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Polymers

12 min read

Addition polymers

A polymer is a very large molecule made by joining many small molecules, the monomers. In addition polymerisation the monomers are alkenes (or similar molecules with C=C). The double bonds open and the monomers join end to end. No other molecule is formed, so the polymer has the same empirical formula as the monomer.

Making polymersAddition polymerisation: many ethene molecules, each with a double bond, join into a long chain called poly(ethene), with no other product. Condensation polymerisation: a molecule with two amine groups and a molecule with two carboxylic acid groups join to make nylon, and a small molecule of water is lost at each link.Addition: ethene → poly(ethene)n CH₂=CH₂heat, pressure, catalyst—(CH₂—CH₂)ₙ—Condensation: diamine + diacid → nylon + waterH₂N–R–NH₂+HOOC–R′–COOHnylon: –NH–CO– (amide) links+ H₂O
MonomerPolymerProperties and uses
Ethene, CH₂=CH₂Poly(ethene) (polythene)Flexible: bags, bottles, pipes
PropenePoly(propene)Strong: ropes, crates, carpets
Chloroethene, CH₂=CHClPVCRigid or flexible: pipes, cables, floor covering
TetrafluoroethenePTFE (Teflon)Non-stick coatings
Phenylethene (styrene)Poly(styrene)Packaging and insulation

Condensation polymers

In condensation polymerisation two different monomers, each with two functional groups, join and a small molecule (usually water or HCl) is lost at each link. Polyamides (such as nylon) are made from a diamine and a dicarboxylic acid (or diacid chloride) and have amide links. Polyesters (such as Terylene, PET) are made from a diol and a dicarboxylic acid and have ester links. Proteins are natural polyamides made from amino acids.

PolymerMonomersLinkSmall molecule lostUses
Nylon-6,6Hexane-1,6-diamine and hexanedioic acidAmideWaterRopes, clothes, bearings
Terylene (PET)Ethane-1,2-diol and benzene-1,4-dicarboxylic acidEsterWaterDrinks bottles, fibres
ProteinAmino acidsPeptide (amide)WaterMuscle, enzymes
Many small molecules become one long chain.
Try itLength of a polymer chain

The average relative molecular mass of a sample of poly(ethene) and the mass of one monomer unit (CH₂CH₂ = 28) give the number of units n.

Plastics and the environment

TypeBehaviour on heatingExamples
ThermoplasticsSoften and can be remouldedPolythene, PVC, nylon
Thermosetting plasticsDo not soften: cross-linked chainsBakelite, melamine resin
Check yourself Which type of reaction makes poly(ethene)?
Check yourself In nylon, the monomers are linked by…
Check yourself Which is a thermosetting plastic?

Key terms

Monomer
A small molecule that joins with others to make a polymer.
Addition polymer
A polymer made by joining alkene monomers without loss of any small molecule.
Condensation polymer
A polymer made with the loss of a small molecule at each link.
Biodegradable
Able to be broken down by living organisms.

Summary

  • Addition polymers come from alkenes; condensation polymers lose a small molecule at each link.
  • Nylon is a polyamide; Terylene is a polyester; proteins are polyamides.
  • Thermoplastics soften on heating; thermosets do not.
  • Plastic waste needs reducing, recycling and biodegradable alternatives.
Amines Environmental Chemistry