# NCERT Solutions for Class 12 Chemistry Chapter 15 - Polymers

##### Question 1:

Is (—NH—CHR—CO)n a homopolymer or a co-polymer ?

It is a homopolymer because the repeating structural unit has only one type of monomer, i.e.,
H2N—CHR—COOH.

##### Question 2:

What are polymers?

Polymers are high molecular mass substances formed by the combination of a large number of simple molecules. They are also called macromolecules. For example, polythene, bakelite, etc.

##### Question 3:

Write the names of monomers of the following polymers.

Monomers of polymers

1. Hexamethylene diamine and adipic acid
2. Caprolactam
3. Tetrafluoroethene
##### Question 4:

Classify the following as addition and condensation polymers : Terylene, bakelite, polythene, teflon.

Terylene : Condensation polymer
Bakelite : Condensation polymer
Polythene : Addition polymer
Teflon : Addition polymer

##### Question 5:

Explain the difference between Buna-N and Buna-S.

Buna-N is a copolymer of buta-1, 3-diene and acrylonitrile. Buna-S is a copolymer of buta-1, 3-diene and styrene.

##### Question 6:

Arrange the following polymers in increasing order of their intermolecular forces:
Nylon-6,6, Buna-S, Polythene

Buna-S, Polythene, Nylon 6, 6

##### Question 7:

Explain the terms polymer and monomer.

Polymer is a high molecular mass macromolecule formed by the combination of a large number of simple molecules. Monomer is a simple molecule capable of undergoing polymerisation to form polymers.

For example, polythene is a polymer and its simple molecule ethene is a monomer.

##### Question 8:

What are natural and synthetic polymers? Give two examples of each type.

Natural polymers are high molecular mass macromolecules obtained from natural sources (plants and animals). For example, (i) Proteins; (ii) Nucleic acids Synthetic polymers are man made high molecular mass macromolecules which are prepared in laboratories. For example, (i) Polythene; (ii) Nylon

##### Question 9:

Distinguish between the terms homopolymer and copolymer and giving one example of each.

Homopolymer is a high molecular mass macro molecule (polymer) which is formed from one type of monomers. For example, polythene is a homopolymer because it is formed from ethene monomer. Copolymer is a high molecular mass macromolecule (or polymer) which is formed from two or more different monomers. For example, nylon-6,6 is a copolymer formed form hexamethylene diamine and adipic acid.

##### Question 10:

How do you explain the functionality of a monomer?

Functionality is the number of bonding sites in a monomer.

##### Question 11:

Define the term polymerisation.

Polymerisation is a process of formation of high molecular mass polymer from one or more monomers by linking together of repeating structural units with covalent bonds.

##### Question 12:

Is (NH-CHR-CO)n, a homopolymer or copolymer?

It is a homopolymer because the repeating structural unit has only one type of monomer i.e.,
H2N—CHR—COOH.

##### Question 13:

In which classes, the polymers are classified on the basis of molecular forces?

On the basis of molecular forces present between the chains of various polymers, these are classified as:

1. Elastomers e.g., natural rubber, buna-S
2. Fibres e.g., dacron, nylon-6,6
3. Thermoplastics e.g., polythene, teflon
4. Thermosetting plastics e.g., bakelite, melamine.
##### Question 14:

How can you differentiate between addition and condensation polymerisation?

In addition polymerisation, the molecules of same or different monomers combine to form a large polymer without the elimination of some other molecules.

In condensation polymerisation, the molecules of two or more bifunctional monomers undergo a series of condensation reactions with the elimination of simple molecules (water, ammonia, alcohol, etc.).

##### Question 15:

Explain the term copolymerisation and give two examples.

Copolymerisation is a process in which two or more monomers combine to form a polymer. The copolymers contain a multiple units of each monomer in the chain.
Examples :

1. 1, 3-butadiene and acrylonitrile
2. 1, 3-butadiene and styrene
##### Question 16:

Write the free radical mechanism for the polymerisation of ethene.

The polymerization takes place through the generation of an initiator, which is a molecule which decomposes to form free radicals. The commonly used initiator is t-butyl peroxide.

##### Question 17:

Define thermoplastics and thermosetting polymers with two examples of each.

Thermoplastics are the polymers which can be easily softened repeatedly on heating and hardened on cooling. Therefore, it can be used again and again. For example,

1. polythene
2. polyvinyl chloride

Thermosetting polymers are those which undergo permanent change on heating. They become hard and infusible on heating and cannot be softened again. For example,

1. Bakelite
2. Melamine formaldehyde.
##### Question 18:

Write the monomers used for getting the following polymers:

1. polyvinyl chloride
2. Teflon
3. Bakelite
1. Polyvinyl chloride : CH2 == CHCl vinyl chloride
2. Teflon : CF2 == CF2 Tetrafluoroethene
3. Bakelite : C6H5OH (phenol) and HCHO (formaldehyde).
##### Question 19:

Write the name and structure of one of the common initiators used in free radical addition polymerisation.

Benzoyl peroxide :

##### Question 20:

How does the presence of double bonds in rubber molecules influence their structure and reactivity?

Natural rubber is a linear cis-1,4-polyisoprene in which double bonds are present between C2 and C3 of isoprene units. The cis configuration about double bonds does not allow the chains to come closer for effective intermolecular attraction due to weak intermolecular attractions. Therefore, the natural rubber has coiled structure and shows elasticity and is non-crystalline.
On the other hand, all trans configurations occur in gutta-percha which is trans-polyisoprene. These zig-zag chains pack more closely in gutta-percha and hence it is highly crystalline and non-elastic and is more hard and brittle than rubber.

##### Question 21:

Discuss the main purpose of vulcanization of rubber.

The main purpose of vulcanization of rubber is to improve upon the properties of natural rubber to improve its utility. For example, natural rubber becomes soft and sticky when heated at high temperature ( > 335 K) and brittle at low temperature ( < 283 K). Moreover, natural rubber has high water absorption capacity, low elasticity and tensile strength and wear and tear resistance. The properties of natural rubber are improved by vulcanization in which it is heated with sulphur at a temperature of 373–415 K, when cross links are formed. Because of cross links, rubber becomes hard, tough with greater tensile strength. The vulcanized rubber has excellent elasticity, low water absorption capacity and resistant to oxidation and other organic solvents.

##### Question 22:

What are the monomeric repeating units of nylon-6 and nylon-66?

derived from hexamethylene diamine and adipic acid.

##### Question 23:

Write the names and structures of the monomers of the following polymers:

1. Buna-S
2. Buna-N
3. Dacron
4. Neoprene.

##### Question 24:

Identify the monomers in the following polymer structure:

##### Question 25:

How is dacron obtained from ethylene glycol and terephthalic acid?

Poly $\mathrm{\beta }$-hydroxybutyrate-co-$\mathrm{\beta }$-hydroxyvalerate (PHBV).