Polymers (A-level only) Revision Guide
Topic Overview
Polymers are large molecules composed of repeating subunits called monomers. In AQA A-Level Chemistry, you will explore both addition and condensation polymerisation, focusing on the mechanisms, reaction conditions, and the properties of the resulting polymers. Addition polymers are formed from alkenes via a free-radical mechanism, while condensation polymers involve the stepwise reaction between monomers with functional groups such as -OH, -COOH, and -NH₂, eliminating small molecules like water or HCl. Understanding the distinction between these two types is crucial for predicting polymer structures and their applications.
The topic extends to the properties of polymers, including how chain length, branching, and cross-linking affect crystallinity, melting point, and mechanical strength. You will also learn about biodegradable polymers, such as poly(lactic acid) (PLA) and polyhydroxyalkanoates (PHAs), and their environmental significance. This knowledge is essential for modern materials science, linking chemical principles to real-world issues like plastic waste and sustainable materials. Mastery of polymers also underpins further study in organic chemistry and materials engineering.
Key Concepts
Core ideas you must understand for this topic
- →Addition polymerisation: Alkenes undergo chain-growth polymerisation via a free-radical mechanism, forming polymers like poly(ethene) and poly(chloroethene) (PVC). The repeating unit is identical to the monomer except for the double bond.
- →Condensation polymerisation: Monomers with two functional groups (e.g., dicarboxylic acids and diols or diamines) react to form polyesters or polyamides, releasing a small molecule (e.g., water or HCl) each time a bond forms.
- →Structure-property relationships: Increased chain length and crystallinity raise melting point and tensile strength; branching reduces crystallinity; cross-linking (e.g., in vulcanised rubber) makes polymers thermosetting rather than thermoplastic.
- →Biodegradable polymers: Polyesters like PLA and PHAs can be hydrolysed by water or enzymes, offering alternatives to non-biodegradable plastics. Their degradation depends on the ester linkage's susceptibility to hydrolysis.
What You Need to Demonstrate
Key skills and knowledge for this topic
- Formation of condensation polymers from dicarboxylic acids and diols, dicarboxylic acids and diamines, or amino acids.
- Identification of repeating units in polyesters (e.g., Terylene) and polyamides (e.g., nylon 6,6 and Kevlar).
- Identification of the linkages between repeating units.
- Explanation of the nature of intermolecular forces between molecules of condensation polymers.
- Explanation of why polyesters and polyamides can be hydrolysed while polyalkenes cannot.
- Discussion of the advantages and disadvantages of different methods of polymer disposal, including recycling.
Marking Points
Key points examiners look for in your answers
- Formation of condensation polymers from dicarboxylic acids and diols, dicarboxylic acids and diamines, or amino acids.
- Identification of repeating units in polyesters (e.g., Terylene) and polyamides (e.g., nylon 6,6 and Kevlar).
- Identification of the linkages between repeating units.
- Explanation of the nature of intermolecular forces between molecules of condensation polymers.
- Explanation of why polyesters and polyamides can be hydrolysed while polyalkenes cannot.
- Discussion of the advantages and disadvantages of different methods of polymer disposal, including recycling.
Examiner Tips
Expert advice for maximising your marks
- 💡Practice drawing the repeating unit from given monomer structures and vice versa.
- 💡Be prepared to identify the specific type of linkage (ester or amide) in a given polymer chain.
- 💡Ensure you can explain the chemical basis for biodegradability in condensation polymers compared to the inert nature of polyalkenes.
- 💡Review the environmental and economic implications of polymer disposal methods.
- 💡When drawing polymer structures, always show the repeating unit in brackets with an 'n' outside to indicate many units. For addition polymers, ensure the bonds extend from the brackets to show continuation.
- 💡In condensation polymerisation, identify the small molecule eliminated (usually H₂O or HCl) and check that the monomers have complementary functional groups (e.g., -COOH with -OH or -NH₂).
- 💡For property questions, link molecular features (chain length, branching, cross-linking) to macroscopic properties (melting point, strength, elasticity). Use specific examples like poly(ethene) (HDPE vs LDPE) to illustrate.
Common Mistakes
Pitfalls to avoid in your exam answers
- Confusing the repeating unit of a condensation polymer with the monomer structure.
- Failing to identify the correct functional groups involved in the condensation reaction.
- Incorrectly drawing the linkage (e.g., ester or amide bond) between repeating units.
- Misunderstanding the difference in chemical reactivity between addition polymers (polyalkenes) and condensation polymers (polyesters/polyamides) regarding hydrolysis.
- Misconception: All polymers are made by addition polymerisation. Correction: Condensation polymerisation is equally important and produces polymers like nylon and polyester, with a small molecule by-product.
- Misconception: The repeating unit of an addition polymer is exactly the same as the monomer. Correction: The monomer's C=C double bond becomes a single bond in the polymer; the repeating unit is the monomer minus the double bond (e.g., ethene → -CH₂CH₂-).
- Misconception: Biodegradable polymers decompose quickly in any environment. Correction: Many require specific conditions (e.g., industrial composting with heat and microbes) to break down; they may persist in landfills.
Frequently Asked Questions
Common questions students ask about this topic