  With the remarkable success of the LVK consortium in detecting binary black
hole mergers, it has become possible to use the population properties to
constrain our understanding of the progenitor stars' evolution. The most
striking features of the observed primary black hole mass distributions are the
extended tail up to 100M$_\odot$ and an excess of masses at 35M$_\odot$.
Currently, isolated binary population synthesis have difficulty explaining
these features. Using the well-tested BPASS detailed stellar binary evolution
models to determine mass transfer stability, accretion rates, and remnant
masses, we postulate that stable mass transfer with super-Eddington accretion
is responsible for the extended tail. Furthermore, that the excess is not due
to pulsation-pair instability, as previously thought, but due to stable mass
transfer. These systems are able to merge within the Hubble time due to more
stable mass transfer with extreme mass ratios that allows the orbits to shrink
sufficiently to allow for a merger. These finding are at odds with those from
other population synthesis codes but in agreement with other recent studies
using detailed binary evolution models.
