CBSE Class 12 Chemistry Chapter 15 Polymers NCERT Solutions

Solution: A polymer is a large, complex molecule, also known as a macromolecule, that has a high molecular mass, typically ranging from $(10^3\; -\; 10^7\; u)$. It is constructed from many smaller, repeating structural units called monomers, which are linked together by strong covalent bonds. Polymers can occur naturally, such as proteins and cellulose, or be synthesized artificially, like polythene, rubber, and nylon 6,6.

A monomer, on the other hand, is a simple, reactive molecule that serves as the basic building block for polymers. Monomers undergo a process called polymerization, where they chemically bond with numerous other monomer molecules to form the long chains characteristic of polymers. Examples of monomers include ethene, propene, styrene, and vinyl chloride.

Solution: Polymers can be broadly classified into two main categories based on their origin:

Natural Polymers: These are polymers that are found in nature and are produced by plants and animals. They are essential components of living organisms. Examples include:

• Proteins (e.g., silk, wool)
• Cellulose (a major component of plant cell walls)
• Starch (an energy storage molecule in plants)
• Natural rubber (latex from rubber trees)

Synthetic Polymers: These are polymers that are manufactured by humans through chemical processes. They are widely used in various applications due to their diverse properties. Examples include:

• Plastics like Polythene (polyethylene)
• Synthetic fibres such as Nylon 6,6 and Polyester
• Synthetic rubbers like Buna-S (Styrene-Butadiene Rubber)

Solution: The distinction between homopolymers and copolymers lies in the type of monomers they are composed of:

Homopolymer: A homopolymer is a polymer formed from the polymerization of only a single type of monomer. This means that all the repeating structural units within the polymer chain are identical. For instance, polythene is a homopolymer because it is exclusively made up of repeating ethylene units.

Copolymer: A copolymer is a polymer formed from the polymerization of two or more different types of monomers. The resulting polymer chain contains repeating units derived from these different monomers. For example, Buna-S is a copolymer synthesized from two distinct monomers: 1,3-butadiene and styrene. The arrangement of these different monomer units can vary, leading to different copolymer structures.

Solution: The functionality of a monomer refers to the number of specific sites on the monomer molecule that are capable of participating in the polymerization reaction, i.e., the number of binding or reactive sites available for forming covalent bonds with other monomers. This property is crucial in determining the type of polymerization that can occur and the structure of the resulting polymer.

For example:

• Monomers like ethene ($CH\_2=CH\_2$) and propene ($CH\_2=CH-CH\_3$) have a functionality of one, as they typically polymerize through their double bond, forming linear chains.
• Monomers such as 1,3-butadiene ($CH\_2=CH-CH=CH\_2$) and adipic acid ($HOOC-(CH\_2)\_4-COOH$) have a functionality of two. 1,3-butadiene can polymerize via its double bonds, and adipic acid has two carboxyl groups that can react, allowing them to form linear polymer chains by linking with other difunctional monomers.

Solution: Polymerization is the chemical process by which small, reactive molecules called monomers combine with each other in a large number to form a very large molecule known as a polymer. This process involves the formation of strong covalent bonds between the monomer units, resulting in a high molecular mass macromolecule, typically in the range of $(10^3\; -\; 10^7\; u)$. The repeating structural units within the polymer are derived directly from the monomers. Polymerization can occur through various mechanisms, such as addition polymerization or condensation polymerization, depending on the nature of the monomers and the reaction conditions.

Solution: The polymer represented by the formula $+NH-CHR-CO)\_n$ is a homopolymer. This is because the repeating unit $(-NH-CHR-CO-)$ is derived from a single type of monomer. This monomer is an amino acid, specifically $NH\_2-CHR-COOH$, which contains both an amino group ($NH\_2$) and a carboxyl group ($COOH$). During polymerization (specifically, condensation polymerization of amino acids), these functional groups react to form peptide bonds ($-NH-CO-$), creating a chain where each repeating unit originates from the same amino acid structure.

Solution: Polymers can be classified into different classes based on the magnitude and type of intermolecular forces of attraction present between their long molecular chains. These forces significantly influence the physical properties of the polymers, such as their elasticity, strength, and melting point. The main classes based on intermolecular forces are:

1. Elastomers: These polymers have weak intermolecular forces and possess the unique ability to regain their original shape after being stretched. The polymer chains are held together by weak van der Waals forces, allowing them to stretch considerably. Example: Vulcanized rubber.
2. Fibres: Fibres are polymers that exhibit strong intermolecular forces, such as hydrogen bonding or dipole-dipole interactions. These strong forces cause the polymer chains to align parallel to each other, resulting in high tensile strength and low extensibility. Examples include Nylon and Polyester.
3. Thermoplastic polymers: These polymers soften upon heating and can be molded into different shapes, and they harden upon cooling. This process is reversible. The intermolecular forces in thermoplastic polymers are intermediate between elastomers and fibres. Examples include Polythene and PVC (Polyvinyl chloride).