Polyesterification acid-catalyzed reaction

Consider the polyesterification of a diacid and a diol to illustrate the general form of the kinetics of a typical step polymerization. Simple esterification is a well-known acid-catalyzed reaction and polyesterification follows the same course [Otton and Ratton, 1988 Vancso-Szmercsanyi and Makay-Bodi, 1969]. The reaction involves protonation of the carboxylic acid,... 

Let us consider the polyesterification of a diacid and a diol to illustrate the kinetic behavior of a typical step polymerization. Like simple esterification (Sykes, 1986), polyesterification is an acid-catalyzed reaction that can be represented by a sequence of reactions as shown by Eqs. (5.4)-(5.6). In these equations, the wavy lines (www) are used to signify that these equations apply, irrespective of the size of the molecular species. Equation (5.4) represents pfotohation of oxygen in carbon-oxygen double bond which leads to a more positive carbon atom for subsequent addition of a nucleophile, in this case www-OH [Eq. (5.5)], followed... 

Second-order rate constants for some acid-catalyzed polyesterifications and polyamidations obtained by the above method are shown in Table 5.3. The Arrhenius parameters A and E of the equation k = Aqxp —E/RT) are also tabulated for those reactions that have been studied kinetically at more than one temperature. 

Case 1 Polymerization without Added Strong Acid Consider esterifieation — the formation of a polyester from a glycol and a dibasic acid. The progress of reaction is easily followed by titrating the unreacted carboxyl groups in samples removed from the reaction mixture. This polyesterification and other simple esterifications are acid-catalyzed. In the absence of an added strong acid, a second molecule of the acid being esterified acts as the... 

The kinetics of step-growth polymerization can be derived from a polyesterification reaction that follows the same course as all acid-catalyzed esterifications. ... 

In spite of being the first reaction ever studied [95], esterification has been under investigation ever since, and much knowledge has accumulated, even if some points are stUl less clear. The basic kinetic model for polyesterification was established by Flory and is summarized in his classic book [5j. Esterification was shown to be acid-catalyzed, it is first-order with respect to hydroxyls and, with respect to carboxyls, its order is either one in the presence of foreign strong protic acids, or two in their absence [Eq. (52)]. 

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...

Several authors suggested mechanisms for esterifications catalyzed by titanium tetraalk-oxides. Bolotina et al.16,221,2221 who studied the polyesterification of 2-ethylhexyl phtha-late with 2-ethylhexanol found the same reaction order with respect to catalyst, acid and alcohol, namely 1 they suggested the following rate-determining step ... 

Caglioti et al.201 suggested a mechanism for the action of hexachlorocydotriphos-photriazene in the polyesterification of carboxylic acids with phenols. Higashi291 catalyzed the reaction of various aromatic acids and alcohols by poly(ethyl phosphate). Both Caglioti201 and Higashi291 studied the influence of tertiary amines on the reactivity. 

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... 

Polyesters have been obtained in organic medium by polyesterification of hydroxy acids,328,329 hydroxy esters,330 stoichiometric mixtures of diols and diacids,331-333 diols and diesters,334-339 and diols and cyclic anhydrides.340 Lipases have also been reported to catalyze ester-ester interchanges in solution or in die bulk at moderate temperature.341 Since lipases obviously catalyze the reverse reaction (i.e., hydrolysis or alcoholysis of polyester), lipase-catalyzed polyesterifications can be regarded as equilibrium polycondensations taking place in mild conditions (Scheme 2.35). 

Consider the condensation polyesterification reaction between ethylene glycol, H0-(CH2)2-0H, and terephthalic acid, HOOC-Ph-COOH, each of which has an initial concentration of 1.0 mol/liter. Calculate the number average and weight average degrees of polymerization at 1, 5, and 20 hours. The forward reaction rate constant for the polymerization reaction is 10.0 liter/mol hr, and second-order, catalyzed kinetics can be assumed. 

Figure 5.2 Later stages of catalyzed polyesterification of adipic acid and 1,10-decamethylene glycol (Problem 5.2). (Note that the reaction time of zero corresponds to 82% conversion of the original COOH groups present.)...

Problem 5.6 Calculate the conversion that would be obtained in 1 h for reversible polyesterification in an equimolar system catalyzed by externally added strong acid. It is given that Cold = 5x 10 " s and K = 1. Compare with the conversion that would be obtained if the reaction were carried out in an irreversible manner by removing water from the system. 

In this scheme, the reaction of the protonated form (1) of the carboxylic acid with the hydroxy compound to give the addition intermediate (2) is usually taken as the ratecontrolling step. This mechanism is usually extrapolated to proton-catalyzed direct polyesterification. 

The assiunption that functional group reactivity is independent of chain length can be verified kinetically by following a polyesterification. The simple esterification is an add-catalyzed process in which protonation of the acid is followed by interaction with the alcohol to produce an ester and water. If significant polymer formation is to be achieved, the water must be removed continuously from the reaction to displace the equilibriiun, and the water eUminated can be used to estimate the extent of the reaction. Alternatively, the rate of disappearance of carboxylic groups can be mea-smed by titrating aliquots of the mixture. 

The polyesterification becomes a much more economically feasible reaction when it is catalyzed by an external acid. The self-catalyzed polymerization is not a useful reaction from the practical viewpoint of producing high polymers in reasonable reaction times. 

These results and discussion in favor of the p-toluene sulfonic acid (pTSA) as an effective catalyst for polyesterification of dimer acid and butanediol strongly supports the proton-catalyzed nature of polyesterification. Usually, the proton-catalyzed mechanism for esterification is extrapolated to proton-catalyzed polyesterification [104]. The polyesterification of dimer acid and butanediol involves protonation of the dicarboxylic acid by the reaction of protonated species with the hydroxy group of glycol to yield the polyester. The proton catalyzing the protonation of carboxylic acid is provided by the carboxyl group of the monomer, i.e., dimer acid, and by pTSA in absence and presence of added catalyst, respectively. 

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. 

Kinetics of Step Poiymerizations Step polymerizations can be divided into two broad categories, those catalyzed by some externally added chemical such as an acid, and those that are self-catalyzed. For simplicity, the self-catalyzed polyesterification reaction will be considered further. The reaction can be written. 

Two cautions are in order about the preceding example. First, by writing an irreversible rate expression, we have assumed that any molecule of condensation is being continuously and efficiently removed from the reaction mass so that there is no depolymerization. Second, not all step-growth reactions are of second order. Some polyesterifications, for example, are catalyzed by their own acid groups and are, therefore, first order in hydroxyl concentration, second order in acid, and third order overall. The rate may also be proportional to the concentration of an added catalyst (usually acids or bases for polycondensations), if used. 

Obtain the expression that relates to time for a linear polyesterification that is catalyzed by its own acid groups, that is, second order in COOH and first order in OH. Assmne an irreversible reaction at constant volume and [A]o = [B]o-... 

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