Mechanisms of formation of plastics [German version]

Plastics are formed by three different reaction mechanisms: addition polymerization, polycondensation and polyaddition.

Type of reaction Explanation
Addition polymerization Addition polymerization involves joining several small molecules together into a chain to form a large molecule. In this case, the molecules are merely arranged in succession. The product retains the same composition as the starting materials and no secondary products are eliminated. If the starting materials consist of identical molecules, the product is known as a homopolymer, while if they consist of different molecules, the product is known as a copolymer. The addition polymerization reaction is initiated by heat, pressure and catalysts.
Addition polymerization gives rise, for example, to polystyrene (PS), polypropylene (PP) and polyethylene (PE)
Polycondensation In polycondensation, differing starting materials are combined to yield a single molecule with elimination of a secondary product (usually water). If the reaction yields only linear chains, polycondensation may give rise to thermoplastics. However, if the individual molecules of the starting materials link together at several points, a three-dimensional structure is obtained, giving rise to thermosets.
Polyaddition Polyaddition is defined as the joining together of several molecules of different starting materials with migration of hydrogen atoms, but without secondary products being formed. Polyaddition gives rise, for example, to polyurethane (PU), which is highly environmentally friendly and is used for strapping tapes in packaging applications.