Introducing Spiroacetal Units to Tune Thermomechanical Properties and Enhance Chemical Recyclability of Poly(butylene adipate- co -terephthalate)

Abstract

Poly(butylene adipate-co-terephthalate) (PBAT) is a widely used biodegradable polyester. Still, the range of applications may be further broadened by extending the property range and improving recyclability. To address this issue, we gradually exchanged the adipate and terephthalate units, respectively, of PBAT with rigid spiroacetal units. These units are incorporated by adding a biobased spiroacetal diester monomer, synthesized from glyoxylic acid, bio-pentaerythritol, and methanol, in the polycondensations. The synthesis exhibits a low environmental factor (E-factor = 0.47) with acid catalyst and methanol recycling, indicating environmental sustainability. Increasing contents of the spiroacetal units lead to increased glass transition temperatures (Tgs) and reduced crystallinity and melting points, consistent with the introduction of bulky cycloaliphatic structures into the backbone. Hence, the resulting polyesters show widely tunable Tgs (–20 to 63 °C) and viscoelastic properties while retaining the original molecular weight of PBAT. The spiroacetal units also introduce predetermined cleavage sites into the polyester backbone, which undergo selective cleavage of ester bonds adjacent to the spiroacetal units in neat methanol at room temperature, accompanied by rapid physical disintegration. Theoretical calculations reveal that the carbonyls of these ester groups are more electrophilic and have a lower barrier toward methoxide attack than the terephthalate ester groups, providing a molecular-level explanation for the observed bond selectivity. These results establish electronic differentiation between ester linkages as a molecular-design principle for programming selective deconstruction of the polyester backbones. Thus, the comonomer content controls not only the material properties but also the density of chemically programmed cleavage sites and, consequently, the degradation behavior of the polymer. The degradation products are readily recovered by filtration and are then re-polymerized. Moreover, the chemically recycled material shows an almost identical composition and performance to the virgin copolyester, demonstrating the effectiveness of the closed-loop chemical recycling.