The Role of Hydrosilanes in the Preparation of Silicones

Abstract

Abstract Silicone polymers (PDMS) are widely used for a large range of applications from automotive to cosmetic industries owing to their unique properties, such as electrical resistance, biocompatibility, and gas permeability. Conventionally, linear silicone polymers are produced by acid or base-catalyzed equilibration, leading to broad molecular weight dispersities, and high concentrations of a controversial monomer that requires removal. Therefore, in the first part of the research, we developed a new hydrolytic route to linear silicone polymers that is both simple and inexpensive. The small hydrosilane tetramethyldisiloxane (HSi(CH3)2OSi(CH3)2H) was polymerized to hydride-terminated PDMS (HSi-PDMS-SiH) using only water as a reagent, a process that has industrial potential. The product can further be converted into silicone block copolymers, elastomers, foams and resins. We broadened the studies to consider the incorporation of different functional groups (vinyl and alkoxy groups) into silicones, including developing methods to control their precise locations in polymers and to synthesize dendritic hyperbranched silicone polymers in a controllable manner. The ability of silanes to behave as functional reducing agents was examined. A series of model compound studies, including reduction with benzyl disulfide and tetrasulfide, were conducted to illustrate the mechanism of S-S bond cleavage using a catalytic amount of B(C6F5)3. The effect of sterics on the rates of hydrolysis of the product S-Si compounds allowed consideration of the use of this strategy to protect for thiol groups. In addition, the relative reactivities of hydrosilane with different functional groups including mercaptan (SH), disulfide (SS) and ethoxy (SiOEt) were also mapped out as follows: S-H>S-S>SiOEt. These investigations allowed us to exploit hydrosiloxanes for the reduction of the sulfur crosslinks in used rubber tires; current reuse methodologies either utilize harsh conditions or occur in low yields.