Polyesterification

The rate constants obtained by the above method for uncatalyzed polyesterification of adipic acid with different glycols are shown in Table 5.2. The Arrhenius parameters A and E of the equation k — Aexp —E/RT) are also presented in Table 5.2 for those reactions that have been studied kinetically at more than one temperature. Note that the concentration units of the rate constants are in terms of moles per kilogram, which is a more convenient measure of concentration than the usual moles per liter because the volume of the system decreases significantly due to reaction. 

If a strong acid, such as sulfuric acid or p-toluene sulfonic acid, is added to a polyesterification system, [H ] in Eq. (5.23) then represents the 

Problem 5.2 How would you modify Eq. (5.33) for a case of polyesterification where k is not negligible relative to k l 

The natural log term on the right side of Eq. (P5.2.3) is the contribution of the uncatalyzed reaction. Its relative importance increases as k/k increases. When k/k is very small, Eq. (P5.2.3) converts to Eq. (533). 

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The oxidation reactor effluent and methanol ate sent to the esterification reactor, which operates at up to 250°C and a pressure sufficient to maintain the Hquid phase. This latter is about 2500 kPa (25 atm). The oxidation products are converted to methyl -toluate and dimethyl terephthalate without a catalyst. Excess methanol is suppHed, and steam and vaporized methanol ate removed and enter a methanol recovery column. The esterification products flow to a cmde ester column, which separates the toluate from the terephthalate. The overhead stream of methyl -toluate is returned to the oxidation reactor, and the bottoms stream of dimethyl terephthalate goes to a primary distillation. The distillate is dissolved in methanol, crystallized, and sohd dimethyl terephthalate is recovered. The dimethyl terephthalate can then be either recrystallized or distilled to yield the highly pure material needed for the polyesterification reaction. 

The polyesterification reaction is normally carried out in stainless steel vessels ranging from 8,000—20,000 L, heated and cooled through internal cods (Fig. 1). Blade agitators revolving at 70—200 rpm ate effective in stirring the low viscosity mobde reactants, which ate maintained under an inert atmosphere of nitrogen or carbon dioxide during the reaction at temperatures up to 240°C. 

Table 6. Gel Points of Polyesterification Reaction Systems with Stoichiometric Reactants ...

Organic titanates perform three important functions for a variety of iadustrial appHcations. These are (/) catalysis, especially polyesterification and olefin polymerization (2) polymer cross-linking to enhance performance properties and (J) Surface modification for adhesion, lubricity, or pigment dispersion. 

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. 

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

Inventory of the Main Catalysts Used in Esterifications and Polyesterifications 65... 

Activation Parameters of Non-catalyzed and Protonic Acid-catalyzed Esterifications and Polyesterifications. 83... 

In his classic work on polyesterification Flory1,2 established many of the basic principles of step-growth polymerization ... 

Since Flory s works many other articles have been published on this subject and the two most important assumptions that have been made in interpreting the experimental results on both esterification and polyesterification are ... 

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. 

Inventory of the main catalysts used in esterifications and polyesterifications, Chap. 4. 

Main Techniques Used in Kinetic Studies of Polyesterifications... 

An inert atmosphere is required in all cases and a vacuum line is often used for the removed of water at the end of the polyesterification reaction. 

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. 

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. 

The volume of the water removed in esterifications through azeotropic distillation287-289 and also in polyesterifications is determined6,19,264,292). This method is difficult to carry out because it is necessary to quantitatively collect the water which is released and to determine its volume or its weight. It is important to estimate as accurately as possible the amount of water remaining in the distillation column. 

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. 

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

At the temperatures at which polyesterifications are performed a stream of dry nitrogen or a vacuum line effectively removes the water. 

More and more authors use Eq. (8) which gives AH and AS a more theoretical base than Eq. (7). However, it must be remembered that relation (8) has been established for reactions in the gaseous phase or at the best in highly diluted phase. This is not the case with many of the polyesterification systems studied where concentrations are high and expressed in term of mole per kilogram and not, as normally, in mole per liter. The consequences of these approximations are not clear and it is obvious that a considerable amount of additional work must be carried out on this subject. 

During the course of a polyesterification the volume and the weight of the reaction mixture vary because condensation water is released. In most cases, the progress of the reaction is followed by titration of the acid groups at definite intervals the carboxy group concentration is expressed in equivalents per kilogram. Consequently, several authors tried to find out if the weight decrease due to the elimination of water must be taken into account. 

Many compounds have been used as polyesterification catalysts some of which are reported in Table 2. The efficiencies of these catalysts are analyzed in Chap. 6. 

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