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

Abstract

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

Synthesis of cyclic polymers and flaws of the Jacobson–Stockmayer theory

“…In a previous study dealing with the usefulness of dibutyltin bisphenoxides as ROPPOC catalysts for lactides 26 two important trends were elaborated. The four catalysts used in that work, BuSnOPh, BuSnPCl, BuSnPCN and BuSnOPF (Scheme 2) are based on a series of phenols with increasing acidity.…”

“…The syntheses and properties of dibutyltin bisbenzyloxide, dibutyltin bis benzyl mercaptide, dibutyltin bisphenoxide, dibutyltin bis(4-cyanophenoxide), dibutyltin bispentafluoro-phenoxide and dibutyltin bis( pentafluoro-thiphenolate) have previously been reported (for formulas see Scheme 2). 25,26…”

Ring‐expansion polymerization of meso‐lactide catalyzed by dibutyltin derivatives

“…Later, it was possible to catalyze a ROPPOC process with dibutyltin bis(pentafluorothiophenolate), which yielded an optically pure cyclic poly( l ‐lactide). But, at Lac/Cat ratios >200/1 side reactions occurred to a significant extent . A fourth and more successful approach, which was recently reported by the authors, was ring expansion polymerization (REP) of l ‐lactide.…”

“…But, at Lac/Cat ratios >200/1 side reactions occurred to a significant extent. 10 A fourth and more successful approach, which was recently reported by the authors, was ring expansion polymerization (REP) of L-lactide. The first example was realized with 2,2-dibutyl-2-stanna-1,3-dithiolane (DSTL) as catalyst (Scheme 1).…”

Cyclic poly(lactide)s via the ROPPOC method catalyzed by alkyl‐ or aryltin chlorides

“…Only the dibutyltin derivatives of pentafluorophenol (SnOPF, Scheme 2) or pentafluorothiophenol (SnSPF) gave satisfactory results. 9,10 This raised the question, if commercial alkyltin chlorides are also capable of catalyzing a ROPPOC process as outlined in Scheme 3. Hence, it was the purpose of the present study to find out, if alkyl-or aryltin chlorides can be used to prepare cyclic poly(l-lactide)s. This study was limited to alkyl and diphenyl tin chlorides for two reasons.…”