Synthesis of polyesters

Failure to remove the alcohols generated in either of the equilibrium condensation steps will reduce the efficiency of the polymerization process. This effect can be explained by Le Chatelier s principle, which was discussed in Chapter 3. The volatile alcohols produced during polymerization act as a chemical stress on the product side of the reaction, which inhibits polymerization. Another implication of the equOibrium nature of this polymerization process is seen in the molecular veight distribution of the final polymer. AH polyesters contain a fe v percent of lo v molecular veight oligomers, regardless of the polymerization process. 

Removing these oligomers from the polymer accomplishes nothing as they will eventually reform from the high polymer. The reformation of the oligomers occurs because the system responds to their absence by creating more, in accord with Le Chatelier s principle. 

In addition to the desired polymerization reaction, the dialcohol reactants can participate in deleterious side reactions. Ethylene glycol, used in the manufacture of polyethylene terephthalate, can react with itself to form a dialcohol ether and water as shown in Fig. 24.4a). This dialcohol ether can incorporate into the growing polymer chain because it contains terminal alcohol units. Unfortunately, this incorporation lowers the crystallinity of the polyester on cooling which alters the polymer s physical properties. 1,4 butanediol, the dialcohol used to manufacture polybutylene terephthalate, can form tetrahydrofuran and water as shown in Fig. 24.4b). Both the tetrahydrofuran and water can be easOy removed from the melt but this reaction reduces the efficiency of the process since reactants are lost. 

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Glycols such as neopentyl glycol, 2,2,4-trimethyl-l,3-pentaiiediol, 1,4-cyclohexanedimethanol, and hydroxypivalyl hydroxypivalate are used in the synthesis of polyesters (qv) and urethane foams (see Foamed plastics). Their physical properties are shown in Table 1 (1 6). 

Prior to 1975, benzene was catalytically oxidized to produce maleic anhydride, an intermediate in synthesis of polyester resins, lubricant additives, and agricultural chemicals. By 1986 all commercial maleic anhydride was derived from oxidation of / -butane. It is expected that / -butane will remain the feedstock of choice for both economic and environmental reasons. 

Many other reactions have been reported for the synthesis of polyesters, such as reactions between dicarboxylic acid salts and dialkyl halides, reactions between... 

Phenolic compounds are weaker nucleophiles and better leaving groups than aliphatic alcohols. They do not yield polyesters when reacted with carboxylic acids or alkyl carboxy lates. The synthesis of polyesters from diphenols is, therefore, generally carried out through the high-temperature carboxylic acid-aryl acetate or phenyl ester-phenol interchange reactions with efficient removal of reaction by-product (Schemes 2.10 and 2.11, respectively). 

The carboxy-hydroxy reaction (direct esterification) is the most straightforward method of polyester synthesis. It was first reported in the 1930s by Carothers10 12 and is still a very widely used method for the synthesis of polyesters from diacids and diols (Scheme 2.12) or from hydroxy acids (Scheme 2.13). Direct... 

Although low-molar-mass aliphatic polyesters and unsaturated polyesters can be synthesized without added catalyst (see Sections 2.4.1.1.1 and 2.4.2.1), the presence of a catalyst is generally required for the preparation of high-molar-mass polyesters. Strong acids are very efficient polyesterification catalysts but also catalyze a number of side reactions at elevated temperature (>160°C), leading to polymers of inferior quality. Acid catalysts are, therefore, not much used. An exception is the bulk synthesis of hyperbranched polyesters reported in Section 2.4.5.1, which is carried out at moderate temperature (140°C) under vacuum in the presence of p-toluene sulfonic acid catalyst. The use of strongly acidic oil-soluble catalysts has also been reported for the low-temperature synthesis of polyester oligomers in water-in-oil emulsions.216... 

Many ester-forming reactions reported in the literature cannot be applied to the synthesis of polyesters due to side reactions, incomplete conversions, or non-quantitative yields. Some examples of nonconventional polyester syntheses are listed below. Most of them lead to oligomers rather titan polymers and require expensive reactants or special reaction conditions, which make them of little practical interest. 

Enzymes are generally classified into six groups. Table 1 shows typical polymers produced with catalysis by respective enzymes. The target macromolecules for the enzymatic polymerization have been polysaccharides, poly(amino acid)s, polyesters, polycarbonates, phenolic polymers, poly(aniline)s, vinyl polymers, etc. In the standpoint of potential industrial applications, this chapter deals with recent topics on enzymatic synthesis of polyesters and phenolic polymers by using enzymes as catalyst. 

