Polycondensation equilibrium

In 1959 and 1960, Challa published the first results of quantitative experiments on the poly condensation equilibrium in PET [22, 41, 42], He determined the polycondensation equilibrium constant K at different temperatures and average degrees of polycondensation and found that this parameter depends only slightly on temperature, but increases significantly with increasing degree of polycondensation. The monomer BHET was found not to follow the principle of equal reactivity. 

Later, Fontana [43] performed experiments on transesterification and reinterpreted Challa s results. He concluded that the value of the polycondensation equilibrium constant is close to 0.5, being independent of temperature or degree of polycondensation and that the normal Flory-Schuz distribution does hold in the PET system. In Figure 2.8, the polycondensation equilibrium constant K from different sources [22, 43, 44] is shown as a function of the average degree of polycondensation, Pn. 

Challa, G., The formation of polyethylene terephthalate by ester interchange I. The polycondensation equilibrium, Macromol. Chem. Phys., 38, 105-122 (1960). 

Reversible reactions involve both formation of polyester and the reverse reaction of the polymer with the liberated low-molecular-weight product. The so-called polycondensation equilibrium occurs, of course, when the rates of the forward and reverse reactions are the same. This equilibrium determines the extent of conversion to polymer, d the position of the equilibrium determines the molecular weight of the product. In the case of monofunctional reactants, the equilibrium governs only the yield of the product. 

The preparation of polycondensation products in the laboratory as weU as on an industrial scale roughly foOows procedures employed for low-molecular condensations. The reactants are brought together in appropriate reaction vessels and mixed with the catalyst. The polycondensation is started by heating the reaction mixture, and the reaction is continued under close temperature control imtil the desired degree of polymerization is reached. Reaction conditions have to be adjusted in such a way that premature gel formation is avoided. Likewise, stoichiometric proportions of the reactants have to be chosen carefully, or monofunctional additives have to be introduced, so as to assure that the desired extent of the reaction is reached. Finally, provisions have to be made to eliminate reaction product D in the general scheme of the polycondensation equilibrium reaction... 

Fontana, C. M., Polycondensation equilibrium and die kinetics of catalyzed transesterification in the formation of polyethylene terephthalate, J. Polym. ScL, Part A-1, 6, 2343-2358 (1968). 

The polycondensation equilibrium, as represented by Equation 2.5, is also part of the more complex mechanism of the water-initiated polymerization of lactams (Equation 2.3), which entails hydrolytic ring opening (Equation 2.11), condensation (Equation 2.12), and addition (Equation 2.13), as the principal equilibrium reactions [15]. 

If the equilibrium constants are not too large, the polycondensation equilibrium is also actually reached over the usual reaction time scales and yields. In such cases, the equilibrium constants may be related to either the yield in groups or the yield in molecules. 

The synthesis of polyamides of the nylon series has the advantage that the fraction of monomers and oligomers at the polycondensation equilibrium is very small. Consequently monomers and oligomers do not have to be removed from the polyamide, which is in contrast to polyamide production by lactam polymerization. On the other hand, the nylon polycondensation is not as straight-forward a process as the lactam polymerization. For historical reasons, PA 6,6 dominates the market in England and the USA, whereas in West Germany and Japan, PA 6 plays the dominant role. 

The melt-water vapor equilibrium is shifted because the spinning apparatus is very dry. The polycondensation equilibrium is thus displaced toward higher degrees of polymerization. 

A general theory of polycondensation equilibrium in silicic acid solutions was proposed by Stbber (94). From it he deduced the concentration of monomer in equilibrium with polymers of different degrees of condensation. Further data are needed to check the validity of the complex equations that were developed. 

Barandiaran, M. J., and Asua, J. M., Low molecular species in the reaction of dimethyl terephthalate with ethylene glycol, J. Polym. Set, Polymer Chem. Ed., 27, 4241, 1989. Fonatana, C. M., Polycondensation equilibrium and the kinetics of the catalyzed transesterification in the formation of polyethylene terephthalate. J. Polymer Sci., A-1, 6, 2343, 1968. 

