Condensation Polymerisation or Step Growth Polymerisation

Building chains step by step

The second big family is condensation (step growth) polymerisation. Instead of monomers simply adding on to a growing radical chain, here the polymer is built by a repeated condensation reaction between monomer units that each carry two (or three) reactive functional groups (bi- or tri-functional monomers).

• Each condensation step typically releases a small molecule as a by-product — commonly water, an alcohol, or hydrogen chloride.
• Crucially, the product of every single step is itself still a bi-functional molecule, capable of reacting further at both ends. Since every step is essentially independent of the ones before it, and simply extends the chain by one more link, this route is called step growth polymerisation.
• Repeating this many times over builds up a high molecular mass condensation polymer.

Worked case: terylene (Dacron)

A classic example is the reaction between ethylene glycol (ethane-1,2-diol) and terephthalic acid (benzene-1,4-dicarboxylic acid). The –OH groups of the diol and the –COOH groups of the diacid react repeatedly, esterifying at each end of the growing chain and eliminating water at every step, to build the long polyester chain known as terylene or Dacron: …