Functionality (chemistry)

In organic chemistry, functionality is often used as a synonym for functional group. For example, a hydroxyl group can also be called a HO-function.[1][2]

Functionalisation means the introduction of functional groups, for example

• the functionalisation of a surface[3] (e.g. silanization for the specific modification of the adhesion of a surface)
• the functionalization of nanoparticles of a metal or metal oxide to stabilize such nanoparticles[4] or
• the so-called C-H functionalization,[5] which means the substitution of a C-H bond by a functional group, bonded at the same carbon atom

According to IUPAC, the functionality of a monomer is defined as the number of bonds that a monomer's 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.[6]

From the average functionality of the used monomers the reaching of the gel point can be calculated as a function of reaction progress.[7] Side reactions may increase or decrease the functionality.[8]

However, IUPAC definition and the use of the term in organic chemistry differ with respect to the functionality of a double bond.[6][9] 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.