Biodegradable polyesters reactions

The concept of transesterifications was used for polymerization reactions by Hedrick and colleagues [76]. Various biodegradable polyesters were synthesized with the l,3-dimethylimidazol-2-ylidene carbene in THF at 25 °C. Polymers such as poly(e-caprolactone) were obtained with no need of organometallic catalysts, as in classical methods. Poly(ethylene terephthalate) (PET) 97 was synthesized in the ionic liquid 98, which functions as the reaction medium and, at the same time, as a precatalyst that is activated (99) with KOt-Bu. Dimethyl terephthalate (DMT) 100 was condensed with an excess of ethylene glycol 101 to generate 102. The melt condensation of 102 was performed under vacuum using a heating ramp to 280 °C. 

So far, biotransformations of various combinations of dicarboxylic acid derivatives and glycols to biodegradable polyesters have been reported. Dicarboxylic acids as well as its derivatives, activated and nonactivated esters, cyclic acid anhydride, and polyanhydrides, were found to be employed as useful monomers for the enzymatic synthesis of polyesters under mild reaction conditions. 

Using special reactions, it has also been shown that, a degradable polymer can be produced as well i.e., by transesterification, new biodegradable polyesters can be synthesised from polybutylene adipate-co-succinate and polyethylene terephthalate [21]). 

To produce a terephtalate adipate copolyester, dimethyl terephthalate, adipic acid, 1,4-butanediol, and glycerol are mixed together with tetrabutyl orthotitanate as a catalyst (44). The reaction mixture is then heated to a temperature of 180°C for 6 h and at 240°C. The excess dihydroxy compound can be removed by distillation in vacuo. Then hexamethylene diisocyanate is slowly added to the mixture. The diisocyanate act as chain extenders. From this material, a biodegradable polyester film can be produced. In addition, the s mtheses of related copolyesters have been described in detail (45). It has been found that the addition of chalk can achieve an additional improvement in the biodegradability. 

Still another starch-polyester graft copolymer and a chemically modified starch-polyester graft copolymer composition has been described. The starch-polyester graft copol5mers are synthesized by the reaction of starch with biodegradable polyester polymers in the presence of maleic acid and glycerol as plasticizer. Examples of biodegradable polyesters are summarized in Table 7.6. 

Bikiaris, D.N. and Achillas, D.S. (2006) Synthesis of poly(alkylene succinate) biodegradable polyesters. Mathematical modelling of the esterification reaction. Polymer, 47, 4851 -4860. 

Various combinations of dicarboxylic acid and their activated derivatives with glycols have been reacted enzymatically to generate biodegradable polyesters under mild reaction conditions (Table 12.2), in accordance with the following general scheme of dehydration polycondensation mode (Scheme 12.3) [7]. 

The enzymatic synthesis of biodegradable polyesters from activated diesters was achieved under nuld reaction conditions. The polymerization of bis(2,2,2-trichloroethyl) glutarate and 1,4-butanediol proceeded in the presence of porcine pancreas hpase (PPL) at room temperature in diethyl ether to produce polyesters with a molecular weight of 8.2 x 10 [52]. The polycon-... 

PBS is one of the most important biodegradable polyesters synthesized by polycondensation between succinic acid and butanediol. The reaction proceeds in two steps. First esterification occurs between the diacid and the diol, and then polycondensation takes place under high temperature to form high-molecular-weight PBS (Fig. 7.6). 

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