Reactions of Functional Groups

Kinetic considerations are of paramount importance in understanding the mechanism of step-growth polymerization. As stated in Chapter 1, chain-growth polymerizations take place in discrete steps. Each step is a reaction between two functional groups for instance, in a polyesterification reaction it is a reaction between -COOH and -OH. The increase in molecular weight is slow. The first step is a condensation between two monomers to form a dimer  

A dimer can react next with another monomer to form a trimer  

The kinetics of step-growth polymerization can be derived from a polyesterification reaction that follows the same course as all acid-catalyzed esterifications.  

Reaction of the protonated carboxylic acid group with the alcohol  

The above polyesterifications, like many other reactions, are equilibrium reactions. They must be conducted in a way that allows the equilibrium to shift to the right to attain high molecular weights. One way is by continual removal of the byproducts. In such situations the reactions take place at nonequilibrium conditions and there is no Ka. 

Principles of Polymer Chemistry, DOI 10.1007/978-l-4614-2212-9 7, 1st and 2nd editions Kluwer Academic/Plenum Publishers 1995, 2000, 

7 Step-Growth Polymerization and Step-Growth Polyrntrs 

Since the unsaturation in MA is present in some MA derivatives, attention will be directed to the reaction of either MA or its simple derivatives such as maleic acid, esters, or amides. These derivatives, as can be seen in this chapter, are relatively easy to obtain. A fact to remember is that often the reactivity of its derivative is less than MA. However, this is hardly a detriment, since many times a number of other factors such as ease of handling, convenience, and available routes to the desired end take precedence. In this connection, reactions at the anhydride function are useful since ester, amic acids, and imides are made by the participation of the anhydride function. This possibility of independent manipulation lends MA versatility and uniqueness. Therefore, in this chapter, in addition to the reactions of MA, reactions of common derivatives of MA are used where synthetically or mechanistically pertinent. 

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A corresponding equation exists for the ratio of equilibrium constants. The utility of equations of this form lies in the fact that the p value is characteristic of a particular reaction of functional groups, while the o value is characteristic of the nonreactive functional groups. Once a values have been determined for one class of reactions, they may be used for another class of reactions. Hence a knowledge of k0 and p for... 

In the condensation polymerisation or step growth polymerisation, the polymer molecules are built up through many separate reaction of functional groups. 

It should be Dointed out in conclusion that the results obtained can be used for the further develooment of the theory of reactions of functional groups of macromolecules in different processes including both intra-and intemolecular interactions of polymer chains. 

The control of the synthesis of polymers is crucial to obtain the final bulk properties of the polymers needed for the end application. The use of enzymes in polymer synthesis has been demonstrated to allow control of polymer properties such as average molecular weight and dispersity, avoid the use of toxic intermediates, enable the selective reaction of functional groups and allow the use of unstable intermediates. 

Table II. Comparison of calculated and experimentally derived values of the final extents of intramolecular reaction for RA2 + RBf polymerisations, under conditions of the random reaction of functional groups (p° o)...

Extrapolation of pj. g to the limit of zero pre-gel intramolecular reaction for given reaction systems shows that post-gel intramolecular reaction always results in network defects, with significant increases in Mg above Mg. Such post-gel intramolecular reaction is characterised as pg g. The variation of pg g with intramolecular-reaction parameters shows that even in the limit of infinite molar mass, i.e. no spatial correlation between reacting groups, inelastic loops will be formed. The formation may be considered as a law-of-mass-action effect, essentially the random reaction of functional groups. Intramolecular reaction under such conditions (p2 ) must be post-gel and may be treated using classical polymerisation theory. 

With few exceptions, the reactions of functional groups separated by a chain of two or more saturated atoms from carbazole nitrogen show no deviation from their normal behavior. 

The structure and properties of a network polymer are determined by the relation between the inter- and intramolecular reactions of functional groups. The latter gives rise to ineffective cycles, and has been studied fairly well3-5 7°.71-8i-85.i°8 u2 Their occurrence in the system results in the gelation point being shifted towards higher conversions, higher sol fraction, fewer elastically active chains, and smaller equilibrium modulus of the network. 

In aliphatic compounds, reactions of functional groups are often modified very significantly by an adjacent carbonyl group. As would be expected from the discussion in the preceding section, the reactions of certain substituents a and y to pyridine-like nitrogen atoms in azole rings are similarly influenced. Such effects on substituents can be classified into six groups. 

This review has been concerned with the photochemistry of heterocyclic systems photoreactions which are more correctly designated as reactions of functional groups have, in general, been omitted or not seriously considered. Thus, the formation of pinacols by the photoreduction of such heterocyclic ketones as 3-acetylpyridine 453 and the keto sulfone (422)454 has not been included, nor has cis-trans isomerization been reviewed. 

The degradation reactions of polymers have been widely reviewed 525). In the absence of air, thermal reactions are the important degradation route. They may involve reactions of functional groups on the chain without chain scission, typified for example by the dehydrochlorination of PVC, or reactions involving chain scission, often followed by depropagation and chain-transfer reactions to yield complex mixtures of products. This latter route would be typical of the degradation of poly(methyl methacrylate), which depolymerizes smoothly to its monomer, and of polystyrene, which produces a wide range of tarry products. 

Usually, mechanistic proposals based on product studies are supplemented by independent evidence from similar reactions (i.e. arguments based on analogies). Such proposals might also be supported by a knowledge of characteristic reactions of functional groups and properties of reactive intermediates (e.g. acidities of C—H groups, or relative stabilities of carbocations), and the proposals are a basis for the more detailed investigations by methods discussed in later chapters. 

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