Methane diphenyl diisocyanate, commonly abbreviated as MDI (Methylene diphenyl 4,4'-diisocyanate), is an aromatic diisocyanate. It exists in three different isomeric forms, 2,2´-MDI, 2,4´-MDI and 4,4´-MDI, the latter being however the most used and qualified as "pure MDI" or simply MDI.

MDI is abundantly used combined with different polyols, glycols, and/or diamines in the production of polyurethane. It is the diisocyanate that captures the largest market share.

The chemical synthesis of MDI starts from the condensation reaction between formaldehyde and aniline in an acidic medium:[2]​.

2(Phe)-NH + HCHO → HN-Phe-CH-Phe-NH + HO

The reaction product is then reacted with phosgene to give the diisocyanate:.

HN-Phe-CH-Phe-NH + 2(Cl)CO → OCN-Phe-CH-Phe-NCO + 4HCl.

The product is a mixture of polymeric MDI and a mixture of isomers from which the desired isomer (usually 4,4´-MDI) is purified.[3]​ [4]​

Currently, efforts are being made to try to replace the phosgenation process[3][5][6]​ in the synthesis of isocyanates, due to the high toxicity of phosgene.

MDI develops a chemistry similar to that of the rest of isocyanates. However, having an aromatic nature, the reactivity is greater than in the case of aliphatic diisocyanates (such as hexamethylene diisocyanate, HDI or isophorone diisocyanate, IPDI), giving rise to highly exothermic reactions, and can react very violently when reacted with diamines. The polymerization speed with dibases is greater than that presented by aliphatic diisocyanates.

MDI is mainly used in the synthesis of polyurethane. Especially due to its reactivity, it is used in the synthesis of polyurethane foams, where a short polymerization time is required. MDI is also used in the synthesis of elastomers[7]​ and in polyurethane-based adhesives[8]​.

Due to its low vapor pressure, MDI presents a lower risk of inhalation or contamination during handling than other diisocyanates such as toluene diisocyanate (TDI) or hexamethylene 1,6-diisocyanate (HDI). However, it is one of the most reactive diisocyanates and its contact can cause irritation and even allergic reactions. Despite its low vapor pressure and solid state, small traces in the environment can cause serious respiratory problems. The proper use of relevant personal protective equipment such as gloves, glasses, gowns and mask is essential.