Chain termination
In polymer chemistry, chain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt. Chain termination contributes to chain length, end-group structure, and other properties such as thermal stability and toughness.
There are multiple types of chain termination depending on the type of polymerization that is being used. Free radical, cationic, and anionic polymerizations are among the most common types of addition polymerizations and all employ different mechanisms for termination of chain growth. In free radical polymerizations, recombination and disproportionation are two termination mechanisms that are commonly observed. In recombination, two free radical ends combine to form a covalent bond, while in disproportionation, a hydrogen atom is transferred between two radical ends to form two molecules. In cationic polymerization, termination can occur via halide addition, β-elimination, or chain transfer. There are no such termination mechanisms for anionic polymerization. Instead, termination is commonly induced by the presence of trace impurities such as oxygen gas, carbon dioxide, or water molecules. Because the anionic chain ends are stable, they continue to grow in a process known as 'living' polymerization, and growth is halted once the monomer is depleted.
Chemical reaction in which a chain carrier is converted irreversibly into
a non-propagating species without the formation of a new chain carrier.