UC Santa Barbara

Polymers containing a backbone of repeated Si–O bonds, referred to as siloxanes or silicones, are a ubiquitous class of materials due to their collection of unique properties. The exceptionally wide Si–O bond angle and high deformability of the backbone bonds results in a low glass transition temperature of –120 °C and a crystallization temperature below room temperature. Additionally, the high bond dissociation energy gives rise to decomposition temperatures above 300 °C and results in a chemically inert polymer. The combination of these features result in a massive temperature window while maintaining inert. With a market size of over $4 billion, polydimethylsiloxane (PDMS) is the most common siloxane and has been the subject of decades of research. These research and development efforts have led to the material being used in numerous environments ranging from cosmetics to aerospace. The utility of PDMS arises from the sophisticated methods used to tune the polymer chemistry, which result in a range of properties appropriate for numerous different applications. Robust chemical reactions, like hydrosilylation, have led to an extremely broad scope of functional groups that can be synthetically bonded to the siloxane backbone. This same strategy has been used to attach polymers, polymerization initiators, or polymerizable moieties to the siloxane chain, leading to a broad scope of possible siloxane-based copolymer architectures. One architecture that is lacking in synthetic versatility, however, is graft copolymers with a siloxane backbone and polymeric grafts based on vinyl monomers. Chapter 2 of this dissertation outlines a methodology to synthesize graft copolymers with siloxane backbones using a combination of thiol–ene click chemistry and atom-transfer radical polymerization. Optimization of polymerization conditions enabled the synthesis of graft copolymers with a range of grafting densities and molecular weights > 5 MDa. Degradation of the siloxane backbone enabled characterization of the polymerized grafts, showcasing dispersities around 1.2 and molecular weights close to the targeted values. In order to form a freestanding polymeric material, the siloxane must be crosslinked. The network architecture is an important parameter that can be manipulated to control the properties of the material. Additionally, novel functionality can be incorporated through the usage of a dynamic crosslinker. An example of such a material is Silly Putty, which is PDMS crosslinked through borosiloxane bonds. The method of synthesizing this material, however, has remained largely unchanged since the 1940s and remains a time and energy intensive process with limited tunabilty. In Chapter, 3 we demonstrate a novel methodology to synthesize polyborosiloxane networks using hydrosilylation in minutes. While the current state-of-the-art is limited to end-functionalized PDMS chains, our developed method gives access to backbone-functionalized PDMS starting materials, which drastically increases the degree of tunability over the final network properties. In Chapter 4, we expand upon this method by synthesizing boronate esters containing varying functional groups to investigate the impact of the crosslinker chemical environment on the stress relaxation dynamics. We demonstrate an unprecedented level of control over the rate of stress relaxation for polyborosiloxane materials while maintaining a facile and robust crosslinking method. The newly developed methodologies for graft copolymers and functional polymeric networks will enable future investigations into the properties of these materials and their incorporation into complex composite systems to expand upon the current availability of silicone materials.