Chemically recyclable polyester thermosets from activated adipic acid and renewable polyols.

This study outlines a method for producing chemically recyclable crosslinked polyesters using renewable polyols - glycerol and sorbitol - combined with adipic acid (AA), which is transformed/activated into a polyanhydride mixture prior to use. A three-step procedure was designed: i) an acid-catalyzed reaction of AA with non-toxic isopropenyl acetate or acetic anhydride to form a crosslinking mixture (CLM) made of adipic-acetic mixed polyanhydrides; ii) a solvent- and additive-free process where glycerol and/or sorbitol was reacted with the CLM to achieve a pre-polymer, and iii) a casting/molding of the liquid viscous pre-polymer to yield a thermoset as the end product. Different thermosets (8 examples) were prepared by changing the reagents ratio. These solids were thoroughly characterized by tensile tests, DMA, HR-MAS and solid-state NMR, TGA, DSC, and FT-IR. The formation of cross-linked polyesters was confirmed in all cases, but mechanical properties varied significantly from one specimen to another. Interestingly, a tensile strength up to 18 MPa - approximately an order of magnitude higher than similar polymers - was achieved when sorbitol and the CLM were used in a 1:1 wt% ratio. The chemical recycle of the resulting polymers was achieved via methanolysis with quantitative recovery of the monomeric units.