Polyalkylene

Polyalkylenes, or polyolefins, are polymers derived from alkylene monomers, chiefly ethylene and propylene. The class includes polyethylene (PE), polypropylene (PP), polybutene, and polyisobutylene, among others. They are produced by addition polymerization, forming long chains of carbon atoms interrupted by hydrogen.

The repeating unit for polyethylene is -CH2-CH2-, for polypropylene -CH2-CH(CH3)-. Polymers can be linear or branched; tacticity (the arrangement of side groups) strongly affects density and crystallinity. Isotactic and syndiotactic PP are crystalline and rigid, while atactic PP is more amorphous.

Synthesis is mainly by catalytic polymerization of olefin monomers using Ziegler–Natta or metallocene catalysts, and by high- or low-pressure processes. Controlling catalysts, temperature, and comonomers yields materials with a range of molecular weights, densities, and branching.

Applications include packaging films and containers, piping and fittings, automotive components, and fibers (especially PP). The combination of light weight, chemical resistance, and processability makes polyalkylenes widely used in many industries.

Environmental note: Polyalkylenes are relatively inert and resistant to many chemicals, but recycling options depend on grade and contamination. HDPE and PP are commonly mechanically recycled, while chemical recycling is under development.