Lactic acid monomer

DUactide (5) exists as three stereoisomers, depending on the configurations of the lactic acid monomer used. The enantiomeric forms whereia the methyl groups are cis are formed from two identical lactic acid molecules, D- or L-, whereas the dilactide formed from a racemic mixture of lactic acid is the opticaUy iaactive meso form, with methyl groups trans. The physical properties of the enantiomeric dilactide differ from those of the meso form (6), as do the properties of the polymers and copolymers produced from the respective dilactide (23,24). 

Problem 22.34 Polylactic add (PLA) has received much recent attention because the lactic acid monomer [CH3CH(0H)C00H] from which it is made can be obtained from carbohydrates rather than petroleum. This makes PLA a more environmentally friendly polyester. (A more in-depth discussion of green polymer synthesis is presented in Section 30.8.) Draw the structure of PLA. 

Polylactic add (PLA) A synthetic polymer formed from lactic acid monomers which degrades by hydrolysis. 

Gu et al. reported that a high-molecular-weight biodegradable lactic acid polymer was successfully synthesized in two steps. First, the lactic acid monomer was oligomerized to low-molecular-weight hydroxyl-terminated prepolymer the molecular weight was then increased by chain extension using 1,6-hexamethylene diisocyanate as the chain extender. The results showed that the obtained polymer had an M of 27,500 g-mol" and an Af of 116,900 g mol after 40 min of chain extension at 180°C [13]. 

Song et al. [67] prepared PLA/CNT nanocomposites by one step based on in situ polycondensation of the commercially available lactic acid monomer in the presence of purified CNTs. The TEM image (Fig. 11.5) of core/shell nanostructures clearly indicates that the coating of grafted polymer was uniform both on the CNT s sidewall and tip. It was suggested that the incorporation of CNT into PL A will improvement its solubility and biocompatibility as it may promise a good future in biomedical systems and the development of bio-nanomaterials. The researchers also suggested that the current method of PLA/CNT nanocomposites preparation should be favored in industrialization as it takes less steps and cheaper. 

Definition Synthesized from lactic acid monomer which is mostly produced by carbohydrate fermentatin of corn dextrose Properties Dens. 1.250 kg/m m.p. 130-180 C Environmental Biodeg. 

In contrast, the chemical process could lead to various ratio of l- and o-lactic acid. Indeed, the chemical reactions leading to the formation of the cychc dimer, the lactide, as an intermediate step to the production of PLA, could lead to macromolecular chains with L- and o-lactic acid monomers. This mechanism of ring-opening polymerization (ROP) from the lactide explains the formation of two enantiomers. This ROP route has... 

Belonging to the family of aliphatic polyesters, poly(lactic acid) or polylactide (PLA) is composed of lactic acid repetitive units, which is the simplest ot-hydro)y acid with an asymmetric carbon atom. Interestingly, the L-lactic acid monomer, and more recently the D-lactic acid monomer, can be straightforwardly obtained by bacterial fermentation from renewable resources (namely starch), making both monomers and therefore the resulting polymers environmentally friendly. Polycondensation of lactic acid and ring-opening polymerization (ROP) of lactide (LA), i.e. cyclic diesters of lactic acid, are currently used to prepare PLA polymers (Scheme 4.1). 

The chiral nature of lactic acid results in distinct forms of polylactide, namely, poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and poly(DL-lactide) (PDLLA), which are synthesized from the L-, D-, and DL-lactic acid monomers, respectively, or from the corresponding L,L-lactide, D,D-lactide, and DL-lactide, respectively [15]. 

Lactide (3,6-dimethyl-l,4-dioxane-2,5-dione) is a cyclized dimer of lactic acid (anhydrous lactic acid), and it is used in the manufacture of PLA resin (see Chapters 1 ) for the plastic industry. The compound exists as a translucent white powder or clear flakes, is odorless at room temperature but emits an acrid plastic odor when heated. LA decomposes when exposed to moisture to form the linearized lactic acid dimer followed by hydrolysis to lactic acid monomer [22]. 

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. 

Direct polycondensation in the bulk through the reaction of hydroxy and carboxy functionalities of the lactic acid monomer with the elimination of water molecule has all the drawbacks associated with the character of an equilibrium step-growth polymerization. In addition, employing this polymerization method, the stereoregularity of the polymer cannot be controlled. This polymerization technique involves the use of a catalyst and reduced pressure. " ... 

Polymers produced by classical chemical synthesis from renewable bio-derived monomers A good example is polylactate, a biopolyester polymerized from lactic acid monomers. The monomer itself is produced by fermentation of carbohydrate feedstock. 

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