Date of Award
Doctor of Philosophy in Chemistry
Organocatalysis is a powerful tool for polymer synthesis. It has been widely demonstrated that organocatalytic systems enable precise control over polymer microstructure, provide competitively fast reaction rates compared with metal-based catalysts, and effect a broad assortment of polymerization mechanisms. The added value of metal-free polymerizations is that they can be utilized in sensitive applications intolerant to the presence of residual metal-based catalysts.
The initial focus of the present dissertation was placed on mechanistic studies in organocatalytic ring-opening polymerization (ROP) of cyclic esters alongside with subsequent development of new organocatalytic systems for ROP. ROPs of this kind can be mediated by a thiourea-based hydrogen-bond donating catalyst and a strong organic base. The two cocatalysts activate the monomer and initiator for the reaction to commence. The question is: how do the four species interact? The binding between thiourea and bases was investigated – an interaction that had not been previously considered. An array of binding constants between thiourea and various bases was obtained. Importantly, the binding constants proved to correlate with the δ- valerolactone ROP rate depending upon the base used for the polymerization. The theory paved a way to the assessment of weaker bases in ROP.
With the original theory working, a new goal was selected – to investigate the binding between thiourea and weak alkylamine bases. A range of binding constants was measured for various thiourea and alkylamine cocatalyst pairs. The correlation between the binding constants and the rate of L-lactide ROP was non-existent. However, enthalpy and entropy of cocatalyst binding were found to correlate with the L-lactide ROP rate. The more entropically favorable cocatalyst interactions yielded higher rates of L-lactide ROP. Additionally, the enthalpy and entropy of the thiourea-alkylamine binding exhibited enzyme-like compensation behavior similar enzyme-substrate analogues.
Kinetic investigations demonstrated that thiourea-alkylamine mediated ROP of L-lactide exhibited a second-order rate dependence in thiourea. This observation prompted us to assess the effect of two thiourea motifs tethered in one molecule on the ROP rate. The new bis-thiourea catalyst provided exquisite control over ROP, yielded well-defined polymers (narrow polydispersities, predictable molecular weights), was able to polymerize a host of cyclic ester monomers, and brought a significant rate acceleration for polymerizations even at small catalyst loadings, compared with monothiourea catalyst.
Seeking active and selective H-bonding catalysts, attention was attracted by the widely available triclocarban, formerly used as an antibacterial soap component. Triclocarban contains a urea functionality that renders the compound a potential H-bond donating catalyst. The examination of triclocarban as a ROP catalyst proved its efficiency for hydrogen-bonding ROP of a broad scope of monomers in different solvents. Having the ROP conditions optimized, extremely active triclocarban-based cocatalyst pairs were discovered. The polymers produced under triclocarban-mediated ROPs display precise macromolecular architecture. Mechanistically, it was proposed that the nature of the catalytic intermediate - neutral hydrogen-bonded or charged imidate - may change depending on the strength of base used in ROP. The quest for optimization and discovery of potent H-bond donating organic catalysts for ROP, rivaling the metal-based counterparts, continues.
Further, attempts were undertaken in stereoselective organocatalytic ringopening polymerization (SROP) of lactide. The SROP of lactide is an attractive approach for the generation of polylactides with tunable materials properties. This reaction exemplifies mechanistic control at the molecular level, conferring different bulk properties to the resulting polymer. Conversely, analysis of the bulk material allows for the detailed understanding of the molecular level processes that gave rise to particular material properties. A series of small molecule H-bond mediated catalysts were developed for SROP of rac-lactide. Catalytic scaffolds leading to low, moderate, and high stereoselectivity were identified through structure-activity relationships. Future generation catalysts will be made building on these early observations.
Kazakov, Oleg, "Fundamental Investigations in Organocatalytic Ring-Opening Polymerization: A Trek to New Catalysts" (2018). Open Access Dissertations. Paper 761.
Available for download on Monday, July 27, 2020