Problem 108 - 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 cannot act as monomer? (a) \(\mathrm{H}_{2} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) (b) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\overrightarrow{\mathrm{CH}}_{2} \mathrm{OH}\) (c) \(\mathrm{NH}=\mathrm{CH}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

Option (d) cannot act as a monomer due to lack of reactive sites.

Step 1: Define Monomer Criteria

A monomer must have a certain reactive part such as a double bond or a functional group that can participate in a polymerization reaction to link with other monomers. We'll analyze each option to see if it has the necessary reactive groups.

Step 2: Analyze Option (a)

The compound \( \mathrm{H}_{2} \mathrm{N}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) is ethylenediamine, which has two amino groups (-NH\(_2\)) but no reactive double bond or additional functionalities that contribute to forming polymer chains via traditional polymerization processes.

Step 3: Analyze Option (b)

The compound \( \mathrm{CH}_{3}-\mathrm{CHOH}-\overrightarrow{\mathrm{CH}}_{2} \mathrm{OH} \) has a hydroxyl group \(-OH\) which can participate in condensation polymerization, typically seen in polyester formation. Thus, it can act as a monomer.

Step 4: Analyze Option (c)

The compound \( \mathrm{NH}=\mathrm{CH}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) has multiple amino groups and the presence of a double bond. This gives it potential functionality to act as a monomer in certain polymerization reactions.

Step 5: Analyze Option (d)

The compound \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) is ethanolamine, which has a simple amine group but lacks the reactive functionality (like double bonds or multiple functional groups) typically needed for polymerization.

Step 6: Determine Non-Monomer

Based on the analysis, option (a) \( \mathrm{H}_{2} \mathrm{N}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) and option (d) \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) do not have structures that allow them to form polymers readily. However, (d) is more distinct in its lack of traditional reactive sites needed for common polymerization reactions.

Key Concepts

Monomers

Monomers are the basic building blocks of polymers. Think of them as the individual beads that make up a necklace. Each monomer is a small and simple molecule that can join together with other monomers to form a long chain, much like how beads are strung together.

To act as a monomer, a molecule must have certain features, such as a reactive part like a double bond or particular functional groups, which enable them to connect with other monomers. These reactive features are essential for the polymerization process, where monomers link up to form polymers. In summary, if a molecule lacks these intrinsic reactive characteristics, it cannot act efficiently as a monomer.

Reactive Functional Groups

The backbone of polymer chemistry is based on reactive functional groups. These are specific parts of a monomer's structure, like anchors, which allow the linking process to occur seamlessly.

Important reactive groups include:

• Double Bonds: Serve as active sites for addition reactions.
• Hydroxyl Groups \(-OH\): Enable condensation reactions, often forming esters or ethers.
• Amino Groups \(-NH_2\): Utilized in forming different linkages, such as amides in nylon.

These groups are decisive in characterizing the polymer's properties and dictate how readily a monomer can join in polymer chains.

Types of Polymers

Polymers can be classified into various types based on different criteria:

• Natural vs. Synthetic: Natural polymers include DNA and proteins, while synthetic ones involve plastics like polystyrene.
• Thermoplastic vs. Thermosetting: Thermoplastics are moldable upon heating (like PVC), whereas thermosets harden permanently upon being heated (such as bakelite).
• Biodegradable vs. Non-Biodegradable: Biodegradable polymers break down in natural environments, while non-biodegradable ones persist over time.

These classifications help determine the specific applications and handling methods for different polymer types.

Copolymers

Copolymers are polymers made from two or more different types of monomers. Imagine mixing different colors of beads to make your necklace. Each monomer type brings its own properties, resulting in a versatile polymer capable of various functions.

Types of copolymers include:

• Random Copolymers: The different monomers are arranged in no definite order.
• Block Copolymers: Large blocks of each monomer type are linked together.
• Alternating Copolymers: Monomers are arranged in a strict alternating sequence.

Copolymers enable the customization of polymers with tailored properties for specific applications.

Homopolymers

Homopolymers consist of only one type of monomer repeated through the entire chain. Imagine a necklace entirely made of beads of the same color.

Some features of homopolymers include:

• Simplicity: Only one monomer type contributes to the polymer, leading to uniform properties.
• Common Examples: Polyethylene, made solely from ethylene monomers, is a widely used homopolymer in packaging materials.
• Consistency: Due to their uniform structure, homopolymers exhibit predictable mechanical properties.

In essence, homopolymers are straightforward yet highly valuable in producing consistent materials.