Peter J. Roth

Pressure-sensitive adhesives are used widely in tapes and labels but create recycling challenges because their cross-linked polymer network is insoluble in solvents. When plastics and paper are recycled, they are soaked in hot aqueous alkaline solutions. Processing breaks adhesives into lumps of insoluble gel (“stickies”) that clog machinery, contaminate repulped paper, and divert recyclable material to landfill. Previously, dibenzo[c,e]oxepin-5(7H)-thione (DOT) was copolymerized with an acrylate monomer to introduce a cleavable linkage that was degradable only in amine solutions, which limited its utility in present-day recycling. Here, we address the stickies problem by copolymerizing hydrophilic poly(ethylene glycol) methyl ether acrylate (PEGA) with DOT and methyl acrylate (MA) to create a water-soluble copolymer that swells in aqueous NaOH solutions to facilitate chemical degradation by hydrolysis. Sparse cross-linking was achieved by the photo-cross-linker 4-acryloyloxy benzophenone (ABP). The PEGA:MA monomer ratio, cross-linking density, and chain length were varied to optimize the balance of viscous and elastic properties needed for good adhesion (achieving peel force: 4.5 ± 0.7 N/25 mm, adhesion energy: 140 ± 10 Jm2–, and shear holding time: 5 ± 1 min). A copolymer with a low gel fraction and a loose cross-linked network resulted in higher tack adhesion energies but lower shear holding times. With relative humidity <50%, water sorption is low, regardless of PEGA:MA ratio. Under higher relative humidities, there is greater sorption but water diffusivity slows. A cross-linked poly(PEGA-MA-ABP-DOT) network degrades into soluble molecular fragments in 0.5 M NaOH solutions at room temperature─leading to zero gel and avoidance of stickies. In an adhesive, substrate detachment was observed after only 4 min in an alkaline bath at room temperature, reducing energy demand. In contrast, a nonhydrophilic butyl acrylate-DOT adhesive, which did not swell in water, did not detach. Our poly(PEGA-MA-ABP-DOT) copolymer combines adequate adhesion with rapid, alkaline-triggered removal, providing a promising route toward greater recyclability.

Depolymerization, the complete degradation of a polymer into its monomers, is of great interest for the development of circular materials. But current depolymerizations require high temperatures, suffer from low monomer recovery, and/or require a separate repolymerization step. Herein, unsubstituted 1,2-dithiolane, which is unstable in pure form, was successfully prepared, stored, and polymerized in solution. Poly(1,2-dithiolane), i.e., poly(trimethylene disulfide), was prepared through radical and cationic ring-opening polymerization procedures in solution, but also, leveraging the monomer instability, upon evaporation of a monomer solution. Poly(1,2-dithiolane) depolymerized efficiently to monomer when heated to 90 °C. Uniquely, the monomer was gaseous at this temperature and spontaneously repolymerized into a solid polymer upon cooling. The resulting polymer could be sublimed repeatedly. The widespread applicability was demonstrated through depositing insoluble hydrophobic waterproof coatings, the thermal removal of such coatings, and the purification of a contaminated polymer through sublimation. Additionally, 1,2-dithiolane was found to undergo radical ring-opening copolymerization with styrene, acrylates, vinyl acetate, and methyl methacrylate, introducing cleavable S−S and C−S bonds into the backbone. Key requirements for the development of other sublimable polymers are discussed and anticipated to lead to the development of tailored depolymerizable circular materials.

Depolymerization, the complete degradation of a polymer into its monomers, is of great interest for the development of circular materials. But current depolymerizations require high temperatures, suffer from low monomer recovery, and/or require a separate repolymerization step. Herein, unsubstituted 1,2-dithiolane, which is unstable in pure form, was successfully prepared, stored, and polymerized in solution. Poly(1,2-dithiolane), i.e., poly(trimethylene disulfide), was prepared through radical and cationic ring-opening polymerization procedures in solution, but also, leveraging the monomer instability, upon evaporation of a monomer solution. Poly(1,2-dithiolane) depolymerized efficiently to monomer when heated to 90 °C. Uniquely, the monomer was gaseous at this temperature and spontaneously repolymerized into a solid polymer upon cooling. The resulting polymer could be sublimed repeatedly. The widespread applicability was demonstrated through depositing insoluble hydrophobic waterproof coatings, the thermal removal of such coatings, and the purification of a contaminated polymer through sublimation. Additionally, 1,2-dithiolane was found to undergo radical ring-opening copolymerization with styrene, acrylates, vinyl acetate, and methyl methacrylate introducing cleavable S–S and C–S bonds into the backbone. Key requirements for the development of other sublimable polymers are discussed and anticipated to lead to the development of tailored depolymerizable circular materials.

