Step-growth polymerization

Step-growth polymerization is a polymerization process that involves a chemical reaction between multifunctional monomer molecules. In a step-growth reaction, the growing chains may react with each other to form even longer chains. This applies to chains of all lengths. Thus, a monomer or dimer may react in just the same way as a chain hundreds of monomer units long. This is in contrast to a chain-growth polymerization, where only monomers may react with growing chains (In chain-growth polymerization, two growing chains can't join together the way they can in a step-growth polymerization).

Irreconcilable Differences: One variety of step-growth polymerization is called condensation polymerization and the product a condensation polymer because the chemical reaction by which the monomer molecules bond is often a condensation reaction that produces a small molecule byproduct. A multifunctional monomer is a molecule that has more than one potential reactive site by which it can form intermolecular chemical bonds. The easiest way to visualize a step growth polymerization is a group of people holding hands to form a human chain: each person has two hands (=reactive sites).

A pioneer in step-growth polymerisation is Wallace Carothers who invented nylon, a condensation product of hexamethylene diamine and adipic acid. Each monomer has two functional groups (two amino groups or two carboxyl group) and so each monomer can form an amide link with each of its neighbour.

The functionality of a monomer is the number of reactive sites. A functionality of 2 will yield a linear polymer. Examples:

• hexamethylene diamine and adipic acid create nylon
• terephthalic acid and ethylene glycol create PET

In theory the polymerisation will continue to result in a single macromolecule. The relationship between the extent of the reaction and the average number of monomer units in a polymer chain is given by the Carothers equation. High molecular weight polymer is formed only at high degrees of conversion (extent of reaction). In practise the average length of the polymer chain is limited by such things as the purity of the reactants and the viscosity of the medium.

A monomer with functionality 3 will introduce branching in a polymer and will ultimately form a cross-linked macrostructure. The point at which this three-dimensional structure is formed is known as the gel point because it is signalled by an abrupt change in viscosity. One of the earliest so-called thermosets is known as bakelite.

It is not always water that is released in step-growth polymerization: in acyclic diene metathesis or ADMET dienes polymerize with loss of ethylene.