Addition Polymers

Addition Polymers

Introduction:

Polymers are long chain giant organic molecules are assembled from many smaller molecules called monomers. Polymers consist of many repeating monomer units in long chains. A polymer is analogous to a necklace made from many small beads (monomers).

Many monomers are alkenes which react by addition to their unsaturated double bonds. The formation of polyethylene from ethylene (ethene) may be illustrated in the graphic on the left as follows:

The electrons in the double bond are used to bond two monomer molecules together. This is represented by the red arrows moving from one molecule to the space between two molecules where a new bond is to form.

Note that in the complete polymer, all of the double bonds have been turned into single bonds. No atoms have been lost and you can see that the monomers have just been joined in the process of addition.

A simple representation is -[A-A-A-A-A]-.

Polyethylene is used in plastic bags, bottles, toys, and electrical
insulation.

LDPE - Low Density Polyethylene: The first commercial polyethylene process used peroxide catalysts at a temperature of 500 C and 1000 atomspheres of pressure. This yields a transparent polymer with highly branched chanis which do not pack together well and is low in density. Today most LDPE is used for blow-molding of films for packaging and trash bags.

HDPE - High Density Polyethylene: An alternate method is to use Ziegler-Natta aluminum titanium catalysts to make HDPE which has very little branching, allows the strands to pack closely, and thus is high density. It is 3 times stronger than LDPE and more opaque. About 45% of the LDPE is blow molded in milk and squeezable bottles.

Link to Polyethylene - with Chime molecule - Macrogalleria at U. Southern Mississippi