Intermolecular =C-H•••O=C Hydrogen Bonding and Conformational Preference in Crystalline Poly(butylene 2,5-furandicarboxylate)

Besides maximizing packing efficiency, molecular interactionsalsoplay an indispensable role upon conformational ordering for crystallizationof long-chain polymers. Herein, structural evolution upon heatingof biobased poly-(butylene 2,5-furandicarboxylate) (PBF) is followedby temperature-dependent infrared (IR) spectroscopy and wide-angleX-ray diffraction (WAXD) to elucidate the molecular interactions andconformational preference in its crystalline state. By estimatingthe group distances according to atomic coordinates of its crystalstructure, the stronger intermolecular interaction than the van derWaals force is speculated to exist in crystalline PBF, which is fullyconfirmed by the changes of IR absorptions associated with -C-Hand C-O groups upon heating as an abnormal -C-H center dot center dot center dot O-Chydrogen bonding. The combination of temperature-dependent -C-H band position and crystal lattice expanding supports a good thermostabilityof this interaction up to melting, fully defining its nature as stronghydrogen bonding rather than the generally reported weak H center dot center dot center dot O-Cbonding in biodegradable polyesters. By further analyzing the confirmationalordering in its crystalline state, it is believed that such intermolecular-C-H center dot center dot center dot O-C hydrogen bonding stabilizesthe energetically inaccessible syn-syn conformerof 2,5-furandicarboxylic moieties for PBF crystallization.