Novel organoboron polymers were prepared by alkoxyboration polymerization of diisocyanates with mesityldimethoxyborane. The polymers obtained have boronic carbamate in their repeating units and can be expected as a novel type of reactive polymers. First, alkoxyboration polymerization between mesityldimethoxyborane and 1,6-hexamethylene diisocyanate was examined, and the best reaction condition was optimized to be 140 degrees C in bulk. Both aliphatic and aromatic diisocyanates gave the corresponding polymers. When aromatic diisocyanates were employed, the reactions required severe reaction conditions (150 degrees C). The polymers prepared from aromatic diisocyanates have a high stability toward air. Among various boron monomers, mesityldimethoxyborane was found to be the most suitable monomer for the present polymerization system.