Lactides

C, b.p. 150 C/25mm. Prepared from l-lactic acid. It is partially converted to lactic acid by water. o-Lactide is similar. DL-Lactide crystallizes in colourless needles, m.p. 124-5 "C, b.p. l42°C/8mm. Obtained from DL-lactic acid. 

Lactic acid tends to pass into the lactide I [ when heated in... 

Lactides, intermolecular cyclic esters, are named as heterocycles. Lactams and lactims, containing a —CO—NH— and —C(OH)=N— group, respectively, are named as heterocycles, but they may also be named with -lactam or -lactim in place of -olide. For example. 

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). 

Polymer—polymer iacompatibiHty encapsulation processes can be carried out ia aqueous or nonaqueous media, but thus far have primarily been carried out ia organic media. Core materials encapsulated tend to be polar soHds with a finite degree of water solubiHty. EthylceUulose historically has been the sheU material used. Biodegradable sheU materials such as poly(D,L-lactide) and lactide—glycoHde copolymers have received much attention. In these latter cases, the object has been to produce biodegradable capsules that carry proteias or polypeptides. Such capsules tend to be below 100 p.m ia diameter and are for oral or parenteral administration (9). 

Several parenteral microencapsulated products have been commercialized the cote materials ate polypeptides with hormonal activity. Poly(lactide— glycohde) copolymers ate the sheU materials used. The capsules ate produced by solvent evaporation, polymer-polymer phase separation, or spray-dry encapsulation processes. They release their cote material over a 30 day period in vivo, although not at a constant rate. 

Noncrystalline aromatic polycarbonates (qv) and polyesters (polyarylates) and alloys of polycarbonate with other thermoplastics are considered elsewhere, as are aHphatic polyesters derived from natural or biological sources such as poly(3-hydroxybutyrate), poly(glycoHde), or poly(lactide) these, too, are separately covered (see Polymers, environmentally degradable Sutures). Thermoplastic elastomers derived from poly(ester—ether) block copolymers such as PBT/PTMEG-T [82662-36-0] and known by commercial names such as Hytrel and Riteflex are included here in the section on poly(butylene terephthalate). Specific polymers are dealt with largely in order of volume, which puts PET first by virtue of its enormous market volume in bottie resin. 

Polylactic acid, also known as polylactide, is prepared from the cycHc diester of lactic acid (lactide) by ring-opening addition polymerization, as shown below ... 

The actual time required for poly-L-lactide implants to be completely absorbed is relatively long, and depends on polymer purity, processing conditions, implant site, and physical dimensions of the implant. For instance, 50—90 mg samples of radiolabeled poly-DL-lactide implanted in the abdominal walls of rats had an absorption time of 1.5 years with metaboHsm resulting primarily from respiratory excretion (24). In contrast, pure poly-L-lactide bone plates attached to sheep femora showed mechanical deterioration, but Httie evidence of significant mass loss even after four years (25). 

Poly(lactide-coglycolide). Mixtures of lactide and glycolide monomers have been copolymerised in an effort to extend the range of polymer properties and rates of in vivo absorption. Poly(lactide- (9-glycolide) polymers undergo a simple hydrolysis degradation mechanism, which is sensitive to both pH and the presence of ensymes (32). 

Similar to pure polyglycoHc acid and pure polylactic acid, the 90 10 glycolide lactide copolymer is also weakened by gamma irradiation. The normal in vivo absorption time of about 70 days for fibrous material can be decreased to less than about 28 days by simple exposure to gamma radiation in excess of 50 kGy (5 Mrads) (35). 

The crystallinity of poly(lactide- (9-glycoHde) samples has been studied (36). These copolymers are amorphous between the compositional range of 25—70 mol % glycoHde. Pure polyglycoHde was found to be about 50% crystalline whereas pure poly-L-lactide was about 37% crystalline. An amorphous poly(L-lactide-i (9-glycoHde) copolymer is used in surgical cHps and staples (37). The preferred composition chosen for manufacture of cHps and staples is the 70/30 L-lactide/glycoHde copolymer. 

Copolymers of S-caprolactone and L-lactide are elastomeric when prepared from 25% S-caprolactone and 75% L-lactide, and rigid when prepared from 10% S-caprolactone and 90% L-lactide (47). Blends of poly-DL-lactide and polycaprolactone polymers are another way to achieve unique elastomeric properties. Copolymers of S-caprolactone and glycoHde have been evaluated in fiber form as potential absorbable sutures. Strong, flexible monofilaments have been produced which maintain 11—37% of initial tensile strength after two weeks in vivo (48). 

Braided Synthetic Absorbable Sutures. Suture manufacturers have searched for many years to find a synthetic alternative to surgical gut. The first successful attempt to make a synthetic absorbable suture was the invention of polylactic acid [26023-30-3] suture (15). The polymer was made by the ring-opening polymerization of L-lactide [95-96-5] (1), the cycUc dimer of L-lactic acid. 

The lac resin is associated with two lac dyes, lac wax and an odiferous substance, and these materials may be present to a variable extent in shellac. The resin itself appears to be a polycondensate of aldehydic and hydroxy acids either as lactides or inter-esters. The resin constituents can be placed into two groups, an ether-soluble fraction (25% of the total) with an acid value of 100 and molecular weight of about 550, and an insoluble fraction with an acid value of 55 and a molecular weight of about 2000. 

Throughout the 1990s a large portion of the research and development effort for hot melt adhesives focused on developing adhesives that are either environmentally friendly or functional [69,81,82]. Environmentally friendly attributes include biodegradability, water dispersibility (repulpability), renewability, and water releasability. Biodegradable adhesives have been developed based on starch esters [83-86] and polyesters such as poly (hydroxy butyrate/hydroxy valerate) [87], poly(lactide) [88-91], and poly(hydroxy ether esters) [92-94]. All but the... 

Lactic acid and levulinic acid are two key intermediates prepared from carbohydrates [7]. Lipinsky [7] compared the properties of the lactide copolymers [130] obtained from lactic acid with those of polystyrene and polyvinyl chloride (see Scheme 4 and Table 5) and showed that the lactide polymer can effectively replace the synthetics if the cost of production of lactic acid is made viable. Poly(lactic acid) and poly(l-lactide) have been shown to be good candidates for biodegradeable biomaterials. Tsuji [131] and Kaspercejk [132] have recently reported studies concerning their microstructure and morphology. 

Table 5 Comparison of Properties of Lactide Polymers with Polystyrene and Poly(vinyl chloride) ...

Polymer/Property Polystyrene 95/5 Lactide/Caprolactone 95/15 Lactide/Caprolactone Flexible PVC... 

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