Polyesterification acid catalysis

Mares et al. [52] recently concluded that hydrogen-ion catalysis during this esterification was not likely and that catalysis by undissociated acid occurred together with a reaction in which, kinetically at least, no form of catalysis was involved. On the other hand, Vansco-Szmercsanyi et al. [53] concluded that polyesterification of maleic, fumaric and succinic acids with ethylene glycol or 1,2-propylene glycol was catalysed by protons. Much work clearly remains to be done on defining the detailed mechanism of catalysis by the acidic species during esterifications. 

Ester formation is a standard organic reaction between an alcohol and a carboxylic acid, which is an equilibrium reaction that has been shown to occur under catalysis by either acid or base. In polyesterification involving an organic acid, the substrate is itself the catalyst. It has been noted (Pilati, 1989) that, among the many reaction mechanisms, Scheme 1.1 is the most likely for acid-catalysed esterification, with the second reaction being the rate-determining step. 

Because many catalysts used in polyesterification reactions are liable to hydrolyse and thus lose their catalytic activity, frequently the first part of the polyesterification reaction (when water is massively eliminated from the reaction mass), is done without a catalyst (the catalysis is assured by the acidic carboxyl groups). After the distillation of the majority of water (3-6 hours) a specific catalyst is added (tin, titanium, lead or manganese catalyst, the best results being obtained with tin catalysts). Using a two-step polyesterification reaction, the catalyst is protected against hydrolysis and assures a good catalytic activity at the end of polyesterification reaction, when the concentration of carboxyl groups is very low, and it is very important to obtain a final low acidity. 

Polyesterifications of various diacids/diols and linear hydroxy acids were studied using lipase-catalysis. The different diols studied include 1,4-butaiw, 1,6-hexane, and 1,8-octane diol. The diacids studied include succinic, glutaric, adipic, and sebacic acid. The different hydroxy acids studied were 6-hydroxyhexanoic acid, 10-hydroxydecanoic acid, 12-hydroxydodecanoic acid, and 16-hydroxyhexadecanoic. The effect of the chain length of the diol and diacids and hydroxy acids on the build-up of polymer chains by lipase-catalyzed polycondensation reactions was studied. The effect of reaction parameters on product molecular weight averages and chain dispersity were assessed. 

The acid-catalyzed polyesterification of ethylene glycol and adipic acid has been studied by Flory. The data are plotted in Figure 3.5. This figure also reveals that Eq. (3.7.8) holds after about 80% conversion. More extensive experimental data [24-27] do not, however, confirm Flory s conclusions. It has been argued that only a limited amount of adipic acid dissociates in ethylene glycol (0.390 moles per mole ethylene glycol), and only this acid contributes to the catalysis in the polymerization without strong acid. Hence, instead of Eq. (3.7.2), it has been proposed that... 

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