  In this paper we report a theoretical study of the effects of the presence of
boron in growing carbon nanotubes. We employ a well established Tight Binding
model to describe the interactions responsible for the energetics of these
systems, combined with the Molecular Dynamics simulation technique and
Structural Relaxation calculations. We find, in agreement with the previous
theoretical/experimental work of Blase {\em et al.} [{\em Phys. Rev. Lett.}
{\bf 83}, 5078 (1999)], that boron favors (n,0) (zig-zag) tubular structures
over (n,n) (arm-chair) ones by stabilizing the zig-zag edge. Furthermore, it is
shown that boron has the effect of delaying the tube closure process, a fact
which could explain the improved aspect ratio experimentally observed in
nanotubes synthesized in the presence of boron. Our dynamical simulations lead
us to propose a mechanism through which this extension of the closure time can
be explained.
