Direct Coupling of Biobased Bifunctional Hydroxy-Aldehyde Monomers to Chemically Recyclable Alipharomatic Polyesters via Dehydrogenative Polycondensation

A series of biobased alipharomatic polyesters using vanillin-derived bifunctional hydroxy-aldehyde monomers (M1)s as the starting materials are synthesized through direct dehydrogenative polycondensation with commercially available Milstein catalyst. The thermal properties of the resulting P(M1)s can be precisely tuned by altering the alkylene length of the monomers, with a maximum T_g value reaching 37.9 °C. Compared to the reported vanillic acid-based polyesters, the synthesis of M1 is less labor-intensive, and the polymerization conditions are milder than those of conventional polycondensation. Additionally, experimental results and mechanism elucidations revealed four different types of ester linkages in the P(M1) chain. The ruthenium dihydride complex has been identified as the true active species required for promoting the polymerization of hydroxy-aldehyde monomers into polyesters. Moreover, this catalytic system also exhibits depolymerization capability toward resulting polyester back into vanillin-derived diol (M2), which can then be repolymerized into a polyester P(M2) with an identical structure to that of P(M1), thus demonstrating a rare example where two different monomers yield the same polymer product. This study presents a green and economical approach to the preparation and recycling of widely used alipharomatic polyesters.