A patent application filed by Alexandria BASF in the United States recently claimed that the group has developed a new process to produce pure MDI and polymerized MDI by gas phase phosgene process. Firstly, aniline was reacted with formaldehyde to produce a mixture of pure MDA and polymerized MDA. The reaction temperature is set, so the crude MDA can be converted into gas. After that, the gaseous crude MDA was converted into gas phase by phosgenation. The ratio of aniline to formaldehyde is 7.5. 88-99.9% of the crude MDA mixture was pure MDA, only 0.1-12% was polymerized MDA.