A non-isocyanate route (NIR) of making polyureas of high molecular weight has been found through transureation of N,N'-diphenyl-4,4′-methylenediphenylene biscarbamate (4,4′-DP-MDC) with a variety of diamines and mixed diamines. The preparation of 4,4′-DP-MDC was achieved readily by carbonylation of 4,4′-methylenedianiline (4,4′-MDA) with diphenyl carbonate (DPC) using organic acids as catalysts. It was found that the highest yield (99%) of pure 4,4′-DP-MDC can be isolated in a toluene solution under mild conditions co-catalyzed by benzoic acid and tertiary amine.