A cyclic xanthate is shown to undergo thiocarbonyl addition-ring-opening (TARO) homo- and copolymerization, affording a labile dithiocarbonate backbone functionality. Both C–S bonds are cleavable through aminolysis and hydrolysis.

Polythioesters are promising as sustainable materials with improved properties. But, despite the industrial importance of radical polymerization, polythioester homopolymers are not readily accessible through this method. Using dibenzo-[c,e]-oxepin-5-(7H)-thione (DOT) as a thionolactone monomer, this study shows that the attempted radical homopolymerization suffers from low (3-13%) conversion. Conversely, addition of diethyl vinylphosphonate (DEVP), a judiciously chosen comonomer, led to near-quantitative DOT conversion at low (1H-¹H COSY, ¹H-¹³C HMBC, and ¹H-³¹P HMBC NMR spectroscopy revealed that DEVP was inserted as single units and enabled the identification of DEVP repeat units sandwiched between two DOT groups (δ_P = 24.5 ppm) and those located at the α end group between an AIBN-derived fragment and a DOT repeat unit (δ_P = 23.7 ppm). The proportion of single DEVP repeat units at the α end group increased with a decreasing DEVP copolymerization feed, demonstrating the importance of the secondary DEVP-based radical in effectively initiating the polymerization of DOT. Polymers that carried, on average, less than one internal DEVP unit and consisted mostly of AIBN-DEVP-initiated DOT homopolymers were prepared by tuning the DEVP feed ratio. The polymers were fully degradable through aminolysis and by heating (in bulk) to 140 °C, with a higher DEVP content found to increase the formation of dibenzo-[c,e]-thiepin-5-(7H)-one (DTO) thiolactone product during thermal degradation.

The development of degradable polymeric vesicles is essential for next-generation controlled delivery systems and responsive materials, particularly for therapy and diagnostics. However, achieving precise control over their synthesis and degradation behavior remains a significant challenge. Herein we report the synthesis of degradable diblock copolymer vesicles via radical ring-opening polymerization-induced self-assembly (rROPISA) in aqueous media. This approach employs radical ring-opening copolymerization (rROP) of a thionolactone with a water-miscible vinyl monomer to introduce cleavable thioester groups within the membrane-forming polymer chains. More specifically, a water-soluble poly(N,N-dimethylacrylamide) (PDMAC) precursor is chain-extended by statistically copolymerizing dibenzo[c,e]oxepane-5-thione (DOT) with 2-methoxyethyl acrylate (MEA) using a reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization formulation. The disparity in reactivity between DOT and MEA is addressed using a comonomer-starved feed strategy, with MEA acting as a cosolvent for the water-insoluble DOT. This controlled feeding approach ensures a more uniform distribution of DOT repeat units within the hydrophobic core block, significantly improves copolymer composition control, enables DOT incorporation of up to ∼4 mol %, and enhances overall comonomer conversion. Compared to conventional one-shot batch syntheses, this strategy yielded vesicles with superior degradation profiles. Morphological characterization using cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) confirmed the formation of well-defined vesicles. Hydrolytic degradation studies conducted in mildly basic aqueous media led to vesicle disintegration, as indicated by cryo-TEM, DLS, and SAXS analyses. Importantly, size exclusion chromatography (SEC) analysis revealed the formation of significantly shorter copolymer chains and final molecular weight distributions comparable to that of the nondegradable PDMAC precursor, indicating efficient hydrolytic degradation of the membrane-forming P(MEA-stat-DOT) block. This study provides valuable insights regarding the rational design of degradable vinyl-based diblock copolymer vesicles, which augurs well for potential biomedical applications.