What is Polymerisation (Chain Growth)?

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Grade Level:

Class 12

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Definition

What is it?

Polymerisation (Chain Growth) is a process where small, simple molecules called monomers join together one after another to form a very large molecule called a polymer. Think of it like adding beads to a chain, where each new bead attaches to the previous one to make the chain longer. This process happens very quickly once it starts.

Simple Example

Quick Example

Imagine you have many small ladoos (monomers) and you want to make a long mala (polymer) for a celebration. In chain growth polymerisation, you start with one ladoo. Then, you quickly add another ladoo to it, then another to that, and so on, making the mala grow longer very fast. Each new ladoo adds to the end of the existing chain.

Worked Example

Step-by-Step

Let's understand how polyethylene, a common plastic used for carry bags, is formed through chain growth polymerisation from ethene monomers (CH2=CH2).

Step 1: Initiation – A special starting molecule, called an initiator (like a free radical), breaks a bond in an ethene monomer. This creates a reactive ethene unit with an unpaired electron, ready to start the chain. For example, if we have 'I.' as an initiator, I. + CH2=CH2 -> I-CH2-CH2.
---Step 2: Propagation (Chain Lengthening) – This reactive ethene unit (I-CH2-CH2.) quickly adds to another ethene monomer. The double bond in the new ethene monomer breaks, and it joins the growing chain, extending it. I-CH2-CH2. + CH2=CH2 -> I-CH2-CH2-CH2-CH2.
---Step 3: This process repeats very rapidly. Each time, a new ethene monomer adds to the end of the growing polymer chain. I-(CH2-CH2)n. + CH2=CH2 -> I-(CH2-CH2)n+1.
---Step 4: Termination – Eventually, two growing chains or a growing chain and an initiator fragment meet. Their unpaired electrons combine, stopping the chain growth and forming the final stable polyethylene polymer. For example, I-(CH2-CH2)n. + . (CH2-CH2)m-I -> I-(CH2-CH2)n-(CH2-CH2)m-I.

Answer: Many ethene monomers link together in a continuous, rapid chain reaction to form a long polyethylene polymer.

Why It Matters

Chain growth polymerisation is crucial for making most of the plastics we use daily, from water bottles to car parts. Understanding this process helps engineers design stronger materials for EVs and space technology, and develop new biomaterials for medicine. Learning this can open doors to careers in material science, chemical engineering, and research.

Common Mistakes

MISTAKE: Thinking that all monomers join at the same time to form the polymer. | CORRECTION: In chain growth, monomers add one by one, sequentially, to a growing active site, similar to how a train adds one bogie at a time.

MISTAKE: Confusing chain growth polymerisation with step growth polymerisation. | CORRECTION: Chain growth polymerisation involves an active site that grows rapidly, adding monomers one at a time, without producing small by-products. Step growth polymerisation involves monomers reacting in pairs, slowly building up larger molecules, often releasing small molecules like water.

MISTAKE: Believing that initiation, propagation, and termination steps happen slowly or separately. | CORRECTION: Once initiated, the propagation step in chain growth is extremely fast, making the polymer grow very quickly. All three steps occur in a continuous, rapid sequence.

Practice Questions

Try It Yourself

QUESTION: Which type of polymerisation involves monomers adding to an active site one by one? | ANSWER: Chain growth polymerisation

QUESTION: Polypropylene, used in ropes and carpets, is formed from propene (CH2=CH-CH3) monomers. Is its formation an example of chain growth or step growth polymerisation? Explain why in one sentence. | ANSWER: Chain growth polymerisation. Because the propene units add sequentially to a growing active site without the loss of any small molecules.

QUESTION: Imagine you are making a polymer. If you have 1000 monomer units and the initiation step creates 5 active sites, how many polymer chains could potentially be formed (assuming ideal termination)? | ANSWER: 5 polymer chains. Each active site will grow into one polymer chain.

MCQ

Quick Quiz

Which of the following is a key characteristic of chain growth polymerisation?

Monomers react slowly in pairs to form dimers, then trimers, and so on.

Small molecules like water are always released during the polymer formation.

Monomers add rapidly to an active site, one at a time, to lengthen the polymer chain.

It typically forms polymers with low molecular weights.

The Correct Answer Is:

In chain growth polymerisation, an active site rapidly adds individual monomers, extending the polymer chain quickly. Options A and B describe step growth polymerisation, and option D is generally incorrect for chain growth polymers which often have high molecular weights.

Real World Connection

In the Real World

You see chain growth polymers everywhere in India! The plastic chairs at your local chai shop, the PVC pipes used for plumbing in your home, the packaging for your favourite snacks, and even the insulation around electrical wires are all made using this process. Indian companies use this technology to manufacture these essential items, contributing to our daily lives and infrastructure.

Key Vocabulary

Key Terms

MONOMER: A small, simple molecule that can join with others to form a polymer. | POLYMER: A very large molecule made up of many repeating monomer units. | INITIATION: The first step in chain growth polymerisation where an active site is created. | PROPAGATION: The step where monomers rapidly add to the active site, lengthening the polymer chain. | TERMINATION: The final step where the growth of the polymer chain stops.

What's Next

What to Learn Next

Great job understanding chain growth polymerisation! Next, you should explore 'What is Polymerisation (Step Growth)?'. This will help you compare the two main types of polymerisation and understand how different polymers are made for various applications.