Under isothermal crystallization (T-c) from the melt, polyacetals spaced by 12, 18, 19, or 23 methylenes develop two or three distinctive layered polymorphs. The polymorphs formed in the lowest T-c range are kinetically favored (hexagonal and Form I) and characterized by highly nucleated small axialites up to T-c very close to their melting point. In the higher range of T-c, a thermodynamically more stable Form II develops that melts at 5-8 degrees higher temperatures and forms large spherulites. Form I and Form II overlap in a very small range of T-c. While the overall crystallization kinetics of Form I display the usual negative temperature coefficient, an inversion of the dependence of the rate of Form II with temperature occurs when approaching from above the narrow T-c range where Form I and Form II coexist. The inversion is attributed to a competition in nucleation between Form I and Form II. Just before inception of Form II, the crystallization rate is so low that it becomes basically extinguished. The degree of crystallinity recovers when pure Form II develops with a small increase in T-c. Although in the overlapping range, the growth rates of Form I are significantly lower than those of Form II, compared at a fixed undercooling, the rates of Form I are one order of magnitude higher than those of Form II. The difference is attributed to a two to six times higher energy barrier for nucleation of Form II, calculated from analysis of growth rate data according to surface nucleation theory. Such a difference explains the observed variation in nucleation density between the two polymorphs. A minimum in the growth rate of Form I of PA-12, consistent with the effect of "self-poisoning", occurs at T-c approaching the melting point of the hexagonal phase from above.