Exploiting Catalytic Steric Hindrance for Enhanced Tishchenko Polymerization: Toward Biorenewable Aromatic Polyesters

Abstract

Tishchenko reaction represents a highly atom-efficient method for synthesizing esters via aldehydes disproportionation, which holds significant promise in sustainable chemistry for polymerizing biorenewable dialdehydes into polyesters. However, previous Tishchenko polymerization attempts merely yielded oligomers with very low molecular weights due to intramolecular cyclization. To address this important challenge, we propose an efficient approach to leverage catalytic steric hindrance to achieve higher molecular weights. The investigation of six catalysts revealed that Al(OEt)₃, with its three ethoxy groups initiating, imparts significant steric hindrance around the metal center and effectively prevents cyclization termination at the initial stage of polymerization, resulting in aromatic polyesters with a record M_w of 26.6 kg/mol. This breakthrough signals the potential of enhancing catalytic steric hindrance to overcome the low-molecular-weight limitation of Tishchenko polymerization. Building upon this discovery, we synthesized a series of biorenewable meta-substituted dialdehyde monomers (M1–M4) derived from vanillin to afford aromatic polyesters P(M1)–P(M4). These polymers offer tunable thermal properties through molecular weight and side chain flexibility adjustments, exhibiting high thermal stability (T_d,5% > 208 °C) and controllable glass transition temperatures (1–58 °C), and are degradable under mild conditions. This work highlights the importance of developing new methods to utilize readily available bioderived molecules, which is of great value for the sustainable economy.