Askelkasvupolymerointi

Askelkasvupolymerointi, also known as step-growth polymerization, is a fundamental polymerization mechanism where monomers react with each other to form larger molecules in a stepwise fashion. Unlike chain-growth polymerization, where a small number of active centers propagate by adding monomers sequentially, step-growth polymerization involves the reaction between functional groups of monomers. This process can occur between monomers, oligomers, and polymers, and each reaction step involves the formation of a new covalent bond and typically the release of a small molecule, such as water or alcohol, as a byproduct.

The degree of polymerization increases gradually throughout the reaction. Initially, the reaction mixture consists mainly of monomers and dimers, trimers, and other short-chain oligomers. As the reaction progresses, these oligomers react with each other, leading to longer and longer polymer chains. The molecular weight distribution in step-growth polymerization is generally broad, and high molecular weights are achieved only at high conversion rates.

Common examples of polymers formed via step-growth polymerization include polyesters, polyamides (like nylon), polyurethanes, and polycarbonates. The mechanism is dictated by the functionality of the monomers. Monomers with two functional groups can react to form linear chains, while monomers with three or more functional groups can lead to branched or cross-linked polymer structures. The rate of polymerization is often dependent on the concentration of reactive functional groups.