Kinetics of Condensation Polymerization

The generic condensation polymerization begins with monomers AMA and BMB and produces molecules of the forms A A, B B, and A B. Each step of the reaction generates a longer polymer by the step-growth mechanism of Equation (13.2) and produces 1 mol of condensation product AB. 

Suppose that the reactivity of the A and B endgroups is independent of the chains to which they are attached. This is a form of the equal reactivity assumption that is needed for almost all analytical solutions to polymer kinetic problems. If it is satisfied, we can ignore the details of the polymerization and just concentrate on the disappearance of the endgroups. For a batch system, 

The initial condition for this ODE is based on Equation (13.3), [A]0 = S tBjo-The solution is 

We see from Equations (13.16) and (13.17) that the conversion of endgroups obeys the kinetics of a simple, second-order reaction the second-order reaction having perfect initial stoichiometry in the case of Equation (13.17). 

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We discuss the reaction kinetics of condensation polymerization, but addition polymerization can be discussed in much the same way. Consider the formation of a polyester from a monomer diacid, HOOC-X-COOH, and a monomer dialcohol, HO-Y-OH, where X and Y represent two hydrocarbon chains. The first step in the polymerization is... 

The free amino group of the amino ester may then react analogously with another molecule of the monomer, etc. The kinetics of the polymerization are in harmony with a mechanism of this sort. The final polypeptide may contain up to 300 or more structural units. While the polymerization of N-carboxyanhydrides is closely analogous to the addition polymerizations of ethylene oxide and of other cyclic substances, definition unfortunately classifies it as a condensation polymerization inasmuch as carbon dioxide is eliminated in the process. 

The largest-volume polymers are polyolefins, and the kinetics of olefin polymerization are fairly similar to the ideal addition process just considered. All these olefins form condensation products to form a very long-chain alkane such as... 

Formaldehyde polymerization has been studied in the liquid state, in solution of protic or aprotic solvents and in the gaseous state where gaseous formaldehyde forms directly crystalline polymer. It has been studied with anionic and cationic initiators and by high energy radiations. Although there are more than 100 MM lbs. of poly form aldehyde produced per year, very few papers have been published that are actually concerned with the kinetics of formaldehyde polymerizations. The reason for this lack of detail is understandable when one realizes how difficult it is to obtain pure formaldehyde (with impurities of less than 100 p.p.m). Even pure formaldehyde undergoes side reactions and self condensation which cause new introduction of impurities. 

Stepwise Polymerization. Although condensation polymers account for only about one-fourth of synthetic polymers (bulkwise), most natural polymers are of the condensation type. As shown by Carothers in the 1930s 2, ), the chemistry of condensation polymerizations is essentially the same as classic condensation reactions that result in the synthesis of monomeric amides, urethanes, esters, etc. the principle difference is that the reactants employed for polymer formation are bifunctional (or higher) instead of monofunctional. Although more complicated situations can occur, we will consider only the kinetics of simple polyesterification. The kinetics of most other common condensations follow an analogous pathway. 

Flory s equal reactivity principle, which has been validated on mechanistic and experimental grounds, has greatly simplified an otherwise complicated kinetic analysis of condensation polymerization. This... 

Condensation polymerization also has a special place in polymer science history. The first truly synthetic polymer, Bakelite, was developed in 1907, as the condensation product of phenol and formaldehyde [66]. Meanwhile, Wallace Carothers pioneered polyester synthesis in the 1930s at Dupont and developed a series of mathematical eqnations to describe the kinetics, stoichiometry, and molecular weight distribution of condensation polymerizations. 

Decavanadate species, written generally as Vio02s (OH). are the predominant V(V) species at moderate acidities [lOJ and may also form by polymerization of [VOjfOH ) precursors. (See, e.g.. Fig. 5.) Structures of several common polyanions are shown in Figs 6a and 6b. More compact polyanions (Fig. 6a) are formed when the kinetics of condensation via A,v or oxolation mechanisms are rapid, whereas slower kinetics result in more open structures (Fig. 6b) and generally allow the formation of clear gels when an acid is added. 

Similarly, the polymerization of polyurethane does not involve the evolution of a condensation product, even though its kinetics can be described by that of condensation polymerization. Clearly, it is not correct to classify polymers according to the scheme discussed earlier. It is now established that there are two classes of polymerization mechanisms ... 

The kinetics of this type of polymerization are the same as for simple condensation for this reason, the use of the term polycondensation is perhaps more appropriate. Unless kinetic evidence suggests otherwise, polymerizations involving the formation of chain polymers from cyclic compounds, following ring scission, are classed as condensation polymerizations. Some important con-... 

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

Hydrolysis and Polycondensation. As shown in Figure 1, at gel time (step C), events related to the growth of polymeric chains and interaction between coUoids slow down considerably and the stmcture of the material is frozen. Post-gelation treatments, ie, steps D—G (aging, drying, stabilization, and densification), alter the stmcture of the original gel but the resultant stmctures aU depend on the initial stmcture. Relative rates, of hydrolysis, (eq. 2), and condensation, (eq. 3), determine the stmcture of the gel. Many factors influence the kinetics of hydrolysis and... 

The study of PF polymerization is far more difficult than that of methylolation due to the increased complexity of the reactions, the intractability of the material, and a resulting lack of adequate analytical methods. When dealing with methylolation, we saw that every reactive ring position had its own reaction rate with formaldehyde that varied with the extent of prior reaction of the ring. Despite this rate sensitivity and complexity, all reactions kinetics were second-order overall, first-order in phenol reactive sites and first-order in formaldehyde. This is not the case with the condensation reactions. 

A generalized kinetic treatment of the array of processes occurring in condensation polymerization might appear hopelessly complex. In the polyesterification of a hydroxy acid, for example, the first step is intermolecular esterification between two monomers, with the production of a dimer... 

The combined results of kinetic studies on condensation polymerization reactions and on the degradation of various polymers by reactions which bring about chain scission demonstrate quite clearly that the chemical reactivity of a functional group does not ordinarily depend on the size of the molecule to which it is attached. Exceptions occur only when the chain is so short as to allow the specific effect of one end group on the reactivity of the other to be appreciable. Evidence from a third type of polymer reaction, namely, that in which the lateral substituents of the polymer chain undergo reaction without alteration in the degree of polymerization, also support this conclusion. The velocity of saponification of polyvinyl acetate, for example, is very nearly the same as that for ethyl acetate under the same conditions. ... 

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