Abstract
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
Highlights
Controlling the stereoselectivity of a polymerization is an important tool of controlling polymer tacticity
We agree with the conclusion of Bibal et al that TU/amine base binding can be inhibitory to ROP5,6 but submit that: 1) the phenomenon is much more general than first proposed; 2) the magnitude of the interaction may be a good predictor of cocatalyst activity; and 3) the point at which cocatalyst binding becomes counterproductive to catalysis is significantly higher than once believed
Initiation of a VL (1.0 mmol) ring-opening polymerization (ROP) catalyzed by TCC/MTBD (0.05 mmol each) from 1-pyrenebutanol (0.02 mmol) and subsequent addition of a second monomer portion (1.0 mmol) exhibits overlapping UV and refractive index traces in the gel permeation chromatogram (GPC) of the resulting polymer, suggesting end group fidelity and a chain end that is susceptible to chain-extension
Summary
The field of ROP with TU-based catalysts has experienced substantial growth and witnessed a number of breakthroughs capable of advancing the field since its recent genesis. Utilization of TU catalytic systems in ROP of cyclic esters showcased, as a significant achievement, an opportunity to carry out traditional ROP without metal-based catalysts. Exquisite precision in polymerization control was a great benefit in the generation of polymeric materials for fine applications (microelectronics, biomedical devices) despite the manageable deficiencies, such as only a fledgling monomer pool for effective ROP by organocatalysts, inability to conduct enantioselective polymerization, and economic difficulties associated with organocatalytic ROP implementation on a wide industrial scale. The state of the art of TU H-bonding catalysis changed substantially in the past decade. The monomers pool for controlled ROP deepened significantly, the TU catalytic systems underwent designer changes targeted to introduce faster yet controlled catalysis unfathomable before in the slate of organocatalytic systems for ROP.. The importance of stereoselective ROP lies in the ability thereof to afford a variety of polymer architectures depending on the arrangement of stereocenters in the polymer backbone. . Polymerization of rac-LA can afford an array of stereoregular polymers, such as syndiotactic, heterotactic, and isotactic kinds. . Polymerization of rac-LA can afford an array of stereoregular polymers, such as syndiotactic, heterotactic, and isotactic kinds.11, 12 All of these polymeric species are differentiated on the basis of stereocenters sequenced in the polymer chain. The variable physical properties of PLAs differing in the sequence of stereocenters in the polymer backbone are very valuable from the standpoint of industrial applications of such polymers..
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