Polyolefins

Polyolefins are a family of thermoplastic polymers produced from olefin (alkene) monomers, principally ethylene and propylene. They are saturated hydrocarbons with nonpolar, simple hydrocarbon backbones and are among the most widely produced and used plastics in the world. The properties and applications of polyolefins arise from differences in tacticity, branching, and crystallinity.

The two principal polyolefins are polyethylene (PE) and polypropylene (PP). PE includes high-density (HDPE), linear low-density (LLDPE), and low-density (LDPE) varieties, which differ in branching and density and thus stiffness and toughness. PP is generally more rigid and offers higher temperature resistance. Less common polyolefins include polybutene and various copolymers. Polyolefins are typically semi-crystalline to crystalline and exhibit excellent chemical resistance, good moisture barriers, and ease of processing.

Manufacturing of polyolefins relies on catalytic polymerization of ethylene and propylene. LDPE is produced mainly by high-pressure radical polymerization, while HDPE, LLDPE, and PP are produced via coordination polymerization using Ziegler–Natta or metallocene catalysts, enabling control of tacticity and microstructure. Processing methods encompass extrusion, injection molding, blow molding, and film extrusion, with properties tunable through comonomers, branching, and crystallinity.

Applications are broad: packaging films and containers, bottles, pipes and fittings (notably HDPE), automotive components, electrical insulation, and PP fibers for textiles. Recycling is widespread for polyolefins (codes 2 and 4 in most schemes), though contamination and mixed streams pose challenges. Growth of bio-based feedstocks and advances in recyclability continue to influence the environmental footprint of polyolefins.