Condensation and Addition Polymers

Polymers are very large molecules made by joining many small molecules called monomers. Many plastics are polymers. Understanding how monomers join helps you predict the polymer’s structure and properties.

Addition polymerisation

- Monomers: alkenes (they have a C=C double bond).
- What happens: the double bond opens and links the molecules into a long chain.
- Products: only one product — the polymer (no small molecules lost).
- Naming: poly(name of alkene), e.g. poly(ethene).

Example:

$$n\,\mathrm{CH_2{=}CH_2} \rightarrow [-\mathrm{CH_2-CH_2}-]_n$$

Spotting addition polymers

The repeat unit looks like a saturated chain (single bonds only) and usually contains only carbon and hydrogen.

Condensation polymerisation

- Monomers: molecules with two functional groups each (one at each end).
- What happens: each time a link forms, a small molecule is removed (often water, sometimes HCl).
- Two key types for IGCSE:
Polyesters: from a dicarboxylic acid (–COOH at both ends) and a diol (–OH at both ends). Link formed is an ester, –COO–.
Polyamides: from a dicarboxylic acid and a diamine (–NH2 at both ends). Link formed is an amide, –CONH–.

Forming an amide link: $\text{-COOH} + \text{-H}_2\text{N-} \rightarrow \text{-CONH-} + \text{H}_2\text{O}$

Examples
- Nylon: a polyamide made from a diamine and a dicarboxylic acid; contains –CONH– links and releases water when linking.
- PET (a common polyester for drinks bottles): made from a diol and a dicarboxylic acid; contains –COO– links. PET can be broken back into its monomers and re-polymerised (chemical recycling).

Key differences

• Monomers: addition uses alkenes; condensation uses di-functional monomers
• By-product: addition has none; condensation produces small molecules (e.g. water)
• Links: addition makes a carbon chain; condensation makes ester (–COO–) or amide (–CONH–) links
• Examples: poly(ethene) vs. nylon/PET

Natural connection

Proteins are natural polyamides. Amino acids (with –NH2 and –COOH) join by condensation to form peptide (amide) links, releasing water.