Self-catalyzed polycondensation of glycolic acid: Influence of catalysts and synthesis of high-Tm cyclic polyglycolides

The current work comprises three stages. First, the self-catalyzed polycondensation (SCP) of glycolic acid (GA) was studied in bulk or in suspension at 190 °C or at 205 °C. Cyclic poly(glycolic acid)s, cPGAs, with number average molecular weights (M n ́s) up 6 700 g mol −1 and dispersities below 2.0 were obtained. These cPGAs possess an unusual molecular weight distribution with a considerable predominance of cycles having degrees of polymerization of 28, 32 and 36. The cycles were formed in the solid state under thermodynamic control which favors the formation of extended-ring crystals. These cPGAs are an off-white, brittle porous mass that is easy to grind, yielding a flowable, crystalline powder suitable for 3D printing with laser sintering (SLS). In the second stage, the influence of non-toxic catalysts such as, Mg, Zn, Ti, Sn(II), Zr and Bi salts or complexes was studied. Compared to the results obtained with self-catalyzed polycondensation in bulk, the best metal catalysts increased the molecular weight by a factor of two to three. However, when compared to the results obtained with self-catalysis in 1,2-dichlorobenzene, the increase was limited to approximately 20%. The third stage examined the influence of 4-toluene sulfonic acid (TSA), which was found to favor the formation of the most perfect crystals with crystallinities up to 84% and the highest reported melting temperatures (up to 245 °C). This indicates the existence of a high T m morphology, analogous to that of polylactide. Small-angle X-ray scattering (SAXS) measurements indicate that significant growth in crystal thickness is mainly responsible for these effects.