AbstractAs a novel methodology for the preparation of organoboron polymers, this paper describes “hydroboration polymerization” and “haloboration polymerization”. A polyaddition between diene andtert‐hexylborane (thexylborane) produced a polymer consisting of C‐B bonds in the main chain. The resulting organoboron polymers can be regarded as a polymer homologue of trialkylborane and can be expected as a novel type of reactive polymers. For example, the reaction of organoboron polymer with carbon monoxide followed by the oxidative treatment produced a polyalcohol by the migration of C‐B bonds. Similarly, the reaction with potassium cyanide produced a polyketone. On the other hand, hydroboration polymerization of dicyano compounds such as isophthalonitrile with t‐butylborane produced an air‐stable boron‐containing polymer (polycyclodiborazane) having B‐N four‐membered rings via dimerization of iminoborane species. In haloboration polymerization, i.e., a polyaddition between diacetylene and boron tribromide, a linear poly(organoboron halide) was obtained. This polymer still has B‐Br bonds in its structure and was used not only as a novel reactive polymer but also