Investigation of Novel Thiol "Click" Reactions
Date of Award
Doctor of Philosophy (PhD)
Polymers and High Performance Materials
Charles E. Hoyle
Polymers and High Performance Materials
The research herein describes the nucleophile catalyzed thio-Michael addition to electron poor enes as a integral reaction in the canon of thiol-ene click reactions. This dissertation includes chapters of the kinetics and spectroscopic evaluation of the nucleophile catalyzed thio-Michael addition reaction and resulting products; the use of nucleophile catalyzed thio-Michael addition for the rapid synthesis of star polymers; and the physical and mechanical properties of networks prepared with a combination of the photo-cured and nucleophile cured reactions of multi-acrylates with multi-functional thiols. The first fundamental study discusses a proposed anionic chain mechanism for the nucleophile catalyzed thio-Michael addition to electron poor alkenes. Traditional base catalyzed mechanisms show the deprotonation of the thiol by a weak base such as triethyl amine. Results show that nucleophilic amines, such as hexyl amine, with similar pKa values as the weak bases have faster rates of reaction, indicating that base strength alone is not responsible for the apparent increase in rates. Results demonstrate that the effect of nucleophilicity has a greater role than basicity (pKa) in the rates of reaction. An anionic chain mechanism is proposed involving the initiation of the thio-Michael reaction by an initial attack of a nucleophile onto an electron poor double bond creating a super-strong enolate anion which carries out the subsequent base catalyzed thio-Michael addition. The second study reports the facile formation of star polymers using the nucleophile catalyzed thio-Michael addition reaction of polymers prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization and a tri-acrylate monomer. The nucleophilic catalyst employed for the thio-Michael addition reaction has shown to have a dual purpose: to catalyze the Michael addition and to prevent the disulfide formation commonly seen in the reduction step of the RAFT end group. Acrylates are commonly used for the preparation of polymer networks due to their wide commercial availability, tunable mechanical properties, and sensitivity to photopolymerization. Photo-cured multi-acrylate systems produce films with inhomogeneous micro-structures leading to broad glass transition temperatures (T g ). Incorporation of thiols into these systems narrows the Tg s but quantitative addition (1 to 1) of thiol to acrylate does not occur due to the competitive acrylate homopolymerization. The nucleophile catalyzed thio-Michael addition reaction promotes the quantitative addition of thiols to acrylates resulting in very narrow T g s. The third study discusses the use of sequential thio-Michael reaction followed by the photo-cured reaction. This process allows tunability of mechanical and physical properties of resulting films. In the fourth study, the nucleophile catalyzed thio-Michael addition reaction is used for preparation of multi-functional alkynes. Alkynes, like alkenes, react rapidly and quantitatively with thiols in a photocured system in a 1:2 ratio. A series of poly-functional branched materials was prepared by clicking two thiol groups to one terminal alkyne proceeded quantitatively, in the absence of solvent, rapidly and with no evidence of side products. The fifth study demonstrates the preparation of a series of multi-functional alkyne monomers (f=4,6,8) that were subsequently photopolymerized with a series of multifunctional thiols (f=2,3,4). Mechanical and physical properties showed an increasing correlation between gel point and functionality. Additionally, this study demonstrated the utility of tailoring the T g values by increasing the functionality of starting monomers. High sulfur content materials have shown to have high refractive index values. In the final study, networks were prepared from commercially available dialkyne and dithiols, consisting only of sulfur and hydrocarbon. Sulfur content in some films reached nearly 50% and, as a result, refractive index values were determined to be greater than 1.65. Data from this study shows a linear relationship between the weight% sulfur and the refractive index in sulfur containing crosslinked hydrocarbon networks. (Abstract shortened by UMI.)
Chan, Justin William, "Investigation of Novel Thiol "Click" Reactions" (2009). Dissertation Archive. 150.