A polymer is a large molecule composed of identical chemical units joined together like links in a chain. If all these links are identical to one another, the molecule is a homopolymer. Polyethylene is a homopolymer in which all the links are molecules of ethylene, a combustible gas with a low molecular weight. The ethylene molecule has a backbone of two carbon atoms with only four hydrogen atoms attached to them. A double bond unites the two carbon atoms. Since a single bond suffices for a union of two carbon atoms, the extra bond is available for uniting the ethylene molecules together to form polyethylene.
R.O. Gibson and E.W. Fawcett stumbled upon the first polyethylene in 1933. Their discovery became known as low density polyethylene (LDPE) after the subsequent development of other types of polyethylene. Its manufacture requires a high temperature and a high pressure. Polymerization takes place when one of the double bonds between the ethylene carbons is broken, so that ethylene becomes a free radical that can easily unite with other ethylene units. The result is not a straight chain, but a chain with many side branches. Because of these side chains, the resultant polyethylene is weaker than it would be if all the ethylene units formed a perfectly straight chain.
The production of high density polyethylene (HDPE) takes place at significantly lower temperatures and pressures than LDPE. Instead, HDPE relies on suitable catalysts to effect polymerization. The resultant polyethylene has longer chains and fewer branches than HDPE. It also has a higher molecular weight than LDPE. Consequently, HDPE is a stronger polyethylene. Whereas LDPE is used for such products as shopping bags and shipping envelopes, the stronger HDPE serves as a material for milk bottles and picnic tables. Karl Ziegler played an important role in the development of HDPE.
Like HDPE, ultrahigh molecular weight polyethylene (UHMWPE) has few side chains, but its principal chain is considerably longer. It may have a molecular weight up to 30 times greater than that of HDPE, according to "UHMWPE Biomaterials Handbook," by Steven Kurtz. Its resistance to wear makes it an ideal material for artificial hips, according to the same source. Because of its resistance to fracture, it can replace metal in such products as machine gears and bulletproof vests.