Lactides organic catalysts

Coulembier O, Dove AP, Pratt RC, Sentman AC, Culkin DA, Mespouille L, Dubois P, Waymouth RM, Hedrick JL (2005) Latent, thermally activated organic catalysts for the on-demand living polymerization of lactide. Angew Chem Int Ed 44 4964-4968... 

Myers, M. Connor, E. F. Glauser, T Mock, A. Nyce, G Hedrick, J. L. Phosphines Nucleophilic organic catalysts for the controlled ring-opening polymerization of lactides. 7. Polym. ScL, Part A Polym. Chem. 2002,40,844-851. 

Scheme 1. General strategy for ROP of lactides using organic catalysts.

Early-on it was discovered that these Salen compounds, and the related six-coordinate cations [6], were useful as catalysts for the polymerization of oxiranes. These applications were anticipated in the efforts of Spassky [7] and in the substantial work of Inoue [8]. Subsequently, applications of these compounds in organic synthesis have been developed [9]. Additional applications include their use in catalytic lactide polymerization [10], lactone oligomerization [11], the phospho-aldol reaction [12], and as an initiator in methyl methacrylate polymerization [13]. 

The following cases should be distinguished caramelization in the presence of (a) an inorganic acid, (b) an organic acid as the catalyst, and (c) either a-, y-, or < hydroxycarboxylic acid as the catalyst. In the last case, such acids may form lactides or lactones, respectively, under the caramelization conditions. Our preliminary results suggested that such acids participate in the formation of secondary aromas similarly to 0 -amino acids. This problem will be discussed separately in the section devoted to certain food aromas in which carbohydrates are involved. In either case when organic acids are... 

The first ROP of lactide with organic bases was reported with 4-(dime-thylamino) (DMAP) and 4-pyrrolidino-pyridines (PPY) as the catalysts. ... 

The specifications and allowed impurity levels of lactide monomer for PLA are defined by the polymerization mechanism and the applied catalyst. PLA is commercially produced by ROP of lactides in bulk. The tin(II)-catalyzed process offers good control over molecular weight and reaction rate provided that it is performed in the absence of impurities such as water, metal ions, lactic acid, or other organic acids. Purification of crude lactides is therefore indispensable for the industrial manufacture of high molecular weight PLA (M > lOOkg/mol). In fact, lactide is the ultimate form of lactic acid, in its dehydrated and purest form. 

Optically active lactones are valuable building blocks in organic synthesis (4) and in the preparation of optically active biodegradable polymers (7,5). Several chemical methods for producing these compounds and their corresponding polymers have been explored (6) but unfortunately all of these methods are either experimentally cumbersome or afford the lactones with only modest enantioselectivities. Examples of chemically prepared optically active polyesters include poly(a-phenyl-P-propiolactone) (7), poly(a-ethy(-a-phenyl-P -propiolactone) (S, 9), poly(a-methyl-a-ethyl-P-propiolactone) (70) and poly(lactic acid) (77, 72). Use of enantioselective polymerization catalysts to carry out stereoelective polymerizations of racemic lactones has produced mixed results. For example, stereoelective polymerization of [/ ,S]- P-methyl-P-propiolactone with a catalyst from Zn ( 2115)2 and [7 ]-(-)-3,3-dimethyl-l,2-butanediol showed only a small enantiomeric enrichment in the final polymer (75). Stereoselective copolymerizations of racemic (LL/DD monomers) and meso (LD monomer) lactides using chiral catalyst that gives heterotactic and syndiotactic PLA, respectively have also been studied (77). 

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