH(OTf)2, TfOH). Solvent: CH2Cl2.
• Copolymerization conditions: Typical feed ratios [chloral or bromal]:[anhydride]:[catalyst] = 100:100:1. Anionic example: Et3N-catalyzed chloral/GA at 25 °C for 15 min gave >99% conversion. Cationic example: InBr3-catalyzed chloral/GA at 25 °C for 47 h gave high conversion. Polymerization conducted under Ar in a glovebox; workup by dissolution/precipitation protocols tailored to anionic vs cationic products.
• Mechanistic scope: Both anionic and cationic mechanisms accessible for chloral/anhydride; bromal copolymerization realized via cationic mechanism but not anionic (likely due to weaker inductive effect of Br). Alternation quantified via 1H/13C/HSQC NMR using defined integral ratios; MALDI-TOF MS used to assess end groups and sequence; DOSY NMR verified single polymer component.
• Kinetics and control: First-order decay in chloral concentration observed. Anionic (Et3N): k_obs ≈ 0.9 min^-1 at 25 °C; Mn increased linearly with conversion; Đ ~1.2–1.4. Cationic (InBr3): k_obs ≈ 0.041 h^-1 at 25 °C; Mn plausibly linear with conversion; Đ ~1.4–1.5. Water and 1,4-benzenedimethanol (BDM) used to regulate initiation and Mn: extra water decreased Mn in both mechanisms; improved AD in anionic; increased activity in cationic by generating more initiating species. Alternative bases (DBU, MTBD) accelerated anionic polymerization but increased transesterification and AD. Brønsted acids afforded high-AD, high-Mn products in cationic mode.
• Temperature effects: Increasing temperature (25→40–100 °C) reduced Mn (more transesterification, higher initiation efficiency) but increased AD (up to 100% under cationic conditions at 100 °C). Cationic activity increased with temperature.
• Materials characterization: NMR (1H, 13C, HSQC, DOSY), GPC (THF, PS calibration), MALDI-TOF MS (DHB matrix), DSC for Tg, TGA for Td (5% mass loss), mechanical tensile testing (stress–strain). Long-term stability assessed by NMR and GPC after 8 months at room temperature.
• Thermodynamics: Determined equilibrium monomer concentrations at 80–140 °C for chloral/GA (cationic catalysis). Van't Hoff analysis gave ΔH and ΔS°, permitting estimation of ceiling temperatures (Tc) at given initial concentrations, indicating chemical reversibility.
• Chemical recycling: Depolymerization of GA–chloral polyester using 2 wt% Sn(Oct)2 at 180 °C with distillation to recover chloral, followed by sublimation at 70 °C to recover GA. Re-polymerization of regenerated monomers to confirm closed-loop CRM. Tested monomer recovery from polyester admixed with common plastics (LDPE, PP, HDPE, PVC, PS, PET, PA, SBS) via sequential distillations and sublimation.
• Representative procedures: Detailed glovebox preparation, reagent charges (e.g., CH2Cl2 0.4 mL; GA 4.1 mmol; chloral 4.1 mmol; Et3N or InBr3 0.041 mmol), reaction times and temperatures, and purification steps.

• Dual-mechanism copolymerization and performance:
  • Anionic (Et3N) chloral/GA at 25 °C, 15 min: >99% conversion; Mn 52.6 kDa; Đ 1.3; AD 66% (Table 1, entry 1).
  • Cationic (InBr3) chloral/GA at 25 °C, 47 h: 96% conversion; Mn 79.1 kDa; Đ 1.5; AD 98% (Table 1, entry 2).
  • At 100 °C (cationic), complete alternating sequence (AD 100%) achievable (e.g., chloral/3-MGA; entries 3, 5–6).
• Kinetics and control:
  • Et3N-catalyzed anionic: first-order in chloral; k_obs ≈ 0.9 min^-1 at 25 °C; Mn grows linearly with conversion; Đ 1.2–1.4. AD increases with conversion (50%→66%).
  • InBr3-catalyzed cationic: first-order in chloral; k_obs ≈ 0.041 h^-1 at 25 °C; Đ 1.4–1.5. MALDI-TOF shows dicarboxylate end groups and complete alternation at low Mn.
• Tuning by water/initiators and temperature:
  • Anionic: Increasing water (2→15%) decreases Mn (14.3→2.5 kDa) and increases AD (66%→81%); slows homo-propagation vs alternating growth.
  • Cationic: Increasing water (2→15%) decreases Mn (31.8→5.6 kDa) and increases activity (e.g., 88% conversion in 36 h at 8% water); Mn higher than theoretical indicates low initiation efficiency.
  • Higher temperature reduces Mn but increases AD in both mechanisms; cationic AD reaches 100% at 100 °C.
• Monomer and catalyst scope:
  • Broad range of anhydrides (GA, 3-MGA, DGA, 2,2-DMGA, CPDA, CHDA, DPA) with chloral: AD 50–100%; Mn up to 79.1 kDa; Đ 1.1–1.5; Tg 28–148 °C (Table 1).
  • Bromal copolymerization succeeds via cationic catalysis (AD 95–100%; Mn 29.0–40.2 kDa; Đ 1.5) but not via anionic.
  • Alternative bases (DBU, MTBD) give rapid anionic polymerization with higher AD (71–73%) but lower Mn (25.9–32.1 kDa) vs Et3N; Brønsted acids (NH(OTf)2, TfOH) enable high-AD cationic products (95–97%) with high Mn (59.5–72.8 kDa).
• Thermal properties:
  • Tg decreases with higher AD: e.g., GA–chloral from 45 °C (AD 66%) to 28 °C (AD 100%). Overall Tg tunable 28–148 °C depending on monomer/AD/halogen.
  • Td (5% mass loss) ranges 173–264 °C.
  • Chlorine-substituted polyesters have lower Tg than bromine analogs (e.g., 3-MGA–chloral Tg 32 °C vs 3-MGA–bromal Tg 54 °C).
  • Polymers stable at room temperature for 8 months (no Mn decrease, no small-molecule release).
• Mechanical properties (tensile):
  • DGA–chloral: σb = 66.1 ± 3.3 MPa; εb = 4.6 ± 0.3% (entry 7).
  • GA–chloral: σb = 46.1 ± 2.3 MPa; εb = 4.3 ± 0.8% (entry 1).
  • 3-MGA–chloral: σb = 48.4 ± 2.1 MPa; εb = 7.5 ± 0.3% (entry 4).
  • GA–bromal: σb = 40.9 ± 4.2 MPa; εb = 8.0 ± 0.5% (entry 18).
  • Comparable to commercial polystyrene (σb 41.0 ± 1.6 MPa; εb 3.8 ± 0.4%) and PVC (σb 35.2 ± 6.4 MPa; εb 3.3 ± 0.7%).
• Flame retardancy and processing:
  • Halogen-rich polyesters are transparent after hot pressing and exhibit self-extinguishing behavior similar to PVC; GA–chloral extinguishes within ~0.2 s after ignition is removed.
• Thermodynamics of reversibility:
  • Equilibrium monomer concentrations: 0.42 M (80 °C), 0.73 M (100 °C), 1.33 M (120 °C), 2.34 M (140 °C). Van’t Hoff: ΔH = −34.8 kJ mol^-1; ΔS° = −91.1 J mol^-1 K^-1. Ceiling temperatures: Tc ≈ 158 °C at 3.5 M; Tc ≈ 91 °C at 1 M.
• Chemical recycling to monomer (CRM):
  • GA–chloral polyester depolymerized with 2 wt% Sn(Oct)2 at 180 °C: chloral recovered by distillation in 94% yield (2.5 g from 4.0 g polymer) in 4 h; GA recovered by sublimation at 70 °C for 10 h in 90% yield (1.2 g). Regenerated monomers re-polymerize to similar Mn and structure.
  • Monomer recovery from mixtures with common plastics (LDPE, PP, HDPE, PVC, PS, PET, PA, SBS) successful: chloral 90% yield (1.2 g) after sequential distillation; GA 75% yield (0.5 g) by sublimation.

