Diol-Diacid Aliphatic Polyesters

Aliphatic polyesters derived from the polycondensation of diols with aliphatic dicarboxylic acids are another important class of synthetic biodegradable polymers. Since 1994, Showa Highpolymer in Japan has been producing a family of aliphatic polyesters known as Bionolle , obtained from butanediol and succinic acid to give polybutylene succinate (PBS), and the copolymer polybutylene succinate adipate (PBSA) obtained from butanediol, succinic acid and adipic acid. Polyethylene succinate (PES) was also produced. 

Property PBSU 1000 PBSU Co. 2000 PBSU Co. 3000 LDPE HDPE 

HDPE High-density polyethylene LDPE Low-density polyethylene MER Melt flow rate PBSU Polybutylene succinate Reproduced with permission from T. Fujimaki, Polymer Degradation and Stability, 1998, 59, 209. 1998, Elsevier Science [53]  

PBS is a highly crystalline polymer (melting point around 114 °C) and its properties are suitable for thermally stable injection moulded and thermoformed products. Its main use today is in Japan, as a component of mulch films and compost bags with a market volume of approximately 2,000 tonnes/year. 

Since nonbiodegradable aromatic polyesters like PET provide excellent material properties [55], with respect to easily degradable aliphatic polyesters, a number of aliphatic/aromatic copolyesters were studied and developed in order to produce materials which combined good mechanical properties and biodegradability. Major polyester producers in Europe and the USA brought aliphatic/aromatic copolyesters for biodegradable applications to the market. 

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Completely aliphatic polyesters, made from aliphatic diacid and aliphatic diol components), are not of major industrial importance because of their low melting temperatures and poor hydrolytic stability. (Low-molecular-weight aliphatic polyesters are used as plasticizers and prepolymer reactants in the synthesis of polyurethanes see Secs. 2-12e, 2-13c-2). 

The next approach to incorporate the 12F-diol into a polyurethane matrix was reaction of the m- 12F-diol with aliphatic diacid chlorides (where x = 3 or 4) to give low molar mass polyesters (141) ... 

The largest-volume polyether used is obtained from propylene oxide polymerized under basic conditions. Polyester polyols are produced from a number of different materials involving diacids and diols to give the ester linkage. Aliphatic polyesters generally are used for elastomers to impart chain flexibility. 

Aliphatic polyesters may present a crystalline a process, and as a consequence the notations (3 and y are adopted for the glass-rubber and subglass relaxations, respectively. Although fully amorphous polymers cannot be achieved by quenching, it is possible to obtain polyesters with different degrees of crytallinity by copolymerization with a noncrystallizable diol. For example, the polyester of 1,6-hexanediol condensed with adipic acid is about 60% crystalline, while the polyester of this diacid with 2,5-hexanediol is completely amorphous. By varying the l,6-hexanediol/2,5-hexanediol... 

Typical diol-diacid type biodegradable aliphatic polyesters, PBA and PBS, were degraded in a diluted toluene solution by the immobilized lipase CA which exclusively formed the cyclic oligomers. Their composition was partially dependent on the degradation conditions, such as the enzyme concentration and temperature. On the other hand, in a more concentrated condition, the cyclic oligomer was readily repolymerized by the ring-opening polymerization. 

Lipase-catalyzed synthesis of aliphatic polyesters via diacid/diol polycondensation reactions have been explored by us and others at temperatures of 90°C and below (4,5). The substrates used in previous studies are a,a)-linear aliphatic diacid and/or diol monomers with six or more carbons. Herein we report a new method that allowed the first lipase-catalyzed synthesis of high molecular weight PBS fi om practical monomer precursors. 

PGS is a bioresorbable elastomeric polymer and extensively evaluated for various biomedical applications such as soft and hard tissue engineering and controlled drug delivery [8]. In a similar way, a number of aliphatic polyester elastomers for various biomedical applications were prepared from diacid monomers such as citric acid and a-ketoglutaric acid with aliphatic diols and triols using thermal polycondensation reactions [9-12]. 

Aliphatic polyesters and polyesters formed from aromatic diacids and aliphatic diols eg, poly(ethylene terephthalate) and poly(butylene terephthalate) (3) cannot decompose by reversion of the polymerization, because the water, or alcohol, eliminated in the synthesis is no longer available. They initially decompose by /3-C—H transfer reactions via a six-membered transition state ... 

It is of interest to note that from that time nearly all the synthetic pioneering work on polymers has been carried out in industrial laboratories. This started with Carothers (Du Pont) in 1930, who studied the formation of aliphatic polyesters from diols and diacids but abandoned them in favour of aliphatic polyamides (nylons) when the polyesters were not suitable for fibres (wool and silk proteins are polyamides). It is continuing at the present time with the development of high performance aromatic polymers such as Nomex (Du Pont), Kevlar (Du Pont) and PEEK (ICI). 

The synthesis of aliphatic polyesters with high molecular weight, in order to achieve satisfactory mechanical properties, is considered as being one of the most difficult problems to be solved. Till today this can be achieved only by either using techniques such as ringopening polymerization of cyclic monomers (lactones) or with the use of chlorides of acids, which are very expensive and inappropriate for industrial scale use [14,15]. The production of high molecular polyesters using diacids and diols can proceed only by the addition of chain extenders or branched comonomers as is the case of Bionolle [16]. 

For his first experiments in polycondensation, Carothers utilized aliphatic diacids and diols. The poor mechanical properties of the corresponding condensation polymers prevented further investigations on aliphatic polyesters. However, polyesters acquired a renewed interest when relationships between molecular structure and physical—particularly mechanical—properties were precisely established. 

Polyesters are the most important class of synthetic fibers. In general, polyesters are produced by an esterification reaction of a diol and a diacid. Carothers (1930) was the first to try to synthesize a polyester fiber by reacting an aliphatic diacid with a diol. The polymers were not suitable because of their low melting points. However, he was successful in preparing the first synthetic fiber (nylon 66). In 1946, Whinfield and Dickson prepared the first polyester polymer by using terephthalic acid (an aromatic diacid) and ethylene glycol. 

Hie most representative member of this class of polyesters is the low-molar-mass (M 1000-3000) hydroxy-terminated aliphatic poly(2,2/-oxydiethylene adipate) obtained by esterification between adipic acid and diethylene glycol. This oligomer is used as a macromonomer in the synthesis of polyurethane elastomers and flexible foams by reaction with diisocyanates (see Chapter 5). Hydroxy-terminated poly(f -caprolactonc) and copolyesters of various diols or polyols and diacids, such as o-phthalic acid or hydroxy acids, broaden the range of properties and applications of polyester polyols. 

Depending on dieir structure, properties, and syndietic methods, degradable polyesters can be divided into four groups poly(a-esters), poly(fi-esters), poly(lactones), and polyesters of aliphatic diols and diacids. 

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