New Routes to Functional Siloxanes: Applications of the Thermal Azide-Alkyne Cycloaddition for the Silicone Chemist

Abstract

Silicone oils (polysiloxane) and elastomers are a class of hydrophobic polymers with an extensive range of uses. While the high hydrophobicity can be beneficial in a variety of applications, it is not universally the case. Modification strategies for both fluid and elastomeric polydimethylsiloxane (PDMS) must be employed to create silicones with the appropriate properties for a given application, including enhanced hydrophilicity. Derivatization of PDMS leads to functional silicones with unique properties and added value. Strategies have been developed to modify both fluid and elastomeric PDMS, however, they all have varying degrees of drawbacks: the use of sophisticated equipment or expensive catalysts, restrictions to certain types of solvents, cumbersome multi-step synthetic procedures and surface reversion are some of the challenges faced. There is an opportunity to develop a simple and generic method for the controlled functionalization of PDMS. The Sharpless concept of ‘Click’ chemistry was an ideal approach to solving some of these challenges. Following nature’s lead, these reactions that are modular, wide in scope, high yielding, have simple reaction conditions and generate inoffensive byproducts. Herein, a synthetic method to functionalize silicones using the thermal Huisgen 1,3-dipolar cycloaddition of azides to alkynes is described. Initial exploration focused on the creation of inherently reactive elastomers that could be modified with a model hydrophilic moiety, poly(ethylene glycol). This was extended to the creation of amphiphilic multi-functional polysiloxanes and amphiphilic networks. Furthermore, the ‘Click’ approach was used to solve challenges faced in applications where silicones find use. The method described overcomes silicone modification challenges. The cycloaddition reaction is tolerant to many reaction conditions, is orthoganol to a variety of chemical reactions, does not require the use of a catalyst, the starting functional groups and bonds formed are stable and the reaction is high yielding, positioning the Huisgen ‘click’ reaction is an exceptional synthetic tool for the silicone chemist.