Polymerization of Lactic Acid

The mechanism for the polymerization of lactide can be cationic, anionic, coordination or free radical polymerization (Auras, Rafael) as discussed below  

A degradable high molecular weight polydactic acid) can be produced by the following mechanism (1) Having a terminal end group of one of the 

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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). 

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. 

Bipyridine-centered triblock copolymers of the type BA-bpy-AB were prepared by a combination of ATRP and ROMP [159]. 4,4 -Bis(hydroxymelhyl)-2,2/-bipyridine was employed for the polymerization of lactic acid, LA or CL in the presence of Sn(Oct)2 in bulk at 130 and 110°C, respectively. The hydroxyl end groups were converted to tertiary or secondary bromo esters by reaction with 2-bromoisobutyryl bromide or 2-bromopropionyl bromide. The reaction yields were very high (> 80%) but not quantitative. These products were used as macroinitiators for the ATRP of MMA or tBuA in the presence of CuBr/HMTETA. 4,4/-bis(Chloromethyl)-2,2 -bipyridine was employed to promote the ATRP of MMA or styrene followed by the addition... 

Polylactide is a degradable polyester, formed by the ring-opening polymerization of lactide or the condensation polymerization of lactic acid. Lactide is produced from lactic acid, which derives from the fermentation of D-glucose, which is usually harvested from high-starch-content crops, such as com or sugar beet (Fig. 1). 

PLA was first produced by Pelouze (2), in 1845, through condensation polymerization of lactic acid. Although PLA was first synthesized more than 160 years ago, its commercial feasibility and usefulness has not been prominent until recently, when an economical way of manufacturing the monomer for PLA was discovered and commercialized (3). 

The second method is azeotropic condensation polymerization of lactic acid, which produces high-molecular weight PLA without using chain-extenders or esterification-promoting adjuvants. This type of polymerization needs high reaction rates and thus uses catalysts however, due to the use of catalysts, the PLA produced by this method is not suitable for some applications, such as medical, since any residual catalyst offers toxicity within the polymer, which is harmful for medical applications. In addition to toxicity, residual catalyst degrades PLA in further processing (39). On the other hand, the level of residual catalyst can be reduced with the use of sulphuric acid (55,56). 

Ajioka, K. Enomoto, K. Suzuki, and A. Yamaguchi, The Basic Properties of Poly Lactic Acid Produced by The Direct Condensation Polymerization of Lactic Acid, Journal of Environmental Polymer Degradation, Vol. 3 (8), p. 225-234,1995. 

Ajioka, M., Enomoto, K., Suzuki, K., Yamaguchi, A. The basic properties of polyQactic acid) produced by the direct condensation polymerization of lactic acid. I. Environ. Polvm. Dearad.. 1995, 3, 225-234. 

Figure 12.20 Ring opening polymerization of lactic acid dimer yielding polylactide (reproduced from Wikipedia Polylactic Acid).

Ajioka M, Inomoto K, Susuld K, Yamaguchi A (1995) Basic properties of polylactic add produced by the direct condensation polymerization of lactic acid. Bull Chem Soc Jpn 68 2125-2131... 

Synthesis PL A is a thermoplastic aliphatic polyester which is formed by condensation polymerization of lactic acid, as mentioned in the preceding. Lactic acid is isolated from tapioca, corn and other plant root starches, sugarcanes, or other resources. Bacterial fermentation is normally used to produce lactic acid... 

The polymerization of lactic acid to lactide or high-molecular-weight lactic acid-based polymers can be conducted in several ways ... 

SCHEME 25.1 Poly(lactic add) formed by condensation polymerization of lactic acids. 

The fourth step in the manufacturing process is the polymerization of lactic acid to polylactide, or PLA. PLA can be polymerized via direct polycondensation reaction through azeotropic dehydration. High molecular weight polymers are difficult to synthesize due to the equilibrium between the free acids, water, and the polymer. Dean-Stark trap can be used to remove excess water during the reaction. m-Xylene can be added to lactic acid that added together in a flask at 138 C for 30 hours. After water is removed, the Dean-Stark trap can be replaced with a molecular sieve to recycle the azeotropic mixture. The resultant mixture can be polymerized to PLA at 138°C (Kim and Woo 2002). 

Due to abundantly available feedstock and low cost, poly lactic acid (PLA) is one of the most promising bio-based polymers. PLA is obtained by the controlled polymerization of lactic acid monomers which in turn are obtained from renewable resources such as sugar feedstock, wheat, maize, com, and waste products from food or agriculture industry by fermentation (Siracusa et al., 2008). Properties of PLA vary according to the Z - to - D lactylenantiomeric ratio. Table 8 lists some important properties of PLA. 

Many drawbacks have arisen during the direct hydrogenation of lactic acid to PPG, for instance, catalyst deactivation due to the polymerization of lactic acid and the formation of undesirable propionic acid. In addition, hydrogenation is known to have a lower productivity. 

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