How is a polymer formed?

Polymerization requires chemical reactions, and chemical reactions happen as a consequence of collisions of molecules; if monomers never encounter each other, they can never react! Therefore, we would expect that the rate of a reaction has dependence on collision frequency. This is an especially important consideration for polymers, because polymers can be really big which means they diffuse slowly, leading to a lower rate of collision, providing fewer opportunities for reactions. However, because the polymer is large and diffuses more slowly, each encounter between reactants actually has a longer duration, which favors a reaction. These two effects — less overall collisions but longer collision duration — are assumed to balance each other out. Analyses of polymer reaction kinetics suggest that this is a reasonable assumption in most cases. We are therefore going to make this very key assumption called the principle of equal reactivity, which is that reactivity does not vary as a function of polymer size. This assumption underlies all of our future analysis of polymerization, and is important to keep in mind.

Now to actually make a polymer in the first place, you have to have the correct degree of functionality. Clearly, for a polymer to form and grow, 1) there has to be an initial reaction between monomers, and 2) there has to be reaction between monomers and the growing polymer molecule. Consider the two molecules in Figure 2.1, where A and B are able to react together to form a bond. Because each molecule only has one A or B group, once they react, there is no more functionality left to continue the polymerization. Thus, these molecules don’t have enough functionality to form a polymer.

Figure 2.1: Reaction between monomers without functionality

Source: Lauren Zarzar

What if instead you had molecules with both an A and B group, as shown in Figure 2.2. Now, after A and B react, there is still enough functionality on the molecules to continue adding monomers to the polymer chain. There is sufficient functionality for a polymer to form.

Figure 2.2: Reaction between monomers with functionality of two

Source: Lauren Zarzar

In Figure 2.2 the monomers have a functionality of two, and so the polymer that forms is a linear polymer (We first learned about skeletal structures in Lesson 1). But what if the monomer had even more functionality? Consider Figure 2.3 where the monomer has a functionality of 4. Now we see that we can get formation of branched polymers and/or network polymers.

Figure 2.3: Reaction between monomers with functionality of four

Source: Lauren Zarzar

So identifying the functionality in a monomer — and whether it has enough to form a polymer in the first place — is going to be key. But let’s say you already know that your monomer can form a polymer — what happens next? How does a monomer actually add to another monomer, or add to a dimer, or add to an existing polymer? There are two general mechanisms for how this can happen: step growth polymerization and chain growth polymerization.