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Silicones 7: Polymer Chemistry, Thermochemistry, and Kinetics

Heat and Chemical Resistant Silicone Rubber

You may recall from your studies of carbon-based materials, that segment of chemistry we call organic chemistry, the term polymers. Polymers are giant molecules, long sequences of chemical units bonded together. Some polymers are natural in origin:

  • Cellulose consists of repeated glucose units,
  • Natural rubber consist of repeated units of isoprene.
  • Proteins consist of a sequence of amino acids of a predetermined length and sequence,
  • DNA and RNA consist of phosphate backbones with predetermined sequences of four nucleic acids.
  • Some polymers are synthetic:
    • Polyethylene consists of long sequences of ethylene units,
    • Nylon consists of a long sequence of alternating units of adipic acid and hexamethylene diamine formed into a continuous polyamide.

Silicone polymers consist of repeated units of dimethyl silicone:


Chemists Segragate Polymers into two Classifications

Addition Polymer - Natural Rubber

Chemical structure of cis-polyisoprene, the main constituent of natural rubber. Synthetic cis-polyisoprene and natural cis-polyisoprene are derived from different precursors, isopentenyl pyrophosphate and isoprene. Image used with permission (CC BY-SA 3.0; Smokefoot).

Addition polymers form by adding one small molecule, the monomer, to the end of a forming chain of monomer units. Natural rubber and polyethylene are addition polymers.

Condensation Polymer - Cellulose

Condensation polymers form in a series of condensation reactions in which two molecules react with the elimination of a small molecule such as water. Cellulose, DNA, RNA, nylon and the silicones are condensation polymers. cellulose

Structure of Cellulose.

What you need to recall about Thermochemistry and Reaction Kinetics

Do you recall the section on thermochemistry? We learned that some chemical reactions were endothermic, requiring energy input for the reaction to proceed. Others were exothermic, releasing energy as they proceeded. We learned that the hydrocarbons in the ground are a source of energy because their oxidation is exothermic. Even a few cherries in a bowl, a banana, a slice of cheddar cheese are sources of energy because their oxidation, too, is exothermic.

But isn't there a problem of undersanding here? If a gallon of gasoline and a bowl of cherries both oxidize in an exothermic process, why don't they do so in the tank of our car or in the bowl on the table. Why do the forests, full of combustible wood, stand with majesty and serenity, and only burn when struck by lightning's torch? Our understanding of chemical kinetics deals with this problem. Chemical reactions require collisions - the physical impact of the reacting molecules. But not all collisions result in reactions. The collisions must be vigorous enough, have sufficient energy for reaction to take place.

That energy is called activation energy, the energy required to take our reactants to a physical condition called the activated complex. So gasoline and oak trees and bing cherries do not burst into flame out in the open because the energy of collision between the cellulose or the hydrocarbons is insufficient to reach the level of the activated complex.

But our bodies seem to convert the cherries and bananas and bread to energy without reaching extreme heat conditions. How do our bodies produce energy? How do we achieve the activated complex.

We must understand the process called catalysis, the ability of substances to speed up reactions without themselves being consumed. Catalysts lower activation energy, they allow the activated complex to be reached under milder reaction conditions. The catalysts in our bodies are called enzymes. Look at the figure below. It depicts the work of a catalyst, reducing the activation energy for a process.

Catalysis and Activation Energy

Activation Energy with Catalysis

General Electric's silicone development depended on understanding polymer chemistry. The preparation of (CH3)2SiCl2(9) could only be mastered if the chemists and engineers understood exothermic reactions with high activation energies.