According to IUPAC, the functionality of a monomer is defined as the number of bonds that a monomers repeating unit forms in a polymer with other monomers. Thus in the case of a functionality of f = 2 a linear polymer is formed by polymerizing (a thermoplastic). Monomers with a functionality f' ≥ 3 lead to a branching point, which can lead to cross-linked polymers (a thermosetting polymer). Monofunctional monomers do not exist as such molecules lead to a chain termination.
From the average functionality of the used monomers the reaching of the gel point can be calculated as a function of reaction progress. Side reactions may increase or decrease the functionality.
However, IUPAC definition and the use of the term in organic chemistry differ with respect to the functionality of a double bond. In polymer chemistry, a double bond possesses a functionality of two (because two points of contact for further polymer chains are present, on each of the two adjacent carbon atoms), while in organic chemistry the double bond is a functional group and thus has a functionality of one.