Difunctional

Difunctional is a term used in chemistry to describe a molecule, monomer, or reagent that contains two functional groups capable of reacting. The two reactive sites may be the same or different, and they enable processes such as chain growth, linking, or crosslinking when the species participate in the same reaction system. Difunctionality contrasts with monofunctionality (one reactive site) and higher functionality (three or more sites).

Common examples include diols such as ethylene glycol (two hydroxyl groups), dicarboxylic acids such as terephthalic acid, and diisocyanates such as toluene diisocyanate. These difunctional units are integral to forming polymers, polyesters, polyurethanes and epoxies, where the two reactive sites can link with complementary groups on other molecules.

In polymer synthesis, difunctional monomers tend to produce linear or moderately branched polymers, depending on stoichiometry and reaction type. When monomers with more than two reactive sites are used, networks and crosslinked materials can form, leading to gels and elastomers. End-group functionality can be preserved to enable further reactions, making difunctional building blocks valuable for sequential or step-growth polymerizations.

The term is often used interchangeably with bifunctional; some texts distinguish the two to emphasize two reactive sites on a single molecule. In practice, the exact behavior of a difunctional compound depends on the chemistry of the functional groups and the reaction conditions.