polyadditio

Polyaddition is a type of step-growth polymerization where monomers add to one another in such a way that the repeating unit in the polymer contains the same atoms as the repeating units of the monomers, with no atoms lost. This process is characterized by the absence of small molecule byproducts, such as water or HCl, which are typically eliminated in other step-growth polymerization mechanisms like polycondensation.

The reaction involves the addition of functional groups on one monomer to functional groups on another monomer, or within the same monomer if it possesses multiple reactive sites. For example, a diisocyanate monomer can react with a diol monomer to form a polyurethane. In this reaction, the isocyanate group (-N=C=O) on the diisocyanate reacts with the hydroxyl group (-OH) on the diol, forming a urethane linkage (-NH-COO-). The repeating unit of the polyurethane polymer is formed by combining the structures of the diisocyanate and diol monomers without the release of any small molecules.

Common examples of polymers formed via polyaddition include polyurethanes, polyureas, and some types of epoxy resins. The key feature distinguishing polyaddition from polycondensation is the conservation of all atoms from the reacting monomers in the final polymer structure. This often leads to polymers with high molecular weights and specific desirable properties, making polyaddition a significant polymerization technique in polymer chemistry and material science. The reaction mechanism is typically bimolecular and proceeds through a series of steps as monomers link together.