Porraspolymeeriytymisessä

Porraspolymeeriytymisessä, often translated as step-growth polymerization, is a fundamental process in polymer chemistry where monomers react with each other to form larger molecules, progressively increasing their molecular weight. Unlike chain-growth polymerization, where a chain reaction propagates rapidly, step-growth polymerization involves a series of discrete reaction steps between functional groups on monomer molecules. These steps can occur between two monomers, a monomer and a growing chain, or two growing chains.

Key characteristics of step-growth polymerization include the formation of a low molecular weight species as a byproduct in many cases, such as water or methanol, when the reacting functional groups eliminate these small molecules. The polymerization proceeds through a series of condensation or addition reactions. The molecular weight of the polymer increases gradually throughout the reaction, and high molecular weights are typically achieved only when the conversion of monomers is very high, often exceeding 99%.

Common functional groups involved in step-growth polymerization include hydroxyl, carboxyl, amino, and isocyanate groups, leading to polymers like polyesters, polyamides, polyurethanes, and polycarbonates. The rate of polymerization is generally slow and is often dependent on the concentration of both functional groups and the catalyst, if used. Characteristically, all monomer molecules are converted into polymer molecules, with no unreacted monomers remaining at high conversion. The reaction mechanism involves the formation of dimers, trimers, and oligomers, which then react further to form high molecular weight polymers.