Problem 77 - A Complete Resource Book in Chemistry for JEE Main

Question

Polyacrylonitrile, characterized by the repeating unit is made from which of the following monomer? (a) \(\mathrm{CH}_{2}=\mathrm{CHCN}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\) (c) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCN}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{CH}_{2}=\mathrm{CHCN}\) is the monomer for polyacrylonitrile.

Step 1: Understanding the Repeating Unit

Polyacrylonitrile is a polymer made from repeating units. Typically, the repeating unit structure can be expressed in the form of \(\text{-CH}_2-\text{CHR}-\) where R is a cyano group (\(\text{-CN}\)). This indicates that the monomer must have an alkene group to allow polymerization.

Step 2: Analyzing Each Option

We need to determine which given option can form the repeating unit found in polyacrylonitrile: (a) \(\mathrm{CH}_{2}=\mathrm{CHCN}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\) (c) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCN}\)

Step 3: Identifying the Monomer

Option (a) \(\mathrm{CH}_{2}=\mathrm{CHCN}\) has a carbon-carbon double bond, which is essential for forming polymers through addition polymerization. The presence of the cyano group (\(\mathrm{CN}\)) precisely matches the repeating unit requirements for polyacrylonitrile. None of the other options contain a double bond that could participate in such polymerization.

Step 4: Conclusion

The correct monomer for polyacrylonitrile is the one that matches the requirements of the repeating unit and can undergo addition polymerization. Thus, option (a) \(\mathrm{CH}_{2}=\mathrm{CHCN}\) is the monomer used to make polyacrylonitrile.

Key Concepts

Monomers

In polymer chemistry, a monomer is a small molecule that can link with other identical molecules to form a polymer. Think of monomers as the individual building blocks. When these building blocks are bonded together, they create long chains or networks through a process known as polymerization.
Monomers are crucial in determining the properties and characteristics of the resultant polymer. The specific arrangement of atoms and the functional groups present in the monomer play a pivotal role. For instance:

• Monomers with a carbon-carbon double bond are typical in addition polymerization, allowing the formation of long chains as these double bonds break and form new single bonds.
• The presence of certain functional groups, like a cyano group \((\text{-CN})\), can impact the reactivity and functionality of the polymer.

In our case, the monomer \(\mathrm{CH}_{2}=\mathrm{CHCN}\) becomes critical since its double-bonded structure allows it to polymerize into polyacrylonitrile.

Addition Polymerization

Addition polymerization is a process where monomers with unsaturated bonds, typically double or triple bonds, combine to form a polymer. It’s like a chain reaction. The presence of the double bond initiates the reaction, and as each monomer adds, the chain grows longer.
This mechanism involves a few key steps:

• Initiation: A reactive center is formed, often by breaking a double bond to form a reactive site.
• Propagation: The reactive site moves along the chain as successive monomers link together.
• Termination: The reaction stops when reactive sites are neutralized or when reactants are depleted.

In the polymerization of polyacrylonitrile, the double bond in \(\mathrm{CH}_{2}=\mathrm{CHCN}\) initiates the addition, breaking to connect with another \(\mathrm{CH}_{2}=\mathrm{CHCN}\) monomer, thus extending the chain. This chain reaction continues, creating a long backbone typical of polyacrylonitrile.

Polymer Repeating Units

A polymer consists of many repeating units, the specific structure of which is derived from the original monomers. These repeating units are the basis of the polymer’s macroscopic properties, such as strength, flexibility, and thermal stability.
For example, in polyacrylonitrile, the repeating unit is \-\text{CH}_2-\text{CH(CN)}-\. This structure forms because the monomer \(\mathrm{CH}_{2}=\mathrm{CHCN}\) undergoes addition polymerization to yield a straight chain with repeating patterns.

• Each unit is linked by single covalent bonds that were once part of the original double bonds of the monomer.
• The cyano group \((\text{-CN})\) on each repeating unit imparts particular characteristics, such as rigidity and resistance to chemical environments.

Recognizing polymer repeating units helps in predicting how a polymer will behave under different conditions, guiding its practical applications in industries ranging from fabrics to composite materials.