The Kinetics of Polycondensation Reactions

The kinetics of polycondensation reactions might be expected to be similar to those found in condensation reactions of small molecules (evidence suggests that rate coefficients are independent of polymer size). Polyesterification reactions between dibasic carboxylic acids and glycols can be catalysed by strong acids. In the absence of added catalyst, it has been suggested that the acidic monomer should act as a catalyst, whereupon the rate of reaction should be given by... 

Saunders, J. H. andF. Dobinson, The Kinetics of Polycondensation Reactions, Chap. 7 in Comprehensive Chemical Kinetics, Vol. 15, C. H. Bamford and C. F. H. Tipper, eds., American Elsevier, New York, 1976. 

The kinetics of polycondensation reactions is similar to that of ordinary condensation reactions. Since the average chain length is related to conversion in linear polycondensation by (9.7), and conversion is given as a function of time by the kinetic expression, x is directly related to the reaction time, and can thus be controlled by limiting the reaction time. Similarly, the time to reach a gel point is related by the rate expression, (9.18) and (9.19). 

The kinetics of polycondensation hy nucleophilic aromatic substitution in highly polar solvents and solvent mixtures to yield linear, high molecular weight aromatic polyethers were measured. The basic reaction studied was between a di-phenoxide salt and a dihaloaromatic compound. The role of steric and inductive effects was elucidated on the basis of the kinetics determined for model compounds. The polymerization rate of the dipotassium salt of various bis-phenols with 4,4 -dichlorodiphenylsulfone in methyl sulfoxide solvent follows second-order kinetics. The rate constant at the monomer stage was found to be greater than the rate constant at the dimer and subsequent polymerization stages. 

This chapter will attempt to survey critically the major areas of experimentally determined kinetic data which are available on polycondensation reactions and their mechanisms, and to emphasize the mechanistic similarities of many of the reactions. The statistics of polycondensation reactions will be touched on only to the extent that it helps understand the reactions and their kinetics. The general approach to the subject of kinetics is designed to be of primary use and interest to those polymer chemists who are concerned with the synthesis of products having desired properties, and with the understanding of their synthetic processes. 

Emulsion systems, while widely used in the polymerization of unsaturated monomers, are used rarely for polycondensation. The emulsion system is one in which two (or more) liquid phases are present, md in which polymerization occurs entirely in the bulk of one of the phases and is almost exclusively kinetically controlled. It thus represents a transition from solution polymerization to interfacial polymerizations. In the case of polycondensation reactions, emulsion polymerization has not been studied in detail. Results thus far indicate that molecular weight and molecular weight distribution are subject to the same statistical considerations as apply to solution and melt polymerizations. 

The kinetics of polycondensation and polyaddition reactions follow the same general scheme, but both differ sharply from the kinetics of addition or chain polymerization. 

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 ... 

Many publications have appeared on the kinetics of transesterification, dealing with either PET or model compounds. A selection of these papers is summarized in Table 2.5. The overall reaction order of polycondensation is 3, being 1 each for ester, alcohol, and catalyst [43], The reaction rate of poly condensation is generally limited by the rate of removal of EG from the reaction mixture. A... 

The understanding of the SSP process is based on the mechanism of polyester synthesis. Polycondensation in the molten (melt) state (MPPC) is a chemical equilibrium reaction governed by classical kinetic and thermodynamic parameters. Rapid removal of volatile side products as well as the influence of temperature, time and catalysts are of essential importance. In the later stages of polycondensation, the increase in the degree of polymerization (DP) is restricted by the diffusion of volatile reaction products. Additionally, competing reactions such as inter- and intramolecular esterification and transesterification put a limit to the DP (Figure 5.1). 

According to the principles of polycondensation, all of the above reactions will also take place during SSP. The conditions for the latter, however, are different as this process is carried out at lower temperatures in a non-homogeneous environment. In order to examine the kinetics of SSP, some assumptions have to be made to simplify the analysis. These are based on the idea that the reactive end groups and the catalyst are located in the amorphous regions. Polycondensations in the solid state are equilibrium reactions but are complicated by the two-phase character of the semicrystalline polymer. 

Since the time of Flory, only a few papers have appeared in the hterature in which the kinetics of A2B condensation reactions are treated. A purely theoretical paper was recently pubhshed by Moller et al. where Flory s theory of AjjB polycondensations was expanded to describe the distribution of molecules containing arbitrary numbers of branching units [43]. In another paper, Hult and Malmstrom studied the kinetics of a reacting system based on 2,2-bis(hy-droxymethyljpropionic acid [44]. 

The kinetics of template polymerization depends, in the first place, on the type of polyreaction involved in polymer formation. The polycondensation process description is based on the Flory s assumptions which lead to a simple (in most cases of the second order), classic equation. The kinetics of addition polymerization is based on a well known scheme, in which classical rate equations are applied to the elementary processes (initiation, propagation, and termination), according to the general concept of chain reactions. 

The calculation of the gel point or the molecular weight distribution becomes difficult if the functional groups in monomers differ in reactivity (26, 27). Polycondensation of glycerol with symmetrical dibasic acid or of an unsymmetrical dibasic acid with symmetrical tribasic alcohol may serve as examples. The reaction probabilities of various functional group , necessary for the calculation of molecular weight distributions and gel points, have to be determined either from complete chemical analysis or from the kinetics of the respective reactions. 

In a polymerization process the chain length distribution or molar mass distribution (MMD) is influenced by a large number of factors and conditions the kinetics of the reaction plays a very important role. The calculation of the resulting MMD is thus very complicated. For one of the simplest cases, a step reaction with polycondensation, a first-order approach is given here. As an example we take a hydroxy acid HO-R-COOH, which, upon condensation, forms the chain -[-O-R-CO-]n. 

You are working for a company that many years ago conducted a number of polycondensation reaction experiments between diol and diacid monomers. You need data on the kinetics of one of these reactions from the archives, but much was lost in a flood. All you find are the results of the original experiments, where small aliquots of sample were withdrawn from the reaction vessel, quenched to low temperature to stop the reaction and titrated to determine the amount of unreacted acid functional groups. The results... 

Polycondensation reactions are conducted under a variety of reaction conditions. The initial features of kinetic control may vary considerably according to the system used. For example, some are limited primarily by the kinetics of the chemical reactions, whereas others have as the limiting rate processes the diffusion of the reactants. The former are those in... 

Most of the reported work on the kinetics of catalysed polycondensation by transesterification is quite recent and in the main deals with the preparation of PET, though related work on other esters from ethylene glycol has been reported. Reinisch et al. [34] found that the melt polycondensation leading to poly(ethylene sebacate) in 3—4 mm films under vacuum, with manganous acetate as catalyst, was second order from p = 0.70 to p = 0.97. Rate coefficients are listed in Table 5 for this reaction, for the formation of poly(ethylene sebacate-co-terephthalate) and for PET. 

An ever-increasing number of polycondensation reactions is being studied for preparative purposes. Some kinetic data are available on a few of them. In some cases many steps are involved, making reliable kinetic analysis difficult. In many other cases we may expect more data in the future, as the importance of the reactions becomes clearer. The following examples are not all-inclusive, but do illustrate the range of recent studies of polycondensation systems. 

ADMET has been shown to be a step-growth polycondensation reaction [31[. The kinetics of step-growth polymerization and consequences thereof are completely different than those of chain polymerizations. Since ROMP and many other single-site transition metal-catalyzed polymerizations discussed in this book proceed... 

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