Closed-Loop Recyclable Aliphatic Poly(ester-amide)s with Tunable Mechanical Properties

The design and synthesis of closed-loop recyclable polymers is a promising solution to address the large negative effects of plastic pollution problem and massive economic loss associated with single-use plastics. We demonstrate that ring-opening polymerization (ROP) of 6-alkyl-substituted morpholine-2,5-dione (MDs) leads to closed-loop recyclable aliphatic poly(ester-amide)s (PEAs) with tunable mechanical properties. The controlled ROP of these MDs was achieved using benzyl alcohol as an initiator and DBU/TU as a catalyst, affording various PEA homo- and copolymers with different molar masses and compositions. All these PEAs are amorphous and thermally stable with Td,5% values in the range of 267–292 °C. Their glass transition temperatures (Tg) are in the range of 58–142 °C, being affected by the structure of pendent alkyl groups, the hydrogen bonding between amide groups, and the composition of copolymers. Tensile tests revealed that the structure of pendent alkyl groups exerts a significant effect on the mechanical property of PEAs, and they are brittle (n-butyl or cyclohexyl substituted) or ductile (n-hexyl or n-octyl substituted) plastics. In addition, the mechanical properties of PEAs could be finely adjusted by the copolymerization of different MD monomers. Of importance, both homopolymers and copolymers of these PEAs could be thermally depolymerized by sublimation to recover the corresponding monomers in high purity and efficiency. Given their good and adjustable thermal/mechanical properties, and excellent recyclability, these PEAs show promise as new close-loop recyclable polymers.