Catalyst lactide

Keywords Carbon dioxide and epoxides copolymerization Group 13 metal catalysts Group 3 metal catalysts Lactide polymerization Polycarbonate Polylactide... 

Di-[di-(trimethylsilyl)amine]stannate has been used to prepare biocompatible poly (lactide-co-glycolide) containing up to an 88% lactide composition. When the molar ratio of catalyst/lactide was 41.9 17.9, respectively, apolymer was formed which had a molecular weight of 164,700 daltons having a polydispersity of 1.8. AU other ratios generated molecular weights less than 78,000 daltons. 

Coordination numbers ranging from five to seven were observed, and they appeared to be controlled by the steric bulk of the supporting amidinate and coligands. Complexes 2-5 and 7 were found to be active catalysts for the pol3uneiization of D,L-lactide (e.g., with 2 and added benzyl alcohol, 1000 equiv of D,L-lactide were polymerized at room temperature in <1 h). The neutral complexes 2, 5, and 7 were more effective than the anionic complexes 3 and 4. ... 

Oxazolones (azlactones) are a form of activated lactones, so they are included in this section. CAL-B is an effective catalyst for the DKR of various racemic four-substituted-5 (4H)-oxazolones, in the presence of an alcohol, yielding optically active N-benzoyl amino acid esters as illustrated in Figure 6.24 [40]. Enantioselective biotransformations of lactides [72,73] and thiolactones ]74] have also been reported. 

The solution to this problem has been to isolate the lactide and to polymerize this directly using a tin(ii) 2-(ethyl)hexanoate catalyst at temperatures between 140 and 160 °C. By controlling the amounts of water and lactic acid in the polymerization reactor the molecular weight of the polymer can be controlled. Since lactic acid exists as d and L-optical isomers, three lactides are produced, d, l and meso (Scheme 6.11). The properties of the final polymer do not depend simply on the molecular weight but vary significantly with the optical ratios of the lactides used. In order to get specific polymers for medical use the crude lactide mix is extensively recrystallized, to remove the meso isomer leaving the required D, L mix. This recrystallization process results in considerable waste, with only a small fraction of the lactide produced being used in the final polymerization step. Hence PLA has been too costly to use as a commodity polymer. 

Enzymatic synthesis of aliphatic polyesters was also achieved by the ringopening polymerization of cyclic diesters. Lactide was not enzymatically polymerized under mild reaction conditions however, poly(lacfic acid) with the molecular weight higher than 1 x 10" was formed using lipase BC as catalyst at higher temperatures (80-130°C). Protease (proteinase K) also induced the polymerization however, the catalytic activity was relatively low. 

Metal hahdes in imidazolium ionic hquids offer unique enviromnents able to facihtate dehydration reactions. Under such conditions certain metal halides are able to catalyze formal hydride transfer reactions that otherwise do not occur in the ionic liquid media. We have now discovered two systems in which this transformation has been observed. The initial system involves the conversion of glucose to fractose followed by dehydration the second system involves the dehydration of glycedraldehyde dimer followed by isomerization to lactide. CrCls" anion is the only catalyst that has been effective for both systems. VCI3" is effective for the glyceraldehyde dimer system but not for glucose. 

Diiminatc zinc complexes are highly active catalysts in the copolymerization of epoxides and C02. Complexes that are catalytic are of the form ZnLX, where X is alkoxide, acetate, or bis(tri-methylsilyl)amide. Changing the ligand geometries of the complexes allows variation in the catalytic behavior and activity.941 The polymerization of lactide with diiminate zinc has also been studied.942... 

Polymers are formed via two general mechanisms, namely chain or step polymerisation, originally called addition and condensation, respectively, although some polymerisations can yield polymers by both routes (see Chapter 2). For example, ring opening of cyclic compounds (e.g., cyclic lactides and lactams, cyclic siloxanes) yield polymers either with added catalyst (chain) or by hydrolysis followed by condensation (step). Many polymers are made via vinyl polymerisation, e.g., PE, PP, PVC, poly(methyl methacrylate) (PMMA). It could be argued that the ethylenic double bond is a strained cyclic system. 

Ag(carbene)2][AgCl2] (carbene = l,3-dimesitylimidazol-2-ylidene) Was also reported for use as a pre-catalyst for the polymerization of L-lactide with low PDI and in the absence of monomer racemization.69... 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

Jensen et al. reported the stereoselective polymerization of D,L-lactide with dibenzyloxidezinc(2,4,6-trimetylphe-nyimidazol-2-ylidene), which was synthesized as shown in Scheme 39.100 Surprisingly, a mixture of the heterocarbene and benzyl alcohol was a better catalyst for polylactide formation than the zinc complex, and unlike 50 the mixture produced heterotactically enriched polylactide. 

For applications in ring-opening catalysis of chiral lactides, pyrazolylborates have been suitably modified to yield chiral zinc complexes. An example of such a chiral pre-catalyst is the highly sterically encumbered tris(indazolyl)bo-rate methylzinc complex 107, introduced by Chisholm et al., whose C -symmetric structure is shown in Figure 53.165... 

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). 

The ethylzinc compound 143, created from a bulky diaminophenol by treatment with ZnEt2, as shown in Scheme 89, was synthesized for the purpose of creating a well-defined zinc catalyst for the polymerization of lactides.206... 

The polymers were made according to procedures described in Ref. 15. A representative example is given here. A 1-liter round-bottom flask equipped with an overhead mechanical stirrer was flame dried under a vacuum to remove moisture from the interior surfaces of the vessel. When cool, the vacuum was released by the introduction of dry nitrogen gas. To the flask was added, in a dry nitrogen glovebox, 500.0 grams (3.469 moles) of pure L-lactide. Thai, 0.35 ml of a 0.33 molar catalyst solution (stannous octoate in toluene) containing 1.155 x 10 moles of catalyst was Current address Los Alamos National Laboratory, Los Alamos, NM 87544... 

Shueh ML, Wang Y-S, Fluang B-H, Kuo C-Y, Lin C-C (2004) Reactions of 2,2 -methylenebis(4-chloro-6-isopropyl-3-methylphenol) and 2,2 -ethylenebis(4,6-di-tert-butylphenol)with Mg"Bt2 efficient catalysts for the ring-opening polymerization of e-caprolactone and L-lactide. Macromolecules 37 5155-5162... 

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