  Among the different dynamical processes that take place in polymers, methyl
group rotation is perhaps the simplest one, since all the relevant interactions
on the methyl group can be condensed in an effective mean-field one-dimensional
potential. Recent experimental neutron scattering results have stimulated a new
revival of the interest on methyl group dynamics in glasses and polymer
systems. The existence of quantum rotational tunnelling of methyl groups in
polymers was expected for a long time but only very recently (1998), these
processes have been directly observed by high-resolution neutron scattering
techniques. This paper revises and summarizes the work done on this topic over
last ten years by means of neutron scattering. It is shown that the results
obtained in many chemically and structurally different polymers can be
consistently described in the whole temperature range -- from the quantum
tunnelling limit to the classical hopping regime -- as well as in the
librational spectrum, in terms of the Rotation Rate Distribution Model (RRDM),
which was first proposed in 1994. This model introduces a distribution of
potential barriers for methyl group rotation, which is associated to the
disorder present in any structural glass. The molecular and structural origin
of the barrier distribution in polymers is discussed on the basis of a huge
collection of investigations reported in the literature, including recent fully
atomistic molecular dynamics simulations.
