Monomer – Ethylene
Ethylene (IUPAC name is ethene) is a hydrocarbon which has the formula C2H4. It is a colourless flammable gas, widely used in chemical industry, and its worldwide production exceeds that of any other organic compound.
Ethylene is produced in the petrochemical industry by steam cracking. In this process, gaseous or light liquid hydrocarbons are heated to 750–950 °C, inducing numerous free radical reactions followed by immediate quench to stop these reactions. This process converts large hydrocarbons into smaller ones and introduces unsaturation. By repeated compression and distillation, ethylene is separated from the resulting complex mixture .
Ethylene is a rather stable molecule that polymerises only upon contact with catalysts. The conversion is highly exothermic.
Coordination polymerisation (one kind of addition polymerisation) is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist of titanium (III) chloride and aluminium alkyl, the so-called Ziegler – Natta catalysts. Another common catalyst is the Phillips catalyst, prepared by depositing chromium (VI) oxide on silica. Using catalysts allows the polymerisation of ethylene to happen in low pressure, which will result in the production of high density polyethylene (HDPE).
Polyethylene can be also produced through free radical polymerisation, but this route has only limited utility and typically requires high pressure apparatus. This process is used to produce low density polyethylene (LDPE), which requires a small amount of oxygen or organic peroxide as an initiator.
Physical properties: Depending on the crystallinity and molecular weight, a melting point and glass transition temperature varies strongly with the type of polyethylene. For HDPE, the melting point is typically in the range 120 to 180 °C. For LDPE, it is is typically 105 to 115 °C.
Chemical properties: They have excellent chemical resistance, meaning that it is not attacked by strong acids or strong bases and resistant to gentle oxidants and reducing agents.