Condensation polymers, kinetics

Molecular-Weight Distribution of Condensation Polymers Kinetics and Mechanism of Nylon 66 Polyamidation Amidation... 

Kricheldorf H R and Denchev Z (1999) Chemistry of transreactions in condensation polymers kinetics, mechanisms and peculiarities, in Transreactions in Condensation Polymers (Ed. Fakirov S) Wiley-VCH, Weinheim, Germany, pp. 1-78. 

In this chapter we will emphasize condensation polymers. Since most of these are formed from stepwise kinetics, we will also focus on this kinetic process. 

There is a large, but not total, overlap between the terms condensation polymers and stepwise kinetics and the terms addition (or vinyl) polymers and chain kinetics. In this section we will look at each of these four terms and illustrate the similarities and differences between them. 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

Internal esters (lactones) and internal amides (lactams) are readily polymerized through a chainwise kinetic process forming polyesters and polyamides, clearly condensation polymers with respect to having noncarbons in the backbone, but without expulsion of a by-product ... 

Although Diels-Alder polymerizations involve a reaction of an unsaturated molecule, and the polymer does not have a structure typical of a condensation polymer, the characteristics of the polymerization reaction are those of a condensation polymerization. The kinetics of a Diels-Alder polymerization should be those of a typical condensation polymerization and involve a series of individual reactions, and not a chain type mechanism. A typical second-order rate expression, -dC/dt = kC2 should hold for a good portion of the polymerization reaction, where C is the concentration of both diene and dienophile ends. Typically, this rate would break down only when the molecular mobility is extremely low or when shielding of functional groups occurs in dilute solutions. 

The reaction does not involve elimination of any small molecules, and thus according to Carothers could be classified as addition polymers. However, the polymers are structurally more similar to condensation polymers than to addition polymers. The repeating unit contains functional groups (or is heteroatomed). The formation of the two polymers also proceeds through stepwise kinetics. 

Several additional reactions have been used to prepare condensation polymers, although relatively little is known about their kinetics. Some are indicated here, to illustrate more fully the scope of poly condensations. 

Although the overlap of terms is great, many exceptions exist. For example, the formation of polyurethanes typically occurs through stepwise kinetics. The polymers are classified as condensation polymers, and the backbone is heteratomed, yet no byproduct is released when the isocyanate and diol are condensed. The formation of nylon 6, a condensation polymer, from the corresponding internal lactam occurs through chain-growth kinetics. 

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. 

If chain termination occurs only by coupling or combination, each polymer molecule consists of two kinetic chains which grew independently and were joined together following their mutual termination. The polymer formed is analogous to a di-chain (/ = 2) condensation polymer (Chapter 5). If P represents the probability of continuation of either chain from one of its units to the next, (1 - P) is the probability that a given unit in the chain reacts by termination and the latter can be equated to the ratio of terminated to total units. Since two units are involved in each termination step, one can then write... 

The small molecule literature has some precedent for hydrolysis reactions under pH neutral conditions to be second order in water [18-21]. The hydrolysis mechanism requires some catalytic species either to polarize the carbonyl group or to aid in moving a proton from one oxygen to another. In the absence of added catalyst, as would be the case for a clean polymer, another molecule of water serves this purpose. We can think of condensation polymer hydrolysis as a water-catalyzed hydrolysis. Since two molecnles of water are required in the rate-determining step (one as the reactant and one as the catalyst), the kinetics are second order in water. The water concentration in the polymer is proportional to the RH, so the kinetics are second order in RH. 

Pickett JE, Coyle DJ (2013) Hydrolysis kinetics of condensation polymers under humidity aging conditions. Polym Degrad Stab 98 1311-1320... 

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... 

The polymerization of j3-carboxymethyl caprolactam has been observed to consist of initial isomerization via a second-order kinetic process followed by condensation of the isomer to polymer ... 

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