Ethylene use falls into two main categories: 1) as a monomer, from which longer carbon chains are constructed, and 2) as a starting material for other two-carbon compounds. The first of these is the single largest use of ethylene, consuming about one-half of the annual output. polymerization (the repetitive joining of many small molecules (molecule) into larger ones) of ethylene gives polyethylene, a polymer having many uses, particularly in the production of packaging films, wire coatings, and squeeze bottles. When polymerization is carried out at high pressures and temperatures, the product is called low-density polyethylene and has properties different from the high-density polyethylene formed by polymerization under Ziegler-Natta catalytic conditions (see industrial polymers (industrial polymers, chemistry of)). Another use of ethylene as a monomer is in the formation of linear α-olefins (olefin). The oligomerization catalysts are similar to the Ziegler-Natta polymerization catalysts. Linear α-olefins have a number of applications, including the preparation of linear low-density polyethylene.