Dilactide polymerization

The living nature of PCL obtained in the presence of Zn(OAl-(OPri)2)2 has been used to prepare both di- and triblock copolymers of e-caprolactone and lactic acid (42,43). Treatment of the initial living PCL with dilactide afforded a PCL-PLA diblock with M /Mn = 1.12, with each block length determined by the proportions of the reactants, i.e., the ratio of [monomer]/[Zn]. While the living diblock copolymer continued to initiate dilactide polymerization, it failed to initiate e-caprolactone polymerization. To obtain a PCL-PLA-PCL triblock, it was necessary to treat the living PCL-PLA-OAIR2 intermediate with ethylene oxide, then activate the hydroxy-terminated PCL-PLA-(OCH2CH2)nOH with a modified Teyssie catalyst (Fig. 5). 

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). 

Stannous octoate has the advantage of having been used to prepare polymers (Silastic, Capronor) for which substantial toxicological data are now available (6,48). Stannous octoate-initiated polymerization has been used to prepare copolymers of e-caprolactone with other lactones, including diglycolide, dilactide, 6-valerolactone, e-decalactone, and other alkyl-substituted e-caprolactones. Conducting... 

In addition to solvent uses, esters of lactic acid can be used to recover pure lactic acid via hydrolysis, which in-tum is used to make optically active dilactide and subsequently polylactic acid used for drag delivery system.5 This method of recovery for certain lactic acid applications is critical in synthesis of medicinal grade polymer because only optically active polymers with low Tg are useful for drug delivery systems. Lactic acid esters themselves can also be directly converted into polymers, (Figure 1), although the commercial route proceeds via ring-opening polymerization of dilactide. 

Kowalski A, Duda A, Penczek S (2000) Kinetics and mechanism of cyclic esters polymerization initiated with tin(II) octoate. 3. Polymerization of L,L-dilactide. Macromolecules 33 7359-7370... 

The ring-opening polymerization of dilactide (dimeric cyclic ester of lactic acid) allows the preparation of high molecular weight, optically active polyesters of lactic acid. The configuration of the asymmetric carbon atoms of the monomer is retained when the polymerization is initiated with SnCl4 or Et2Zn, for example ... 

Ring-Opening Polymerization of Dilactide with Cationic Initiators in Solution... 

Chisholm, M.H., and Eilerts, N.W., Single Site Metal Alkoxide Catalysts for Ring-Opening Polymerizations. PoIy(dilactide) Synthesis Employing HB93-BuV>z)3 Mg(Oet). 

For the production of polymers or their derivatives, the technology for producing the dilactide (the internal diester) is critically important. The processes include a multistage evaporation followed by polymerization to a low molecular weight prepolymer, which is then catalytically converted to the dilactide. The dilactide is purified in a distillation system by partial condensation and recycling. This diester can be used to synthesize high molecular weight polymers and copolymers. " ... 

Poly(lactic acid) (PLA) is formed by the ring-opening polymerization of dilactide (Fig. 6.1), the dimerization product of lactic acid. As two enantiomeric isomers of lactide exist, PLA is available as the fully crystalline or the fully amorphous form, depending on the relative levels of the optical isomers present in the molecule. By tuning their relative amount it is also possible to mimic the mechanical properties of different tissues. 

Poly(L-lactic acid) (PLLA) is a semicrystalline polymer with a crystallinity of about 37% and a melting temperature around 175 °C it is therefore preferred in applications where high mechanical strength and tonghness are required (e.g. for sutures and orthopedic devices). In contrast, the polymerization of racemic dilactide leads to poly(D,L-lactic acid) (PDLLA), which is an amorphons polymer used in, e.g., craniofacial hxation applications and drng delivery systems. Further combinations have been employed to match stiffness and degradation kinetics, snch as poly(L-lactide-co-D,L-lactide) (PLDL). 

Jedlinski, Z., Walach, W., Kurcok, P., Adamus, G., 1991. Pol3mierization of lactones. 12. Polymerization of L-dilactide and L,D-dilactide in the presence of potassium methoxide. Die Makromolekulare Chemie 192 (9), 2051—2057. 

Table I. Effect of Polymerization Conditions on the Co-Polymerization Parameters (r, T2) of Dilactide and Caprolactone...

Rafler G, Lang J, Johmarm M, Bechthold I (2001), Technological relevant aspects of kinetics and mechanism of ring-opening polymerization of L,L-Dilactide , Macromol. Mater. Eng., 286, 761-768. 

The bulk polymerization of the racemic dilactide may be carried out under vacuum in the presence of a tin or zinc salt. Under these conditions the configuration of the asymmetric carbons is maintained (12). In each growth step two dilactide units are added. The resulting polymer is considered atactic although there are restrictions in the possible stereosequences (13). Poly (DL-lactide) is amorphous by X-ray diffraction measurement and has a glass transition... 

---