Polycondensation Reactions with Substitution Effects

When the reaction of a functional group changes the reactivity of its neighbors, a substitution effect exists. 

A first-shell substitution effect (ESSE) is the simplest kind of departure from ideal behavior. It is the situation where reactivity is affected only by the reaction of the functional groups attached to the same monomer unit. FSSEs are encountered in many polycondensations, as reactivities of functional groups in monomers are often different from the reactivities of end groups in polymers because of mutual steric, resonance, or electrostatic interactions. 

Eor instance, a glycol HO-X-OH shows no substitution effect in an esterification reaction if the reactivity of the hydroxyls in -COO-X-OH is the same a single kind of group -OH needs to be considered and the description of the reaction scheme is much simpler. 

A second-shell substitution effect (SSSE) occurs when the reactivity of a functional group is affected by the reaction of all the groups attached not only to the same monomer unit, but also to the units linked to that one. This behavior has been recognized in urea/formaldehyde formation (see Section 3.3.4.2). 

Linear polycondensations AXA + BYB with FSSEs in both root units X and Y are examples found in some important processes, such as the esterification of tereph-thalic acid and ethylene glycol leading to poly(ethylene terephthalate) (PET). Four different rate constants k are needed to describe forward reactions, and possibly an equal number of apparent rate constants (k , i = 1... 4) are required to describe reverse reactions  

---

This example illustrates several difficulties encountered in modeling reversible polycondensation reactions with substitution effects. A major problem is the possible change in the nature of bonds because another bond connecting a different unit has been destroyed. There is no closed set of rate equations in terms of the concentrations of functional groups [AXA], [BYB], [Zp], [Z j, [Zb], and [Zp], even with the help of the two stoichiometric restrictions [X] = [AXA] - - [Zp] + [Za] + [Zp] and [Y] = [BYB] - - [Z ] -I- [Zp] + [Zp]. Introduction of more complex chemical entities does not alleviate the problem. 

Description of Reactions in Polycondensations of Several Monomers with Substitution Effects... 

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. 

---