Mechanisms of termination
In polymer chemistry, there are several mechanisms by which a polymerization reaction can terminate depending on the mechanism and circumstances of the reaction. A method of termination that applies to all polymer reactions is the depletion of monomer. In chain growth polymerization, two growing chains can collide head to head causing the growth of both of the chains to stop. In the case of radical or anionic polymerization, chain transfer can occur where the radical at the end of the growing chain can be transferred from the chain to an individual monomer unit causing a new chain to start growing and the previous chain to stop growing. With step-growth polymerization, the reaction can be terminated by adding a monofunctional species containing the same functionality as one or more of the types of monomer used in the reaction. For example, an alcohol R'-OH can be used to stop a reaction between a polyisocyanate and a polyol because it will react with the isocyanate functionality R-N=C=O to produce R-(N-H)-(C=O)-O-R' which is then no longer reactive with the polyol.