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What is Polymerisation (Step Growth)?
Grade Level:
Class 12
AI/ML, Physics, Biotechnology, FinTech, EVs, Space Technology, Climate Science, Blockchain, Medicine, Engineering, Law, Economics
Definition
What is it?
Step-growth polymerisation is a process where small molecules (monomers) react with each other to form larger chains (polymers) by repeatedly joining together, usually releasing a small molecule like water in each step. Think of it like building a long train by attaching one compartment at a time, each time releasing a small 'waste' piece.
Simple Example
Quick Example
Imagine you are making a long garland for Diwali. You have many small flowers (monomers). You join one flower to another, then that pair to another flower, and so on. Each time you join two flowers, a small leaf or stem piece might fall off (the small molecule released). This continuous joining to make a long chain is like step-growth polymerisation.
Worked Example
Step-by-Step
Let's see how Nylon-6,6 is formed, which is a common example of step-growth polymerisation.
1. We start with two different small molecules (monomers): hexamethylenediamine and adipic acid.
---2. Hexamethylenediamine has two -NH2 (amino) groups, one at each end. Adipic acid has two -COOH (carboxyl) groups, one at each end.
---3. When one -NH2 group from hexamethylenediamine reacts with one -COOH group from adipic acid, they join together.
---4. During this joining, a molecule of water (H2O) is released. This is called a condensation reaction.
---5. The newly formed molecule still has reactive ends: one -NH2 group and one -COOH group.
---6. This new molecule can then react with another hexamethylenediamine or adipic acid molecule, releasing more water.
---7. This process repeats again and again, like adding bricks one by one to build a wall, forming a very long chain called Nylon-6,6.
---ANSWER: A long polymer chain (Nylon-6,6) is formed by the repeated condensation reaction of hexamethylenediamine and adipic acid, with water being released at each step.
Why It Matters
Step-growth polymerisation is crucial for making many everyday plastics and materials, from the clothes we wear to parts in our mobile phones. Engineers use this knowledge to design new, stronger materials for EVs and even medical implants. Understanding it can open doors to careers in material science and chemical engineering.
Common Mistakes
MISTAKE: Thinking that all monomers must be the same for step-growth polymerisation. | CORRECTION: Step-growth polymerisation often involves two DIFFERENT types of monomers, each with two reactive functional groups, like in Nylon-6,6 formation.
MISTAKE: Confusing step-growth with chain-growth polymerisation, especially regarding the release of small molecules. | CORRECTION: In step-growth, a small molecule (like water) is usually released in each joining step. In chain-growth, monomers simply add to a growing chain without releasing anything.
MISTAKE: Believing that all the monomers react at once to form the polymer. | CORRECTION: In step-growth, monomers first react to form dimers, then trimers, then tetramers, and so on. The polymer grows gradually, step by step, not all at once.
Practice Questions
Try It Yourself
QUESTION: What small molecule is most commonly released during step-growth polymerisation? | ANSWER: Water (H2O)
QUESTION: If you are making a polyester by reacting a diol (molecule with two -OH groups) and a dicarboxylic acid (molecule with two -COOH groups), what type of polymerisation is this? | ANSWER: Step-growth polymerisation (specifically, condensation polymerisation, which is a type of step-growth)
QUESTION: Imagine you have 100 units of monomer A and 100 units of monomer B. If each A reacts with a B, releasing one water molecule, and this continues until a long chain is formed, how many water molecules would be released to form a polymer chain with 200 monomer units (100 A and 100 B)? | ANSWER: 199 water molecules would be released. (Each bond formation releases one water molecule. To join 200 units, 199 bonds are formed.)
MCQ
Quick Quiz
Which of the following is a key characteristic of step-growth polymerisation?
Monomers add to a rapidly growing active site without releasing any by-product.
Small molecules like water are typically released during each bond-forming step.
It always involves only one type of monomer.
The polymerisation reaction is very fast and forms high molecular weight polymers immediately.
The Correct Answer Is:
B
Option B correctly describes step-growth polymerisation, where condensation reactions release small molecules like water. Options A and D describe chain-growth polymerisation, and Option C is incorrect as step-growth often uses two different monomers.
Real World Connection
In the Real World
Many common plastics are made using step-growth polymerisation. For example, the PET plastic used in water bottles and soft drink bottles is a polyester formed this way. Also, the strong nylon fabric used in school bags, ropes, and even some car parts is a polyamide, also made through step-growth polymerisation. This process helps create durable materials essential for daily life in India.
Key Vocabulary
Key Terms
MONOMER: A small molecule that can react with other monomers to form a polymer | POLYMER: A large molecule made up of many repeating smaller units (monomers) | CONDENSATION REACTION: A chemical reaction where two molecules combine to form a larger molecule, with the loss of a small molecule like water | FUNCTIONAL GROUP: Specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules (e.g., -OH, -COOH, -NH2) | NYLON-6,6: A common synthetic polyamide polymer made via step-growth polymerisation, used in textiles and plastics.
What's Next
What to Learn Next
Now that you understand step-growth polymerisation, you should explore 'What is Chain-Growth Polymerisation?'. This will help you compare and contrast the two main types of polymer formation and fully grasp how different plastics and materials are made.
