How chain stereoconfiguration and molecular weight influence Poly(propylene oxide) crystallization

We report on the synthesis and crystallization behavior of poly(propylene oxide) (PPO) with tunable molar masses and defined stereoconfiguration (PPO-R and PPO-S), obtained through controlled oxyanionic ring-opening polymerization using hexaethylene glycol (EG6) as the initiator, with an equimolar mixture of potassium acetate (KOAc) and 18-crown-6 ether (18C6). This method provides access to well-defined enantiopure PPO samples across a broad range of number-average molecular weights (Mn), allowing for the independent evaluation of how molecular weight and stereoconfiguration influence crystallization. Morphology, thermal transitions, structural features, and crystallization kinetics were analyzed using Polarized Light Optical Microscopy (PLOM), Differential Scanning Calorimetry (DSC), in situ Wide- and Small-angle X-ray Scattering (WAXS/SAXS), and, for the first time in PPO, thermal fractionation via Successive Self-nucleation and Annealing (SSA). Both PPO-R and PPO-S display increasing thermal transitions with Mn, eventually reaching a plateau. Although they crystallize into identical orthorhombic unit cells, the two enantiomers show small yet consistent and reproducible kinetic differences across all techniques used: PPO-R crystallizes faster at low Mn, while PPO-S does so at high Mn. This crossover, related to a specific Mn value, though unexpected for two enantiomeric polymers forming identical lattices, was consistently observed by different experimental techniques across nucleation, spherulitic growth, overall crystallization rate, and SSA fractionation. This confirms the effect is real and experimentally reliable. We provide a mechanistic interpretation suggesting that stereoconfiguration could be influencing melt dynamics, likely through subtle differences in chain diffusion and entanglement onset. Racemic PPO-R:S blends were prepared at both low and high Mn. No stereocomplexation was observed; however, these blends exhibited lower melting transitions and slower crystallization kinetics than the enantiopure samples, possibly due to packing frustration between chains of opposite helicities. Overall, molecular weight and stereoconfiguration are effective parameters for tuning PPO crystallization kinetics, thereby enabling PPO-based blends and copolymers with controlled crystallization rates and expanded processability in biodegradable polymer systems.