Ring-opening polymerization of lactides

The preparation of PGA given below (Scheme 2.51) is a polycondensation of glycolic acid followed by a solid-state postpolycondensation.374 (Reproduced from ref. 374. Copyright 2000 Elsevier Science Ltd, by permission of the copyright owner.) This method is different from the commonly used ring-opening polymerization of lactide (see Section 2.3.6). 

Polyester syntheses have been achieved by enzymatic ring-opening polymerization of lactide and lactones with various ring-sizes. Here, we focus not only on these cyclic esters but also other cyclic monomers for lipase-catalyzed ringopening polymerizations. Figure 8 summarizes cyclic monomers for providing polyesters via lipase catalysis. 

Chisholm et al. synthesized organozinc compounds with bulky biphenolates as catalysts for the ring-opening polymerization of lactides.196 The protonolysis of diethyzinc by the biphenols, in the presence of diisopropylmethanol, afforded the polycyclic, trimetallic zinc-di(ethylzinc) pre-catalyst 135, which polymerizes /m -lactide to polylactide, enriched in isi- and. sir-tetrads (Scheme 85). 

Recent Developments in Metal-Catalyzed Ring-Opening Polymerization of Lactides and Glycolides Preparation of Poly lactides, Polyglycolide, and Poly(lactide-co-glycolide)... 

Polylactide is a degradable polyester, formed by the ring-opening polymerization of lactide or the condensation polymerization of lactic acid. Lactide is produced from lactic acid, which derives from the fermentation of D-glucose, which is usually harvested from high-starch-content crops, such as com or sugar beet (Fig. 1). 

The most effective, and commercially applied, method to produce polylactide is via the ring-opening polymerization of lactide. This process is initiated by metal complexes and proposed to occur via a coordination-insertion mechanism, as illustrated in Fig. 2. The most common initiators for this polymerization are Lewis acidic metal alkoxide or amide complexes. Key initiator criteria are sufficient Lewis acidity to enable binding and activation of the lactide unit and a labile metal alkoxide (or amide) bond so as to enable efficient insertion. 

Dechy-Cabaret, O., Martin-Vaca, B., and Bourissou, D. (2004) Controlled ring-opening polymerization of lactide and glycolide. Chemical Reviews, 104, 6147. 

Ligand Design in Enantioselective Ring-opening Polymerization of Lactide... 

Monomeric amide or aryloxide derivatives [Ca(X)(Tp pr)] (X = CH[CMeNC6H3-2,6- Pr2]2, CH[CMeNC6H4-2-OMe]2) have been prepared and characterized and employed for the ring-opening polymerizations of lactide.19,20 [Ca(Tp,pr)N(SiMe3)2]-THF has been synthesized and structurally characterized.21... 

H4-2-OMe]2) species have been employed for the ring-opening polymerizations of lactide.33... 

Scheme 7-2 The NHC catalysed ring-opening polymerization of lactide.

The NHC complex (l-ethyl-3-methylimidazol-2-ylidene)silver(i) chloride is an ionic liquid, and was foimd to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight [94]. Here, the ionic liquid is acting as a source of the NHC by the thermal decomposition of the silver imidazolylidene complex cation. 

Base for C-H or N-H Deprotonation of Precursors to Carbene or Amide Ligands in the Preparation of Metal Complexes. KHMDS has been used as a base for the deprotonation of N-H or B-H bonds in imine-based ligands or trispyrazolyl borate-derived ligands to synthesize calcium complexes, which were used as initiators of the ring-opening polymerization of lactides. A first example of a two-coordinate, neutral In singlet carbene complex has been prepared via an in situ deprotonation of the imine ligand precursor with KHMDS in the presence of In I to afford the In(I) complex in 36% unoptimized yield as pale yellow, thermally stable crystals. ... 

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