Semi-Crystalline and Amorphous Polyesters Derived from Biobased Tri-Aromatic Dicarboxylates and Containing Cleavable Acylhydrazone Units for Short-Loop Chemical Recycling

Recycling polymers by site-specific scission into short-chain oligomers/polymers, followed by recoupling these to form the original polymer presents an energetically more favorable shorter-loop chemical recycling in comparison to recycling into monomers. Here, we present the synthesis and polymerization of tri-aromatic diesters to prepare polyesters with acylhydrazone units as weak structural links. Two diester monomers were prepared by combining methyl 5-chloromethyl-2-furoate, obtainable from 5-chloromethylfurfural, with potentially biobased hydroquinone and resorcinol, respectively. The two diesters have a central phenyl ring flanked by two furan rings, and were polymerized with 1,6-hexanediol and 1,4-butanediol, respectively, together with controlled amounts of monofunctional ethyl levulinate to form telechelic ketone-terminated polyesters. Subsequent reactions of these telechelic polyesters with adipic dihydrazide yielded the corresponding chain-extended polyesters with increased molecular weights ([eta] = 0.29-0.52 dL g(-1)) with acylhydrazone units in the backbone. Thermogravimetric analysis showed a high thermal stability of the polyesters with thermal decomposition only above 275 degrees C. The polyesters containing the linear hydroquinone units were found to be semi-crystalline materials with melting points at 158 and 192 degrees C, respectively, while those containing the kinked resorcinol units were fully amorphous with glass transition temperatures at 35 and 44 degrees C, respectively. Initial investigations of the chemical recyclability of the polyesters demonstrated that acylhydrazone units could be selectively cleaved to recover the original telechelic ketone-terminated polyesters, which could again be chain-extended to obtain a recycled polymer with molecular weights and properties very similar to those of the original material.