What is the difference between addition and condensation polymerisation?

Addition polymerisation involves monomers with a C=C double bond (alkenes) that join together without losing any atoms; the polymer is the only product. Condensation polymerisation involves monomers with two functional groups (e.g., -COOH and -OH) that react, eliminating a small molecule like water or HCl each time a bond forms. The polymer and the small molecule are both products.

How do you draw the repeating unit of a polymer from a monomer?

For addition polymers, take the monomer (e.g., ethene CH₂=CH₂) and break the double bond, leaving single bonds on each carbon. The repeating unit is -CH₂-CH₂- (with bonds extending from the brackets). For condensation polymers, identify the linkage (e.g., ester or amide) and show the atoms that repeat, including the functional group. For example, from ethane-1,2-diol and hexanedioic acid, the repeating unit is -OCH₂CH₂OCO(CH₂)₄CO-.

Why do some polymers have higher melting points than others?

Melting point depends on the strength of intermolecular forces between polymer chains. Longer chains have more points of contact, increasing van der Waals forces. Crystalline regions (where chains pack closely) also raise melting point. Branching reduces crystallinity, lowering melting point (e.g., LDPE vs HDPE). Cross-linking creates a network that prevents melting (thermosetting polymers).

What are biodegradable polymers and how do they work?

Biodegradable polymers are designed to break down in the environment through hydrolysis or microbial action. Examples include poly(lactic acid) (PLA) and polyhydroxyalkanoates (PHAs). They contain ester linkages that are susceptible to hydrolysis in the presence of water and enzymes. However, many require specific conditions (e.g., industrial composting) to degrade effectively.

How do you name polymers from their monomers?

For addition polymers, the name is 'poly(monomer name)'. For example, from ethene → poly(ethene); from chloroethene → poly(chloroethene). For condensation polymers, the name often reflects the linkage: polyesters from diols and dicarboxylic acids, polyamides from diamines and dicarboxylic acids (e.g., nylon-6,6 from hexane-1,6-diamine and hexanedioic acid).

What is the difference between thermoplastic and thermosetting polymers?

Thermoplastic polymers (e.g., poly(ethene), nylon) have linear or branched chains that can be melted and reshaped repeatedly because intermolecular forces are weak enough to break upon heating. Thermosetting polymers (e.g., Bakelite, vulcanised rubber) have extensive cross-linking, forming a rigid 3D network that cannot be remelted; they decompose upon heating.

Polymers (A-level only)

AQA

A-Level

This topic covers the formation and properties of condensation polymers, including polyesters and polyamides. It also addresses the environmental impact of both addition and condensation polymers, focusing on biodegradability and the challenges associated with their disposal and recycling.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

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

Before You Start

Prior knowledge that will help with this topic

•Alkenes and their reactions (electrophilic addition, free-radical addition) – essential for understanding addition polymerisation.
•Organic functional groups (carboxylic acids, alcohols, amines) – needed for condensation polymerisation.
•Bonding and intermolecular forces (van der Waals, hydrogen bonding) – explains polymer properties like melting point and solubility.

Key Terminology

Essential terms to know

Condensation polymerization mechanisms
Structural identification of repeating units and linkages
Intermolecular forces and physical properties
Biodegradability and hydrolysis of polyesters and polyamides

Likely Command Words

How questions on this topic are typically asked

Draw

Explain

Identify

Discuss

Ready to test yourself?

Practice questions tailored to this topic

Polymers (A-level only)

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Key Terminology

Likely Command Words

Ready to test yourself?

Related Topics in AQA A-Level Chemistry

Acids and bases (A-level only)

Alcohols

Aldehydes and ketones (A-level only)

Alkanes

Topic Synopsis

Key Concepts & Core Principles

Exam Tips & Revision Strategies

Common Misconceptions & Mistakes to Avoid

Examiner Marking Points

Polymers (A-level only)

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between addition and condensation polymerisation?

How do you draw the repeating unit of a polymer from a monomer?

Why do some polymers have higher melting points than others?

What are biodegradable polymers and how do they work?

How do you name polymers from their monomers?

What is the difference between thermoplastic and thermosetting polymers?

Before You Start

Key Terminology

Likely Command Words

Ready to test yourself?

Related Topics in AQA A-Level Chemistry

Acids and bases (A-level only)

Alcohols

Aldehydes and ketones (A-level only)

Alkanes