Rediscovering Silicones: MQ Copolymers

Organic solvent-soluble trimethylsiloxysilicate polymers (MQ copolymers) were prepared with controlled molecular weight using kinetically controlled, sulfuric acid-catalyzed, hydrolysis/condensation reactions of tetraethoxysilane (TEOS, tetraethyl orthosilicate) and subsequent (end capping) reactions with hexamethyldisiloxane. We have simplified what is a complex, variable-dependent, industrial preparative procedure that involves competitive sol growth and end-capping reactions to one that separates these reactions into discrete steps and can be carried out with control in a common academic lab. These MQ copolymers are model compounds of commercial MQ resins that are important commodity materials with unique impact in a range of applications. Although they are recognized for their broad utility, they are not widely appreciated as (or even considered) unusual molecules with special structures and properties. The number-average molecular weight of the copolymers reported here can be controlled from similar to 2000 to similar to 15 000 g/mol and their composition is similar to 70 mol % silica (Q), yet they are readily soluble in hydrocarbon solvents and liquid silicones. In addition to reporting a synthetic procedure, we make comments and conjectures concerning their polycyclic molecular structures based on their characterization using IR, H-1, Si-29, and diffusion NMR, dynamic light scattering, gel permeation chromatography, and transmission electron microscopy. Derivatives of these materials that contain vinyldimethylsilyl- and hydridodimethylsilyl-M units were prepared with the objectives of demonstrating that our synthetic approach is chemically versatile and preparing MQ copolymers that are reactive toward hydrosilylation. We expect that these inorganic/organic hybrid materials,, which could be termed molecular nanoparticles, could serve as a platform upon which chemical derivatization will permit rational implementation in a wider range of applications and technologies than is currently served by commercial MQ resins.