Polyethylene and polyvinyl chloride, or PVC, are plastics that form through a mechanism called radical polymerization. The reaction conditions used for each, however, are different, as are the structures, properties and uses of the finished materials. Both polymers are chemicals you use all the time in your daily life.
PVC is formed from long chains of carbon atoms where every other carbon atom has a chlorine atom attached to it. Polyethylene, by contrast, is a large chain of carbon atoms with only hydrogen atoms attached; there are no atoms of chlorine, oxygen or any other elements. Furthermore, while PVC always has the same basic structure, polyethylene is divided into several different types based on the degree of branching from the main chain in each polymer. Some types of polyethylene, such as low-density polyethylene, are highly branched, while other types have a more unbranched structure.
Both polyethylene and PVC are manufactured through radical polymerization, in which a type of compound called a peroxide splits into two radicals. One of these two radicals attacks the double-bonded carbon group, which now becomes a radical and can go on to attack other double-bonded carbon groups in turn. PVC, however, is made from subunits of vinyl chloride. Each vinyl chloride monomer has a pair of double-bonded carbons with a chlorine atom attached to one of them. Polyethylene is made from ethylene subunits. Special catalysts are used when making polyethylene to ensure the chain stays unbranched, whereas no catalyst is needed with PVC.
Both polyethylene and PVC are waterproof, but PVC is more so. Additionally, PVC is much more fire-resistant than polyethylene, because the chlorine atoms it releases during a fire inhibit the process of combustion. PVC in its native form is brittle and hard, so it needs to be softened and made flexible by adding other compounds called plasticizers. Polyethylene's properties depend on the type. LDPE is much softer and more malleable than linear polyethylenes such as high-density polyethylene, or HDPE.
PVC is one of the most popular plastics in the manufacture of plumbing components. It's also used to make garden hoses, raincoats and vinyl leather bags. Polyethylene has an almost innumerable variety of uses. LDPE is used for plastic films and shopping bags, while the sturdier HDPE is used for everything from large containers to plastic milk jugs and children's toys. Ultra-high molecular weight polyethylene, or UHMWPE, is so strong that it is used in some bulletproof vests and ice skating rinks.