Investigation of Novel Thiol Click Reactions

The thio-Michael addition reaction is traditionally considered a base catalyzed reaction which involves high catalyst concentrations and long reaction times. This reaction utilizes potent, simple nucleophiles to catalyze the reaction, decreases the catalyst concentration and greatly increases the reaction times. The free radical mediated thiol-ene click reaction uses light or heat and an initiator to catalyze the rapid and quantitative addition of thiols to most electron rich enes without the formation of side products and in the absence of solvent. Recently, the thiol-ene click reaction has been exploited for these reasons in materials science and organic synthesis. 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. This dissertation also discusses the less researched thiol-yne reaction, which provides the addition of two thiol groups to one alkyne group. Mechanistically, a thiyl radical adds to an alkyne group creating a very reactive thio vinyl radical, which, in turn, abstracts a hydrogen from another thiol creating a new thiyl radical. The resulting thio vinyl group, which shows higher reactivity than the initial alkyne, reacts rapidly with a second thiyl group. Additional chapters in this dissertation will discuss the formation of multi-functional materials (16 \u3e functionality \u3e 8) in a sequential nucleophile catalyzed thio-Michael addition followed by the thiol-yne reaction; the mechanical and physical properties of films prepared with multi-functional alkynes and multi-functional thiols; and the linear relationship of refractive index and sulfur content in polysulfide networks made possible by the thiol-yne reaction. 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 (Tg). Incorporation of thiols into these systems narrows the Tgs 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 Tgs. 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 polyfunctional 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 Tg 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