  We present analytical and numerical study of discrete breathers identified as
localized deformations of valence angles accompanied by change of valence bonds
in crystalline polyethylene (PE). It is shown that such breathers can exist
inside the optic wave number band and can propagate along the macromolecule
chain with subsonic velocities. Analytical results are confirmed by numerical
simulation using Molecular Dynamics procedure. We examine also stability of the
breathers relative to thermal excitations and mutual collisions as well as to
collisions with acoustic non-topological and topological solitons. The
conditions of breathers existence depend strongly on their generating
frequency, relationship between stiffnesses of valence bonds and valence angles
as well as on type of nonlinear characteristics of intrachain interactions.
