Reactivity and flexibility enhancement of polylactide diol: one-pot synthesis and properties of poly(ε-caprolactone)-polylactide-poly(ε-caprolactone) triblock diol

The low reactivity, easy hydrolysis, semi-crystallization and rigidity of polylactide (PLA) polyols inhibit their applications, especially in polyurethane (PU) preparation. This work proposes a poly(ε-caprolactone)-polylactide-poly(ε-caprolactone) (PCL-PLA-PCL) triblock diol to enhance the reactivity and flexibility of PLA diol while maintaining degradability, thereby expanding its applications. However, when synthesizing an oligomeric PCL-PLA-PCL triblock diol by cationic ring-opening polymerization catalysed by trifluoromethanesulfonic acid (TfOH) or methanesulfonic acid (MSA), numerous secondary hydroxyl groups (OH) were detected. Unlike synthesizing a high-molecular-weight polymer, monomer amount is limited during oligomerization, leading to distinct competition between initiation and propagation and yielding a PLA diol residue with secondary OH. Several factors, including temperature and the amounts of catalyst and monomer, were investigated to improve the primary OH content. Leveraging the inhibitory effect of excessive TfOH on the propagation rate of CL under limited monomer addition, PCL-PLA-PCL diol with a primary OH content of up to 90 % was obtained. On the contrary, excessive MSA, with a relatively weaker acidity, cannot effectively increase the primary OH content, whereas an excessive monomer amount remains helpful. The findings on the differences between synthesizing a high-molecular-weight PCL-PLA-PCL block copolymer and an oligomeric one may facilitate the construction of more high-performance block co-oligomers for drug delivery and PU applications. Compared with PLA diol, the PCL-PLA-PCL diol (∼2000 g/mol, CL: LA = 1: 1, with 90.1 % primary OH) has no crystallization behaviour, has reduced viscosity and much increased reactivity, making it a promising alternative to pure PLA diol for broader PU applications.