Thermoset networks are important in a range of industries such as dental care, composites and coatings, providing excellent mechanical strength and durability. However, cross-linking causes them to become insoluble and therefore makes their reuse, separation from high value components and recycling challenging. Existing recycling techniques can be broadly encompassed into two categories; mechanical recycling, where networks are broken down into fibres or fillers for reuse and thermal processing, where networks are typically used as fuel. However, chemical recycling of materials into useful chemicals is a desirable alternative. Furthermore covalently adapatable networks have emerged as promising candidates for reprocessable thermosets. Herein we report the synthesis of acetal based methacrylate thermosets with adaptable, degradable and reprocessable properties from commodity reagents. Firstly a pH responsive methacrylate cross-linker was synthesised from 2-hydroexthyl methacrylate and tri(ethylene glycol) divinyl ether (A1, Scheme 1). Under acidic conditions the cross-linker readily hydrolysed to the commodity chemicals tri(ethylene glycol) and acetaldehyde. Cross-linked methacrylate networks were subsequently synthesised containing A1 via redox initiated free radical polymerisation. Networks containing the labile acetal moiety were shown to degrade and solubilise on exposure to mild solutions of sulfonic acid derivatives. The polymeric product of which can be collected and reprocessed to give cross-linked networks similar in structure to the virgin material. Furthermore, the acetal groups impart the networks with adaptable properties via acetal metathesis and transacetalisation reactions between cross-linkers. We demonstrated this by repeated re-joining of mechanically separated lap shear substrates, the bond strength was ~60 % that of the original material.

Acetals for adaptable, degradable and reprocessable thermosets

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ACS Fall 2022

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