Chiral aliphatic polyester

Polylactic acid (PLA) is the second common biopolymer that is produced by microbial fermentation. It is thermoplastic aliphatic polyester that can be synthesized from biologically produced lactic acid polymerized by ring opening polymerization. Lactic acid is a chiral molecule existing as two stereoisomers, L- and D-lactic acid, which can be produced by different ways, i.e., biologically or chemically synthesized [Averous, 2008). 

It is an aliphatic polyester derived from renewable resources, such as com starch, tapioca roots, chips or starch, or sugarcane. Polylactic acid or polylactide (PLA) can withstand temperatures up to 110 °C [69]. PLA is soluble in chlorinated solvents, hot benzene, tetrahydrofuran, and dioxane [70]. It can be processed like other thermoplastics into fiber (for example, using conventional melt spinning processes) and film. Due to the chiral nature of lactic acid, several distinct forms of polylactide exist ... 

Poly(lactic acid) (PLA) (Fig. 1.16) is an aliphatic polyester polymerised by lactic acid which is made by fermentation of natural raw materials, for example, com starch and sugarcanes. Due to the chiral nature of lactic acid and its effects on the polymer s characteristics, the biodegradability and mechanical properties of PLA can be tailored by varying the proportion of different forms. Meanwhile, PLA can also copolymerise with other monomers or blend with other polymers to improve some properties of the material, eg, flexibility. PLA and PLA-based copolymers are the most popular biodegradable materials for the production of absorbable sutures (Li, 1999) (Fig. 1.16). 

Poly-R-hydroxyalkanoates, PHAs, are produced as storage granules or inclusion bodies by a wide variety of bacteria under conditions of metabolic stress in the presence of excess carbon. Normally the PHAs produced are a family of aliphatic polyesters with n-alkyl groups at the R-position, which is a chiral center that exists only in the [R] absolute configuration in these biopolymers, as follows ... 

Poly(j8-malic acid) is an aliphatic polyester of the poIy(hydroxyacid)-type which is water soluble regardless of pH. It is now considered as a promising carrier to make polymeric prodrugs because of the presence of a carboxyl pendant group [200-203]. Malic acid is a chiral compound like lactic acid. Various racemic and optically active poly(j -malic acids), PMLA, as well as their sodium salts PMLA Na have been synthesized with different enantiomeric excess, jc being the percentage of L-malic acid units in the main chains [204]. 

A manifold of dendrimers have been presented in the literature ranging from polyamidoamine, polyfpropylene imine), aromatic polyether and polyester, aliphatic polyether and polyester, polyalkane,polyphenylene, polysilane, and phosphorus dendrimers. Combinations of different backbones as well as architectural modifications have also been presented. For example, the incorporation of chirality in dendrimers, copolymers of linear blocks with dendrimer segments (dendrons), and block copolymers of different dendrons has been described. Numerous applications have been proposed for dendrimers such as biotemplates, liquid membranes, catalysts, or in medical applications. ... 

Their general structure is shown in Chapter 5, but chirality is introduced into the main chain structure. Chirality is most conveniently incorporated into the flexible aliphatic units of the main chain. For example, polymer 23 consists of a fairly common polyester type backbone however, in this case two chiral centres are incorporated within the dipropylene glycol unit. 

The molecular structure of polylactic acid (PLA) is schematically presented in Fig. 2.2. PLA, linear aliphatic thermoplastic polyester, is prepared from lactic acid. Lactic acid (2-hydroxy propionic acid) is one of the simplest chiral molecitles and exists as two stereo isomers, L- and D-lactic acid (Fig. 2.3). 

Tg measurements have been performed on many other polymers and copolymers including phenol bark resins [71], PS [72-74], p-nitrobenzene substituted polymethacrylates [75], PC [76], polyimines [77], polyurethanes (PU) [78], Novolac resins [71], polyisoprene, polybutadiene, polychloroprene, nitrile rubber, ethylene-propylene-diene terpolymer and butyl rubber [79], bisphenol-A epoxy diacrylate-trimethylolpropane triacrylate [80], mono and dipolyphosphazenes [81], polyethylene glycol-polylactic acid entrapment polymers [82], polyether nitrile copolymers [83], polyacrylate-polyoxyethylene grafts [84], Novolak type thermosets [71], polyester carbonates [85], polyethylene naphthalene, 2,6, dicarboxylate [86], PET-polyethylene 2,6-naphthalone carboxylate blends [87], a-phenyl substituted aromatic-aliphatic polyamides [88], sodium acrylate-methyl methacrylate multiblock copolymers [89], telechelic sulfonate polyester ionomers [90], aromatic polyamides [91], polyimides [91], 4,4"-bis(4-oxyphenoxy)benzophenone diglycidyl ether - 3,4 epoxycyclohexyl methyl 3,4 epoxy cyclohexane carboxylate blends [92], PET [93], polyhydroxybutyrate [94], polyetherimides [95], macrocyclic aromatic disulfide oligomers [96], acrylics [97], PU urea elastomers [97], glass reinforced epoxy resin composites [98], PVOH [99], polymethyl methacrylate-N-phenyl maleimide, styrene copolymers [100], chiral... 

Poly(lactic acid), PLA, is probably the most investigated and commercially used aliphatic biodegradable polyesters. It can be obtained from lactic acid in several different ways. Lactic acid is a chiral molecule which exists as two stereoisomers naturally occurring L-lactic acid and D-lactic acid (Scheme 6.2). 

Lactic acid is a linear, aliphatic thermoplastic polyester with a rigidity and clarity similar to polystyrene and poly(ethylene terephthalate) (PET Martin and Averous, 2001). It is a hydroxycarboxylic acid having a chiral center on its second carbon. Because of the presence of fxmctional groups in a 3-carbon molecule, a lot of chemical reactivity can be incorporated. The carboxylic acid group is mildly acidic, and the stereochemistry of the second carbon is very important in the PLA chemistry (Garlotta, 2001 Gupta and Kumar, 2007). The major characteristics of lactic acid are presented in Table 14.2. Lactic acid comprises two optical isomers of lactic acid L (+)-lactic acid and D(-)-lactic acid. 

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