Synthesis of polyesters is generally achieved by a polycondensation reaction. The general equation for the reaction of a diol with a diacid is: :(n+1) R(OH)2 + n R'(COOH)2 → HO[ROOCR'COO]nROH + 2n H2O. Polyesters can be obtained by a wide range of reactions of which the most important are the reaction of acids and alcohols, alcoholysis and or acidolysis of low-molecular weight esters or the alcoholysis of acyl chlorides. The following figure gives an overview over such typical polycondensation reactions for polyester production. Furthermore, polyesters are accessible via ring-opening polymerization. : Azeotrope esterification is a classical method for condensation. The water formed by the reaction of
alcohol and a
carboxylic acid is continually removed by
azeotropic distillation. When melting points of the monomers are sufficiently low, a polyester can be formed via direct esterification while removing the reaction water via vacuum. : Direct bulk polyesterification at high temperatures (150 – 290 °C) is well-suited and used on the industrial scale for the production of aliphatic, unsaturated, and aromatic–aliphatic polyesters. Monomers containing
phenolic or
tertiary hydroxyl groups exhibit a low reactivity with
carboxylic acids and cannot be polymerized via direct acid alcohol-based polyesterification. The high-temperature melt synthesis between bisphenol diacetates and aromatic dicarboxylic acids or in reverse between bisphenols and aromatic dicarboxylic acid diphenyl esters (carried out at 220 to 320 °C upon the release of acetic acid) is, besides the acyl chloride based synthesis, the preferred route to wholly aromatic polyesters. The reaction is carried out at lower temperatures than the equilibrium methods; possible types are the high-temperature solution condensation, amine catalysed and interfacial reactions. In addition, the use of activating agents is counted as non-equilibrium method. The equilibrium constants for the acyl chloride-based condensation yielding arylates and polyarylates are very high indeed and are reported to be 4.3 × 103 and 4.7 × 103, respectively. This reaction is thus often referred to as a 'non-equilibrium' polyesterification. Even though the acyl chloride based synthesis is also subject of reports in the patent literature, it is unlikely that the reaction is utilized on the production scale. The method is limited by the acid dichlorides' high cost, its sensitivity to hydrolysis and the occurrence of side reactions. The high temperature reaction (100 to > 300 °C) of an diacyl chloride with an dialcohol yields the polyester and hydrogen chloride. Under these relatively high temperatures the reaction proceeds rapidly without a catalyst:
Silyl method In this variant of the HCl method, the carboxylic acid chloride is converted with the
trimethyl silyl ether of the alcohol component and production of trimethyl silyl chloride is obtained
Acetate method (esterification) :
Ring-opening polymerization :
Aliphatic polyesters can be assembled from
lactones under very mild conditions, catalyzed
anionically,
cationically,
metallorganically or enzyme-based. A number of catalytic methods for the copolymerization of epoxides with cyclic anhydrides have also recently been shown to provide a wide array of functionalized polyesters, both saturated and unsaturated. Ring-opening polymerization of lactones and lactides is also applied on the industrial scale.
Other methods Numerous other reactions have been reported for the synthesis of selected polyesters, but are limited to laboratory-scale syntheses using specific conditions, for example using dicarboxylic acid salts and dialkyl halides or reactions between bisketenes and diols. Instead of acyl chlorides, so-called activating agents can be used, such as
1,1'-carbonyldiimidazole,
dicyclohexylcarbodiimide, or
trifluoroacetic anhydride. The polycondensation proceeds via the
in situ conversion of the carboxylic acid into a more reactive intermediate while the activating agents are consumed. The reaction proceeds, for example, via an intermediate
N-acylimidazole which reacts with catalytically acting sodium alkoxide: into two main categories: a) equilibrium polyesterifications (mainly alcohol-acid reaction, alcohol–ester and acid–ester interchange reactions, carried out in bulk at high temperatures), and b) non-equilibrium polyesterifications, using highly reactive monomers (for example acid chlorides or activated carboxylic acids, mostly carried out at lower temperatures in solution). The acid-alcohol based polyesterification is one example of an equilibrium reaction. The ratio between the polymer-forming ester group (-C(O)O-) and the condensation product water (H2O) against the acid-based (-C(O)OH) and alcohol-based (-OH) monomers is described by the equilibrium constant
KC. :K_C = \frac{[...\ce{-C(O)O -}...][\ce{H2O}]}{[\ce{-C(O)OH}][\ce{-OH}]} The equilibrium constant of the acid-alcohol based polyesterification is typically
KC ≤ 10, what is not high enough to obtain high-molecular weight polymers (
DPn ≥ 100), as the number average degree of polymerization (
DPn) can be calculated from the equilibrium constant
KC. :DP_n ~ = ~ \sqrt[2]{K_C} + 1 In equilibrium reactions, it is therefore necessary to remove the condensation product continuously and efficiently from the reaction medium in order to drive the equilibrium towards polymer. The condensation product is therefore removed at reduced pressure and high temperatures (150–320 °C, depending on the monomers) to prevent the back reaction. With the progress of the reaction, the concentration of active chain ends is decreasing and the viscosity of the melt or solution increasing. For an increase of the reaction rate, the reaction is carried out at high end group concentration (preferably in the bulk), promoted by the elevated temperatures. Equilibrium constants of magnitude
KC ≥ 104 are achieved when using reactive reactants (
acid chlorides or
acid anhydrides) or activating agents like
1,1′-carbonyldiimidazole. Using these reactants, molecular weights required for technical applications can be achieved even without active removal of the condensation product. == History ==