Karen A. Beitler
Monomers build into polymers and then connect with other molecules to form the things we eat, the organisms we are. A monomer is a small molecule that through chemical bonding can become a larger molecule. An example of a monomer is methylene (CH
2
). Bond some CH
2
-CH
2
groups and you get ethylene (ethene), the simplest alkene and the most produced organic compound in the world
2
. Ethylene is a natural hormone in plants and a sweet smelling and tasting gas. Ethylene can be found in natural gas and petroleum. In nature this gas promotes fruit ripening, leaf fall and inhibits growth
3
. Polymerization of ethylene, or adding more monomers, makes polyethylene. Monomers that are bonded together without loss of any atoms of the monomer are named by placing 'poly' in front of the monomer name. Polyethylene terephthalate (PET) is what soda bottles and laundry detergent containers are made from. Ethylene is a starting monomer for other two-carbon compounds; the most common are ethanol (manufactured alcohol) and ethylene glycol, the starting material for polyester, also known as anti-freeze. High density Polyethylene (HDPE) is used for milk jugs, shampoo bottles and landfill liners, while garbage bags, tape and disposable diapers are made from Low Density Polyethylene (LDPE). Poly vinyl chloride (PVC) is formed into pipes, siding and shower curtains; polypropylene (PP) is use for chip and cookie bags as well as Tupperware
4
. Ethylene is also converted to acetic acid and vinyl chloride or added to benzene to make plastics and synthetic rubber.
Synthetic, or man-made, polymers are commonly called plastics. Polystyrene, first made in the 1800s, seemed to have limited uses, until scientist learned to adjust the formula weight and inject gases into the mixture of monomers in the liquid state
5
. Polystyrene can be found in packing foam and disposable cups. Polymers play important roles in both living and non-living things, they are as simple sugar, fat, amino acids and plastics and form living organisms as complex as human beings.
Natural polymers are used as emulsifiers, thickens and stabilizers in the food and medical markets. Natural polymers are derived from the simplest organic compound; a hydrocarbon
6
. Hydrocarbons contain only hydrogen and carbon and can take many shapes. Carbon has an atomic number of six. Six protons in its nucleus, six electrons flying around them. The six electrons are arranged in two orbits, two in the first, which is called 'filled', and four in the second. The second orbit or 'p' orbit is happy as an octet, which means the atom is stable and will remain that way when there are eight electrons circling the nucleus; that means carbon is a friendly atom and is looking to share electrons with four other atoms. Hydrogen has an atomic number of one, like carbon; hydrogen would like a full outer shell as well. Hydrogen atoms are abundant and four of them are willing to bond with carbon making the molecule Methane (CH
4
)
7
. What is special about carbon is that carbon likes to bond to many different molecules and form various structures; like rings and branched chains. Carbon also forms double bonds and seems to have a limitless capacity for creating new molecules.
Some common natural polymers are gelatin (denatured collagen), keratin (the strong protein in skin, hair, nails, hooves, horns and teeth), silk (protein fiber), wool (hair follicles of animals in the Caprinae family), rubber (derived from plant sap), chitin (shells of shellfish) and DNA (a double helix of polymer chains)
8
. Chains of monomers comprise just about every reaction on earth. The longer the chain the more tangled it becomes and the less mobile the molecule becomes.
Polymers are linked, which makes them slow moving molecules. The larger a polymer is the less movement there is in a molecule
9
. This makes some molecules harder, more brittle and easily shattered and others difficult to break down (like rubber). Some polymers are made by a process called dehydration synthesis. Dehydration synthesis is a way that molecules are joined and a water molecule is removed
10
. When the bond between monomers is broken by enzymes there is addition of a water molecule, this is called hydrolysis.
Hydrolysis is the breakdown of a molecule into smaller components with the addition of water; hydro=water, lysis=to breakdown. Enzymes are known to catalyze, or speed up, reactions by lowering the amount of energy needed to start a reaction. They are therefore the biological catalysts that assist in the breakdown of polymers by reducing start up energy for a reaction
11
. Enzymes are very large protein molecules that resist being altered themselves; therefore they can be used over and over again. Enzymes are, however, specific to particular reactions. Amylases are enzymes that breakdown starch into sugar. They are present in the mouth and pancreas of mammals and are a common component of seeds. Natural polymers owe their diversity to carbon. The abundance of different combinations of molecules is possible because carbon is a molecule with four bonding sites that are always looking to be filled. Carbon is a very friendly molecule that loves to grab onto other molecules to form new bonds. Add water and its special properties into the mix and the combinations are endless.