The enzymatic synthesis of polyesters from activated diesters was achieved under mild reaction conditions. The polymerization of bis(2,2,2-trichloroethyl) glutarate and 1,4-butanediol proceeded in the presence of PPL at room temperature in diethyl ether to produce the polyesters with molecular weight of 8.2 x 10. Vacuum was applied to shift the equilibrium forward by removal of the activated alcohol formed, leading to the production of high molecular weight polyesters. The polycondensation of bis(2,2,2-trifluoroethyl) sebacate and aliphatic diols took place using lipases BC, CR, MM, and PPL as catalyst in diphenyl ether. Under the... 

Lipase-catalyzed synthesis of polyesters from cyclic anhydrides and oxi-ranes was reported. The polymerization took place by PPL catalyst and the molecular weight reached 1 x 10" under the selected reaction conditions. During the polymerizahon, the enzymatically formed acid group from the anhydride may open the oxirane ring to give a glycol, which is then reacted with the anhydride or acid by lipase catalysis, yielding the polyesters. 

In vitro synthesis of polyesters using isolated enzymes as catalyst via non-biosynthetic pathways is reviewed. In most cases, lipase was used as catalyst and various monomer combinations, typically oxyacids or their esters, dicarboxylic acids or their derivatives/glycols, and lactones, afforded the polyesters. The enzymatic polymerization often proceeded under mild reaction conditions in comparison with chemical processes. By utilizing characteristic properties of lipases, regio- and enantioselective polymerizations proceeded to give functional polymers, most of which are difficult to synthesize by conventional methodologies. 

Various combinations of dicarboxylic acid derivatives and glycols enzymatically afforded polyesters under mild reaction conditions. Dicarboxylic acids as well as derivatives, activated and non-activated esters, cyclic acid anhydride, and polyanhydrides, were found to be useful as monomer for the enzymatic synthesis of polyesters. 

Some proteases show an esterase activity, especially in their catalytic activity for regioselective acylation of sugars. By utilizing this property, enzymatic synthesis of polyester containing sugar group in the backbone was demonstrated... 

Fig. 12. Enzymatic synthesis of polyester macromonomer and telechelics by terminator method...

Matsumura S. Enzymatic Synthesis of Polyesters via Ring-Opening Polymerization. Vol. 194, pp. 95-132. 

Figure 5.1 Reaction mechanism for the synthesis of polyesters (e.g. poly (ethylene terephthalate)) via molten (melt)-state polycondensation... 

This section describes the synthesis of oxazolidine esters used as polymer hardeners that cannot be synthesized using chemical catalysis, the synthesis of polyurethane polymers with methods that avoid the use of isocyanates and the enzymatic synthesis of polyesters with low molecular weight dispersity. 

Binns, F., Harffey, P., Roberts, S.M. and Taylor, A., Studies leading to the large scale synthesis of polyesters using enzymes. J. Chem. Soc. Perkin Trans. 1999, 2671. 

Although some organometallic compounds can allow for the synthesis of polyesters of a high molecular weight, control of the ROP process usually remains a problem. As an example, in the case of Lewis acid catalyst, molecular weights are difficult to predict and the molecular weight distribution is broad. is... 

Synthesis of Polyester Resins Described in Table II. The monomers and 3 wt % of xylene were placed in a 1-L breakaway reaction flask equipped with a thermometer, a Liebig condenser, a mechanical stirrer, an N line and a heating mantle. The mixture was slowly heated under 210 C, with stirring being started as soon as... 

The DuPont research team turned from the synthesis of polyesters to tackle, more successfully, the synthesis of the first synthetic fiber material, nylon, which approached, and in some cases exceeded, the physical properties of natural analogs (Section 4.7). The initial experience with polyesters was put to good use in the nylon venture. 

Since quenching the reaction or adding a stoichiometric excess of one reactant is seldom economical, the commercial practice is to add a specific amount of a monofunctional reactant in the synthesis of polyesters, nylons, and other similar polymers. In these cases, a functionality factor / is used that is equal to the average number of functional groups present per reactive molecule. While the value of / in the preceding examples has been 2.0, it may be reduced to lower values and used in the following modified Carothers equation ... 

Varma IK, Albertsson A-C, Rajkhowa R, Srivistava RK (2005) Enzyme catalyzed synthesis of polyesters. Prog Polym Sci 30 949-981... 

Pentanediol (PDO) holds promise for being used in the synthesis of polyesters. It has been synthesized from GVL in the presence of a copper chromite catalyst. At 150 °C and 20.3-30.4 MPa hydrogen pressure, 78.5% PDO was produced together with 8.1% 1-pentanol. ... 

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