We present a study of the nucleation mechanism that allows the decay o
f the metastable phase (trans-cisoid) to the stable phase (cis-transoi
d) in quasi-one-dimensional nondegenerate polymers within the continuu
m electron-phonon model. The electron-phonon configurations that lead
to the decay, i.e., the critical droplets (or transition state), are i
dentified as polarons of the metastable phase. We obtain an estimate f
or the decay rate via thermal activation within a range of parameters
consistent with experimental values for the gap of the cis configurati
on. It is pointed out that, upon doping, the activation barriers of th
e excited states are quite smaller and the decay rate is greatly enhan
ced. Typical activation energies for electron or hole polarons are alm
ost-equal-to 0.1 eV and the typical size for a critical droplet (polar
on) is about 20 angstrom. Decay via quantum nucleation is also studied
and it is found that the crossover temperature between quantum nuclea
tion and thermal activation is of order T(c) less-than-or-equal-to 40
K. Metastable configurations of nondegenerate polymers may provide exa
mples for mesoscopic quantum tunneling.