Problem 105 - A Complete Resource Book in Chemistry for JEE Main

Question

Polymerisation is the process by which monomers are converted into polymers. Both polymers and macromolecules are high molecular mass substances but still they have one important difference. For example, polythene and chlorophyll are macro molecules but polythene alone is a polymer because it has a repeating structural unit. Polymers are classified on the basis of structure, synthesis and molecular force existing between the polymer molecules. Identify the incorrect statement: (a) Kelvar is a nylon - polymer obtained by condensation of terephthalic acid and 1,4 - diamino benzene (b) Lexan is a nylon \(-\) polymer prepared by condensation of acrylic acid and vinyl cyanide. (c) Nomex is used in fire resistant protective clothing. (d) Vinyon is a co- polymer of vinyl chloride and vinyl acetate.

Step-by-Step Solution

Answer

Statement (b) is incorrect.

Step 1: Analyze the Statements

Read through each statement and understand the compounds mentioned, specifically focusing on the nature of each polymer and their synthesis methods.

Step 2: Evaluate Statement (a)

Statement (a) mentions Kelvar being a nylon polymer from the condensation of terephthalic acid and 1,4-diaminobenzene. This is correct because Kelvar is indeed a type of aramid (a class of nylon) derived in this way.

Step 3: Evaluate Statement (b)

Statement (b) claims Lexan is a nylon polymer from the condensation of acrylic acid and vinyl cyanide. This is incorrect because Lexan is actually a type of polycarbonate made from bisphenol A and phosgene, not related to acrylic acid or vinyl cyanide.

Step 4: Evaluate Statement (c)

Statement (c) states Nomex is used in fire-resistant clothing. Nomex is indeed known for its use in fire-resistant protective gear, making this statement correct.

Step 5: Evaluate Statement (d)

Statement (d) describes Vinyon as a copolymer from vinyl chloride and vinyl acetate, which is correct because these are the monomers that form Vinyon.

Key Concepts

Polymerisation

Polymerisation is the chemical process where small molecules called monomers join together to form a larger chain-like or network molecule known as a polymer. This process can occur via different mechanisms, the most common being addition polymerisation and condensation polymerisation.

• Addition Polymerisation: Monomers with double bonds react in a chain-like fashion, adding onto each other without a byproduct.
• Condensation Polymerisation: Monomers link together by eliminating a small molecule such as water as a byproduct.

This essential process is responsible for creating a wide variety of materials, from plastics like polythene to natural polymers like proteins. Understanding polymerisation helps in designing materials with specific properties for diverse applications.

Monomers and Polymers

Monomers are the building blocks of polymers. These small molecules possess functional groups that enable them to react and bond to form polymers. Think of monomers as the individual beads that you string together to form a bracelet.
Polymers, on the other hand, are macromolecules, which are large, complex molecules composed of these monomers.

• Examples of Monomers: Ethylene, propylene, styrene.
• Examples of Polymers: Polyethylene, polypropylene, polystyrene.

By choosing different types of monomers and their arrangements, polymers can be tailored to exhibit unique characteristics like flexibility, strength, or chemical resistance.

Polymer Classification

Classifying polymers helps in understanding and predicting their properties and behavior under different conditions. Polymers can be categorized based on:

• Origin: Natural (e.g., cellulose) or Synthetic (e.g., polystyrene).
• Structure: Linear, branched, or cross-linked.
• Synthesis Method: Addition or condensation.
• Mechanical Properties: Elastomers, fibers, plastics.

Each class displays distinct characteristics, making this classification vital in industries ranging from plastics manufacturing to biomedical engineering. This structured approach allows scientists and engineers to select the right polymer for specific applications.

Synthesis of Polymers

The synthesis of polymers involves methods designed to link monomers together in specific sequences and structures. There are two main synthetic routes:

• Addition Polymerisation: Typically involves free radical mechanisms, where monomers with double bonds are converted into long chains.
• Condensation Polymerisation: Involves monomers that react to form bonds while releasing small molecules like water or methanol.

The choice of synthesis method affects both the polymer's properties and applications. For example, condensation polymers are often more heat-resistant due to their stable linkages. Understanding these methods is crucial for producing materials with precise functions.

Molecular Structure of Polymers

The molecular structure of polymers determines their physical and chemical properties. Structural characteristics include chain length, branching, cross-linking, and arrangement.

• Chain Length: Longer chains usually result in higher strength and toughness.
• Branching: Influences density and flexibility. More branching generally leads to less dense and more flexible materials.
• Cross-Linking: Forms strong connections between chains, often resulting in more rigid and robust materials.

By controlling these structural elements, chemists can engineer polymers with desirable properties for specific uses, like tough, durable plastics for construction or flexible, light materials for clothing.