Polyesterification catalysed

B. Paul, Modelisation de la catalyse de polyesterification en masse par les composes a liaisons metal-oxygene, These de l Universite P. et M. Curie, Paris, 1998. 

The kinetics of polycondensation reactions might be expected to be similar to those found in condensation reactions of small molecules (evidence suggests that rate coefficients are independent of polymer size). Polyesterification reactions between dibasic carboxylic acids and glycols can be catalysed by strong acids. In the absence of added catalyst, it has been suggested that the acidic monomer should act as a catalyst, whereupon the rate of reaction should be given by... 

For a consideration of more strictly kinetic aspects of polyesterification by direct esterification, it is convenient to discuss separately the cases of self-catalysis and catalysed esterification. 

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. 

Table 3 lists determined third-order rate coefficients for self-catalysed esterifications and polyesterifications. 

In the presence of an added catalyst such as p-toluenesulphonic acid, simple esterification reactions and polyesterification reactions are second order [48]. Thus the kinetics of the catalysed reaction of lauric acid and lauryl alcohol in a medium of lauryl laurate closely parallels those of the polymer-forming reaction between adipic acid and 1,10-dodecanediol in a medium of polyester product. Second-order rate coefficients for the two reactions were [35], respectively, 45x10 equiv kg" sec and 16 X 10" equiv kg" sec . 

Later work by Stevenson [72] supported this hypothesis. The preparation of PET catalysed by antimony trioxide was studied in thin films on metal surfaces that were carefully selected to avoid catalysis by surface effects or by dissolved metal as mentioned earlier, a large number of metals and their oxides, salts or other derivatives catalyse the polyesterification reaction. On inactive surfaces like silver or rhodium the catalysed polycondensation rate increased with decrease in film thickness. In the absence of added catalyst there was no tendency for the rate to increase with decreasing film thickness. Stevenson proposed that in thin films the catalyst-deactivating component was more readily lost, thereby increasing the reaction rate. 

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. 

Self Catalysed Polyesterification Reactions (Without Catalyst)... 

The mechanism of self catalysed polyesterification is described by the following four classical equilibrium reactions (reactions 8.7-8.10). 

The mechanism of the acid-catalysed (AH = acid catalyst) polyesterification reaction (for example using para toluenesulfonic acid as the catalyst) is presented in the following equilibrium reactions (8.11-8.14). The mechanism is very similar to those of the self catalysed polyesterifications. 

The third-order kinetic equation of the acid catalysed polyesterification is presented in equation 8.15. 

In principle, the mechanism of acid catalysed polyesterification is similar to the mechanism of self catalysed polyesterification with the difference that the acidic carboxyl group used as catalyst is replaced by a strong acidic catalyst (AH). 

For the mechanism of polyesterification reactions catalysed by metallic derivatives (for example titanium or tin compounds) two variants (variant A and variant B) are discussed. 

A variety of other acid-catalysed transformations have also been performed in ILs such as polyesterifications, Boc amine protection, aiyl alleviation,pinacol rearrangements, acetal deprotection, the Hantzsch reaction and Beckmann rearrangements. ... 

Certain step polymerizations are self-catalysed, i.e. one of the types of functional group also acts as a catalyst (e.g. carboxylic acid groups in a polyesterification). In the absence of an added catalyst the rate of reaction for such polymerizations is given by... 

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