Synthetic fibres and plastics are manufactured from petrochemicals. These are chemical products derived from petroleum (crude oil) or natural gas. Through processes like cracking and polymerization, these raw materials are transformed into monomers, which are then linked into long polymer chains to create synthetic materials.

Key chemical structures often involve repeating units, for example, polyethylene: $[-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{]}_n$

Final Answer: petrochemicals