High-molecular compounds (polymers)
Compounds with molar mass of thousands and more are high-molecular compounds (polymers); they are built of many repeating units. The low-molecular substance that forms the polymer is the monomer, the repeating group is the structural unit, the number of units n is the degree of polymerization, and the molar mass of the polymer is M = n · M(unit). Polymerization is the joining of monomers with double bonds without by-products: nCH₂=CH₂ → (–CH₂–CH₂–)ₙ polyethylene; nCH₂=CHCl → (–CH₂–CHCl–)ₙ polyvinyl chloride; nCH₂=CH–CH₃ → polypropylene; nCH₂=CH–CH=CH₂ → polybutadiene rubber. In polycondensation monomers join with release of a small molecule such as water: phenol + formaldehyde → phenol–formaldehyde resin + H₂O; proteins from amino acids; a polyamide (nylon, capron) from a diamine and a dicarboxylic acid. Polymers are natural (starch, cellulose, protein, natural rubber), artificial (made by modifying a natural polymer: acetate fibre) and synthetic (polyethylene, PVC, divinyl and chloroprene rubbers). Plastics, fibres and rubbers are the three main use types. Plastic waste persists in the environment, so sorting and recycling are important.
Sort plastic items by their marks (PET, PE, PP, PVC) and note the amount of household plastic waste over a week; discuss how to reduce it. Never burn plastic.