Meso-lactide

Polylactide is the generaUy accepted term for highly polymeric poly(lactic acid)s. Such polymers are usuaUy produced by polymerization of dilactide the polymerization of lactic acid as such does not produce high molecular weight polymers. The polymers produced from the enantiomeric lactides are highly crystalline, whereas those from the meso lactide are generaUy amorphous. UsuaUy dilactide from L-lactic acid is preferred as a polymerization feedstock because of the avaUabUity of L-lactic acid by fermentation and for the desirable properties of the polymers for various appUcations (1,25). 

Lactide (LA), the cyclic diester of lactic acid, has two stereogenic centers and hence exists as three stereoisomers L-lactide (S,S), D-lactide (R,R), and meso-lactide (R,S). In addition, rac-lactide, a commercially available racemic mixture of the (R,R) and (S,S) forms, is also frequently studied. PLA may exhibit several stereoregular architectures (in addition to the non-stereoregular atactic form), namely isotactic, syndiotactic, and heterotactic (Scheme 15). The purely isotactic form may be readily prepared from the ROP of L-LA (or D-LA), assuming that epimerization does not occur during ring opening. The physical properties, and hence medical uses, of the different stereoisomers of PLA and their copolymers vary widely and the reader is directed to several recent reviews for more information.736 740-743... 

In addition to lactones, lactides and glycolides are highly interesting monomers for evaluation in lipase-catalyzed ROP. Lactide is the cyclic dimer of lactic acid and can occur in three stereo-configurations L-lactide, D-lactide, and D,L- or meso-lactide (Fig. 6). In analogy, glycolide is the cyclic dimer of glycolic acid but is... 

Monomers like glycolide or lactide are prepared by heating the corresponding acids under controlled conditions [53]. For example, lactide is prepared by heating lactic acid at 120 °C until water ceases to distill. The temperature is then increased to 140 °C and the pressure is reduced to 10 torn After heating for several hours at this temperature, the pressure is reduced further and the temperature increased until lactide begins to distill. Dilactide (3,6-dimethyl-l, 4-dioxan-2,5-dione) contains two asymmetric centers and therefore exists as L-lactide, D-lac-tide, meso-lactide, and the racemic mixture D,L-lactide and gives polymers with different properties. 

Fig. 9. Structure of the different stereoforms of the lactide monomer and the resulting repeating unit, the chiral center marked with. a) l-LA, b) D,D-lactide, and c) meso-lactide...

CO. CO , acetaldehyde, acetone, acrylic acid, acetic acid. DL-lactide. meso-lactide. cyclic oligomers up to nonamer. 2.3 pentanedione. iactoyl lactic acid... 

Because of the selection and concentration of the stereoisomeric l-, d-, or meso-lactide it is possible to receive a wide range of molecular weight. 

Since the bulk properties of PLA are highly dependent on the stereoregularity or the tacticity of the polymer, i the development of catalysts for rac-lactide (or meso-lactide) polymerization has been focused on achieving stereoselectivity. 2 2 " Numerous organocatalysts such as A-heterocyclic carbenes (NHCs) and phosphine-based compounds " have been investigated for the controlled ROP of lactide. 

LL, DL and LD bonds, but DD bonds were not degraded [38,39]. Pyrolysis of biotically and abiotically aged PLLA showed that the ratio of meso-lactide to L-lactide was lower in samples aged in biotic media compared to samples aged in abiotic media [40]. This also confirms that microorganisms preferentially degrade the L-form of poly(lactide) or its oligomers. 

A wide variety of chemical catalysts is nowadays available to polymerize monomers into well-defined polymers and polymer architectures that are applicable in advanced materials for example, as biomedical applications and nanotechnology. However, synthetic polymers rarely possess well-defined stereochemistries in their backbones. This sharply contrasts with the polymers made by nature where perfect stereocontrol is the norm. An interesting exception is poly-L-lactide, a polyester that is used in a variety of biomedical applications [1]. By simply playing with the stereochemistry of the backbone, properties ranging from a semicrystalline, high melting polymer (poly-L-lactide) to an amorphous high Tg polymer (poly-meso-lactide) have been achieved [2]. 

Syndiotactic poly(meso-lactide) DLDLDLDL Al-centered R-chiral catalyst... 

The melting temperature (T ) of PLA occurs between 130 and 180°C according to the L-lactide content and the crystals formed during crystallization. The presence of meso-lactide in the PLA structure induces a decrease in the melting temperature according to the equation 8.3 where is the meso-lactide fraction in the matrix and 175 is the melting temperature of PLLA [5]. 

Homodecoupled NMR spectra can be used to quantitatively determine the composition of D-lactide and meso-lactide stereoisomer impurities in polylactide containing predominantly L-lactide [5]. 

Fig. 3.9 Stereoisomerism of lactide and poly(lactic acid), a The three lactide stereoisomers D-lactide, L-lactide, and meso-lactide. b Stereosequences of poly(lactic acid) (Reproduced from Dove et al. [44] with permission of The Royal Society of Chemistry)...

A crude lactide stream produced in the lactide synthesis reactors contains lactic acid, lactic acid oligomers, water, meso-lactide, and further impurities. Two main separation methods, distillation and crystallization, are currently employed for lactide purification. Crystallization may be carried out either by solvent crystallization or melt crystallization. The most used method for production of ultra-pure lactide in laboratory is by repeated recrystallization of a saturated lactide solution in mixtures of toluene and ethyl acetate [15, 23, 24]. Lactide purification using C4-12 ethers [25], and an organic solvent that is immiscible with water to extract the solution with water [26] are also reported. Melt crystallization is more practical in industry for lactide purification. Several types of equipment are described in the literature for melt crystallization [17, 27-30]. This method uses the differences in the melting points of L-, D-, and meso-lactide for separating the different lactides from each other. In a distillation process, the crude lactide is first distilled to remove the acids and water, and then meso-lactide is separated from lactide [11, 31]. Different methods are reported in the literature for distillation purification of lactide [32, 33]. In... 

Figure 11.1 The three diastereomeric structures of lactide (a) 5,S-Lactide (L-lactide), (b) fi,fi-lactide (D-lactide), (c) fi.5-lactide (meso-lactide).

Schmack et al. [126] spun PLA fibers through the reactive extrusion polymerization of L-lactide (92 wt%) and meso-lactide (8 wt%). In many potential textile technological applications (e.g., for nonwoven materials) the fiber forming process is of general importance. An effective polymer synthesis requires also an effective spinning process to reduce the still high cost of the PLA fibers compared with those of established synthetic fibers. 

The ROP route includes polycondensation of lactic acid followed by a depolymerization into the dehydrated cyclic dimer, lactide [shown in Fig. 23.3). The depolymerization is conventionally done by increasing the polycondensation temperature and lowering the pressure and distilling off the produced lactide. Due to the two stereoforms of lactic acid, the corresponding optically active lactide can be found in two different versions. In addition, lactide can be formed from one D- and one L-lactic acid molecule yielding D,L-lactide[meso-lactide) [7]. 

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