Main-chain, stimuli-degradable polymers possess important advantages in polymer waste management and biomedical applications, allowing the on-demand main-chain polymer degradation using certain external stimuli. Among the different triggers proposed to cleave the polymer bonds, light has emerged as a particularly attractive stimulus to induce a photo-mediated main chain polymer degradation, because of its spatiotemporal control, tunable intensity and wavelength, and noninvasive nature [1]. Herein, two novel, main-chain, stimuli-degradable polymer families as proof-of-concept studies to improve polymer sustainability, are presented. First, soft, transparent, photodegradable, and thermo-reversible polymer gels, comprising PEG as the elastic strands, that undergo degradation upon exposure to light, will be discussed [2]. Mechanistic studies revealed a chemical recycling process to obtain the initial reagents as the main photoproducts, enlightening the mechanism of network reformation upon heating the system at mild temperatures, as verified by shear rheology experiments. The hydrogels successfully underwent reversible photodegradation and reformation upon heating, restoring the initial mechanical properties of the polymer network and thus revealing the re-processability of the system. In the second part, photo- and acid-degradable poly(acylhydrazones) synthesized via a step-growth reaction of dicarbonyl and diacylhydrazide comonomers is presented [3]. The photo-sensitivity of the synthesized polymers to light was verified by irradiation studies in aqueous solution, while a mechanistic study shed light on the photodegradation mechanism and the produced photoproducts.

Acknowledgments
The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Faculty Members & Researchers” (Project Number: HFRI-FM17-3346).

The development of polymers with labile bonds has attracted significant interest due to their potential for chemical recycling into oligomers or even monomers, enabling efficient recovery and reuse. Various types of chemical bonds can break upon exposure to external stimuli.[1] Among these, acetal bonds are particularly noteworthy for their degradation under mild acidic conditions. We focused on the synthesis of polymers containing acetal moieties, designed for triggered depolymerization.[2] A solvent-free polyaddition reaction between 1,4-butanediol and 1,4-butanediol divinyl ether was developed and optimized using the heterogeneous catalyst Amberlyst 15 at 100 °C. Optimal reaction conditions, including catalyst loading and reagent ratios, were determined through a Design-of-Experiment approach to achieve high conversion, low polydispersity, and desired molecular weight. The resulting polymer exhibited an amorphous character with thermal stability up to 220 °C and demonstrated pH-responsive behavior, fully hydrolyzing in acidic conditions within 42 days while remaining stable under neutral and basic pH. These findings provide a proof-of-concept for designing pH-responsive materials via solvent-free, scalable processes. The acetal moiety can further be leveraged to create sustainable, recyclable materials, offering potential for new adhesives or novel degradable thermosetting materials with a recycling-by-design approach.

Adhesives are widely used in various applications, from household items and electronics to biomedical products. Most polymeric adhesives are based on infusible and insoluble thermosets like epoxies, polyurethanes, and silicones. They possess high mechanical properties and offer significant resistance to chemicals and heat. However, recent years have highlighted their environmental impact: thermosets are unrecyclable and cannot be easily removed or reused without damaging the underlying substrate. This lack of reusability not only contributes to environmental problems but also leads to financial issues, especially when improperly adhered adhesives must be removed from expensive products.

To address these challenges, developing adhesives that can be debonded on demand is essential. A promising solution is incorporating reversible or dynamic bonds into the adhesives, transforming them into covalent adaptable networks (CANs) or vitrimers. These materials can fully dissociate or soften when exposed to specific stimuli, making them strong candidates for solving the issues associated with traditional adhesives.

In our research, we have created a solvent-free, photocurable adhesive containing disulfide linkages, which is fully recyclable, effectively addressing some of the significant challenges in this field, including the requirement of high-temperature treatment or adding solvents. Furthermore, we have demonstrated that this adhesive can be cured under different wavelengths across the visible to near-infrared spectrum using appropriate photoinitiators.

Poly(methyl methacrylate) (PMMA) is a high-value polymer uniquely suitable for thermochemical recycling due to the high yield of methyl methacrylate (MMA) monomer recovered, typically above 90%. Hurdles to industrial scale-up of current recycling technologies include low waste stream quality and high energy requirements that limit economic feasibility. The application of microwaves for depolymerisation of PMMA represents a potential technology in circumventing the prohibitive energy costs of thermochemical recycling processes while enabling integration of renewable energy sources via direct use of electricity in heating.

PMMA possesses low dielectric loss tangents at room temperature but attains higher losses above its glass transition temperature due to improved mobility of the polar segments and thus undergoes rapid microwave heating up to the thermal decomposition temperature. This study investigates the modification of PMMA with solvents that act as swelling agents to improve the microwave absorbance capacity of PMMA, particularly to accelerate microwave heating at lower temperatures. PMMA was swollen with the selected solvents, and the resultant swollen PMMA materials were characterised by measurement of dielectric properties, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The differences in dielectric loss tangents exhibited by swollen PMMA suggested improved microwave heating at lower temperatures, supported by shifts in glass transition temperature observed with DSC as well as mass loss differences between unmodified and swollen PMMA during heating with TGA. Depolymerisation tests with microwaves also revealed accelerated heating and reduction in time to complete depolymerisation under microwave processing.