THE PIERS-RUBINSZTAJN REACTION: NEW ROUTES TO STRUCTURED SILICONES

Abstract

<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>