Cyclic polylactides via simultaneous ring‐opening polymerization and polycondensation catalyzed by dibutyltin mercaptides

l ‐Lactide is polymerized in bulk at 160 °C either with dibutyltin bis(benzylmercaptide) (SnSBzl), dibutyltin bis(benzothiazole 2‐mercaptide) (SnMBT), or with dibutyltin bis(pentafluorothiophenolate) (SnSPF) as catalysts. SnSBzl yields linear polylactides having benzylthio‐ester end groups in addition to cyclic polylactides, whereas SnMBT and SnSPF mainly or exclusively yield cyclic polylactides. This finding, together with model reactions, indicates that the SnS catalysts promote a combined ring‐opening polymerization and polycondensation process including end‐to‐end cyclization. SnMBT caused slight racemization (3%–5%), when used at 160 °C. With SnSPF optically pure cyclic poly(l ‐lactide)s with high‐molecular weights can be prepared at 160 °C. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3767–3775