Chemically Recyclable Polyester Thermosets from Activated Adipic Acid and Renewable Polyols

Closed-loop chemically recyclable polyester thermosets were synthesized using glycerol and sorbitol and adipic acid as the monomers. Adipic acid was converted/activated into a mixture of polyanhydrides via a benign procedure prior to polymerization. A tensile strength up to 18 MPa was achieved. Methanolysis provided quantitative recovery of the monomeric units which could then be repolymerized.

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 has been designed: 1) an acid-catalyzed reaction of AA with nontoxic isopropenyl acetate or acetic anhydride to form a crosslinking mixture (CLM) made of adipic-acetic mixed polyanhydrides; 2) a solvent- and additive-free process where glycerol or sorbitol, or a combination thereof, is reacted with the CLM to achieve a prepolymer, and 3) a casting/molding of the liquid viscous prepolymer to yield a thermoset as the end product. Different thermosets (eight examples) are prepared by changing the reagents ratio. These solids are thoroughly characterized by tensile tests, DMA, high-resolution magic angle spinning and solid-state NMR, thermal gravimetric analysis, DSC, and fourier transformed infra red (FT-IR) spectroscopy. The formation of cross-linked polyesters is 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—is achieved when sorbitol and the CLM are used in a 1:1 wt% ratio. The chemical recycle of the resulting polymers is achieved via methanolysis with quantitative recovery of the monomeric units.

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