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Thermal cracking. In thermal cracking, high temperatures (typically in the range of 450Â°C to 750Â°C) and pressures (up to about 70 atmospheres) are used to break the large hydrocarbons into smaller ones. Thermal cracking gives mixtures of products containing high proportions of hydrocarbons with double bonds - alkenes.
First, they must start with various raw materials that make up the monomers. Ethylene and propylene, for example, come from crude oil, which contains the hydrocarbons that make up the monomers. The hydrocarbon raw materials are obtained from the cracking process used in refining oil and natural gas (see How Oil Refining Works). Once various hydrocarbons are obtained from cracking, they are chemically processed to make hydrocarbon monomers and other carbon monomers (like styrene, vinyl chloride, acrylonitrile) used in plastics.