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<p>Silicones are a class of polymeric materials broadly used in numerous commercial applications, primarily due to the significant advantages they poses over their carbon-based analogues. The technology utilized to synthesize them is rather mature, and most ‘new’ synthetic strategies involve only incremental changes to the existing norm. The high level of structural control that has become the hallmark of organic synthesis and increasingly of polymer chemistry is essentially absent from silicone chemistry. The origin of this deficiency is the susceptibility of silicone polymers to redistribution (metathesis/rearrangement) under acidic and basic conditions, which will destroy any existing controlled architectures. The Piers-Rubinsztajn reaction, catalyzed by tris(pentafluorophenyl)borane (B(C₆F₅)₃), involves the direct coupling between an alkoxysilane and hydrosilane forming a new siloxane linkage, (R₃Si-OMe + H- SiR’₃ → R₃Si-O-SiR’₃ + Me-H). The reaction avoids any unwanted acidic/basic reaction conditions and has been shown previously to provide an efficient route to precise, well-defined silicones.</p> <p>Herein, the functional tolerance of the Piers-Rubinsztajn reaction is reported. It has been shown that in the presence of Lewis basic functional groups (such as – OH, -NH₂, -SH) unwanted side reactions result. However in the presence of haloalkanes and alkenes the reaction is fully tolerant, leading to the synthesis of over twenty new, well-defined functional silicones.</p> <p>The ability to utilize prepared functional silicones in common organic transformations is also reported. It has been shown that prepared halocarbon- modified silicones can readily be converted to their subsequent azido derivatives and tethered to alkyne-modified poly(oxyethyene) (PEG or PEO) of a variety of molecular weights. This led to the synthesis of over fifteen new, well-defined silicone surfactants. Structure activity relationships have also been reported for the synthesized surfactants, showing that subtle manipulations to the silicone hydrophobe can substantially alter the properties the surfactants possess. The use of thiol-ene click chemistry which involves the reaction between prepared well-defined alkene containing silicones and thiol modified poly(oxyethylene) of varying molecular weights is also reported, providing another route to well- defined silicone based surfactants.</p> <p>The use of the Piers-Rubinsztajn reaction in the synthesis of larger, well-defined silicone based macrostructures is also reported. It has been shown that through alternation between the Piers-Rubinsztajn reaction and platinum catalyzed hydrosilylation, well defined silicone dendrimers can be obtained with relative ease through a combination of both divergent and convergent growth methods.</p> <p>Finally, a new method for the preparation of both silicone elastomers and silicone foams is reported. Through use of the Piers-Rubinsztajn reaction, elastomers can be readily obtained. A detailed analysis of the many factors that may alter the overall properties of the elastomers produced including solvent volume, crosslinker concentration and type and the molecular weight of the starting hydride terminated polydimethylsiloxane (H-PDMS-H) is discussed.</p> <p>Taking advantage of the volatile hydrocarbon byproducts of the Piers-Rubinsztajn reaction, silicone foams can also be prepared using this method. A study analogous to that carried out on the silicone elastomers is also reported, showing that through subtle manipulations to the silicone foam formulations, significant changes to the materials properties can be obtained.</p> / Doctor of Science (PhD)

<p>The development of both silica and silicones has led to enormous improvements in available products over the last 50 years: the compounds have now found practical applications in fields ranging from electronics to biomaterials. Both of these materials have several desirable intrinsic properties. The compounds can be combined as a blend, in a composite or at an interface with other compounds to tune the chemical and physical properties to those desired. On their own, silica and silicones also have many applications. Their utility would be enhanced if it was possible to improve morphological control of the materials independently or together.</p> <p>This thesis explores various parameters and factors that enable the structuring of elastomers, colloids/suspensions, films and foams with the use of unconventional or new organosilicon chemistries. Specifically, amine and boron based catalysts are utilized to catalyze silicone and silica formation at different interfaces to create the materials mentioned above. Potential applications for these materials include drug delivery, GC chromatography and paper-based diagnostics.</p> / Doctor of Philosophy (PhD)

Silicones are widely used polymeric materials due to their unique properties. The material properties of silicones may be altered by incorporating various organic groups. Traditional methods for linear silicone synthesis involve ring-opening polymerization, which leaves the growing chain susceptible to acid or base mediated chain redistribution and the formation of cyclic monomer byproducts. The Piers-Rubinsztajn (PR) reaction is an alternative siloxane synthetic route that avoids the use of tin- or platinum- based, or of Brønsted acid/base catalysts. Siloxane bond formation is catalyzed by tris(pentafluorophenyl)borane (B(C6F5)3) (R’3Si-H + RO-SiR”3 → R’3Si-O-SiR”3 + RH); alkoxysilanes can be replaced with silanols or alkoxybenzenes. The catalytic activity of B(C6F5)3 was shown to be hindered by trace water in solution; water acts as a Lewis base coordinating to B(C6F5)3. Since the hydrate-free form of B(C6F5)3 is required to initiate a PR reaction, water can act as an inhibitor. In a somewhat contradictory fashion, water was also shown to react with hydrosilanes via a B(C6F5)3 catalyzed hydrolysis reaction to give silanols, that themselves are reagents for the process. The reactivity of alkoxysilanes (or aryl ethers) in the PR reaction was found to be much quicker than water. This was exploited in the synthesis of Ax(AB)yAx triblock copolymers. The aryl rich AB core was first synthesized using the PR reaction. Excess silicone condensed via hydrolysis forming the A blocks. This method of exploiting relative reactivity to tune structure was applied to elastomers made using a single linker (eugenol) with multiple functional groups – elastomer morphology was controlled by changing order of addition. The development of aryl dense silicones is of interest for use in electronic devices. Phenylmethyl homopolymers and highly ordered phenyl pendant copolymers (Ph/Si ratio of 0.5-1.5) were synthesized from monomers to give polymers with high refractive indices (1.51-1.59) and Mw up to 170 kDa. Statistically relevant libraries of aryl functional silicones were developed using combinatorial chemistry in order to analyze their structure-property relationship. Incorporating aromatic groups into silicones worked to elevate thermal stability, refractive index and improve the mechanical strength of silicone rubbers. / Thesis / Doctor of Science (PhD) / Silicone fluids and elastomers possess numerous desirable characteristics which leads to their use in a wide range of applications in the automotive, electronics and biomedical fields, among others. Developing techniques to create well defined, ordered, modified silicones with improved optical properties, mechanical strength and thermal stability was the main focus of this thesis. These objectives were accomplished by incorporating aromatic groups into silicones using boron catalysis. Following the initial (intended) Piers-Rubinsztajn reaction, atmospheric moisture was utilized to promote further polymerization. Statistically relevant libraries of silicone elastomers were prepared using both standard and combinatorial chemistry techniques. This library of elastomers permitted the analysis of trends associated with small changes in elastomer formulation, which could not be accomplished using traditional one-by-one reaction methods in a timely fashion. The modified silicone materials exhibited high refractive indices (up to 1.59), elevated stiffness and improved thermal stability (maintain structure up to 500 °C) when compared to previously synthesized polymers.