Polymers

This guide covers Topic 11, Organic chemistry — subtopic 11.8 Polymers — for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. It builds on alkene addition reactions from Petroleum, Alkanes and Alkenes and the functional-group naming from Organic Chemistry: Formulae and Naming.

Where this fits in 0620/5070

This is the final subtopic of Topic 11, and it pulls together the whole organic sequence: addition polymers come from the alkenes covered earlier in the topic, and condensation polymers introduce the idea — carried much further at AS & A Level — that two different functional groups can react together repeatedly to build a long-chain molecule, which is also how proteins, one of the most important natural polymers, are built.

Syllabus coverage

CAMBRIDGE IGCSE CHEMISTRY 0620

Core

• Defining a polymer as a large molecule built from many smaller molecules called monomers (11.8)
• Describing the formation of poly(ethene) as addition polymerisation of ethene monomers (11.8)
• Stating that plastics are made from polymers (11.8)
• Describing how the properties of plastics affect their disposal, and the environmental challenges: landfill, ocean accumulation, toxic gases from burning (11.8)

Supplement / Extended

• Identifying repeat units and/or linkages in addition and condensation polymers (11.8)
• Deducing the structure/repeat unit of an addition polymer from a given alkene, and vice versa (11.8)
• Deducing the structure/repeat unit of a condensation polymer from given monomers, limited to polyamides (dicarboxylic acid + diamine) and polyesters (dicarboxylic acid + diol) (11.8)
• Describing the differences between addition and condensation polymerisation (11.8)
• Describing and drawing the structure of nylon (a polyamide) and PET (a polyester) (11.8)
• Stating that PET can be converted back into monomers and re-polymerised (11.8)
• Describing proteins as natural polyamides, formed from amino acid monomers (11.8)

CAMBRIDGE O LEVEL CHEMISTRY 5070

5070 has no Core/Extended split — every outcome above, Core and Supplement alike, is required for every O Level candidate.

What a polymer is

A polymer is a large molecule built up from many smaller, repeating molecules called monomers, joined together in a long chain. Plastics are materials made from polymers — “polymer” describes the molecular structure, “plastic” describes the resulting material.

Addition polymerisation

Addition polymerisation joins alkene monomers together, opening each C=C double bond so the monomers link directly, with no other product formed — this directly reuses the alkene chemistry from Petroleum, Alkanes and Alkenes.

Worked example. Ethene monomers polymerise to form poly(ethene).

n(CH2=CH2)  --polymerisation-->  (-CH2-CH2-)n

The repeat unit is -CH2-CH2-, repeated n times, with the double bond
"opened up" to form two new single bonds linking each monomer to its neighbours.

Given any alkene monomer, you can deduce the addition polymer’s repeat unit by opening its double bond into two single bonds; given a polymer’s repeat unit, you can work backward to identify the original alkene monomer the same way, in reverse.

0620 vs 5070 scope: 0620 names poly(ethene) formation from ethene as a specific required example. 5070 has no separate poly(ethene)-formation outcome — instead, 5070 candidates are expected to reach the same idea via the general skill of deducing an addition polymer’s structure from any given alkene monomer (as above), rather than recalling poly(ethene) as a named case.

Condensation polymerisation

Condensation polymerisation differs from addition polymerisation in two linked ways: it needs monomers that each carry two reactive (functional) groups, and it produces a small molecule (usually water) as a by-product alongside the polymer — nothing is lost in addition polymerisation, but condensation always releases something. Those two functional groups can sit on two different monomers (e.g. a diamine + a dicarboxylic acid for nylon, or a diol + a dicarboxylic acid for a polyester), or on a single type of monomer that already carries both groups itself (e.g. an amino acid, which has both an amine group and a carboxylic acid group) — proteins are built the second way, from one monomer, not two.

	Addition	Condensation
Monomer(s)	One type, containing C=C	Two types, each with two reactive end groups
By-product	None	Small molecule, usually water
Example	Poly(ethene)	Nylon, PET

Polyamides (like nylon) form from a dicarboxylic acid and a diamine, linked by amide bonds. Polyesters (like PET) form from a dicarboxylic acid and a diol, linked by ester bonds — the same kind of ester link covered in Alcohols and Carboxylic Acids, just repeated many times over instead of forming once.

Nylon’s repeat unit is built by an amide (peptide-type) link, -CO-NH-, forming between the -COOH end of one monomer and the -NH2 end of the other, releasing H2O each time: repeating unit -OC-(chain)-CO-NH-(chain)-NH-, with the amide linkage -CO-NH- appearing twice per repeat unit (once from each end of the diamine).

PET’s repeat unit is built the same way but with an ester link, -CO-O-, forming between a -COOH end and an -OH end, again releasing H2O: repeating unit -OC-(chain)-CO-O-(chain)-O-, with the ester linkage -CO-O- appearing twice per repeat unit.

PET is recyclable at the molecular level: because it was built by condensation (a reversible type of linkage in principle), PET can be broken back down into its original monomers and re-polymerised into new PET — a distinction worth knowing, since not all plastics can be recycled this way.

Proteins: a natural polyamide

Proteins are natural polyamides, built from amino acid monomers. Just as nylon links a diamine and a dicarboxylic acid through repeated amide bonds, a protein links amino acid monomers through the same kind of amide (in this context, called a peptide) bond, repeated along the chain — the same condensation-polymerisation principle occurring in living organisms rather than in a factory.

Plastics and the environment

Because most plastics are chemically very stable — that’s part of why they’re useful — they don’t readily biodegrade, which creates real disposal challenges:

• Landfill — plastics persist for a very long time without breaking down.
• Ocean accumulation — plastic waste that reaches waterways collects in the environment and harms marine life.
• Toxic gases from burning — incinerating certain plastics can release harmful gases, so disposal by burning isn’t a simple solution either.

Common mistakes

• Assuming all polymerisation is “addition.” Only alkene-based polymerisation is addition; anything built from monomers each carrying two reactive functional groups, releasing a small by-product (usually water), is condensation — whether that’s two different monomers (nylon, PET) or a single type of monomer carrying both groups (amino acids, in proteins).
• Forgetting condensation polymerisation releases a by-product. This is the single clearest test question can use to distinguish the two mechanisms — if a small molecule like water is released, it’s condensation.
• Mixing up polyamide and polyester. Polyamides (nylon, proteins) form from a diamine + dicarboxylic acid; polyesters (PET) form from a diol + dicarboxylic acid — same pattern, different second monomer.
• Deducing an addition polymer’s repeat unit incorrectly. The repeat unit comes from opening the C=C bond of the monomer into two single bonds — it is not simply “the monomer copied n times” without that structural change.
• Assuming all plastics can be recycled back to monomers like PET. That specific reversibility is a feature of PET’s condensation chemistry, not a universal property of every plastic.

Quick revision checklist

• Polymer and monomer, defined and distinguished from “plastic”
• Addition polymerisation: one monomer type, no by-product, from an alkene
• (0620 Extended, 5070 required) deducing repeat units from monomers and vice versa, for addition polymers
• Condensation polymerisation: two monomer types, small by-product released
• (0620 Extended, 5070 required) polyamides (nylon) vs polyesters (PET), and their respective monomer pairs
• (0620 Extended, 5070 required) PET’s reversibility back to monomers
• (0620 Extended, 5070 required) proteins as natural polyamides built from amino acids
• Environmental challenges of plastic disposal: landfill, oceans, burning

Related resources

• Petroleum, Alkanes and Alkenes — the alkene monomers addition polymers are built from
• Alcohols and Carboxylic Acids — the ester-forming reaction condensation polymerisation repeats
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.