The solventless reaction of diisopropylaminoborane with n‐butylamine, at room temperature, leads to a mixture of B(sp2)H‐, B(sp3)H2‐, and B(sp3)H3‐containing species. At low temperature, the reaction outcome is completely modified, thus leading selectively to the formation of high‐mass polybutylaminoborane. When extended to a variety of primary amines, under solventless conditions and at low temperature, this reaction provides a new, efficient, and direct metal‐free access to high‐molecular‐mass polyaminoboranes in good to high yields under mild reaction conditions.