Science at a Distance

Physical Structure

The Giant Molecules of Life

Monomers and Polymers


Construction Principles Lipids, Polysaccharides, Proteins and Polynucleotides are the major groups of macromolecules that are found in all living organisms. These giant molecules carry out all the vital functions needed by cells. Macromolecules are involved in processes such as food digestion, information storage, energy manipulation and metabolism. They are complex, huge associations of molecular subunits that appear impossibly difficult to understand. Fortunately they are all built using the same construction principle.

Monomers and Polymers Monomers are small molecules, mostly organic, that can join with other similar molecules to form very large molecules, or polymers. All monomers have the capacity to form chemical bonds to at least two other monomer molecules.
monomers Monomers
Polymers are a class of synthetic substances composed of multiples of simpler units called monomers. Polymers are chains with an unspecified number of monomeric units.
a polymer Polymers
Homopolymers are polymers made by joining together monomers of the same chemical composition or structure.
a polymer consisting of all the same monomer Homopolymers
Heteropolymers are polymers composed of more than one kind of monomer.
a polymer consisting of more than one type of monomer Heteropolymers
Artificial Polymers and Special Properties

Human polymers

One of the first humans to discover, and make, an artificial polymer, was the German chemist Hans von Pechmann. It was probably an accident. In 1899 he found a suspicious, sticky, white substance at the bottom of a flask in which he had been trying to decompose diazomethane. He had no idea what he had made, so he turned the analysis of the material over to Eugen Bamberger and Friedrich Tschirner, who found long chains of -CH2-, which they called "polymethylene".

Some years later (1935) in England, Eric Fawcett and Reginald Gibson had a similar experience. They were trying very hard to make an explosive gas (ethylene) react with a much larger molecule (benzaldehyde), by forcing them together under high pressure. What they got was a useless, (so they thought!), white, waxy solid that couldn't be used for anything interesting or practical. How wrong they were, but nothing much more was done with this "polyethylene" until the start of the Second World War.

Suddenly there was a need for a flexible, non-reactive insulator to go around the cables of a new invention - radar. The British firm Imperial Chemical Industries re-discovered polyethylene and put it into production in 1939.

ethylene Small molecules of the odorless gas ethylene were then, and now, transformed into a polymer called polyethylene by uniting the ethylene monomers into a long chain. Some of these chains can be as long as 10,000 units. In some forms these chains branch, and they all coil and fold. Modern manufacturing methods start with ethylene gas which is heated under very high pressure until it becomes what is known as low-density polyethylene.

This material is a crystalline, transluscent thermoplastic which softens when heated. Today, consumers buy and use polyethylene in a huge number of ways, everything from packaging, garbage bags, soda bottles and containers, around wires (it's original use), and in almost every toy or house ware product on the market. Modern humans are very, very dependent on this particular artificial polymer.

polyvinylchloride

PVC

The second most popular, and useful, artificial polymer is "polyvinyl chloride", PVC. In it's pure form PVC is quite rigid and will not easily catch fire, so it forms the basis of all kinds of pipes, and coverings for such things as siding, windows and doors. When other things, called plasticizers, are added to PVC, the material becomes much more flexible and can be used to produce everything from garden hose to shower curtains.

Such a universal, and safe, material has a dangerous start. The monomer used in it's synthesis is a deadly poisonous gas called vinyl chloride. This gas is made by passing oxygen, hydrogen chloride and ethylene over copper, which acts as a catalyst. After very careful storage and handling, the vinyl chloride is mixed with initiators that begin the polymerization process.

PVC is a homopolymer, which in its pure form is too stiff for most applications. However if a second momomer, vinyl acetate is also incorporated into the chain, a more flexible product is created that has many more uses. In 1930, the Union Carbide Corporation first began making this 'copolymer', called it "Vinylite", and pressed music into it to make phonograph records.

Carbon and Natural Bio-Polymers
Carbon
The carbon atom has six electrons, four in the outermost energy level. Carbon can form four covalent bonds with other atoms and/or molecules. Carbon atoms can link to other carbon atoms to create long carbon strings that form the backbone of many natural organic molecules. It is this special property of carbon atoms that make them so important. Life is based on the chemistry of carbon.

a carbon chain
Natural Bio-polymers There are four major classes of bio-polymers based on the properties of the carbon atom;

Bio-Polymer Type Monomer(s)
Hydrocarbons/lipids homopolymer -CH2- units
Polysaccharides homo- and heteropolymers sugar units
Proteins heteropolymer amino acid units
Polynucleotides heteropolymer nucleotide units


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© 1997, 1998, 1999 Professor John Blamire