Date of Award
Doctor of Philosophy (PhD)
Chemical & Biochemical Engineering
Christopher N. Bowman
Daniel K. Schwartz
Jeffrey W. Stansbury
Cycloadditions are a broad class of reactions where unsaturated species combine to form a cyclic adduct with reduced bond multiplicity. Consequently, unlike condensation reactions, no small molecule species are produced. This permits the formation of high molecular weight species and avoids plasticization. Furthermore, problems plaguing addition polymerization mechanisms such as reactions with water and oxygen are avoided. Despite such inherent advantages and widespread use in small molecule synthesis, cycloadditions have been seldom utilized by the polymer community. The unique reversibility of the Diels-Alder reaction received little mention until 2002, and the Huisgen cycloaddition was not widely utilized until the discovery of its copper catalysis in the same year. This work focuses on the synthesis, mechanisms, and unique attributes of cycloadditions for the fabrication of functional materials. The Diels-Alder reaction was used to synthesize a network polymer with reversible crosslinks. When heated the polymer network depolymerizes and reverts to a liquid. Rheological and spectroscopic measurements demonstrate that the gel point temperature of the material is consistent with the extent of reaction predicted by the Flory-Stockmayer equation. Furthermore, in the gelled state a relaxation process occurs at the rate of bond cleavage in the material. The addition of magnetically susceptible particles causes the material to heat when placed in an electromagnetic field. The self-limiting nature of this heating process enables the material to achieve its native properties over ten cycles of fracture and repair. The reversible nature of the crosslinks also allows the material to function as a photoresist. The reversibility of the Diels-Alder adducts can be selectively eliminated using masked or focused laser light. Upon depolymerization the unexposed material is removed allowing for the fabrication of arbitrary two and three dimensional objects. The photochemical catalysis of the CuAAC reaction via the radical meditated reduction of Cu(II) was also examined as a method of fabricating cycloaddition polymers. This approach affords comprehensive spatial and temporal control of the CuAAC reaction using standard photolithographic techniques. Advantageously, in this process the CuAAC reaction is the rate determining step, and undesirable side reactions of Cu(I) such as disproportionation, reduction of Cu(I) by radicals, and the reaction of Cu(I) with oxygen are avoided, likely by ligand protection. Such interactions explain the high fidelity of patterning in systems where rapid diffusion of the photogenerated catalyst would otherwise be expected.
Adzima, Brian James, "Cycloaddition polymerizations" (2011). Chemical & Biological Engineering Graduate Theses & Dissertations. 11.