  The large number of gravitational wave (GW) detections have revealed the
properties of the merging black hole binary population, but how such systems
are formed is still heavily debated. Understanding the imprint of stellar
physics on the observable GW population will shed light on how we can use the
gravitational wave data, along with other observations, to constrain the poorly
understood evolution of massive binaries. We perform a parameter study on the
classical isolated binary formation channel with the population synthesis code
SeBa to investigate how sensitive the properties of the coalescing binary black
hole population are on the uncertainties related to first phase of mass
transfer and stellar winds. We vary five assumptions: 1 and 2) the mass
transfer efficiency and the angular momentum loss during the first mass
transfer phase, 3) the mass transfer stability criteria for giant donors with
radiative envelopes, 4) the effective temperature at which an evolved star
develops a deep convective envelope, and 5) the mass loss rates of stellar
winds. We find that current uncertainties related to first phase of mass
transfer have a huge impact on the relative importance of different dominant
channels, while the observable demographics of GW sources are not significantly
affected. Our varied parameters have a complex, interrelated effect on the
population properties of GW sources. Therefore, inference of massive binary
physics from GW data alone remains extremely challenging, given the large
uncertainties in our current models.
