Zwitterionic ring opening polymerization of lactide by metal free catalysts: Production of cyclic polymers

Abstract

Tailor made N-heterocyclic carbene (NHC) catalyst precursors namely (þ) and (À) 1-methyl-3-menthoxymethyl imidazolium chloride have been synthesized in high yield with a literature modified procedure. A reaction of catalyst precursor with potassium tert butoxide in situ generates the NHC catalyst. The zwitterionic ring opening polymerization of lactide (LA) mediated by a catalytic system composed of NHC catalyst at 25 C under argon atmosphere led to a cyclic poly(lactide) of a high molecular weight with a narrow molecular weight distribution. The cyclic poly(lactide) was characterized by NMR spectroscopy, Gel Permeation Chromatography (GPC) and Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). The NHC catalysts are active for lactide polymerization in the presence of air and elevated temperatures at 55 C.

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

“…Furthermore, SnMBT can be used without restriction for the preparation of cyclic polyesters from nonchiral monomers such as glycolide and ɛ‐caprolactone (as will be demonstrated in a future publication). Although neither SnMBT nor SnSPF is ideal catalysts, they presents an enormous progress compared to the first ROPPOC approach based on imidazole, and they are far more useful for the preparation of cyclic polylactides than zwitterionic polymerizations . When compared to cyclic catalysts such as DSTL the non‐cyclic catalysts have the advantage that structure and reactivity can be varied over a much broader range, for instance, because mono‐, tri‐, and pentavalent metals can be included.…”

Advanced Developments in Cyclic Polymers: Synthesis, Applications, and Perspectives

“…A high mole ratio of [ l ‐LA]/[catalyst] produces less amount of the macromonomer under the reported conditions and, thus, forms lower‐M w cyclic polymers. Under the same conditions, the polymerization conducted at room temperature produced higher‐M w polymers (M w ∼74 000, MWD 2.02) than at 0 °C (M w ∼20 700, MWD 1.46) …”

“…Considering the depletion of fossil‐based material resources and the ultimate fate of the large‐scale commodity polymers, synthesis of cyclic polymers from renewable resources has become increasingly important. Therefore, within the reported cyclic polymers, cyclic polyesters from biodegradable monomers, such as lactide and ɛ‐caprolactone, have attracted more interest within the community . The use of metal‐coordination initiators catalyzing ROP of cyclic ester monomers is widely recognized as an efficient process to produce linear polyesters with well‐controlled molecular weights, compositions, and microstructures .…”

“…NHC catalysts are generally air and moisture sensitive, and that makes it inconvenient to scale up the process. Zhu, et al have reported that an NHC catalyst produced in situ from 1‐methyl‐3‐menthoxymethyl imidazolium chloride (Figure ) is relatively robust and highly active for the ROP to produce polylactones . The NHC precursor is much cheaper and more active in comparison with the commonly used 1,3‐bisarylimidazolium chloride (Figure .…”

“…The NHC precursor is much cheaper and more active in comparison with the commonly used 1,3‐bisarylimidazolium chloride (Figure . It was first reported that NHC carbene derived from enantiomeric forms of (+)‐1‐methyl‐3‐menthoxymethyl imidazolium chloride precursor displayed higher activity towards l ‐lactide ( l ‐LA, with a product yield of 80 wt %, molecular weight (M w ) of 69 404 and molecular weight density (MWD) of 1.89) than d ‐lactide ( d ‐LA, with a product yield 40 wt %, M w of 27 224 and MWD of 1.64) with significantly high molecular weight . The NHC catalyst also showed high activity for ɛ‐caprolactone and δ‐valearolactone zwitterionic ROP to form corresponding cyclic poly(ɛ‐caprolactone), cyclic poly(δ‐valearolactone), and cyclic poly(ɛ‐caprolactone‐co‐δ‐valearolactone) respectively .…”