Reactivity of functional groups

Generally condensation reactions, such as (5.7)-(5.10), are reversible, so that the eliminated water must be removed if a high polymeric product is to be formed. The rate of a step polymerization is the sum of the rates of reaction between molecules of various sizes, that is, the sura of the rates for reactions such as (5.7)-(5.10). To describe the course of these reactions in terms of reaction kinetics would seem at first sight to be a very complicated task. However, fortunately it is possible to introduce simplifying approximations that make the kinetic problem tractable. 

Consider, for example, a polyesterification in which a dibasic acid condenses with a glycol, as shown by reactions (5.7)-(5.10). No matter what the reactants happen to be (which may be dimers, trimers, tetramers, pentamers. high polymers), the chemical reaction in each step is the same and may be written as 

The kinetic analysis of polycondensation with innumerable separate reactions thus becomes greatly simplified if one assumes that (1) the reactivities 

Monomers bearing functional groups such as -OH, -COOH, -NH2, -NCO, etc., undergo step polymerization. 

The growth of polymer molecules proceeds by a stepwise intermolec-ular reaction (at a relatively slow rate), normally with the elimination of small molecules as by-products of condensation, such as H2O, HCl, NH3, etc., in each step. The molecule never stops growing during the course of the polymerization. 

One basic simplifying assumption proposed by Flory, when analyzing the kinetics of step-growth systems, was that all functional groups can be considered as being equally reactive. This implies that a monomer will react with both monomer or polymer species with equal ease. 

MNH2-R-NH2+ HOOC-R -COOh - hH-NH-R-iSrHCO-R -Caf OH+(2 - 1)H20 wHO-R-OH + NCO-R -NCO — -j-OROCONH-R -NHCOj wHOOC-R-COOH— H04oC-R-CC 0-[ 7H+ ( -1)H20 

25 percent reaction 50 percent reaction 75 percent reaction 

FIGURE 2.1 Diagrammatic representation of a step-growth polymerization. 

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Equal reactivity of functional groups irrespective of the size of the molecule to which the group is attached. 

The concept of equal reactivity of functional groups, Chap. 5. 

One of the main assumptions which have been made in the study of polyesterifications is the concept of equal reactivity of functional groups. It was first postulated by Flory1 who, studying various polyesterifications and model esterifications, found the same orders of reaction and almost the same rate constants for the two systems. He concluded that the reaction rate is not reduced by an increase in the molecular weight of the reactants or an increase in the viscosity of the medium. The concept of equal reactivity of functional groups has been fully and carefully analyzed by Solomon3,135 so that we only discuss here its main characteristics. Flory clearly established the conditions under which the concept of equal reactivity can be applied these are the following ... 

The prindple of equal reactivity of functional groups originates in the comparison by Flory of the results obtained for monoesterifications and polyesterifications. This means that it is particularly important to check whether Flory hypotheses are corred and whether the study of a polyesterification must be limited to the last stages of the reaction. 

Accordingly, the resulting current reflects the rate at which electrons move across the electrode-solution interface. Potentiostatic techniques can thus measure any chemical species that is electroactive, in other words, that can be made to reduce or oxidize. Knowledge of the reactivity of functional group in a given compound can be used to predict its electroactivity. Nonelectroactive compounds may also be detected in connection with indirect or derivatization procedures. 

Compounds containing a (CH2)3 linkage to the POSS core have become more widely studied than many of the alkyl derivatives because of their ready preparation and because the propyl arm is the shortest linker that usually prevents the reactivity of functional groups being enhanced by the nearby silicon... 

Introduction of new precursor architectures brings about new challenges in description and modeling of network formation because the apparent reactivities of functional groups become dependent on the size and shape of the precursors... 

Changes in reactivities of functional groups of hard components (substitution effect). 

CONSTITUTIONAL FACTORS AFFECTING THE REACTIVITY OF FUNCTIONAL GROUPS... 

FIGURE 6.1 Constitutional factors affecting the reactivity of functional groups. (A) The reactivity of W depends on the location of the residue. (B) The amino group of a dipeptide ester reacts with the ester carbonyl to form a cyclic dipeptide amino groups of other peptide esters do not react in this manner. (C, D) Reactions between residues of identical configuration do not occur at the same rates as reactions between residues of opposite configuration. 

Chemical equilibrium or chemical kinetics The reactivity of functional groups Is Independent of the size and topology of the... 

The slow increase in molecular weight was mistakenly thought originally to be due to the low reactivity of functional groups attached to large molecules. It is, however, simply a... 

There are instances where some or all parts of the concept of equal reactivity of functional groups are invalid [Kronstadt et al., 1978 Lovering and Laidler, 1962], The assumption of equal reactivities of all functional groups in a polyfunctional monomer may often be incorrect. The same is true for the assumption that the reactivity of a functional group is... 

The product of a polymerization is a mixture of polymer molecules of different molecular weights. For theoretical and practical reasons it is of interest to discuss the distribution of molecular weights in a polymerization. The molecular weight distribution (MWD) has been derived by Flory by a statistical approach based on the concept of equal reactivity of functional groups [Flory, 1953 Howard, 1961 Peebles, 1971]. The derivation that follows is essentially that of Flory and applies equally to A—B and stoichiometric A—A plus B—B types of step polymerizations. 

The molecular weight distribution and/or PDI has been described for several cases where the assumption of equal reactivity of functional groups is not valid. Unequal reactivity is easily handled by the Macosko-Miller method. For the A—A + B—B + B B system described in the previous section, we simply redefine the relationship between P and y by... 

As mentioned previously, the behavior of systems containing bifunctional as well as trifunctional reactants is also governed by the equations developed above. The variation of wx for the polymerization of bifunctional monomers, where the branching coefficient a is varied by using appropriate amounts of a trifunctional monomer, is similar to that observed for the polymerization of trifunctional reactants alone. The distribution broadens with increasing extent of reaction. The effect of unequal reactivity of functional groups and intramolecular... 

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