Monomerenheter

Monomerenheter, or monomer units, are the smallest repeating structural motifs in a polymer chain. They are derived from the monomer(s) used in polymerization and, together with the linker atoms, form the polymer's backbone. It is important to distinguish between a monomer (the single molecule that can undergo polymerization) and the repeat unit (the portion of the polymer that repeats along the chain). In addition, for chain-growth polymers the repeat unit is often identical to the monomer minus the reactive group; for example, polymerization of ethene yields a polymer whose repeat unit is –CH2–CH2–. In condensation polymers, the repeat unit may incorporate atoms from more than one monomer and may include atoms released as a small molecule during polymerization.

Monomer units determine polymer properties and composition; the sequence and type of repeat units control crystallinity, glass transition, mechanical properties, and thermal behavior. The degree of polymerization (DP) is the number of repeat units in a chain, and the number-average molecular weight Mn = DPn × M0, where M0 is the molar mass of the repeat unit. In copolymers, the chain contains two or more different repeat units in certain ratios, leading to tailored properties.

Examples include polyethylene (monomer: ethene; repeat unit: –CH2–CH2–), polystyrene (monomer: styrene; repeat unit: –CH2–CHPh–), and polyesters formed from diols and dicarboxylic acids where the repeat unit includes ester linkages. The concept also applies to geometric isomerism and tacticity, which affect properties without changing the repeat unit. Monomerenheter are key for describing polymer composition and for calculating molecular weight distributions.