Radicals, often referred to as free radicals, are atoms, molecules or ions with unpaired electrons on an open shell configuration. The unpaired electrons cause them to be highly chemically reactive; the polymerization of vinyl-containing monomers like methyl acrylate is one example. An illustration of radical polymerization is shown in Figure 1. As is generally known, radicals are highly reactive species and generally undesireable in cosmetic products.
In polymer chemistry, ring-opening polymerization is a form of additional polymerization where the terminal end of a polymer acts as a reactive center. In this reactive center, further cyclic monomers join to form a larger polymer chain through ionic propagation. The polymerization of ethylene oxide to form polyethylene glycol (PEG) is an example, as shown in Figure 2.
The opening of the ring is highly energetic and is catalyzed by acid or, more commonly, by base. Residual ethylene oxide must be carefully controlled as it also is undesireable in cosmetic products.
Condensation polymers are any polymers formed through a condensation reaction, releasing small molecules as by-products such as water or methanol. Types of condensation polymers include polyamides, polyacetals and polyesters. The polymer class is interesting in cosmetic chemistry since the products do not have vinyl monomers like those encountered in radical polymer reactions.
Monomers with only one reactive group terminate a growing chain, thereby producing end products with a lower molecular weight. Linear polymers are formed when monomers with two reactive end groups and more than two end groups produce three dimensional polymers that are crosslinked. An example is shown in Figure 3. Since the monomers used in a preparation are generally not highly reactive, they are more tolerated in formulations by cosmetic chemists.