- Step-growth polymerization
Step-growth polymerization is a
polymerizationprocess that involves a chemical reactionbetween multifunctional monomermolecules. 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).
The most common class of step-growth polymerization is called condensation polymerization and the product a
condensation polymer, because the chemical reactionby which the monomer molecules bond is often a condensation reactionthat 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 Carotherswho invented nylon, a condensation product of hexamethylene diamineand adipic acid. Each monomer has two functional groups (two amino groups or two carboxyl group) and so each monomer can form an amidelink 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 diamineand adipic acidcreate nylon
terephthalic acidand ethylene glycolcreate 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 viscosityof 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 pointbecause 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 metathesisor ADMET dienes polymerize with loss of ethylene.
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