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: Identify Key Concepts

Begin by identifying the key terms mentioned in the exercise such as 'Kelvar', 'Lexan', 'Nomex', and 'Vinyon'. Understand that the problem revolves around correct association of these terms with the structures they form or their properties.

Step 2: Analyze Each Option

Examine each statement to verify its correctness based on your knowledge or reference materials: (a) Kevlar is indeed a polymer, but it is not a nylon type and is obtained by the condensation of terephthalic acid and p-phenylenediamine. (b) This statement claims Lexan is nylon, which is incorrect. Lexan is polycarbonate, not formed by condensation of acrylic acid and vinyl cyanide. (c) Nomex is used for fire-resistant clothing, which is correct based on known applications. (d) Vinyon is correctly identified as a copolymer of vinyl chloride and vinyl acetate.

Step 3: Identify the Incorrect Statement

From analyzing each statement, it becomes evident that statement (b) is incorrect. Lexan is not a nylon polymer, nor is it associated with acrylic acid and vinyl cyanide. It is a polycarbonate material made from bisphenol A and phosgene.

Key Concepts

Monomers

Monomers are the building blocks of polymers. These small and simple molecules can bond together in chains to form large and complex molecules. In the process of polymerisation, monomers are repeated numerous times to create polymers. This is somewhat similar to stacking identical bricks to construct a building.

Monomers usually contain a double bond or a functional group, such as a hydroxyl group, capable of reacting with another monomer. Here are some essential points about monomers:

• They are the simplest form of a molecule that can undergo polymerisation.
• Common examples include ethylene, propylene, vinyl chloride, and styrene.
• They often have reactive bonds or functional groups that enable them to join together.

Understanding monomers is crucial because they are the starting point for forming polymers and thus determine the type of polymerisation method needed, such as addition or condensation polymerisation.

Polymers

Polymers are lengthy chains of repeating units derived from monomers. These large molecules encompass materials that we interact with daily, such as plastics, fibers, and rubbers.

Polymers can be naturally occurring, like DNA and cellulose, or synthetic, like polythene and nylon. Here are some critical aspects of polymers:

• They consist of varying numbers of repeating monomer units.
• The properties of polymers, like strength and flexibility, depend on the nature and arrangement of these repeating units.
• They can be categorized by their source, structure, and synthesis methods, among other factors.

For example, polythene is a polymer made by polymerising many ethylene monomers. Unlike macromolecules, not every large molecule with high molecular mass qualifies as a polymer; it must have repeating structural units.

Macromolecules

Macromolecules are exceptionally large molecules essential to various processes critical for life and material production. They consist of high molecular weight and are formed by covalently linking smaller units.

While all polymers are considered macromolecules, not all macromolecules qualify as polymers due to the absence of repetitive structural units. Consider these points about macromolecules:

• Their high molecular weight generally ranges from a few thousand Daltons to several million Daltons.
• Biopolymers like proteins and polysaccharides are natural macromolecules with complex structures and vital biological roles.
• Unlike polymers, macromolecules such as chlorophyll do not need to have repeating units.

The distinction is that while both polymers and macromolecules share high molecular mass, the repetition of units is what differentiates polymers from most macromolecules.

Polymer Classification

Polymers can be classified in several ways, helping in understanding their uses and properties better. The classification can be based on their structure, synthesis process, or the forces between polymer molecules.

Here are key methods of polymer classification:

• By Source: Includes natural polymers like silk, and synthetic polymers like polyester.
• By Structure: Divides polymers into linear, branched, and crosslinked categories based on the architecture of the molecular chains.
• By Synthesis Process: Distinguishes polymers based on the method of polymerisation—addition or condensation.
• By Molecular Forces: Classifies polymers as thermoplastics, thermosetting plastics, and elastomers based on how the molecules interact with each other.

Understanding these classifications helps in selecting polymers for specific applications, such as choosing a material that is lightweight yet strong, or one that is heat-resistant or elastic.