Linear Step Polymerization

We will first consider polymerization in which a linear polymer is formed, and the mechanism is step growth, i.e., linear step polymerization. We already have seen an example of what this could look like (Figure 2.2). Linear step polymerization occurs from polymerization of either bifunctional or difunctional monomers, which have a functionality of two. A difunctional monomer has two of the same reactive groups capable of forming bonds (e.g. two “A” groups), while a bifunctional monomer has two reactive groups, although they are not the same moieties (but can still react to form bonds, e.g. an “A” and “B” group) (Figure 2.4).  We have terminology to describe each of these various combinations of reactions, as depicted in Figure 2.4. For example, if we have monomers that are bifunctional (A-B) and A reacts with B,  we call this type of polymerization “ARB” type. (The “R” represents whatever chemical structure is between the reactive functional groups of the monomer). If we have difunctional monomers that react with themselves (A-A reacts with A-A) we call this “RA2”. And if we have difunctional monomers (A-A and B-B) where A reacts with B, then this is an “RA2+RB2” type reaction.

Figure 2.4 Polymerization of bifunctional and difunctional monomers

Source: Lauren Zarzar

PROBLEM

Consider polycarbonate, formed from the reaction between bisphenol A and phosgene. What “type” of polymerization is this?

Bisphenol A combines with phosgene to form polycarbonate

Source: wikimedia

1. RA₂ + RB₂
2. RA₂
3. RB₂
4. ARB

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ANSWER

A. RA₂ + RB₂

So what exactly are A and B? What functional groups can we use to create polymers, and what sorts of polymers do they produce?