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 ∼2000 to ∼15 000 g/mol and their composition is ∼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, 1H, 29Si, 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.