Two new approaches based on dynamic carboxyl–hydroxyl or hydroxyl–carboxyl transformation for high molecular weight poly(butylene maleate)

Abstract

Synthesis of high molecular weight maleic acid-based polyesters via a green approach is of great significance but also a huge challenge. To date, it is still impossible to obtain high molecular weight poly(butylene maleate) (PBM) through the traditional esterification–polycondensation procedure. Here we report two previously undiscovered green approaches for high molecular weight PBM using a slight excess of maleic acid (MA) to 1,4-butanediol (BDO) or a slight excess of BDO to MA, with p-toluene sulfonic acid as the catalyst. The M_n of the PBM can be up to 92 kDa, 11 times higher than those previously reported. When MA : BDO > 1, the new mechanism relies on the acid-catalyzed dynamic transformation of some carboxyl-terminated oligomers into hydroxyl-terminated oligomers through the elimination of the terminal MA units as the maleic anhydride byproduct under vacuum conditions, and then esterification and transformation reactions of carboxyl–hydroxyl occur iteratively, so high molecular weight PBM was obtained. This mechanism is a general mechanism suitable for the synthesis of high molecular weight unsaturated polyesters from maleic acid and diverse diols. When MA : BDO < 1, the other new mechanism relies on acid-catalyzed dynamic transformation of some hydroxyl-terminated oligomers into carboxyl-terminated oligomers through the elimination of the terminal BDO units as the tetrahydrofuran byproduct under vacuum conditions, and then esterification and transformation reactions of hydroxyl–carboxyl occur iteratively, and high molecular weight PBM was also obtained.