Hydride-Functional Silicones

The hydrosilylation of vinyl-functional siloxanes by hydride-functional siloxanes is the basis of addition cure chemistry used in 2-part RTVs and LTVs.^1,2 The most widely used materials for these applications are methylhydrosiloxane-dimethylsiloxane copolymers which have more readily controlled reactivity than the homopolymers and result in tougher polymers with lower crosslink density. The preferred catalysts for the reactions are platinum complexes such as SIP6830.3 and SIP6832.2. In principle, the reaction of hydride-functional siloxanes with vinyl-functional siloxanes takes place at 1:1 stoichiometry. For filled systems, the ratio of hydride to vinyl is much higher, ranging from 1.3:1 to 4.5:1. The optimum cure ratio is usually determined by measuring the hardness of cured elastomers at different ratios. Phenyl-substituted hydrosiloxanes are used to crosslink phenylsiloxanes because of their greater solubility and closer refractive index match. The following chart gives some examples of starting ratios for common polymers and crosslinkers calculated at 1.5 hydride to vinyl ratio.

The hydrosilylation of olefins is utilized to generate alkyl- and arylalkyl-substituted siloxanes, which form the basis of organic compatible silicone fluids. The hydrosilylation of functional olefins provides the basis for formation of silicone block polymers.

1. Warrick, E. et al. Rubber Chem. Tech. 1979, 52, 437.
2. Dolgov, O. et al. Organosilicon Liquid Rubbers, Int’l Poly. Sci. & Tech. Monograph #1, RAPRA, 1975.

Hydroxyl-functional materials react with hydride-functional siloxanes in the presence bis(2-ethylhexanoate)tin, dibutyldilauryltin, zinc octoate, iron octoate, or a variety of other metal salt catalysts. The reaction with hydroxylic surface groups is widely used to impart water-repellency to glass, leather, paper, and fabric surfaces and powders. A recent application is in the production of water- resistant gypsum board. Application is generally from dilute (0.5-2.0%) solution in hydrocarbons or as an emulsion. The coatings are generally cured at 110-150 °C. Polymethylhydrosiloxane is most commonly employed.

Silanol-terminated polydimethylsiloxanes react with hydride-functional siloxanes to produce foamed silicone materials. In addition to the formal chemistry described above, the presence of oxygen and moisture also influences crosslink density and foam structure.

Polymethylhydrosiloxane is a versatile low cost hydride transfer reagent. It has a hydride equivalent weight of 60. Reactions are catalyzed by Pd⁰; or dibutyltin oxide. The choice of reaction conditions leads to chemoselective reduction, e.g. allyl reductions in the presence of ketones and aldehydes.^4,5,6 Esters are reduced to primary alcohols in the presence of Ti(OiPr)₄.⁷

See brochure “Silicon-Based Reducing Agents”.

3. Larson, G. L., Fry, J. L., “Ionic and Organometallic-Catalyzed Organosilane Reductions”, in Organic Reactions S. E. Denmark, Ed. Volume 71, John Wiley and Sons, pp 1-771, 2008.
4. Lipowitz, J. et al. J. Org. Chem. 1973, 38, 162.
5. Keinan, E. et al. Israel J. Chem. 1984, 24, 82. and J. Org. Chem. 1983, 48, 3545
6. Mukaiyama, T. et al. Chem. Lett. 1983, 1727.
7. Reding, M. et al. J. Org. Chem. 1995, 60, 7884.