Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Polymer is a very large molecule that is made up of repeating small molecular units called monomers. The chemical reaction that unites the monomers is called polymerization.There are homopolymers, copolymers depending upon the type of monomers present in them. Which of the following monomers will give radical polymerization most readily? (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)=\mathrm{CH}_{2}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}\)

Step-by-Step Solution

Answer

The monomer \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) will give radical polymerization most readily.

Step 1: Understand Radical Polymerization

Radical polymerization is a type of chain-growth polymerization where the growing polymer chain is extended by radicals. These radicals are reactive species with unpaired electrons that can rapidly add to molecules with double bonds, initiating a chain reaction.

Step 2: Identify Monomers with Unsaturated Bonds

Look for monomers that contain carbon-carbon double bonds ( C=C ). The presence of these bonds is crucial as they can participate in radical polymerization by breaking one of the bonds to form a new radical site.

Step 3: Analyze Each Option for Reactivity

Examine each given monomer:(a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\): The phenyl group (\(\mathrm{C}_{6}\mathrm{H}_{5}\)) can stabilize radicals through resonance.(b) \(\mathrm{CH}_{2} = \mathrm{CH}_{2}\): Ethylene has no substituents, making it less stable but more reactive to radical attacks due to no hindrance.(c) \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)=\mathrm{CH}_{2}\): The presence of additional \(\mathrm{CH}_{3}\) groups increases steric hindrance.(d) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}\): Methyl group provides some stability, but less than option (a).

Step 4: Decide Based on Reactivity and Stability

The most effective radical polymerization occurs with monomers that have minimal steric hindrance and can easily form stable radicals. Ethylene, \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\), is the most reactive as it lacks substituents that could stabilize the radical or hinder the reaction.

Key Concepts

Monomers

Monomers are the fundamental building blocks of polymers, essentially the repeating units that form the backbone of these large molecules. Each monomer consists of smaller molecules that can join with others to form polymers through a process known as polymerization. Think of monomers as the beads in a long necklace, with the polymer being the completed piece.

There are different types of monomers, which lead to the creation of various kinds of polymers:

• Homopolymers: These are composed of only one type of monomer. For example, when ethylene (\( \mathrm{CH}_2=\mathrm{CH}_2 \)) monomers connect, they form polyethylene, a common plastic.
• Copolymers: These are made up of two or more different types of monomers. They offer the ability to combine properties of different monomers in a single polymer.

Understanding the structure and type of monomers is fundamental in determining how they will behave in polymerization processes, especially in chain-growth mechanisms.

Chain-Growth Polymerization

Chain-growth polymerization is a fascinating process where the growth of a polymer chain continues through successive addition of monomer units to the active site of a growing chain. This type of polymerization is crucial in forming polymers like polyethylene and polystyrene.

In chain-growth polymerization, the chain growth involves three main stages:

• Initiation: This begins with the formation of a radical species that acts as an active site for monomer addition.
• Propagation: Monomers sequentially add to the growing chain, each time extending it by one monomer unit, while the active site remains at the end of the chain.
• Termination: This occurs when the active site is neutralized, halting the polymer growth. Termination can happen through radical recombination or other mechanisms.

Each stage is vital for the polymer development, especially initiation, which determines if the polymerization will effectively proceed. Chain-growth polymerization is highly efficient for forming polymers with high molecular weights in a short time frame.

Carbon-Carbon Double Bonds

Carbon-carbon double bonds play a central role in the efficiency and success of radical polymerization. These double bonds (\( \mathrm{C}=\mathrm{C} \)) provide the necessary reactive site for polymerization to initiate and propagate.

Here's why carbon-carbon double bonds are so crucial:

• They can "open up" and link with other molecules, a process crucial for propagating the polymer chain.
• They offer a site for radical attack, as radicals can efficiently add to double bonds, making them indispensable in radical polymerization.
• Carbon-carbon double bonds may stabilize radical intermediates through delocalization of electrons, allowing the propagation phase to continue smoothly.

During radical polymerization, the double bond in a monomer breaks, forming a radical at the end of the growing chain. This new radical can then react with another monomer, continuing the chain growth process. Their ability to react rapidly and form strong covalent bonds makes carbon-carbon double bonds ideal participants in polymerizations that build large, stable polymers.