Polyethylene (HDPE) ((C₂H₄)n) — Properties, Uses & Safety

Overview

Polyethylene (PE) is the world's most widely produced plastic, with global production exceeding 100 million tons per year. It is a synthetic polymer with the repeating unit (C2H4)n, formed by the addition polymerization of ethylene (ethene) monomers. Depending on the polymerization conditions, polyethylene can be produced in several forms: high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and ultra-high-molecular-weight polyethylene (UHMWPE), each with distinct properties and applications. HDPE has a density of 0.93-0.97 g/cm³ and features mostly linear polymer chains with minimal branching, allowing them to pack closely together in a crystalline structure. This gives HDPE its characteristic rigidity, strength, and chemical resistance. HDPE is used to make milk jugs, detergent bottles, water pipes, cutting boards, and plastic lumber. LDPE, with more branching and lower crystallinity, is softer and more flexible — it is used for plastic bags, food wrap, squeeze bottles, and wire insulation. The discovery of polyethylene was partly accidental — it was first synthesized in 1898 by Hans von Pechmann, and its industrial production began in the 1930s using high-pressure free radical polymerization. The development of Ziegler-Natta catalysts in the 1950s (which earned Karl Ziegler and Giulio Natta the 1963 Nobel Prize in Chemistry) enabled the production of HDPE at lower pressures with greater control over polymer structure. More recently, metallocene catalysts have provided even finer control over polyethylene properties. Polyethylene is chemically inert, resistant to most acids, bases, and solvents, and is an excellent electrical insulator. It is non-toxic and approved for food contact applications. However, the environmental impact of polyethylene waste is a major global concern — it is not biodegradable and persists in the environment for hundreds of years, contributing to ocean plastic pollution. Recycling (HDPE is recycling code #2, LDPE is #4) and development of biodegradable alternatives are active areas of research. Polyethylene has no significant GHS hazards. For chemistry students, polyethylene is the ideal introduction to polymer chemistry, covering addition polymerization mechanisms, the relationship between molecular structure (branching, crystallinity) and physical properties, catalysis in industrial chemistry, and the environmental chemistry of plastics. It demonstrates how a simple monomer like ethylene can be transformed into materials with vastly different properties depending on processing conditions.

IUPAC Name

poly(1-ethylene)

CAS Number

9002-88-4

PubChem CID

6325750

Molecular Formula

(C₂H₄)n

Molar Mass

28060 g/mol

State at STP

solid

Appearance

Translucent to white waxy solid