Direct Polycondensation of Lactic acid

Direct polycondensation of lactic acid is usually performed in bulk by distillation of condensation water with or without a catalyst, while vacuum and temperature are progressively increased. The polymer obtained has a low molecular weight, because it is hard to remove water completely from the highly viscous reaction mixture therefore a polymer of a molecular weight of a few ten thousands is obtained. The polymer of low molecular weight is the main disadvantage of 

The polycondensation system of LA involves two reaction equilibria dehydration equilibrium for esterification and ring-chain equilibrium involving the depolymerization of PLA into lactide (Eq. 1 and 2)  

The polycondensation with OLLA was examined with various catalysts metallic and nonmetallic, organic and inorganic, and homogeneous and heterogeneous. Typical results are shown in Table 3.1. This screening test revealed that tin oxide 

Run Catalyst Catalyst/OLLA (wt%) Temperature ( ) Time (h) Pressure (ton-) 34 Yeild (%) 

Ren et al. [10-11] presented a direct condensation process for the production of high-molecular-weight PLLA. It constitutes two steps pre-polymerization in melt state and chain extending reactions. First, the lactic acid monomer was oligomerized to low molecular weight hydroxyl-terminated prepolymer the molecular weight was then increased by chain extension. The results showed that the obtained polymer had 1.0 - 1.5 dL/g. 

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There are two important methods for PLA synthesis direct polycondensation of lactic acid and ROP of lactic acid cyclic dimer, known as lactide. In direct condensation, solvent is used and higher reaction times are required. The resulting polymer is a material of low to intermediate molecular weight. ROP of the lactide needs catalyst but results in PLA with controlled molecular weight. Depending on the monomer and reaction conditions, it is possible to control the ratio and sequence of d- and L-lactic add units in the final polymer [74,75],... 

Over the past several decades, polylactide - i.e. poly(lactic acid) (PLA) - and its copolymers have attracted significant attention in environmental, biomedical, and pharmaceutical applications as well as alternatives to petro-based polymers [1-18], Plant-derived carbohydrates such as glucose, which is derived from corn, are most frequently used as raw materials of PLA. Among their applications as alternatives to petro-based polymers, packaging applications are the primary ones. Poly(lactic acid)s can be synthesized either by direct polycondensation of lactic acid (lUPAC name 2-hydroxypropanoic acid) or by ring-opening polymerization (ROP) of lactide (LA) (lUPAC name 3,6-dimethyl-l,4-dioxane-2,5-dione). Lactic acid is optically active and has two enantiomeric forms, that is, L- and D- (S- and R-). Lactide is a cyclic dimer of lactic acid that has three possible stereoisomers (i) L-lactide (LLA), which is composed of two L-lactic acids, (ii) D-lactide (DLA), which is composed of two D-lactic acids, and (iii) meso-lactide (MLA), which is composed of an L-lactic acid and a D-lactic acid. Due to the two enantiomeric forms of lactic acids, their homopolymers are stereoisomeric and their crystallizability, physical properties, and processability depend on their tacticity, optical purity, and molecular weight the latter two are dominant factors. 

The synthesis of high-molecular-weight PLA is generally carried out by the ring-opening polymerization of lactide or by direct polycondensation of lactic acid. Tin-based catalysts are typically used in both cases, either alone or in combination with p-toluenesulfonic acid. ° ... 

The polymers are synthesised via two routes direct polycondensation of lactic and glycolic acids or ring-opening polymerisation of the cyclic lactide and glycolide dimers.The nomenclature for polymers prepared by different routes is full of contradictions, but polymers prepared from lactic acid by polycondensation are strictly referred to by the acid, as in poly(lactic acid), and those prepared by ring-opening polymerisation by the dimer, as in polylactide (PLA). ... 

