Polyesterification side reactions

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... 

In a linear polyesterification involving bifunctional monomers exclusively, the hydroxyl and carboxyl groups being present in equal numbers, the number of unreacted carboxyl groups must equal the number of molecules present in the system, provided that no side reactions occur the number of molecules in moles per unit volume is equal, therefore, to co(l—p). If each residue from a glycol and from a di-... 

Various combinations of reactant(s) and process conditions are potentially available to synthesize polyesters [Fakirov, 2002 Goodman, 1988], Polyesters can be produced by direct esterification of a diacid with a diol (Eq. 2-120) or self-condensation of a hydroxy carboxylic acid (Eq. 2-119). Since polyesterification, like many step polymerizations, is an equilibrium reaction, water must be continuously removed to achieve high conversions and high molecular weights. Control of the reaction temperature is important to minimize side reactions such as dehydration of the diol to form diethylene glycol... 

Another possible side reaction under experimental polyesterification conditions is the dehydration of a-diols (Eq. 2.19, Table 2.4). It changes the structure of some constitutional units. Depending on the diol used, cyclic ethers and aldehydes can also be formed. 

Lehtonen et al. (1998) considered polyesterification of maleic acid with propylene glycol in an experimental batch reactive distillation system. There were two side reactions in addition to the main esterification reaction. The equipment consists of a 4000 ml batch reactor with a one theoretical plate distillation column and a condenser. The reactions took place in the liquid phase of the reactor. By removing the water by distillation, the reaction equilibrium was shifted to the production of more esters. The reaction temperatures were 150-190° C and the catalyst concentrations were varied between 0.01 and 0.1 mol%. The kinetic and mass transfer parameters were estimated via the experiments. These were then used to develop a full-scale dynamic process model for the system. 

Toward the end of the reaction, the resin viscosity tends to increase exponentially. Gelation in the reactor is always a threat, due either to what the formulation would theoretically allow by the completion of the polyesterification or to the occurrence of some of the side reactions. After the onset of gelation, it would progress extremely rapidly and would be almost impossible to arrest. Therefore, it is routine to... 

Side reactions that might occur during direct polyesterification include loss of functionality by decarboxylation of an acidic end group, thermal degradation of ester linkE es, the formation of ether linkages, and cyclization reactions. 

During the polyesterification reactions, especially at higher temperatures, it is possible to have some undesired side reactions take place, such as formation of terminal double bonds, formation of aldehydes, formation of polyenes which lead to coloured polyesters and loss of functionality. 

Modeling of alkyd resin production is a rather formidable task because of the high number of distinguishable chemical groups, the branched structure of the polymer, a nonnegligible amount of intramolecular reaction, and side reactions of the double bonds in fatty acids. The usual problems found in previously discussed polyesterifications, namely lack of data for liquid-vapor and Uquid-sohd equilibria and associated mass transfers, are also present. 

Paatero E, Narhi K, Salmi T, Still M, Nyholm P and Immonen K (1994) Kinetic model for main and side reactions in the polyesterification of dicarboxylic acids with diols, Chem Eng Sci 49 3601-3616. 

Sulfur compounds have also been widely studied as activating agents for polyesterification reactions. p-Toluenesulfonyl chloride (tosyl chloride) reacts with DMF in pyridine to form a Vilsmeir adduct which easily reacts with carboxylic acids at 100-120° C, giving highly reactive mixed carboxylic-sulfonic anhydrides.312 The reaction is efficient both for aromatic dicarboxylic acid-bisphenol312 and hydroxybenzoic acid314 polyesterifications (Scheme 2.31). The formation of phenyl tosylates as significant side products of this reaction has been reported.315... 

Fig. 2.4 Reaction pathways in caibodiimide mediated polyesterification. 1 O-acylisourea intermediate, 2 urea, 3 acid anhydride, 4 N-acylpyridinium intermediate, and 5 TV-acyl urea side product Adapted from Ref. [18]...

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