Different Crystalline Chain Conformations Detected by Nuclear Magnetic Resonance Account for Even–Odd Effects in Biodegradable Polyethylene-Like PE-2, N Polyesters

Abstract

Abstract The distinct conformations and motions of diol segments in the crystallites of easily accessible polyethylene-like aliphatic polyesters made from C11, C12, C18, or C19 diacids with 13C2-ethylene glycol (PE-2*,11/12/18/19, respectively) were studied by solid-state 13C nuclear magnetic resonance (NMR). Peak positions of PE-2,11 and –19 were remarkably similar but distinct from corresponding chemical shifts in PE-2,18 and PE-2,12, indicating an even/odd difference in crystal structure. Conformations were probed using centerband-only detection of exchange (CODEX) NMR and static 2D experiments yielding the O13CH2 chemical-shift tensor. Quantum-chemical calculations for the expected all-anti chain conformation were in agreement with the chemical-shift tensor in PE-2*,11 and 19, while the distinct PE-2*,18 2D NMR pattern was matched at energy minima of two gauche CO–CC torsion angles of opposite sign. The resulting doubly-kinked chain conformation in the crystal is in agreement with a recent X-ray structure. Motions of the 13C–13C bonds in the crystallites probed by the 13C–13C dipolar coupling showed limited amplitudes in PE-2*,19. Narrowing of the dipolar spectrum of PE-2*,18 was more pronounced, indicating motions of the C–C bond sweeping ≥36°, while the 13C chemical-shift anisotropy, which probes the O–CH2 bond orientation, remained nearly invariant. These data indicate conformational jumps of diol segments between gauche+ and gauche– in the crystallites of PE-2*,18, with moderate activation energy. The stabilizing entropy increase by Rln2 resulting from this dynamic disorder can account for the ∼10 K higher melting point of PE-2,18 compared to PE-2,19.