The work demonstrates that halogen-rich polyesters formed by reversible copolymerization of chloral (or bromal) with cyclic anhydrides can deliver the key attributes of PVC-like plastics: comparable tensile strength and elongation to PVC and polystyrene, tunable glass transition temperatures across a wide range (28–148 °C), and strong flame retardancy due to high halogen content. Crucially, unlike PVC, these materials undergo efficient chemical recycling to their constituent monomers under thermal conditions using a simple catalyst (Sn(Oct)2), enabling closed-loop circularity. Mechanistic flexibility is central: chloral’s strong inductive effect allows both anionic and cationic pathways, with the anionic route favoring higher activity but lower alternating degree and the cationic route affording near-perfect alternation (and hence lower Tg) albeit with slower kinetics. Process variables (water, initiators, temperature, catalyst basicity/acidity) allow control over molecular weight, dispersity, and alternating degree, which in turn tune thermal and mechanical performance. Thermodynamic measurements establish a finite ceiling temperature and confirm true chemical reversibility, rationalizing high-yield depolymerization at elevated temperature. The ability to recover monomers even from complex plastic mixtures underscores practical feasibility. Collectively, these results directly address the challenge of developing PVC-like plastics compatible with chemical recycling, offering a pathway toward circular plastic economies while maintaining application-relevant properties.

The study introduces a versatile platform for synthesizing halogen-rich, PVC-like polyesters via reversible copolymerization of chloral/bromal with a range of cyclic anhydrides using simple base or acid catalysts under mild conditions. The materials achieve high molecular weights (up to 79.1 kDa), tunable alternating degree (50–100%), broad Tg window (28–148 °C), mechanical properties comparable to PVC and polystyrene, and excellent flame retardancy. Kinetic and thermodynamic analyses confirm true reversibility, enabling closed-loop chemical recycling to monomers in high yield and purity via Sn(Oct)2-catalyzed depolymerization, distillation, and sublimation. While replacing PVC broadly will require overcoming challenges in cost, practicality, and versatility, this work provides a promising blueprint for sustainable, recyclable PVC-like plastics and informs future design of high-performance CRM polymers. Future research should focus on catalyst and process optimization for industrial scalability, moisture-robust initiation control, property tailoring for specific applications, life-cycle assessments, and exploration of bio-based monomer sources.

• Industrial readiness: The authors note that replacing PVC at scale remains difficult at this stage due to considerations of price, practicality, and versatility; more systematic research is needed.
• Moisture sensitivity: In large-scale production, uncontrolled moisture can cause batch-to-batch variability by altering initiation efficiency and molecular weight control, particularly in cationic polymerization.
• Depolymerization conditions: High-temperature depolymerization (180 °C) and use of Sn(Oct)2 are required; energy use and catalyst management need assessment for scale-up.
• Initiation efficiency: Cationic copolymerization shows relatively low initiation efficiency (Mn higher than theoretical), indicating room for catalyst/initiator optimization.
• Reaction rates: Cationic copolymerization can require long times at room temperature; higher temperatures improve rates but can increase transesterification.
• Material scope: While many anhydrides were demonstrated, broader monomer classes and property spaces remain to be explored for application-specific needs.