Part 3: Addition polymers and polymer waste

Convert monomers to repeat units without losing substituents, then compare recycling, energy recovery and degradable materials.

The double-bond carbons form the backbone

Addition polymerisation joins many unsaturated monomers without eliminating a small molecule. For an alkene, change the C=C to a C–C single bond and add continuation bonds through both former double-bond carbons. Every substituent stays attached to the same carbon. Put brackets around one repeat unit with n outside.

Ethene gives [–CH₂–CH₂–]n. Propene gives [–CH₂–CH(CH₃)–]n. Chloroethene gives [–CH₂–CHCl–]n. The methyl group in poly(propene) is a side group; it does not add a third backbone carbon per repeat.

To recover the monomer, identify the two-carbon repeating backbone segment, remove continuation bonds and restore C=C, retaining every side group. Addition polymers with mostly C–C/C–H frameworks are often resistant to chemical attack and persist in the environment.

Industrial uses of alkene chemistry

Alkene feedstocks make a wide range of polymers and chemical intermediates. Hydrogenation also reduces double bonds in unsaturated oils, changing their properties and often increasing their melting range. Connect the application to the reaction rather than memorising product names alone.

There is no single best disposal route

Mechanical recycling sorts compatible polymers, cleans them and remelts suitable materials; contamination and deterioration limit repeated use. Chemical processing can turn waste into feedstocks, but needs energy and separation. Burning recovers energy but releases CO₂ and may release other pollutants.

Halogen-containing polymers can generate acidic gases such as HCl on combustion, requiring controlled processing and gas treatment. A generic “it gives greenhouse gases” misses that additional chemical issue.

Biodegradable polymers can be broken down biologically under specified conditions; photodegradable materials break down under light. “Bio-based”, “biodegradable” and “compostable” are different claims. Degradation may require industrial conditions and must not be assumed for every polymer made from a plant feedstock. Fragmentation alone does not prove complete harmless breakdown.

Worked unfamiliar repeat unit: retain every side group

For 2-methylpropene, CH₂=C(CH₃)₂, one double-bond carbon has two H atoms and the other has two methyl groups. After addition polymerisation, the repeat unit is [–CH₂–C(CH₃)₂–]n. The second backbone carbon has two backbone bonds and two methyl bonds, so it has no H.

The repeat unit contains four carbon atoms overall but only two in the backbone segment contributed by this monomer. Writing a four-carbon straight backbone would change connectivity. When drawing a section of chain, join the continuation bonds of successive units and omit internal brackets; show enough units to demonstrate the repeat.

The monomer and repeat unit have the same atom inventory in addition polymerisation. There is no small-molecule by-product to subtract. Brackets and n show repetition, not an extra atom or a bond back to the start of the same unit.

Recover the monomer by reversing the bond change

From [–CH₂–CH(CH₂CH₃)–]n, identify the two-carbon backbone repeat. Restore a double bond between those carbons and remove the continuation bonds, leaving the ethyl side group in place: CH₂=CHCH₂CH₃, but-1-ene.

For [–CH(CH₃)–CH(CH₃)–]n, both backbone carbons bear methyl groups. Restoring C=C gives CH₃CH=CHCH₃, but-2-ene. The simple repeat-unit connectivity does not by itself recover the original monomer’s E/Z arrangement.

Always use the repeating connectivity supplied, not the longest visible straight line in a convenient drawing. Rearranging side groups onto the backbone produces a different polymer.

Explain the trade-off for a specified waste stream

For clean sorted thermoplastic waste, remelting can retain material value while using less new feedstock. For mixed or contaminated waste, separation may be difficult and the product quality may fall. Feedstock processing breaks polymer chains into useful smaller substances but consumes energy and may produce mixtures needing separation.

Energy recovery makes use of combustion enthalpy but destroys the material and needs emissions control. For chlorine-containing waste, HCl is a specific acidic gas concern; it can be neutralised by an appropriate alkaline treatment. Carbon dioxide emissions still remain a separate issue.

A balanced answer should connect the proposed method to the composition and condition of the waste. “Biodegradable” needs stated environmental conditions and time; “renewable feedstock” alone says nothing about the polymer’s degradation mechanism.

Quick checks

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

Sources