Polymerisation

• Two backbone carbons per monomer, and a bond through each bracket.
• No other product. Two backbone carbons, bonds through the brackets. Saturated backbone, so it neither rots nor burns cleanly.

• 20.1.1 I can describe — describe addition polymerisation as exemplified by poly(ethene) and poly(chloroethene), PVC
• 20.1.2 I can deduce — deduce the repeat unit of an addition polymer obtained from a given monomer
• 20.1.3 I can identify — identify the monomer(s) present in a given section of an addition polymer molecule
• 20.1.4 I can — recognise the difficulty of the disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “The repeat unit of poly(propene) is –[CH₂–CH₂–CH₂]–.” Correct Two backbone carbons, and bonds through the brackets. Only the two carbons of the C=C join the backbone; the CH₃ of propene hangs off as a side group: –[CH₂–CH(CH₃)]ₙ–. And a bond from each backbone carbon must pass through its bracket, with n written after the closing bracket.
• “Addition polymerisation gives the polymer and water.” Correct There is no other product. Every atom of every monomer ends up in the polymer, which is why the polymer has the same empirical formula as its monomer.
• “The repeat unit still has a C=C in it.” Correct The π bond is used up in joining the monomers. The polymer is saturated, whatever the monomer was.
• “The monomer of –CH₂–CHCl–CH₂–CHCl– is ethene and chlorine.” Correct Cut into two-carbon units and put the double bond back into one of them: CH₂=CHCl, chloroethene. The Cl was already on the monomer.
• “Burning PVC releases chlorine, Cl₂.” Correct The chlorine leaves as hydrogen chloride, HCl, an acidic, corrosive and toxic gas.
• “Poly(ethene) does not biodegrade because its molecules are too big.” Correct Size alone is not the reason: starch and proteins are huge molecules too, and they are broken down. The backbone is a saturated, non-polar chain of strong C–C and C–H bonds, unreactive like an alkane, so it is not attacked by water, acids, alkalis or the enzymes of micro-organisms.
• “Addition polymerisation gives the polymer and water.” Repair There is no other product; every atom of the monomers ends up in the polymer.
• “Poly(propene): –[CH₂–CH₂–CH₂]–.” Repair Only the two C=C carbons form the backbone: –[CH₂–CH(CH₃)]ₙ–, with CH₃ as a side group.
• “Poly(ethene): –[CH₂=CH₂]ₙ–.” Repair The π bond has been used to join the units. The polymer is saturated: –[CH₂–CH₂]ₙ–.
• “[CH₂–CHCl]ₙ” — brackets with no bonds through them. Repair A bond from each backbone carbon must extend through its bracket to show the chain continues; then n after the closing bracket.
• “The monomer of –CH₂–CHCl–CH₂–CHCl– is CH₂=CH₂ and Cl₂.” Repair Restore the double bond in one repeat unit: CH₂=CHCl, chloroethene.
• “I cut the chain into three-carbon pieces.” Repair Each monomer contributes exactly two backbone carbons, the two carbons of its C=C, so cut into two-carbon units.
• “Poly(ethene) is not biodegradable because it is a big molecule.” Repair Because its backbone is a saturated, non-polar chain of strong C–C and C–H bonds that water, acids, alkalis and the enzymes of micro-organisms do not attack.
• “Burning PVC gives chlorine, Cl₂.” Repair It gives hydrogen chloride, HCl, an acidic and toxic gas.
• “Burning poly(ethene) is harmless: it only gives CO₂ and water.” Repair Complete combustion gives CO₂, a greenhouse gas; incomplete combustion gives toxic carbon monoxide.
• “Polyethene” and “polychloroethene”. Repair The monomer’s name goes in brackets: poly(ethene), poly(chloroethene).

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. When you reach topic 21 (organic synthesis), re-answer: which alkene would you polymerise to make a given chain? When you reach topic 35 at A Level (condensation polymers), re-answer: how does addition polymerisation differ, and why does it give no other product? Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 20 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A Paper 1 item on this topic can turn on one decision: the right repeat unit among near-misses, the monomer or monomers of a drawn section, or the gas PVC gives on burning. A Paper 2 question asks you to define addition polymerisation, deduce a repeat unit with brackets and n, identify monomers from a section, and explain why poly(alkene)s are non-biodegradable.
• The calculations are topic 2 mole work with the repeat unit as the formula unit: the number of monomers in a chain from its Mr, and the mass or volume of HCl or CO₂ from burning a mass of polymer. You are given Ar values and Vm = 24.0 dm³ mol⁻¹ in the Data section.
• Topic 20 supplies no Paper 3 procedure. Its context, burning a polymer and measuring the acid gas, suits a Paper 5-style plan built on an acid–alkali titration: variables, precision, concordance, and incomplete absorption as the largest error.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.