Industry expert Tony O’Lenick asks: What is the difference between residual monomer concentration in radical polymerization vs. condensation polymerization techniques?
The term polymer describes a large number of classes of molecules consisting of repeating structural units, or monomers, connected by covalent chemical bonds. Polymers are made up of many repeated monomers linked together using various polymerization processes.
One type of process relates to the radical polymerization of reactive vinyl groups to form polymers. An example of this is the polymerization of acrylamide to form polyacrylamide (-CHCHCONH2-). This polymerization reaction is conducted by reacting the double bonds in acrylamide CH2=CH-C(O)NH2.
The level of residual monomer acrylamide needs to be low since it is a neurotoxin. Polyacrylamide is not toxic, but unreacted acrylamide can be present in the polymerized acrylamide. Even within the class of radical polymerization, there are a number of different types. The nature of the monomer, how the process is carried out, and post-treatment methods all affect residual monomer. In this type of polymerization, the residual monomer content is monitored carefully.
In contrast, there are polymers that do not form by reaction of vinyl monomer—an example of which is a polyester. Unlike polyacrylamide, polyesters are examples of a class of polymers called condensation polymers that are formed using classical organic chemistry, not radical chemistry.
In one process, a difunctional acid is reacted with a difunctional alcohol, or for easier reaction conditions, a di-acid chloride and a di-alcohol can be used. For example, a polyester results when polyoxyethylene glycol HO-(CH2CH2)xOH is reacted with dodecanedioic acid HOOC-(CH2)10-COOH. The resulting material is a polyester polymer HO-[(CH2CH2)xOOC-(CH2)10COO]-.
This type of polymer is neither made with vinyl reactants nor radical processes. It is made using common organic reaction techniques; consequently, the question of residual monomer is not very important, but the removal of byproducts becomes the important issue. Without the removal of byproducts, high molecular weight polymers cannot be obtained.
Polymers are made using different processes and raw materials. It is the selection of these that dictate not only functional properties of the polymer, but also the need for additional analysis including residual monomer content.