The biodegradable polymer available in the market today in largest amounts is PEA. PEA is a melt-processible thermoplastic polymer based completely on renewable resources. The manufacture of PEA includes one fermentation step followed by several chemical transformations. The typical annually renewable raw material source is com starch, which is broken down to unrefined dextrose. This sugar is then subjected to a fermentative transformation to lactic acid (LA). Direct polycondensation of LA is possible, but usually LA is first chemically converted to lactide, a cyclic dimer of LA, via a PLA prepolymer. Finally, after purification, lactide is subjected to a ring-opening polymerization to yield PLA [13-17]. 

Azeotropic dehydration and condensation polymerization (route 2 in Figure 8.2) yields directly high molar mass polymers. The procedure, patented by Mitsui Toatsu Chemicals [13, 14], consists of the removal of condensation water via a reduced pressure distillation of lactic acid for 2-3h at 130°C. The catalyst (in high amounts) and diphenyl ester are added and the mixture is heated up to reflux for 30-40 h at 103°C. Polycondensated PLA is purified to reduce residual catalyst content to the ppm range [5,10,15]. 

A process of subjecting a mixture of lactic acid, an aliphatic dihydric alcohol and an aliphatic dibasic acid directly to dehydration polycondensation (9)... 

No one had believed that direct polycondensation of L-lactic acid can give a high molecular weight of PLLA, until 1995 when Mitsui Chemical Co. first succeeded in synthesizing such a PLLA by using a special solution polycondensation technique. With this epoch-making success, many researchers including the present authors made efforts to establish the direct polycondensation of L-lactic acid. As shown in Scheme 1.22, two equilibria exist in the dehydrative polycondensation of L-lactic acid one is hydration/... 

The direct polycondensation technique can be applied to the syntheses of various copolymers that should have different properties and degradability. For example, copolymers of L-lactic acid and phenyl-substituted a-hydro)q acids, such as l- and o-mandelic acids (l-, d-MA) and phenyl-lactic acid (Phe-LA) have been prepared. Polyglactin is also obtainable by the direct co-polycondensation of glycolic acid and L-lactic acid. ... 

PLA can be produced by condensation polymerization directly from its basic building block lactic acid, which is derived by fermentation of sugars from carbohydrate sources such as com, sugarcane, or tapioca, as will be discussed later in this chapter. Most commercial routes, however, utilize the more efficient conversion of lactide—the cyclic dimer of lactic acid— to PLA via ring-opening polymerization (ROP) catalyzed by a Sn(ll)-based catalyst rather than polycondensation [2-6]. Both polymerization concepts rely on highly concentrated polymer-grade lactic acid of excellent quality... 

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). 

Ohara and co-workers reported that the CALB-catalysed polycondensation of alkyl esters of lactic acid as the monomer produces oligoLAs (X = alkyl, n=2-7 in Scheme 12.1) [13].The reaction is perfectly enantioselective only the alkyl D-lactate monomer produced the oligomers. These results provide the first direct evidence that in the lipase-catalysed reaction mechanism the enantioselection is governed by the deacylation step of lipase . 

Poly(lactic acid) (PLA) is produced from the monomer of lactic acid (LA). PLA can be produced by two well-known processes — the direct polycondensation (DP) route and the ring-opening polymerization (ROP) route. Although DP is simpler than ROP for the production of PLA, ROP can produce a low-molecular-weight brittle form of PLA. Generally, several substances are involved in the production of PLA, and these relationships have been summarized in Figure 2.1. The lactic acid for the process is obtained from the fermentation of sugar. Lactic acid is converted to lactide and eventually to PLA. It should be noted that there are two different terms, poly(lactic acid) and polylactide , for the polymer of lactic acid. Both terms are used... 

PLA can be obtained in two ways through direct polycondensation of the hydroxy acid or by ROP of cyclic lactide monomer. The different reaction pathways to PLA are depicted in Scheme 6.3. Different nomenclatures of polymers obtained by the different routes are often observed in the literature those obtained from lactic acid by direct polycondensation are referred to as poly (lactic acid), while those obtained from lactide monomer by ROP are referred to as poly (lactide). The general abbreviation used in both cases is PLA. 

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