Silicones 6. \((CH_3)_2SiCl_2\)

Heat and Chemical Resistant Silicone Rubber

Eugene Rochow set out to make methylsilicone polymer (CH3)2SiCl2. Rochow stumbled through the magnesium-based synthesis scheme introduced first by Kipping then studied by Hyde. By starting with CH₃MgBr, he made (CH₃)₂SiCl₂ (9) and then attempted to make the resins he called methyl silicones:

\[\ce{(CH3)MgBr + SiCl4 -> (CH3)SiCl3 + MgClBr} \nonumber \]

\[\ce{(CH3)MgBr + (CH3)SiCl3 -> (CH3)2SiCl2 (9) + MgClBr} \nonumber \]

Rochow reacted (9) with water just as Hyde had done with (7). He obtained sticky fluids. But when he heated them, they hardened to clear, resinous materials, certainly methylsilicones of the structure shown below:

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(-Si-O-)< face="Arial" size="2">n< face="Arial">
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CH< face="Arial" size="2">₃

But Dr. Rochow needed a better method to make the methylsilicones - some pathway that could be converted to a low cost, commercial process. Successful chemists think by analogy. Dr. Rochow remembered the work of Dr. Alfred Stock, in Germany. Stock believed that a H- atom attached to silicon was just the first member of a like-behaving series of alkyls: CH₃-, C₂H₅-, etc.

Rochow knew that he could make trichlorosilane, HSiCl₃, by reacting:

\[\ce{2Si + 6HCl -> 2HSiCl3 + 2H2} \nonumber \]

Suppose he reacted silicon with methyl chloride, CH₃Cl. He would expect to obtain CH₃SiCl₃, right? If so, could he adjust the ratio of Si and CH₃Cl and obtain the critical intermediate to methyl silicones, (CH₃)₂SiCl₂?

Successful chemists must be observant. Well, Rochow went to the laboratory and treated silicon with CH₃Cl. The experiment was disappointing at first. But when Rochow was cleaning up his flasks with water he noticed the flask had a slippery feel - as if a film of his methylsilicone polymer had formed on its surface!

Successful chemists tend to work long hours. On a spring morning in May, 1940, Eugene Rochow prepared a new experiment. He planned to pass a stream of methyl chloride gas along with some hydrogen chloride across a bed of silicon at high temperature. (From left to right in the accompanying sketch). But this morning he set up using a source of silicon that contained copper metal. He heated a tube containing the Cu-Si to 370 degrees Celsius and ran the gases over the hot silicon all day. He shut the chemical reaction down just before 5PM. But Rochow did not go home.

From Silicon and Silicones, Eugene G. Rochow, Springer-Verlag, Berlin, 1987, p.80.

He found a pool of liquid in the cooled flasks (on the right in the sketch) and treated the liquid with water. "The end result is a clear, resinous body I believe is methyl silicone," Rochow wrote in his laboratory notebook late that night. The chemical reaction to make (9) was a simple one:

2CH₃Cl + Si --> (CH₃)₂SiCl₂ (9)

It turns out, the hydrogen chloride gas served only to etch the surface of the Cu-Si that day. What was more important was the copper. Copper served as a catalyst for the reaction between methyl chloride and silicon.

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< face="Arial" size="2">Rochow Repeating his First Effective Synthesis
Silicones Under the Monogram, H. A. Liebhafsky, John Wiley and Sons, NY, 1978, p.92

You may remember (or you will learn) that a catalyst participates in a chemical reaction by lowering the activation energy for the chemical reaction. Activation energy is the energy required to form the "activated complex" through which all chemical reactions must go on the pathway from starting materials to products. Lowering the activation energy increases the rate of a chemical reaction at any specific temperature - it allows reactions to proceed at practical rates at lower temperatures than without the catalyst.

When Rochow added the water, he made his methyl silicone:

(CH₃)₂SiCl₂ + nH₂O --> n[(CH₃)₂SiO]_n + nHCl

In 1940, Eugene Rochow demonstrated a one step method to make silicone polymers. Much more would need to be done to make Rochow's process practical for the manufacture of high-temperature insulation.

Silicones and Patents

The ChemCases unit on Gatorade discusses United States patents. You might read the patent section on the invention of Gatorade.

General Electric did take one action with Rochow's first methyl silicones. They filed patents, differentiating them in their patent claims from the ethylphenyl silicones made at Corning. These patents led to a controversy between the two companies; Corning, having introduced their silicone work to GE, then sued GE in the patent courts claiming GE's methyl silicone were obvious in light of Corning's own work. Corning lost, the methyl silicones became GE's sole property.