Chemistry · Year 12 · Polymers and Synthesis · Term 4

Addition Polymerization

Investigating the mechanism and properties of polymers formed through addition reactions.

• ACARA Content Descriptions
• ACSCH136

About This Topic

Polymers are large molecules made of repeating monomer units, and they are essential to modern life. Under ACARA AC9S12U17, students compare addition polymers (like polyethylene) and condensation polymers (like polyesters and polyamides). They investigate how the structure of the monomer, such as the presence of double bonds or multiple functional groups, determines the properties of the final material.

This topic is highly relevant to the global challenge of plastic waste and the development of sustainable materials. In Australia, this includes research into biodegradable polymers derived from agricultural waste. Understanding polymer chemistry allows students to evaluate the environmental impact of different materials. Students grasp this concept faster through hands-on 'polymer slime' investigations or by physically linking together to model the process of polymerisation.

Key Questions

1. Explain the mechanism of addition polymerization from monomer to polymer.
2. Relate the structure of the monomer to the properties of the resulting addition polymer.
3. Compare the properties and uses of common addition polymers (e.g., polyethylene, PVC).

Learning Objectives

• Explain the step-by-step mechanism of free radical addition polymerization, identifying initiation, propagation, and termination steps.
• Analyze the relationship between monomer structure, specifically the presence of a carbon-carbon double bond, and the ability to undergo addition polymerization.
• Compare and contrast the physical properties, such as flexibility and melting point, of common addition polymers like polyethylene and PVC, relating these to their monomer structures.
• Evaluate the suitability of different addition polymers for specific applications based on their chemical structure and resulting properties.

Before You Start

Chemical Bonding and Structure

Why: Students need to understand covalent bonds, including single and double bonds, and the concept of molecular structure to comprehend monomer reactivity and polymer formation.

Organic Chemistry Fundamentals

Why: Familiarity with organic molecules, functional groups, and nomenclature is essential for identifying monomers and understanding the reactions involved in polymerization.

Key Vocabulary

Monomer	A small molecule that can be bonded together with other identical or similar molecules to form a larger molecule, called a polymer.
Polymer	A large molecule composed of many repeating subunits (monomers) linked together by covalent bonds.
Addition Polymerization	A type of polymerization reaction where monomers add to one another in such a way that the polymer contains all the atoms of the monomer unit.
Free Radical	An atom or molecule with an unpaired electron, making it highly reactive and capable of initiating a chain reaction like polymerization.
Initiation	The first step in free radical polymerization where a free radical is generated and attacks a monomer molecule, starting the polymer chain.
Propagation	The chain growth step in addition polymerization where the reactive end of the growing polymer chain adds successive monomer units.

Watch Out for These Misconceptions

Common Misconception

Addition polymerisation produces a small molecule byproduct like water.

What to Teach Instead

Only condensation polymerisation produces a byproduct (usually water or HCl). In addition polymerisation, all atoms of the monomer are incorporated into the polymer. Using a 'mass balance' activity where students weigh 'monomers' before and after 'polymerising' them helps clarify this.

Common Misconception

Thermoplastics and thermosetting polymers are the same.

What to Teach Instead

Thermoplastics can be remelted because they have weak intermolecular forces between chains. Thermosetting polymers have strong covalent cross-links that prevent melting. A hands-on activity with 'cooked' vs 'uncooked' noodles can serve as a simple but effective analogy for these structures.

Active Learning Ideas

Role Play: The Human Polymer

Students act as monomers. For addition polymerisation, they 'break' their double bonds (developing arms) to join a long chain. For condensation, they must 'drop' a small object (representing water) to link with a partner, demonstrating the byproduct formation.

20 min·Whole Class

Station Rotations: Polymer Properties

Students test different plastic samples (HDPE, LDPE, PET, Nylon) for density, flexibility, and heat resistance. They then match these physical properties to the molecular structures (branching, cross-linking, or hydrogen bonding) shown on cards at each station.

50 min·Small Groups

Formal Debate: The Bioplastic Solution

Students debate the pros and cons of replacing traditional petroleum-based plastics with biodegradable polylactic acid (PLA). They must use their knowledge of condensation polymerisation and ester linkages to explain how these materials break down in the environment.

45 min·Whole Class

Real-World Connections

• Materials scientists at Dow Chemical Company use their understanding of addition polymerization to design and produce polyethylene, a versatile plastic used in everything from food packaging films to durable pipes.
• Engineers working in the automotive industry select specific grades of PVC, an addition polymer, for car interiors and dashboards due to its flexibility, durability, and resistance to weathering, a direct result of its monomer structure.
• Researchers at CSIRO in Australia are investigating modifications to addition polymerization processes to create more sustainable polymers, aiming to reduce reliance on fossil fuels and address plastic waste challenges.

Assessment Ideas

Quick Check

Provide students with diagrams of three different monomers. Ask them to identify which monomers can undergo addition polymerization and to briefly explain why, focusing on the presence of double bonds.

Discussion Prompt

Pose the question: 'How does the branching in the polymer chain of low-density polyethylene (LDPE) compared to the linear chains of high-density polyethylene (HDPE) affect their properties and uses?' Guide students to discuss flexibility, melting point, and common applications.

Exit Ticket

Ask students to write down the key steps of addition polymerization (initiation, propagation, termination) and provide one sentence describing what happens in each step. They should also name one common addition polymer and its primary use.

Frequently Asked Questions

What is the difference between LDPE and HDPE?

LDPE (Low-Density Polyethylene) has significant branching, which prevents the chains from packing closely, making it flexible and transparent. HDPE (High-Density Polyethylene) has minimal branching, allowing chains to pack tightly, resulting in a stronger, more rigid, and opaque material.

Why is Nylon considered a condensation polymer?

Nylon is formed by the reaction between a diamine and a dicarboxylic acid (or a diacyl chloride). Each time a new amide link is formed, a molecule of water (or HCl) is released, which is the defining characteristic of condensation polymerisation.

What makes a polymer biodegradable?

Biodegradable polymers usually contain functional groups like esters or amides that can be broken down by enzymes or water (hydrolysis) in the environment. Addition polymers like polyethylene have a stable carbon-carbon backbone that most organisms cannot break.

How can active learning help students understand polymers?

Active learning, like the 'Human Polymer' role play, makes the abstract concept of 'breaking bonds to form chains' a physical experience. When students have to 'lose' a water molecule to join a chain, the fundamental difference between addition and condensation polymerisation becomes an action they remember, rather than just a definition they've read.