  We explore a newly proposed channel to create binary black holes of stellar
origin. This scenario applies to massive, tight binaries where mixing induced
by rotation and tides transports the products of hydrogen burning throughout
the stellar envelopes. This slowly enriches the entire star with helium,
preventing the build-up of an internal chemical gradient. The stars remain
compact as they evolve nearly chemically homogeneously, eventually forming two
black holes, which, we estimate, typically merge 4--11 Gyr after formation.
Like other proposed channels, this evolutionary pathway suffers from
significant theoretical uncertainties, but could be constrained in the near
future by data from advanced ground-based gravitational-wave detectors. We
perform Monte Carlo simulations of the expected merger rate over cosmic time to
explore the implications and uncertainties. Our default model for this channel
yields a local binary black hole merger rate of about $10$ Gpc$^{-3}$ yr$^{-1}$
at redshift $z=0$, peaking at twice this rate at $z=0.5$. This means that this
channel is competitive, in terms of expected rates, with the conventional
formation scenarios that involve a common-envelope phase during isolated binary
evolution or dynamical interaction in a dense cluster. The events from this
channel may be distinguished by the preference for nearly equal-mass components
and high masses, with typical total masses between 50 and 110
$\textrm{M}_\odot$. Unlike the conventional isolated binary evolution scenario
that involves shrinkage of the orbit during a common-envelope phase, short time
delays are unlikely for this channel, implying that we do not expect mergers at
high redshift.
