  Using Monte Carlo codes, we follow the collisional evolution of clusters in a
variety of scenarios. We consider the conditions under which a cluster of main
sequence stars may undergo rapid core collapse due to mass segregation, thus
entering a phase of runaway collisions, forming a very massive star (VMS, M >=
1000 Msun) through repeated collisions between single stars. Although
collisional mass loss is accounted for realistically, we find that a VMS forms
even in proto-galactic nuclei models with a high velocity dispersion (many 100
km/s). Such a VMS may be a progenitor for an intermediate-mass black hole (M >=
100 Msun). In contrast, in galactic nuclei hosting a central massive black
hole, collisions are found to be disruptive. The stars which are subject to
collisions are progressively ground down by high-velocity collisions and a
merger sequence appears impossible.
