Chain-growth, step-growth, addition, and condensation polymerization
Chain-growth and step-growth polymerization are often considered to be the two main types of polymerization reactions, under which many other types of polymerizations – if not all – can be classified. Addition and condensation polymerization are often used interchangeably with the two (respective) processes above, but do differ slightly.
Chain-growth polymers (as opposed to step-growth) require initiators to begin the polymerization reaction. Chain-growth polymerization follows the procedure of initiation (allowing monomers to combine/attach to other monomers), propagation (in which the monomers actually attach to the initiated segment), and termination (ending the polymerization). In addition to these steps, chain-growth polymerizations can undergo a process called “chain transfer” where the reactive aspect of the growing chain (most commonly a radical, but may also occur for ionic polymerizations) is transferred to an intermediate species – such as a monomer, solvent or initiator – which terminates the growing chain and begins the growth of a new chain. Chain growth proceeds by adding one monomer at a time.
Step-growth polymers require an active functional group at each end of a monomer, such as -OH, -COOH, or -NH2. The reaction proceeds slowly, monomers reacting with each other and with the growing chains. Even when all the monomers have reacted there are still functional groups at the end of each chain, allowing the polymerization to continue. Step-growth is a slow reaction that typically requires higher temperature than chain-growth and usually results in linear chains, without branching or crosslinking. Most step-growth polymerizations create byproducts, and are thus condensation polymerizations, but a few do not.
Condensation polymers (as opposed to addition polymers) result in a byproduct (often water, sometimes other compounds such as HCl). Because these result from functional groups, all condensation polymers are step-growth polymers (though the reverse is not necessarily true).
Addition polymers react in such a way so that the final product contains all the atoms of the original monomers – each monomer is added to the next to create the polymer. Technically, there are step-growth polymers that proceed through addition polymerization, but the term is usually considered synonymous with chain-growth polymerization. The term polyaddition is sometimes used to refer to step-growth addition polymers, which proceed with independent addition reactions, as opposed to chain-growth.