Dialkylboranes were used as a monomer in hydroboration polymerization of dicyano compounds. The bulk polymerization of(n-Bu(2)BH)(2) with alpha,omega-dicyanoalkanes gave the corresponding poly(cyclodiborazane)s consisting of boron-nitrogen four-membered rings. The obtained polymers were colorless gums and soluble in common organic solvents such as THF or chloroform. The molecular weights of the resulting poly(cyclodiborazane)s were higher than those prepared from monoalkylboranes. Poly(cyclodiborazane) a thus obtained showed a high stability toward air and moisture. The thermal stability was also much improved in comparison with those prepared by hydroboration polymerization of dicyano compounds with monoalkylboranes. After pyrolysis of poly(cyclodiborazane)s, some residue remained. This result indicates a possibility of poly(cyclodiborazane) as a polymeric precursor for the preparation of boron-containing inorganic materials.