Addition and Condensation Polymers

A polymer is a very large molecule built from many small molecules joined together in a long chain. The small molecules are called monomers. Plastics such as poly(ethene) are polymers, and so are many molecules in living things, including DNA and proteins.

(Separate Chemistry only) Polymers in this topic are assessed in GCSE Chemistry, not in Combined Science Trilogy.

There are two ways monomers join, and telling them apart is a core exam skill:

Type	Monomers needed	Small molecule lost?	Assessment
Addition polymerisation	Alkenes (one C=C double bond)	No	All tiers (Separate)
Condensation polymerisation	Monomers with two functional groups	Yes (e.g. water)	Higher Tier (Separate)

Addition polymerisation comes first because it builds directly on alkenes, which react through their C=C double bond. The rest of this lesson works through addition polymers, then the Higher Tier condensation polymers, and finally the natural polymers such as DNA.

In addition polymerisation, many alkene monomers join to make one long polymer. Alkenes can do this because they contain a reactive carbon–carbon double bond (C=C). During the reaction the double bond opens up and the monomers link end to end into a single chain. Crucially, no other molecule is produced — every atom in the monomers ends up in the polymer.

The two named examples are:

Monomer	Polymer
Ethene	Poly(ethene)
Propene	Poly(propene)

The name of the polymer is simply poly( plus the monomer name in brackets. You must be able to recognise addition polymers and their monomers from diagrams, and identify a monomer as an alkene by its C=C double bond. Because the atoms are simply rearranged rather than lost, the polymer's molecular formula is a whole-number multiple of the monomer's.

You must be able to draw the polymer formed from a given alkene monomer and relate the repeating unit to the monomer. The repeating unit has the same atoms as the monomer, but the double bond is opened to a single bond.

Follow three steps, using ethene as the worked example:

1. Draw the monomer with its C=C double bond and everything attached to the two carbons.
2. Change the C=C double bond into a single C–C bond.
3. Draw a bond leaving each of those two carbons (one on the left, one on the right), enclose the unit in brackets, and write n outside to show it repeats many times.

Monomer (ethene):

    H   H
     \ /
      C=C
     / \
    H   H

Repeating unit of poly(ethene):

      H  H
      |  |
   -[-C--C-]-
      |  |  n
      H  H

The repeating unit must have the same number of each atom as the monomer. If your monomer is C₂H₄, the repeating unit must also contain 2 carbons and 4 hydrogens. Always include the two bonds extending out through the brackets and the n.

For poly(propene), the monomer is propene (CH₂=CHCH₃), so the repeating unit carries a –CH₃ group on one of the two carbons; the method is identical.

(Higher Tier only) Condensation polymerisation, including the polyester example, is assessed only at Higher Tier.

Condensation polymerisation is different from addition in two ways: it uses monomers that each have two functional groups, and it loses a small molecule (often water) each time two monomers join. Because a small molecule is removed at every link, the polymer contains fewer atoms than the monomers started with.

The simplest condensation polymers form from two different monomers, each with two of the same functional group. The named example is a polyester made from:

• ethanediol — a molecule with two –OH groups (a diol)
• hexanedioic acid — a molecule with two –COOH groups (a dicarboxylic acid)

$\text{ethanediol}+\text{hexanedioic acid}\to \text{polyester}+\text{water}$

An –OH group on one monomer reacts with a –COOH group on the other, forming a link and releasing a water molecule. Because each monomer has two reactive ends, the chain keeps growing in both directions to build a long polyester. You must be able to explain these basic principles in terms of the functional groups and the repeating unit, but you do not need to memorise the full structural formula.

(Higher Tier only) Amino acids and polypeptides are assessed only at Higher Tier.

Amino acids are monomers that have two different functional groups in the same molecule: an amino group (–NH₂) and a carboxylic acid group (–COOH). Because the two groups can react with each other, amino acids join by condensation polymerisation, losing a water molecule at each link, to form polypeptides.

The named amino acid is glycine, formula H₂NCH₂COOH. You can see both functional groups in it: the H₂N– (amino) at one end and the –COOH (carboxylic acid) at the other.

When different amino acids are combined in a chain, the resulting polymers are proteins. The order in which the amino acids are joined determines which protein is made, which is why a small set of amino acids can build a huge variety of proteins in living things.

A polypeptide is a condensation polymer of amino acids. Different amino acids joined in a chain give a protein.

Not all polymers are man-made. Several polymers essential for life occur naturally, and you must be able to name the monomers they are built from.

DNA (deoxyribonucleic acid) is a large molecule that carries the genetic instructions for the development and functioning of living organisms. Most DNA molecules are two polymer chains made from four different monomers called nucleotides, and the two chains are wound around each other in the shape of a double helix.

The other naturally occurring polymers you must know, with their monomers:

Natural polymer	Monomers
DNA	Nucleotides
Proteins	Amino acids
Starch	Sugars
Cellulose	Sugars

Both starch and cellulose are built from sugar monomers. Proteins, as covered above, are built from amino acids.

1. Confusing addition and condensation

Addition uses alkene monomers with a C=C bond and produces no other molecule. Condensation (Higher Tier) uses monomers with two functional groups and loses a small molecule such as water each time. A question that mentions "loss of water" is describing condensation.

2. Losing atoms when drawing the repeating unit

The repeating unit of an addition polymer has the same atoms as the monomer. If the monomer is C₂H₄, the repeating unit is also C₂H₄. Do not drop hydrogens or leave out the two bonds that extend through the brackets.

3. Forgetting the bonds through the brackets and the n

An addition polymer repeating unit needs a bond leaving each end carbon through the square brackets, plus the letter n outside to show it repeats. A drawing without these is an incomplete answer.

4. Saying condensation loses no molecule

By definition, condensation polymerisation loses a small molecule (water in the polyester and polypeptide examples). Only addition polymerisation forms no other product.

5. Naming the wrong monomer for a natural polymer

Learn the pairs exactly: DNA from nucleotides, proteins from amino acids, starch and cellulose from sugars. Writing "glucose" as the monomer of DNA, or "nucleotides" for starch, loses the mark.