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Condensation Polymerisation

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In condensation polymerisation, reactive groups on both ends of each monomer react with one another. A growing chain also has a reactive group on each end and so the length of the chain can suddenly get a lot larger if two chains join together.

The reactions that take place are usually ones familiar from low molecular weight organic chemistry, such as esterification (formation of a bond between an alcohol and a carboxylic acid). Clearly, two different reactive groups are required - reactions can be done with one monomer bearing two kinds of reactive group (A-B) or two different monomers (A-A and B-B).

Condensation polymerisations are used to make polyesters, nylon, and polyurethane.

The synthesis of Nylon 6,6 by the condensation polymerisation of 1,6-dihexanoic acid (adipic acid) and 1,6-diaminohexane.

The synthesis of poly(ethylene terephthalate) from terephthalic acid and ethylene glycol (1,2-ethanediol). Better known as PET, this polymer is widely used in soft-drink bottles.

The key features of condensation polymerisation are:

  • any two molecular species can react
  • chains start short and steadily grow
  • most monomer gone early in reaction
  • reactions relatively slow, generate little heat

Early in the reaction Midway Late
Some monomer
lots of short chains
Some short chains
lots of middle-sized chains
Some middle-sized chains
lots of long chains