If polycondensation is carried out at low temperature, removal of the liberated water is impossible. In this case, reverse hydrolysis must be taken into account unless equilibrium is shifted towards esterification by an excess of one of the reactants. 

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). 

The main polymerization method is by hydrolytic polymerization or a combination of ring opening as in (3.11) and hydrolytic polymerization as in (3.12).5,7 9 11 28 The reaction of a carboxylic group with an amino group can be noncatalyzed and acid catalyzed. This is illustrated in the reaction scheme shown in Fig. 3.13. The kinetics of the hydrolytic polyamidation-type reaction has die form shown in (3.13). In aqueous solutions, die polycondensation can be described by second-order kinetics.29 Equation (3.13) can also be expressed as (3.14) in which B is die temperature-independent equilibrium constant and AHa the endialpy change of die reaction5 6 812 28 29 ... 

Shorter chain dienes have an increased propensity to form stable five-, six-, and seven-membered rings. This thermodynamically controlled phenomenon is known as the Thorpe-Ingold effect.15 Since ADMET polymerization is performed over extended time periods under equilibrium conditions, it is ultimately thermodynamics rather than kinetics that determine the choice between a selected diene monomer undergoing either polycondensation or cyclization. 

Alkyl esters often show low reactivity for lipase-catalyzed transesterifications with alcohols. Therefore, it is difficult to obtain high molecular weight polyesters by lipase-catalyzed polycondensation of dialkyl esters with glycols. The molecular weight greatly improved by polymerization under vacuum to remove the formed alcohols, leading to a shift of equilibrium toward the product polymer the polyester with molecular weight of 2 x 10" was obtained by the lipase MM-catalyzed polymerization of sebacic acid and 1,4-butanediol in diphenyl ether or veratrole under reduced pressure. ... 

The enzymatic synthesis of polyesters from activated diesters was achieved under mild reaction conditions. The polymerization of bis(2,2,2-trichloroethyl) glutarate and 1,4-butanediol proceeded in the presence of PPL at room temperature in diethyl ether to produce the polyesters with molecular weight of 8.2 x 10. Vacuum was applied to shift the equilibrium forward by removal of the activated alcohol formed, leading to the production of high molecular weight polyesters. The polycondensation of bis(2,2,2-trifluoroethyl) sebacate and aliphatic diols took place using lipases BC, CR, MM, and PPL as catalyst in diphenyl ether. Under the... 

Noteworthy that all the above formulated results can be applied to calculate the statistical characteristics of the products of polycondensation of an arbitrary mixture of monomers with kinetically independent groups under any regime of this process. To determine the values of the elements of the probability transition matrix of corresponding Markov chains it will suffice to calculate only the concentrations Q()- of chemical bonds (ij) at different conversions of functional groups. In the case of equilibrium polycondensation the concentrations Qy are controlled by the thermodynamic parameters, whereas under the nonequilibrium regime of this process they depend on kinetic parameters. 

When monomers with dependent groups are involved in a polycondensation, the sequence distribution in the macromolecules can differ under equilibrium and nonequilibrium regimes of the process performance. This important peculiarity, due to the violation in these nonideal systems of the Flory principle, is absent in polymers which are synthesized under the conditions of the ideal polycondensation model. Just this circumstance deems it necessary for a separate theoretical consideration of equilibrium and nonequilibrium polycondensation. 

A general theory of the equilibrium polycondensation of an arbitrary mixture of monomers, described by the FSSE model, has been developed [75]. Proceeding from rigorous thermodynamic considerations a branching process has been indicated which describes the chemical structure of condensation polymers and expressions have been derived which relate the probability parameters of this stochastic process to the thermodynamic parameters of the FSSE model. 

In an attempt to avoid the polymerization/depolymerization equilibrium that occurs during melt polycondensation, Albertsson and Lundmark (1988) also studied the irreversible reaction of adipic anhydride with ketene. However, they reported very little difference in molecular weights when two ketene syntheses were compared to melt polycondensation and ringopening polymerization using a zinc catalyst (Albertsson and Lundmark, 1988). 

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