Polyesterification kinetics

The catalytic activity of the acid catalyst is due to hydrogen ions [55]. In the presence of a strong acid catalyst (e.g., p-toluene sulfonic acid), hydrogen ions are produced mainly from the added acid. Thus the polyesterification is a second-order reaction. In the absence of an acid catalyst, hydrogen ions are formed from the ionisation of dicarboxylic acid, and the order of the reaction is 2.5 [55]. More complicated rate equations are proposed by considering the reverse reaction, and the effect of dielectric constant of the medium on ionisation of diacid [56, 57]. 

The difference in kinetics may originate from the fact that some studies were carried out under a constant oil bath, and others were carried out at constant reaction temperature. Polyesterification is an endothermic process (even though the polyesterification reaction is exothermic because the condensates are evaporated continuously). Hence there is a large and rapid temperature drop, especially in the early stages of the reaction [56]. Thus the data generated from the experiments at a constant oil-bath temperature are expected to be different from data generated from the experiment at a constant reaction temperature. 

Chen and Wu [57] proposed a rate equation considering that 1) the hydroxyl group is a stronger proton acceptor than the carboxylic acid group 2) variation in the dielectric constant of the reaction mixture affects the dissociation constant of dicarboxylic acid and 3) there is a possibility of reverse reaction due to unremoved water. The rate equation can be expressed as follows  

Considering that a is the conversion of the acid group ([COOH], - [COOH]/ [COOH]q) and r is the ratio of diol to diacid at the beginning of the reaction (r = [OH]y[COOH]o) we obtain  

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In this article we critically review most of the literature concerning non-catalyzed, proton-catalyzed and metal-catalyzed polyesterifications. Kinetic data relate both to model esterifications and polyeste-rificatiom. Using our own results we analyze the experimental studies, kinetic results and mechanisms which have been reported until now. In the case of Ti(OBu)f catalyzed reactions we show that most results were obtained under experimental conditions which modify the nature of the catalyst. In fact, the true nature of active sites in the case of metal catalysts remains largely unknown. 

Determination of carboxy groups. Esterification or polyesterification kinetics is usually followed by this titration which is both easy and accurate. Each sample is dissolved in a solvent or a mixture of solvents (CHCI3, QH /EtOH or MeOH, toluene/EtOH or MeOH...) and then titrated with alcoholic KOH. The end point is determined either with an indicator (in most cases phenolphthalein) or with a pH-meter. An accuracy of about 0.1-1% can generally be achieved. 

Various precautions must be taken when studying polyesterification kinetics above 85% conversion. 

Has the Elimination of Water to be Taken into Account in Polyesterification Kinetics ... 

The following critical study is mainly devoted to polyesterifications. However, several kinetic analyses relate to monoesterifications whose results are particularly useful for the understanding of polyesterification kinetics and mechanisms. In fact, there is a large number of articles concerned with esterifications and polyesterifications. 

Jackson and Kuhfuss from Tennessee Eastman demonstrated that liquid crystalline behavior existed in copolymers based on polyfethylene terephthalate) (PET) and para-hydroxy benzoic acid (ABA). However, intricate details pertaining to the polyesterification kinetics have remained imexamined. 

Copolyesters of poly(ethylene terephthalate) and 4-acetoxybenzoic acid (PET / oxybenzoate) were synthesized by Jackson et al. through high temperature melt transesterification. However intricate details pertaining to the polyesterification kinetics have remained unexamined. In this system, insertion of 4-oxybenzoate moieties, with stiff rod like conformations, into flexible PET chains fosters the development of thermotropic character within a definite range of copolyester composition. 

There is no published literature on the melt polyesterification kinetics of a phase segregated two component system like PET / 80 ABA and PET / 90 ABA. Here we explore the kinetics of a two component system wherein many parallel reactions take place simultaneously. Precipitation has been observed during the synthesis of these polymers. 

This article is a critical review of most of the literature relative to non-catalyzed, base-catalyzed, and miscellaneous-catalyzed epoxy-carboxy esterifications and poly esterifications. Most kinetic data are relative to model esterifications however, some polyesterification kinetics are analyzed. From the analysis of the results reported in the literature and found in our group it has been possible to compare and critize the various general mechanisms which have been proposed. 

Several parameters have to be taken into account in order to explain the differences in reactivity observed when using various polyamide and polyether blocks. Considering the nature of the polyether block, the nature of the functional end-group is one of the major factors influencing the polyesterification kinetics. 

Table I. Kinetic orders of various non-catalyzed polyesterifications of adipic acid with aliphatic primary diols. The first figure in the 3rd column is the overall order and figures in brackets denote orders with respect to acid and alcohol. 2 + 3 means that kinetics has been treated as resulting from the superposition of two reactions with orders 2 and 3, respectively. The range of conversion which has been studied is given in the 2nd column for instance, 80-100 means that kinetics has been studied between 80 and 100% conversion...

This review is devoted to the study of the kinetics and mechanisms of the direct esterification of carboxylic adds with alcohols. The results which are analyzed here concern esterifications and linear polyesterifications between small molecules or oligomers in solution or in bulk. We will examine the following points ... 

Main techniques used in kinetic studies of polyesterifications, Chap. 2. 

Main Techniques Used in Kinetic Studies of Polyesterifications... 

Determination of low molecular weight compounds. The kinetics of monoesterifications or of the first steps of polyesterification can be followed by chromatographic methods. The concentration of each of the species present in the medium can be determined, at least for low molecular weight compounds. 

Since high temperatures and a nitrogen atmosphere are necessary to obtain measurable rates of polyesterification and to remove the reaction water, a loss of volatile reactants can hardly be avoided, especially in early stages of polyesterification. In the last stages, the decrease of the concentration of the volatile reactants can be of the same order of magnitude as their concentration. Consequently, the ultimate points of the kinetic plot have possibly no significance. 

In view of the enormous number of articles and patents concerned with esterifications and polyesterifications, very few kinetic studies on these processes have been reported. In Table 3 (see page 99 to 142) are listed the main kinetic parameters published so far for linear polyesterifications and model esterifications. 

Fiery1 252-254) studied only the last stage of the reactions, i.e. when the concentration of reactive end groups has been greatly decreased and when the dielectric properties of the medium (ester or polyester) no longer change with conversion. Under these conditions, he showed that the overall reaction order relative to various model esterifications and polyesterifications is 3. As a general rule, it is accepted that the order with respect to acid is two which means that the add behaves both as reactant and as catalyst. However, the only way to determine experimentally reaction orders with respect to add and alcohol would be to carry out kinetic studies on non-stoichiometric systems. 

Kinetic studies on the bulk polyesterification of a,o-dicarboxy poly(hexamethylene adipate) with a,polymeric medium. Solomon s mechanism1 can be considered as reasonable. 

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