Molar Mass Characterisation of Polymers via Diffusion-Ordered NMR Spectroscopy: Methodologies for Applied Polymers

Synthetic polymers are inherently distributed in molar mass, and accurately characterising this distribution remains a persistent analytical challenge. While size exclusion chromatography (SEC) is the current gold standard, it requires careful calibration and presents compatibility limitations for several polymer classes. Diffusion-ordered NMR spectroscopy (DOSY) has emerged as a complementary technique, offering solvent flexibility and freedom from column-based artefacts, but its routine adoption for polymer molar mass determination has been limited by unresolved questions regarding calibration reliability, access to genuine distribution information, and applicability across the full molar mass range. This thesis addresses these limitations through a combination of literature review and original research. Following a comprehensive literature review of molar mass determination techniques, tracing their historical development from colligative solution properties through to DOSY itself, this thesis presents three original research chapters. The first investigates the influence of temperature on DOSY molar mass calibrations, identifying convection within the NMR tube as a significant source of error at temperature extremes. Adopting a convection-compensated pulse sequence, alongside other approaches originally developed for small molecule analysis, extends the reliable temperature window for calibration, with the resulting diffusion coefficients validated independently against small-angle neutron scattering data. The second research chapter addresses the extraction of full molar mass distribution information from DOSY data via inverse Laplace transformation (ILT), rather than a single average diffusion coefficient. Using a series of polystyrenes spanning a broad dispersity range, this work establishes that DOSY-ILT reliably reproduces the moments of the molar mass distribution obtained by SEC, up to a dispersity of approximately 2. However, it does not faithfully reconstruct distribution shape at higher dispersity. The standard deviation of the molar mass distribution is shown to correlate more strongly between DOSY and SEC than dispersity itself, supporting its use as a complementary descriptor of distribution width. The third research chapter extends this work to the boundary between oligomeric and polymeric behaviour, using discrete and absolute-mass oligomer libraries of methyl acrylate and methyl methacrylate alongside their polymeric counterparts. This chapter demonstrates that DOSY qualitatively reproduces the coiling transition observed by SEC, and establishes that calibrations built from higher molar mass polymer standards extrapolate more reliably into the oligomeric regime than the reverse, providing practical guidance for calibration standard selection. The thesis closes with conclusion and future directions arising from the topics covered. Together, these chapters demonstrate that DOSY, when appropriately corrected for experimental artefacts and calibrated with due consideration of the molar mass regime under study, constitutes a genuinely viable complement to SEC for polymer molar mass characterisation. This work does not establish DOSY as a full replacement for SEC, particularly regarding distribution shape reconstruction, but substantially advances its reliability and practical applicability as a routine polymer characterisation technique.