Mechanism of Step Reactions

In Sect. 3.1.1 it was established that polymerization reactions must have high precision in repetition in order to yield macromolecules. The two most common reactions for such high-yield polymerizations are step-growth and chain-growth polymerizations. 

These two reactions have largely different reaction paths and lead to different molar-mass distributions which are often rather broad and must be described using the techniques derived in Sect. 1.3. Step-growth polymerization is treated in the remaining part of this Sect. 3.1, chain-growth polymerization is covered in Sect. 3.2. 

One way of two to yield precise molecular composition is the reaction of bifunctional monomers  

Assuming a chemical group A can react only with a second group B, as for an alcohol and an acid on esterification, one can think of two types of such polymerizations. One occurs with identical monomers with oppositely reacting groups on the ends of each molecule, as in A-B, shown in the top two reaction schemes of Fig. 3.6, the other occurs between two different monomers, each with two identical groups on its ends, as shown in the second three reactions of Fig. 3.6. The five major characteristics of step-reaction polymerizations are listed at the bottom of Fig. 3.6 and will be detailed next. Point 5 is particularly troublesome since it will lead to an inactive molecule. 

Step 2 is shown after half of the reaction is over. Larger molecules have now appeared, and the weight fraction of monomers is less than 0.2. Still, one is far from having made polymer molecules. The early decrease in monomer concentration and the late appearance of polymers is typical for the step-growth mechanism. 

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