4.444CH1 · Section 4: Organic chemistry

Synthetic polymers

Key learnings

Addition polymerisation opens C=C
An addition polymer forms when many small monomer molecules (all alkenes, each with a $\text{C=C}$ double bond) join. During polymerisation the double bond opens to a single bond, so the chain has no double bonds left and no other product forms (addition). Ethene ( $\text{CH}_2\text{=CH}_2$ ) makes poly(ethene):

$n\,\text{CH}_2\text{=CH}_2 \rightarrow \text{--(CH}_2\text{--CH}_2\text{)--}_n$

The bracketed part is the repeat unit.

Four named polymers, monomers, uses

Four addition polymers are named in 4CH1:

Polymer	Monomer	Use
Poly(ethene)	ethene, $\text{C}_2\text{H}_4$	bags, bottles
Poly(propene)	propene, $\text{C}_3\text{H}_6$	crates, ropes
PVC	chloroethene, $\text{C}_2\text{H}_3\text{Cl}$	pipes, frames
PTFE	tetrafluoroethene, $\text{C}_2\text{F}_4$	non-stick pans

The repeat unit has the same atoms as the monomer.

•
Disposal: landfill vs burning
Addition polymers are very unreactive (inert), giving two disposal problems:
- Landfill: they are non-biodegradable, so they stay in landfill a very long time.
- Combustion: burning gives carbon dioxide (a greenhouse gas), and burning chlorine-containing polymers such as PVC releases toxic gases (e.g. hydrogen chloride, $\text{HCl}$ ).
Recycling reduces both but the plastics must be sorted first.

Common mistakes

Drawn from real examiner reports.

Leaving C=C in the repeat unit

When drawing a repeat unit, the C=C double bond must be opened to a single bond — leaving a $\text{C=C}$ in the polymer scores zero. The repeat unit must sit in brackets with a bond through each side, labelled $n$ . Copying the monomer unchanged, or omitting the brackets and $n$ , loses marks.

June 2023 Paper 1CR Q10(c)(i): most candidates drew the repeat unit correctly, but some lost a mark by omitting the brackets and n, and a few drew a double bond between the carbons — this scored zero.

Giving burning answers to a landfill Q

Disposal questions specify a method, and the answer must match it. For a landfill question the marks are non-biodegradable and stays in landfill a long time — writing about toxic gases, greenhouse gases, smells or harming wildlife is not creditworthy, as those relate to burning. Answer only the method asked.

June 2023 Paper 1CR Q10(c)(ii): both marks were for non-biodegradable and staying in landfill a long time; answers about toxic/greenhouse gases, smells and harming wildlife were not creditworthy.

Monomer is more than "a small molecule"

"A small molecule" alone does not score. A monomer is a small molecule that joins with many others to form a polymer; for addition polymers it is an unsaturated alkene (has a $\text{C=C}$ double bond). A polymer is a large molecule made from many repeating monomer units. Both need the joining/repeating idea, not just "small/large".

Monomer vs repeat unit (C=C or not)

The monomer and the repeat unit must not be swapped. The monomer keeps its $\text{C=C}$ double bond; the repeat unit has that bond opened to a single bond, with a bond out of each side, in brackets with an $n$ . Giving a monomer where a repeat unit is asked (or the reverse) loses the mark even when the atoms are right.

(common Edexcel chemistry error)

Thinking atoms are lost in polymerising

In addition polymerisation no atoms are gained or lost — only the double bond opens. So the repeat unit has the same atoms as the monomer: ethene $\text{C}_2\text{H}_4$ gives repeat unit $\text{CH}_2$ (the same $1:2$ C:H ratio). Do not add or remove hydrogens going from monomer to repeat unit; the only change is $\text{C=C}$ becoming a single bond.

(common Edexcel chemistry error)

Deducing the monomer: put C=C back

To find the monomer from a section of polymer, take one repeat unit, remove the two side bonds, and put the $\text{C=C}$ double bond back between the two backbone carbons. Candidates forget to restore the double bond, or keep the side bonds. For example, repeat unit $\text{--(CHCl--CH}_2\text{)--}$ gives monomer $\text{CH}_2\text{=CHCl}$ (chloroethene).

Exam tips

Monomer → repeat unit routine
Redraw the alkene with the $\text{C=C}$ in the middle, change the double bond to a single bond, extend a bond out of each end carbon, then enclose it in brackets with an $n$ . The repeat unit keeps every atom of the monomer — check that before moving on.
Repeat unit → monomer routine
Reverse it: take one repeat unit, remove the two outside bonds, and put the double bond back between the backbone carbons. Self-check — the repeat unit's empirical formula equals the monomer's ( $\text{CH}_2$ for both ethene and poly(ethene)).
Match disposal answer to the method
Underline the disposal method named. Landfill wants non-biodegradable and stays a long time; burning wants $\text{CO}_2$ (greenhouse gas) and, for PVC, toxic $\text{HCl}$ . Never mix the lists — burning points score nothing on a landfill question.

Revision notes