  We present a study of the nucleation mechanism that allows the decay of the
metastable phase (trans-cisoid) to the stable phase
  (cis-transoid) in quasi one-dimensional non-degenerate polymers within the
continuum electron-phonon model. The electron-phonon configurations that lead
to the decay, i.e. the critical droplets (or transition state), are identified
as polarons of the metastable phase. We obtain an estimate for the decay rate
via thermal activation within a range of parameters consistent with
experimental values for the gap of the cis-configuration. It is pointed out
that, upon doping, the activation barriers of the excited states are quite
smaller and the decay rate is greatly enhanced. Typical activation energies for
electron or hole polarons are $\approx 0.1$ eV and the typical size for a
critical droplet (polaron) is about $20 \AA$. Decay via quantum nucleation is
also studied and it is found that the crossover temperature between quantum
nucleation and thermal activation is of order $T_c \leq 40 ^oK$. Metastable
configurations of non-degenerate polymers may provide examples for mesoscopic
quantum tunneling.
