Part 3: Addition polymers, repeat units and PVC

Convert between a monomer and a polymer repeat unit, then use chain interactions to explain properties and plasticisation.

From one double bond to a long chain

Addition polymerisation joins many alkene monomers into a long molecule. For a simple monoalkene, the C=C becomes a C–C single bond within the backbone and new bonds join neighbouring units. All the atoms of the monomer are retained in the ideal repeat unit; no small molecule such as water is eliminated.

Do not say the entire double bond disappears and the carbons separate. The two original alkene carbons remain adjacent in the chain. Polymerisation conditions depend on the monomer and process; no single catalyst or pressure suits every addition polymer.

Draw the two backbone carbons first

For propene, begin with CH₂=CHCH₃. Change the double bond to a single bond and extend one bond outwards from each of these two carbons. Keep CH₃ attached to the second carbon. The repeat unit is –CH₂–CH(CH₃)–.

For a polymer equation, put brackets around the unit with bonds passing through the brackets and n outside; write n before the monomer on the reactant side. If asked for exactly one repeat unit, show one unit and its continuation bonds. If asked for several units, repeat the pattern without inventing terminal groups.

Recover the monomer from a chain segment

Find the smallest repeating pattern with the original two alkene-derived backbone carbons. Cut its two continuation bonds and restore C=C between those carbons. Keep the substituents on the same carbons.

For –CH(CH₃)–CH(CH₃)–, the monomer is CH₃CH=CHCH₃, but-2-ene. A repeat unit need not contain a CH₂ group. Redrawing the unit from the other end does not create a different polymer.

Separate bonds within chains from forces between chains

In a simple polyalkene backbone, strong C–C and C–H bonds and the lack of reactive functional groups help explain low chemical reactivity. Breaking down the chain is different from melting the material. On softening or melting, chains gain mobility as intermolecular constraints are overcome; the backbone does not normally break.

Long chains have many points of contact and substantial London attractions. Molecular mass, branching, packing and crystallinity influence properties, so “all polymers have the same melting point” is not meaningful. Their resistance to chemical and biological attack makes many addition polymers durable but also persistent as waste.

Why a plasticiser makes PVC flexible

PVC has polar C–Cl bonds and attractions between chains that restrict movement, giving unplasticised PVC useful rigidity for applications such as window frames and pipes. Plasticiser molecules fit between chains, reducing effective interchain attractions and making sliding easier.

Plasticised PVC is more flexible and can be used in cable insulation and flexible sheeting. The plasticiser does not normally soften PVC by cutting its covalent backbone. Choose a material by linking its structure and interactions to the required property.

Recycling can conserve material, but sorting, contamination and changes in properties can limit reuse. Burning can recover energy but carbon-containing polymers release CO₂; chlorine-containing PVC also needs control of acidic combustion products. These trade-offs do not mean all polymers are biodegradable or all can be recycled together.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources