C4.444SD0 · Section C4: Organic chemistry

Synthetic polymers

Key learnings

Disposal — inert/non-biodegradable, toxic gases when burned
Addition polymers are hard to dispose of for two spec reasons. (1) They are chemically inert / non-biodegradable: microorganisms cannot break them down, so they persist in landfill (which also takes up space). (2) Burning them produces toxic gases: incomplete combustion gives poisonous carbon monoxide (CO), and burning poly(chloroethene)/PVC additionally gives toxic hydrogen chloride (HCl). Recycling reduces both problems but the polymers must be sorted first.
Addition polymerisation — many unsaturated monomers, no by-product
Addition polymerisation: many small unsaturated monomers (alkenes, with a C=C double bond) join to form one long-chain polymer, with no other product formed. Two mark-scheme elements: many monomers join, AND the C=C opens with no by-product — this is what makes it addition, not condensation. A monomer is the small starting molecule; a polymer is the long product; a repeat unit is the group of atoms that repeats along the chain.
Repeat unit — two-carbon backbone, single bond, extension bonds
Every addition-polymer repeat unit is built on the same skeleton: two carbon atoms joined by a SINGLE bond, with an extension bond out on the left and right where it joins its neighbours. The monomer C=C has opened to that single C–C bond — the key change. Then attach the side groups: poly(ethene) keeps four H; poly(chloroethene) swaps one H for Cl; poly(propene) swaps one H for a $\text{CH}_3$ group; PTFE swaps all four H for F. Each carbon must have exactly four bonds.

Common mistakes

Drawn from real examiner reports.

Monomer (reactant) vs polymer (product)

The monomer is the small starting alkene (the reactant); the polymer is the long product. Writing one where the other is asked scores zero. Read the command: "name the monomer" wants the alkene, "name the polymer" wants the poly(...) product. Note too that ethene is a gas at room temperature — a detail candidates often get wrong.

Nov24 1C Q10c

Repeat unit is not the monomer

A repeat unit is NOT the same as the monomer. The monomer has a C=C double bond and no extension bonds; the repeat unit has a single C–C bond and a bond extending at each side. To go from monomer to repeat unit, open the double bond to a single bond and add an extension bond at each end. Confusing the length of the repeat unit with the monomer is reported.

Nov24 1C Q10c

Repeat units need extension bonds

Repeat-unit structures must be fully displayed (every bond shown as a line) and must carry an extension bond going left AND right, showing the unit continues on both sides. Omitting these continuation bonds is the most common error — it makes the drawing look like a molecule, not a repeat unit. Brackets around the unit are optional; the two extension bonds are not.

Jun23 1C Q4bii

Poly(propene): the five-bond carbon

Poly(propene) is the hardest repeat unit to draw because of its CH3 side group: candidates give the backbone carbon five bonds, or add too many hydrogens. Use the two-carbon backbone rule — one backbone carbon carries two H, the other carries one H and one CH3, and each carbon still has exactly four bonds. The CH3 goes on ONE carbon only, never both.

Jun23 1C Q4bii

Vague disposal answers lose marks

A disposal question needs precise, creditworthy reasons, not "they pollute". Use exact ideas: the polymer is non-biodegradable / chemically inert, so microbes cannot decompose it and it persists in landfill; and burning it releases a NAMED toxic gas — carbon monoxide from incomplete combustion, or hydrogen chloride from PVC. "Toxic gas" without naming it is not enough.

Inert does not mean it cannot burn

"Chemically inert / non-biodegradable" describes how a polymer resists decay in the environment — it does NOT mean the polymer cannot burn. Burning is exactly the disposal route that releases the toxic gases (CO, and HCl from PVC). Do not use inertness as a reason that the plastic is harmless; persistence and toxic combustion products are two separate disposal problems.

Exam tips

Convert monomer and repeat unit by routine
Use one routine so you never lose the double-bond mark. Monomer to repeat unit: change the C=C to a single C–C bond, then draw an extension bond at each carbon; keep the side groups where they were. Reverse it to get the monomer: remove the extension bonds and restore the C=C.
Check every carbon has four bonds
Finish any polymer structure by checking each carbon has exactly four bonds — this catches the poly(propene) five-bond error and the extra-hydrogen error. In the repeat unit the double bond has become single, freeing one position for an H or side group.
Repeat unit and monomer share the same Mr
The repeat unit contains the same atoms as one monomer, so its $M_r$ equals the monomer $M_r$ (ethene and the poly(ethene) repeat unit are both $M_r = 28$ ). Work out $M_r$ from the monomer formula and read each $A_r$ from the Edexcel data sheet — note Cl = 35.5, not 35.

Revision notes