Reversible Polycondensation Kinetics

The concentration of a strong acid catalyst added to the reaction mixture for polyesteri cation remains constant throughout the course of polymerization. For the reversible polyesteri cation (as in a closed batch reactor) represented by 

For an equimolar polymerization system, [COO] = [H2O] = pCo and [COOH] = [OH] = (1 - p)Co, where p is the fractional conversion of carboxyl and hydroxyl groups. The rate equation is then obtained as 

Solution of this quadratic equation yields 2 roots, namely, ps = V /( - 1) and [K - K + 1). 

Using Eq. (5.27), the extent of conversion in a closed batch reactor below the upper Umit of equilibrium conversion pE can be calculated as a function of time. The equation applies equally to A—B and stoichiometric A—A plus B—B polymerization where none of the reaction products are removed from the system and none is present initially. 

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

Solution of this quadratic equation yields 2 roots, namely, pg = - 1) 

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In addition to kinetic effects, the choice of solvent may influence the equilibrium in many cases. As an illustration, if water is evolved as a by-product in a reversible polycondensation, a solvent system which has a low solubility for water will be favourable for high molecular weight. When hydrogen chloride is evolved, an increase in the dielectric constant of the medium often favours higher molecular weight. 

Mass fluxes are expressed with an effective mass transport coefficient and a concentration difference between the polymeric phase and the external environment. It is assumed that only monomers are able to leave the matrix because of the low mobility of chain fragments. The adopted kinetic scheme is one of reversible polycondensation + W <-> P + P. Polymer... 

A simplified model (no reaction reversibility) of melamine-formaldehyde formation based on Tomita s kinetic scheme [225] has been presented [227] and afterwards extended to reaction in a CSTR [228], also considering reaction reversibility. There is no experimental validation, but it is noteworthy for the use of a program to calculate the CLD of a nonlinear reversible polycondensation in order to overcome the astronomical number of reaction possibilities in the rates of formation of individual oligomers. 

Challa, G., The formation of polyethylene terephthalate by ester interchange II. The kinetics of reversible melt polycondensation, Macromol. Chem., 38, 123-137 (1960). 

For the polymerization, either in the melt or solid phase, the reaction is driven to the polymer by removing ethylene glycol. The polymerization reaction is typically catalyzed by solutions consisting of antimony trioxide or germanium oxide. Both polycondensation catalysts also catalyze the reverse reaction, which is driven by an excess of ethylene glycol at melt conditions, generally above 255 °C. The polymerization reaction follows second-order kinetics with an activation energy of 22 000 cal/mol [6],... 

The polymerizations shown in Eqs. (2) and (3) actually represent well-known reactions of small molecules, the only distinction being the minimum requirements of difunctionality of each molecule for polymer formation, which makes it possible for the product of each reaction to participate in further reactions. As a rule, the functional groups retain their reactivity regardless of the chain length [11, p. 75], so that these reactions follow the same kinetic rules as for simple molecules however, in contrast to polyaddition reactions, polycondensations suffer from the serious problem of reversibility (e.g., hydrolysis, or depolymerization ) as a result of the possible accumulation of the by-product (e.g., water), and this must be taken into account. In general, because of the unfavorable equilibrium constant for polycondensation reac-... 

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