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.

Poly(ethylene tridecanodiate), also known as poly-(ethylene brassylate) (PEB), is a short−long aliphatic polyester obtained from a renewable source. Cooled from the melt by differential scanning calorimetry or by fast scanning calorimetry, PEBs in a range of molar mass between 27000 and 188000 Da display single crystallization exotherms. On heating, PEBs exhibit two major melt-recrystallization events at ∼40 and ∼60 °C prior to their final melting at ∼70 °C. WAXD patterns collected in situ during heating rule out any polymorphic transition, but SAXS patterns collected at the isothermal crystallization temperatures below and above the highest melt-recrystallization event (∼60 °C) indicate a step increase by one repeating unit of the crystal thickness. The overall isothermal crystallization rate displays two minima at the same temperatures where melt-recrystallization was observed on heating. The minima correspond to the transition between 2−3 repeats (T c = 40 °C) and between 3−4 (T c = 60 °C) monomer repeats in the crystal thickness. A minimum also occurs at the same temperature in the isothermal linear growth rates and is accompanied by a minimum in nucleation density. The observed rate minima at the transition between crystals differing by a quantized thickness are equivalent to the behavior of n-alkanes and low-M w PEO fractions and are also explained by the manifestation of self-poisoning. At T c approaching a rate minimum from above, PEB stems with noninteger repeats attach temporarily to the integer lateral growing surface halting productive growth until they detach or expand to complete the layer. Hence, for this type of polyester, it is the length of the stem approaching the growing surface rather than stems with a different conformation that drives self-poisoning.

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A semi-crystalline precision polymer with a high molar mass (>100 kg mol–1), a moderate dispersity (∼1.6), and an isotactic alcohol pendant on each and every fifth carbon of a linear polyethylene backbone is synthesized through highly regioregular ring-opening metathesis polymerization of (S) or (R)-3-(tert-butyldimethylsiloxy)­cyclopentene followed by olefin hydrogenation and alcohol deprotection. The thermal and semi-crystalline properties of these materials are compared to analogues with atactic alcohol pendants and varying degrees of head-to-tail (HT) regioregularity. For atactic polymers with HT from 77 to 99%, the change in glass transition temperature (T g) is minimally affected (47 ± 2 °C), but the crystalline melting temperature (T m) increases substantially from 96 to 137 °C. When highly isotactic alcohol pendant groups are present (90% enantiopure monomer and HT = 96%), the T g increases slightly to ∼53 °C, but the T m increases dramatically to 190 °C. Although the wide-angle X-ray diffraction patterns of atactic and isotactic materials are similar and infer equivalent crystal unit cell packing, the isotactic sample develops ∼40% crystallinity, which is double that observed for an atactic sample (20%) with high HT = 99%. The thermal stability of all samples was >360 °C. Such investigations present unexplored insights on how isotactic and precision microstructure affect material properties of polymeric systems outside the two-carbon branch periodicity of isotactic polymers from vinyl monomers. Synthesis, structural characterizations, thermal properties in addition to intermolecular hydrogen bonding, and other crystalline structural data are discussed comparatively based on tacticity.