  Studying the formation and evolution of black hole binaries (BHBs) is
essential for the interpretation of current and forthcoming gravitational wave
(GW) detections. We investigate the statistics of BHBs that form from isolated
binaries, by means of a new version of the SEVN population-synthesis code. SEVN
integrates stellar evolution by interpolation over a grid of stellar evolution
tracks. We upgraded SEVN to include binary stellar evolution processes and we
used it to evolve a sample of $1.5\times{}10^8$ binary systems, with
metallicity in the range $\left[10^{-4};4\times 10^{-2}\right]$. From our
simulations, we find that the mass distribution of black holes (BHs) in double
compact-object binaries is remarkably similar to the one obtained considering
only single stellar evolution. The maximum BH mass we obtain is $\sim 30$, $45$
and $55\, \mathrm{M}_\odot$ at metallicity $Z=2\times 10^{-2}$, $6\times
10^{-3}$, and $10^{-4}$, respectively. A few massive single BHs may also form
($\lesssim 0.1\%$ of the total number of BHs), with mass up to $\sim 65$, $90$
and $145\, \mathrm{M}_\odot$ at $Z=2\times 10^{-2}$, $6\times 10^{-3}$, and
$10^{-4}$, respectively. These BHs fall in the mass gap predicted from
pair-instability supernovae. We also show that the most massive BHBs are
unlikely to merge within a Hubble time. In our simulations, merging BHs like
GW151226 and GW170608, form at all metallicities, the high-mass systems (like
GW150914, GW170814 and GW170104) originate from metal poor ($Z\lesssim{}6\times
10^{-3}$) progenitors, whereas GW170729-like systems are hard to form, even at
$Z = 10^{-4}$. The BHB merger rate in the local Universe obtained from our
simulations is $\sim 90 \mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$, consistent with the
rate inferred from LIGO-Virgo data.
