Silicones
Matt Hermes

Chemical Concepts
Let's begin to list the chemical concepts you will reinforce in this silicone rubber unit:

Dr. Eugene Rochow expanded on these simple concepts to envision a material called a silicone polymer. From Rochow's concepts came the development of silicone rubber and products made from silicone rubber.

Micro/Macro
and Symbolic
Representation
Chemistry uses macroscopic, large scale observations to help describe and understand matter at the unseeable, molecular level.

And then we represent both the microscopic and macroscopic with often complex symbolic representation.

Here we describe chemical compounds and chemical reactions by indicating each chemical structure with combinations of letters and numbers. The atoms of the elements have letter symbols representing the element. Subscript numbers indicate the number of atoms in the compound. SiCl4 has a complex meaning. It represents a molecule of the compound silicon tetrachloride consisting of a silicon and four chlorine atoms. But it also represents silicon tetrachloride in bulk - it is the symbol we find on a bottle of the compound. And we get to specify the exact amount of the substance as we write balanced chemical equations.

But again, our capacity to understand a variety of symbolic representations is enormous.

Heat and Chemical Resistant Silicone Rubber
Silicones 2. Organic Silicon Chemistry

At the beginning of the 20th century,  chemists began to make new chemical compounds with silicon directly bound to carbon.   They began to build an inventory of new, uncommon materials.  In contrast to the Si-H and Si-X (X=F, Cl, Br, I) bonds that reacted so rapidly with water,  Si-C bonds in compounds were quite stable. 

Silicon is a tetravalent element, it will form up to four covalent two-electron (single) bonds with other elements.  European chemists combined the highly reactive silicon tetrachloride (SiCl4) with organometallic compounds, such as diethyl zinc and diphenylmercury and prepared tetraethylsilane(1) and tetraphenylsilane(2):

SiCl₄ + 2Zn(C₂H₅)₂   -->  2ZnCl₂ + (C₂H₅)₄Si(1)
(C₂H₅)₄Si   boiling point 154 degrees Celsius

SiCl₄ + 2Hg(C₆H₅)₂   -->  2HgCl₂ + (C₆H₅)₄Si(2)
(C₆H₅)₄Si   boiling point 530 degrees Celsius

These organic silanes do not react with water and are inert to many chemical reagents.  Organic silanes show, through their stability to chemical change, and their ability to withstand the very high temperatures required to boil them, that chemists could consider using them for purposes where stability to temperature and environment are important.

From: The Chemistry of Carbon Compound, James B. Conant, MacMillan, NY, 1936, p.28.

In the history of chemistry there are times when fine scientists come truly close to making critical inventions, only to back  away before taking a final, critical step.  These scientists inadvertently leave to others that next advance, hidden for now like a vein of silver tucked deep in a geological fault. About 1900, soon after the first organosilanes were made in Europe, an English chemist, Frederic Stanley Kipping began four decades of making organic silicon compounds. Kipping treated SiCl4 with various new magnesium-based organometallic compounds, invented by Victor Grignard in 1900.   Chemists made these Grignard reagents(3) easily (see simple apparatus from 1936 text, left): (C2H5)Br + Mg  -->  (C2H5)MgBr(3)

Kipping combined SiCl4 and (3) in various proportions and made a series of new materials.  If Kipping used a large molar excess of the Grignard reagent over the amount of  SiCl4 he could make the same tetraethylsilane(1) that earlier workers had produced.   But Kipping lowered the molar ratio of Grignard reagent vs.SiCl4 and made a series of new organosilanes.  Look at Kipping's experiments as a series of reactions sequentially replacing chlorine atoms on silicon with the organic ethyl group (C2H5-), making: Ethyl trichlorosilane:

SiCl4 +  MgBr(C₂H₅)     -->  MgBrCl + (C₂H₅)SiCl3

Diethyldichlorosilane: (C₂H₅)SiCl3 +  MgBr(C₂H₅)     -->  MgBrCl + (C₂H₅)₂SiCl₂(4)

Triethylchlorosilane:(C₂H₅)₂SiCl₂+   MgBr(C₂H₅)     -->  MgBrCl + (C₂H₅)₃SiCl (5)

Tetraethylsilane: (C₂H₅)₃SiCl +  MgBr(C₂H₅)     -->  MgBrCl + (C₂H₅)₄Si

Kipping recognized his series of ethylchlorosilanes would react with water. Compound (5) gave  (C2H5)3SiOH, but this compound was not stable.  Two molecules would combine by splitting out a molecule of water: 2(C2H5)3SiOH   -->  (C2H5)3SiOSi(C2H5)3 + H2O When Kipping treated (4) with water, he got a mass of sticky stuff.  He characterized his products as "glue like" and unimportant.  He never made the crucial finding that the reaction product of (4) with water would produce the orderly chain structure shown at right.  At the end of his 40 years of silicon investigation, Kipping told a respectful audience," We have considered all the known types of organic derivatives of silicon . . .  the prospect of any immediate and important advance in this section of chemistry does not seem very hopeful".

Figure 19.49, Chemistry: Molecules, Matter and Change, Third Edition Peter Atkins and Loretta Jones, W. H. Freeman, NY, 1997.  Used with permission.