Construction of highly crystalline and recyclable salicylate-based polyesters via amide bonds

This study reports the design of recyclable salicylate-based polyesters with high crystallinity and thermal stability. To address the poor crystallization ability and thermal degradation of salicylic acid–based polyesters, amide structures capable of forming stable hydrogen-bonding networks were introduced to enhance interchain interactions and promote crystallization. Two monomers, 3-methyl-3,4-dihydrobenzo[f][1,4]oxazoline-2,5-dione (3-Me-DH-BOZD) and 3-ethyl-3,4-dihydrobenzo[f][1,4]oxazoline-2,5-dione (3-Et-DH-BOZD), were synthesized from salicylic acid and amino acids. Ring-opening polymerization (ROP) catalyzed by tetrabutylammonium fluoride (TBAF) yielded cyclic oligomers with molecular weights of approximately 2.8 and 3.4 kg/mol, respectively. Differential scanning calorimetry (DSC) revealed a distinct melting point at 211 °C for P(3-Et-DH-BOZD) with a high melting enthalpy of 96.7 J/g, indicating that the introduction of amide linkages significantly enhanced the crystallization capability of the polymer. The polymer could be efficiently depolymerized and recycled in polyethylene glycol under vacuum at 85 °C, achieving a monomer recovery yield of up to 98.5%. Furthermore, when P(3-Et-DH-BOZD) was incorporated into commercial polylactic acid (PLA) as a nucleating agent, the crystallization kinetics were markedly accelerated, and the crystallinity increased from 26.6% to 34.3%, improving processing efficiency. The cyclic amide polyesters constructed from salicylic acid and amino acids thus exhibit high crystallinity, excellent thermal stability, and recyclability, providing a new molecular design strategy for degradable polymer materials.