The reaction of the Lewis base-stabilized phosphinoborane monomer tBuHPBH(2)NMe(3) (2 a) with catalytic amounts of bis(eta(5):eta(1)-adamantylidenepentafulvene)titanium (1) provides a convenient new route to the polyphosphinoborane [tBuPH-BH2](n) (3 a). This method offers access to high molar mass materials under mild conditions and with short reaction times (20 degrees C, 1 h in toluene). It represents an unprecedented example of a transition metal-mediated polymerization of a Lewis base-stabilized Group 13/15 compound. Preliminary studies of the substrate scope and a potential mechanism are reported.