TRANSFORMING TRIATOMIC MOLECULES INTO COMPLEX FUNCTIONAL MATERIALS

Abstract

Perhaps because the dipole moment of CO2 is equal to zero, a widely held misperception is that this triatomic gas is kinetically and thermodynamically stable under most reaction conditions. As a result, only recently has CO2 been exploited as a useful and versatile reagent. Reactions involving it and other neutral triatomic molecules (most notably, CS2) with substrates containing nucleophilic functional groups are based on Lewis acid-base equilibria. This simple chemistry can be used to form a variety of interesting new materials which are metastable or reversible in some cases, and able to be transformed into other materials, in other cases. In this way, molecules with controllable properties can be generated. Such molecules have been employed as cross-linkers, to initiate self-assembly and to allow reversibility in functional soft materials (e.g., gel, ionic liquids, supramolecular structures, catalysts, etc.) that are potentially useful in a wide variety of applications ranging from sensing and sequestration of environmentally deleterious molecules to the development of better materials for art conservation. In our work, greater emphasis will be placed on the chemistries of CO2 and CS2 (X=C=X type molecules in which the electronegativities of the X atoms are significantly larger than that of carbon); both molecules are linear and have large quadrupolar moments. The electrophilic nature of the carbon atoms in molecules such as CO2 and CS2 makes them attractive reagents in organic syntheses as well.

In this dissertation, we investigate how and why these triatomic molecules react with other species and exploit the chemistries to synthesize complex molecules and aggregates, to change drastically the properties of materials, and to devise new processes.

First, the syntheses and properties of a new class of chiral, reversible, room-temperature, ionic liquids (RTILs) are investigated. They are made from easily synthesized, readily available materials and can be transformed reversibly to their nonionic liquid states. The nonionic liquids consist of equimolar mixtures of a N'-alkyl-N,N-dimethylacetamidine (L) and an alkyl ester (N) of a naturally occurring amino acid. When exposed to 1 atm of CO2 gas, 1/1 (mole/mole) L/N solutions become cationic-anionic pairs, amidinium carbamates. Of the 50 L/N combinations examined, all except those involving the methyl ester of tyrosine (which was immiscible with the amidines) form RTIL states under a CO2 atmosphere, and several remain liquids to at least -18 °C. Heating the ionic liquids in air to ca. 50 °C or bubbling N2 gas through them at ambient temperatures for protracted periods displaces the CO2 and re-establishes the nonionic L/N states. Thermal and spectroscopic properties of both the nonionic and ionic phases are reported and compared. Unlike many other ionic liquids, these need not be prepared and handled under scrupulously dry conditions and they can be cycled repeatedly between their high- and low-polarity states.

In addition, the properties of another class of reversible, room-temperature, chiral, ionic liquids (L-A-C) are studied. The L-A-C are prepared by passing CO2 gas through equimolar mixtures of a simple amidine (L) and a chiral amino alcohol (A), L/A, derived from a naturally-occurring amino acid, and they can be returned to their L/A states by passing a displacing gas, N2, through the ionic liquid; the process of passing from uncharged to charged states can be repeated several times without discernible degradation of each phase. All of the 40 L/A combinations examined form room-temperature ionic liquids (most stable to ca. 50 oC under one atmosphere of CO2) and they remain liquids to at least -20 oC. The L-A-C phases are more viscous than their corresponding L/A phases, the conductivities are much higher in the L-A-C phases than in the L/A phases, and the solubility characteristics of the liquids can be modulated significantly by exposing them to either CO2 or N2 gas. The spectroscopic characteristics of the L/A and L-A-C phases have been compared as well. Their reversibility, chirality, broad temperature ranges, tolerance to water, and, ease of preparation should make the L/A and L-A-C phases useful as solvents for several `green' applications.

Also, by adding one equivalent of another triatomic molecule, CS2 to an equimolar mixture of two non-ionic molecules, an amidine and an amine with diverse structures, amidinium dithiocarbamates salts are easily prepared. Many of the salts made in this way are RTILs and are thermally stable to ~80 oC, a temperature significantly higher than the decomposition temperatures of analogous amidinium carbamate RTILs. However, unlike the amidinium carbamates, the amidinium dithiocarbamates do not revert to their amidine/amine mixtures when they are heated. The thermal, rheological, conductance and spectroscopic properties of these RTILs are reported, comparisons between them and their non-ionic phases (as well as with their amidinium carbamates analogues) are made, and the thermolysis pathways of the ammonium dithiocarbamates are investigated. In addition, the above-mentioned in situ-prepared, reversible RTILs have been used as media for the syntheses of cyclic carbonates by addition of CO2 to epoxides.

The similar approach has been explored to develop functional polymers. The physical properties of five siloxane polymers with different types and frequencies of amino functional groups along the polymer side-chains have been changed from flowing liquids to gels and to rubber-like materials by the simple addition or subtraction of CO2 or CS2 at room temperature. The chemical changes, formation of ammonium carbamates and ammonium dithiocarbamates, create materials whose properties are completely different from the parent polymers as a result of the introduction of ionic crosslinks. These materials can be returned to their original forms by heating (in the case of the CO2 adducts) or to their protonated original forms by treatment with an acid (in the case of the CO2 and CS2 adducts). Heating the ammonium dithiocarbamates leads to loss of H2S and permanent (covalent) thiourea crosslinks between the polymer chains. The new materials adhere strongly to other surfaces and can be swelled to several times of their original volumes by different liquids. The rheological, swelling, and physical properties of the new materials have been correlated with the structures of the original polymers to provide a comprehensive picture of how changes at the nanometric length scale are translated to macroscopic changes. At least for the polysiloxanes examined here, the properties of the adducts do not correlate with the molecular weights of the original polymers, but do with the frequency of amino groups. The results demonstrate a simple, new method to crosslink polysiloxanes (and, in principle, a wide range of other polymers), transforming them into materials with totally different and potentially commercially useful properties. Research on swelling-deswelling behavior of crosslinked amino-polysiloxanes and possible applications for chemical spill containment and remediation were carried out as well.