Polyethylene synthesis

Overview

This lesson builds on the unit “Exploring Organic Chemistry Layers” by moving from understanding addition reactions to using that knowledge to predict how polyethylene can be synthesised and why its structure affects key properties. Students will connect molecular processes to observable material behaviour.

Learning intentions

• WALT explain polyethylene formation using an addition polymerisation model.
• WALT describe how repeating units and bonding determine polyethylene’s properties.
• WALT collect and process data from a small investigation to link evidence to particle-level reasoning.
• WALT communicate a conclusion using scientific ideas and an appropriate method.

Success criteria

• I can write a clear addition-reaction pathway from ethene to a polyethylene repeat unit.
• I can justify at least two properties (e.g., density/softening behaviour, stiffness/flexibility, chemical resistance) using molecular structure.
• I can record raw observations clearly, process them, and draw a conclusion related to my purpose.
• I can explain which parts of the method improve the reliability of the data.

Curriculum links

• Science capability: students develop and use scientific knowledge; and communicate findings using evidence and relevant science ideas.
• Earth and Space Science standards are not the focus here; instead, this lesson supports Chemistry within Science by using an investigation process model (purpose, method, evidence, conclusion).
• Achievement standard style expectations from internal science investigations: developing a purpose from a scientific context, following a method, collecting raw data, processing it, interpreting it, and reporting the investigation.

Lesson structure (60 minutes)

1. 0–5 min · Starter: quick model. Teacher shows an ethene-to-polymer diagram (no details yet) and asks: “What must be happening to double bonds during polymerisation?” Students write a 2–3 sentence prediction about bond changes and what a “repeat unit” means.

2. 5–15 min · Mini-teach: addition polymerisation. Teacher explains polyethylene as the product of adding many ethene molecules, forming a long chain from the ethene carbon-carbon double bond opening. Students complete a guided scaffold: identify the functional role of the double bond and construct the repeat unit from a given chain fragment.

3. 15–27 min · Core investigation prep (safe, quick, observable). Teacher introduces a short class investigation using safe materials to generate evidence of polyethylene’s physical behaviour (e.g., softening/shape change when gently heated in a controlled teacher-led way, and comparison with another polymer sample if available). Students in groups set up a simple table for raw observations (mass/size if available, timing, qualitative observations), and confirm variables for a fair comparison (as far as is practical): temperature/heat source, time, sample size, and handling method.

4. 27–40 min · Investigation: observe and record. Teacher demonstrates or monitors a controlled heating/handling sequence and ensures consistent timing and recording expectations. Students collect raw data: time to soften (or time to visible deformation), observations of flexibility, and any surface changes, recording exactly what they see without interpretation.

5. 40–47 min · Processing: from observations to evidence. Teacher models converting observations into processed data (e.g., averages, ranges, a bar chart of “time to deformation”, or a coded scale of flexibility). Students process their group data and identify patterns or lack of patterns.

6. 47–55 min · Interpretation: structure to properties. Teacher prompts links between chain structure (long, saturated backbone; lack of polar functional groups) and properties such as chemical resistance and non-reactivity, plus physical behaviour linked to molecular mobility. Students write a “because… therefore…” explanation for two properties using their evidence plus science reasoning.

7. 55–60 min · Exit ticket: mini report check. Teacher collects a brief written submission and checks for method, evidence, and conclusion alignment. Students submit: one-sentence purpose, one processed evidence point, and one conclusion statement.

Resources

• Ethene and polyethylene diagram cards or printed models (repeat unit and chain growth steps)
• Group investigation worksheet with raw data table and processing space
• Samples of polyethylene and a non-identical comparison polymer (if available) or teacher-provided comparative demonstration materials
• Timers/stopwatches
• Measurement tools (ruler, balance if available) and data recording sheets
• Safety PPE: eye protection and heat-resistant gloves if required by your school process
• Marking scaffold: “Purpose–Method–Evidence–Conclusion” paragraph frame

Assessment

• Formative check during guided polymer model: teacher reviews whether students correctly identify addition/bond opening and the repeat unit.
• Observation of group recording quality: accuracy, completeness, and separation of “raw observation” from “interpretation”.
• Exit ticket assessed against: correct purpose, one processed evidence reference, and conclusion linking to evidence and scientific ideas.

Differentiation

• Support: provide sentence starters for structure-to-properties reasoning (e.g., “Polyethylene is non-polar because…”, “This means… therefore…”).
• Support: give students a partially completed polymerisation sequence with blanks for key terms (double bond, repeat unit, chain).
• Extension: challenge groups to evaluate at least one limitation of their method and propose a specific improvement to reliability (e.g., repeated trials, tighter timing control, consistent sample mass/size).
• EAL/SEN: allow oral rehearsal before writing; use a bilingual word bank if needed for key terms (addition, repeat unit, polymer, bond, evidence).