Total world production of MDI and polymeric MDI is over 7.5 million tonnes per year (in 2017). As of 2019, the largest producer was
Wanhua Chemical Group. Other major producers are
Covestro,
BASF,
Dow,
Huntsman,
Tosoh, Kumho Mitsui Chemicals. All major producers of MDI are members of the International Isocyanate Institute, whose aim is the promotion of the safe handling of MDI and
TDI in the workplace, community and environment. The first step of the production of MDI is the reaction of
aniline and
formaldehyde, using hydrochloric acid as a catalyst to produce
4,4'-Methylenedianiline and other diamine precursors, as well as their corresponding polyamines: : Then, these diamines are treated with
phosgene to form a mixture of isocyanates, the isomer ratio being determined by the isomeric composition of the diamine. Two different reaction mechanisms for this transformation are possible, namely "phosgenations first" and "step-wise phosgenations". : Distillation of the mixture gives a mixture of oligomeric polyisocyanates, known as
polymeric MDI, and a mixture of MDI isomers which has a low 2,4′ isomer content. Further purification entails
fractionation of the MDI isomer mixture. ==Reactivity of the isocyanate group==