We describe a model for polymerization at a surface, in which local kinks within the chain, excited at the free end, may diffuse to the surface, causing termination by disengagement. A model in which kinks do not interact gives a monomodal but broad distribution when kink diffusion is slow. In a more realistic model that permits kink-antikink annihilation, some of the chains escape the kink-controlled population and produce a "living", high molecular weight fraction. A reaction for which this has been characterized is the Wurtz-type reductive coupling of organosilanes, though the phenomenon may be more general.