Lactide depolymerization

Polylactic acid (PLA) has been produced for many years as a high-value material for use in medical applications such as dissolvable stitches and controlled release devices, because of the high production costs. The very low toxicity and biodegradability within the body made PLA the polymer of choice for such applications. In theory PLA should be relatively simple to produce by simple condensation polymerization of lactic acid. Unfortunately, in practice, a competing depolymerization process takes place to produce the cyclic lactide (Scheme 6.10). As the degree of polymerization increases the rate slows down until the rates of depolymerization and polymerization are the same. This equilibrium is achieved before commercially useful molecular weights of PLA have been formed. 

Cyclic oligomers of condensation polymers such as polycarbonates and polyesters have been known for quite some time. Early work by Carothers in the 1930s showed that preparation of aliphatic cyclic oligomers was possible via distillative depolymerization [1, 2], However, little interest in the all-aliphatics was generated, due to the low glass transition temperatures of these materials. Other small-ring, all-aliphatic cyclic ester systems, such as caprolactone, lactide... 

Figure 9.2. Lactide ring formation by depolymerization of low-molecular weight PLA. Reprinted with permission from J. Lunt, Polymer Degradation and Stability, Vol. 59, p. 145,1998, 1998, Elsevier Science Ltd.

The oligomer is then depolymerized to lactide, a cyclic dimer. 

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]. 

Tsuji, H., Fukui, I., Daimon, H. tmd Fujie, K. (2(X)3) Poly(L-lactide) XI. Lactide formation by theimal depolymerization of poly(L-lactide) in a closed system. Polymer Degradation and Stability, 81, 501-509. 

Noda, M. and Okuyama, H. (1999) Thermal catalytic depolymerization of poly(L-lactic acid) oligomer into LL-lactide Effects of Al, H, Zn and Zr compounds as cattilysts. Chemical Pharmaceutical Bulletin, 47, 467- 71. 

Meso-lactide can be obtained from racemic PLA by depolymerization at 180-200°C under reduced pressure and distilling off the meso-lactide (11). Also, lactic acid can be used directly in the presence of catalyst such as tin octanoate, and lithium carbonate (12). 

Each of the aforementioned lactides is usually synthesized by depolymerization of the corresponding oligo(lactic acid) (OLLA) obtained by... 

FIGURE 1.7 Schematic illustration of lactide manufacture by thermal catalytic depolymerization of lactic acid oligomers. 

Some metal cations such as sodium and potassium in the feed increase racemization risk, while other metals (Al, Fe) are catalytically active in transesterification, resulting in competitive polylactide formation [68,69]. Through corrosion, metals may be released in the residue and will build up there [6, 75]. Some patents discuss the presence of acid impurities in the process [6, 7, 67, 78], Mono- and dicarboxylic fermentation acids are responsible for stoichiometric imbalance in the lactic acid polycondensation reaction. Consequently, the composition of the obtained lactic acid oligomer chains can differ from pure PLA, resulting in impeded and incomplete catalytic depolymerization of the oligomers into lactide. In PLA manufacture, degradation reactions play a role, mainly via intramolecular chain scission, and this may also affect lactide synthesis. 

On the one hand, it can be concluded that the lactide synthesis is straightforward in the sense of making a prepolymer and releasing lactide by thermal catalytic depolymerization at low pressure. On the other hand, it can be concluded that the scale-up from a lab-scale process to an economical, large-scale process with high yield and no compromises on stereochemical purity is a complex multifaceted task. 

O Brien has shown that the formation of dark color of lactide was a direct function of the iron content of the material in which the lactide was in contact [86]. Other examples in the patent (Examples 7 and 8) demonstrate the desirability of having low alkali (e.g., sodium) content and minimizing the depolymerization temperature. 

The lactide manufacturing is done by depolymerization of PLA that preferably is in the range of 400-2500 g/... 

Polylactide (PLA) is prepared by the ring-opening polymerization of lactides, such as L,L-lactide, D,D-lactide, and meso-lactide, that are cyclic dimers of lactic acid, with poly (L-lactide) (PLLA), an especially well-known crystalline polymer, being prepared from L,L-lactide [1-3]. This ringopening polymerization is an equilibrium reaction in which the concentration of cyclic monomer is temperature dependent [4]. Therefore, the lactides are regenerated through the thermal depolymerization of PLA. 

MgO even in the lower temperature range. This characteristic antiracemization effect of MgO is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270°C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred causing unzipping depolymerization, resulting in selective L,L-lactide production. 

As mentioned above, Al(OH)3 and MgO are effective degradation catalysts that selectively depolymerized PLLA into L,L-lactide. For MgO in particular, only a small amount (<5 wt%) is required for the depolymerization [55], although the basicity of MgO is poor when compared to other alkaline earth oxides, as shown in the following order BaO > SrO > CaO>MgO [56]. 

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