In globular clusters, dynamical interactions are fairly frequent. A consequence of this is that you end up with a phenomenon called "mass segregation" --- heavy stars sink to the center of the cluster, and lighter ones float to the top. As a consequence, if you get a bunch of stellar mass black holes form (and they remain bound to the globular cluster --- not necessarily guaranteed), they will sink to its center, where they will begin to interact with each other. Over time, they will kick out all lighter stars, removing energy, and shrinking their obrits, until gravitational radiation takes over and causes them to coalesce and all merge together into a single intermediate mass black hole.

In the Galaxy, a similar process could take place in a dense open cluster, but the stellar densities are so much lower that it's a lot harder to get the same dynamics. The presence of gas in an open cluster could contribute to the dynamical friction that leads to BH-BH mergers.