Abstract: A preparation of aryl carbamates can be achieved readily by carbonylation of an aromatic polyamine compound with diphenyl carbonate (DPC) using a combination of an organic acid and a tertiary amine as a catalyst. Aryl carbamate can be converted into 4,4?-diphenylmethane diisocyanate (MDI) by heating it at about 200 to about 230° C. in a non-polar solvent containing inhibitor such as benzoyl chloride. In another application, trans-ureation of biscarbamates with an amine or mixed amines is found to be extremely facile in a polar solvent such as dimethyl sulfoxide (DMSO) and tetramethylene sulfone (TMS) in absence of any catalyst to make polyurea polymers of high molecular weights. Thus, efficient green-chemistry processes based on biscarbamates in making isocyanate products as well as urea prepolymers, urea elastomers and urea plastics have been developed in all in excellent yields without using reactive phosgene or 4,4?-diphenylmethane diisocyanate separately in the trans-ureation polymerizations.