Stereochemistry, Diffusion, and Online Monitoring of Synthetic Polymers

Student thesis: Doctoral ThesisPhD

Abstract

Polylactide (PLA) is a biobased and compostable polymer with applications in products from medical implants to sustainable packaging. The stereochemistry of PLA has a significant impact on the physical and thermal properties of the polymer, and therefore its practical applications. Post-polymerisation modification and control of polymer tacticity would allow polymers with desirable properties such as heat resistance and crystallinity to be produced selectively from low-value PLA. To this end, investigations were made into modifying the stereochemistry of PLA and its monomer, lactide, using various acids and bases, alone and in combination. Racemisation of L-lactide was achieved under catalytic conditions with tris(pentafluorophenyl)borane (BCF) and lutidine, a frustrated Lewis pair-type system, at 10mol%. Ring-contraction side products were identified and suppressed. The thermodynamic parameters for epimerisation of lactide were determined by variable temperature NMR. Ring-opening of lactide by LiHMDS and LDA was observed by DOSY NMR, but without deprotonation. Although effective on lactide, BCF and lutidine were not effective in racemising PLA due to steric bulk preventing close approach to the polymer chain. Racemisation of PLLA was achieved with DBU, TBD and alkali metal amides and alkoxides, although it was always accompanied by chain scission and loss of molecular weight. These results highlight the differences in reactivity between linear PLA and the cyclic monomer, lactide.

Diffusion-ordered spectroscopy (DOSY) is a useful technique for measuring diffusion coefficients and estimating molecular weights of molecules in solution. To date, it has always been performed on static samples, and never in a FlowNMR regime. In this work, effective techniques for obtaining accurate diffusion coefficients on flowing samples have been developed. The effects of changing acquisition parameters and hardware choices on the quality of DOSY data were investigated, and recommended settings for obtaining good quality FlowDOSY data have been derived. A convection compensated pulse sequence is essential for obtaining meaningful data, and is effective up to at least 4 mL min-1. Flow corrections can be used to obtain static diffusion coefficients from FlowDOSY data. The effect of flow on measured diffusion coefficient has been almost entirely eliminated by reducing pump pulsation to <1% with the use of a rotary multi-piston pump. This work adds DOSY to the toolbox of FlowNMR techniques that can be used to obtain detailed information on reacting systems in real time.

Diffusion coefficients from DOSY NMR of polymer samples are related to weight average molecular weight, Mw, by an empirical linear relationship. With a suitable calibration curve produced with samples of known molecular weight, Mw of an unknown polymer sample can be calculated from the diffusion coefficient. Calibration curves for PLA, PMA and PDL with linear, cyclic and star topology were produced from DOSY and GPC data. These calibration curves contain information about the solution properties of polymers with different topologies. For example, the calibration curves for linear and cyclic PLA have significantly different gradients due to the constrained hydrodynamic radii of the cyclic polymers. A crossover point between the two lines can be explained by lack of solvent penetration into the smaller cyclic polymers. Reversible addition-fragmentation chain-transfer (RAFT) polymerisation of methyl acrylate in a flow reactor was monitored by online DOSY and compared with multi-angle light scattering (MALS) data. DOSY gave access to molecular weight of the polymer product with relative errors around 5%, making it at least as accurate as GPC which is commonly used for polymer analysis.

Several reaction monitoring techniques were employed simultaneously to study the ring-opening polymerisation of lactide with either Sn(II) 2-ethylhexanoate or a Zr amine trisphenolate initiator. Changes were made to the experimental setup to minimise sources of contaminants in the flow system tubing that reduced rate and conversion. Polarimetry and homonuclear decoupled 1H NMR were demonstrated to give access to stereochemical information on the monomer and growing polymer. FlowDOSY was used for reaction monitoring for the first time, and showed the polymer diffusion coefficient decreasing as its molecular weight increased. It also confirmed the growing chain was bound to the Zr metal centre of the initiator while the reaction progressed, as expected from the mechanism, but became dissociated to some extent as catalyst activity declined. Online GPC confirmed the increase in polymer molecular weight and gave access to dispersity throughout the reaction. These experiments demonstrated the complementary nature of using multiple techniques to analyse different aspects of a reaction simultaneously.
Date of Award27 Apr 2022
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorUlrich Hintermair (Supervisor) & Matthew Davidson (Supervisor)

Cite this

'