Crystal Clear: A Bottom-Up Strategy for Polymer Structure Determination Using Vibrational Spectroscopy ^†

Determining the crystal structure of semi-crystalline polymers is still quite the challenge, especially when relying on powder X-ray diffraction (PXRD) alone. The (typically) poorly resolved patterns often lead to multiple competing models with indistinguishable powder profiles. Herein, an alternative bottom-up strategy is proposed, using experimental vibrational spectra as a diagnostic tool to iteratively build crystal structures piece by piece, from 1D chain conformation to full 3D packing. The vibrationally guided methodology was applied to poly(trimethylene 2,5-furandicarboxylate) (PTF), a promising biobased polyester. Infrared and inelastic neutron scattering (INS) spectra collected for amorphous and semi-crystalline samples revealed conformationally sensitive bands that were matched against spectra estimated from discrete density functional theory (DFT) models. This allowed for the identification of the chain conformation (ss-tggt) present in the crystalline phase and the rejection of those incompatible with spectroscopic observations. Using this conformer as a building block, a 3D crystal structure was assembled, comprising 2D sheets stabilized by C–H···O bonds and π–π interactions, and then refined using periodic DFT. This model, which greatly resembles that found for the terephthalic counterpart poly(trimethylene terephathalate) (PTT), proved to be able to reproduce experimental INS data. This case study demonstrates the power of vibrational spectroscopy not only as a validation tool but as an active guide in structure determination. By leveraging spectral sensitivity to local conformation and intermolecular contacts, this bottom-up strategy narrows the pool of viable models early in the process, reducing computational cost and ambiguity. This method complements existing techniques such as nuclear magnetic resonance (NMR) crystallography and opens new avenues for understanding structure–property relationships in complex polymer systems.