Polymers (Not for Exams)-Solutions

CBSE Class–12 Subject Chemistry

NCERT Solutions
Chapter – 15
Polymers

In-text Question

1. What are polymers?

Ans. Polymers are high molecular mass macro molecules, which consist of repeating structural units derived from monomers. Polymers have a high molecular mass (103 - 107 u). In a polymer, various monomer units are joined by strong covalent bonds. These polymers can be natural as well as synthetic. Polythene, rubber, and nylon-6,6 are examples of polymers.

2. How are polymers classified on the basis of structure?

Ans. Polymers are classified on the basis of structure as follows:

1. Linear polymers: These polymers are formed of long and straight chains. These polymers are represented as:

For e.g., high density polythene (HDP), polyvinyl chloride, etc.

2. Branched chain polymers: These polymers are basically linear chain having some branches. These polymers are represented as:

For e.g. low density polythene (LDP), glycogen,amylopectin etc.

3. Cross-linked or Network polymers:
These polymers have many cross-linking bonds that give rise to a network-like structure. These polymers contain bi-functional and tri-functional monomers and strong covalent bonds between various linear polymer chains. These polymers are represented as:

Examples of such polymers include bakelite, urea- formaldehyde resin and melamine-formaldehyde resin.

3. Write the names of monomers of the following polymers:

Ans. (i) Hexamethylenediamine [H2N-(CH2)6-NH2] and adipic acid [HOOC-(CH2)4-COOH]

(ii) Caprolactam

(iii) Tetrafluoroethene $(C F_{2} = C F_{2})$

4. Classify the following as addition and condensation polymers: Terylene, Bakelite, Polyvinyl chloride, Polythene.

Ans. Addition polymers: Polyvinyl chloride, polythene.
Condensation polymers: Terylene, bakelite.

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

Ans. Buna-N is a copolymer of 1,3-butadiene and acrylonitrile whereas Buna-S is a copolymer of 1,3-butadiene and styrene. Both are co-polymers.

6. Arrange the following polymers in increasing order of their intermolecular forces.
(i) Nylon-6, 6, Buna-S, Polythene.
(ii) Nylon-6, Neoprene, Polyvinyl chloride.

Ans. Different types of polymers have different intermolecular forces of attraction. Elastomers or rubbers have the weakest while fibres have the strongest intermolecular forces of attraction. Plastics have intermediate intermolecular forces of attraction. Hence, the increasing order of the intermolecular forces of attraction follows the order: Elastomers < Plastic < Fibre.
Therefore, the increasing order of the intermolecular forces of attraction of the given polymers is as follows:
(i) Buna - S < polythene < Nylon-6, 6
(ii) Neoprene < polyvinyl chloride < Nylon-6

Chapter End Question

1. Explain the terms polymer and monomer.

Ans. Polymers are the high molecular mass macromolecules composed of repeating structural units derived from monomers. Polymers have a high molecular mass (103 - 107 u). In a polymer, various monomer units are joined by strong covalent bonds. Polymers can be natural as well as synthetic. Polythene, rubber, and nylon-6, 6 are examples of polymers.
Monomers: Monomers are simple, reactive molecules that combine with each other in large numbers through covalent bonds to give rise to polymers. For example, ethene, propene, styrene, vinyl chloride.
$\begin{array}{l} \underset{E t h e n e, m o n o m e r}{n C H_{2} = C H_{2}} \to_{t r a c e s o f O_{2}}^{350 - 570 K, 1000 - 2000 a t a t m} \underset{P o l y t h e n e, p o l y m e r}{(- C H_{2} - C H_{2} -)_{n}} \end{array}$

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

Ans. Natural polymers are high molecular mass macromolecules and are found in nature. They are formed by plants and animals. Examples include protein, cellulose,nucleic acids,starch,enzymes, resins and rubber.
Synthetic polymers are man made high molecular mass macromolecules. These are prepared in the laboratory. For examples, plastic (polythene), synthetic fibres (nylon-6, 6), synthetic rubber (Buna-S), etc.

3. Distinguish between the terms homopolymer and copolymer and give an example of each.

Ans.

S.No.	Homopolymer	Copolymer
1.	A polymer which is obtained from only one type of monomer molecules.	Polymers whose repeating structural units are derived from two or more types of monomer molecules.
2.	For example: polythene, PVC, teflon, PAN, nylon-6 and polystyrene etc.	For example: Polyester,Buna-S, Buna-N, Bakelite and nylon-6,6 etc.

4. How do you explain the functionality of a monomer?

Ans. The functionality of a monomer is the number of binding sites that is/are present in that monomer. For example, the functionality of monomers such as ethene, vinyl chloride, vinyl cyanide, tetrafluoroethene, styrene, isoprene and propene is one and that of 1, 3-butadiene, phthalic acid, hexamethylene diamine and adipic acid is two.

5. Define the term polymerisation.

Ans. polymerisation is the process of forming high molecular mass (103 - 107 u) macromolecules called polymer, which consist of repeating structural units derived from monomers. In a polymer, various monomer units are joined by strong covalent bonds.

6. Is , a homopolymer or copolymer?

Ans. It is a homopolymer because in it the repeating structural unit has only one type of monomer. The monomer from which it is obtained is a alkyl glycine, NH2- CHR - COOH.

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

Ans. On the basis of magnitude of intermolecular forces of attraction present in polymers, they are classified into the following groups:

Elastomers: In these polymers, the intermolecular forces are the weakest. Example- vulcanised rubber, natural rubber, buna-S and SBR etc.
Fibres: These polymers have quite strong intermolecular forces such as H-bonding or dipole-dipole interactions. For example- nylon-6,6, terylene or orlon etc.
Thermoplastic polymers: These are linear polymers and weak van der Waals' forces acting in the various chains and are in between of the forces present in the elastomers and in the fibres. These can be easily molded into desired shape by heating. For example- polyethene, polystyrene, PVC, teflon etc.
Thermosetting polymers: These are generally cross-linked in nature and formed by condensation polymerisation. These do not soften on heating but rather become hard. For example- bakelite, Urea-formaldehyde, melamine-formaldehyde resins

8. How can you differentiate between addition and condensation polymerisation?

Ans. Addition polymerization is the process of repeated addition of same or different monomer molecule, possessing double or triple bonds to form polymers. For example, polythene is formed by addition polymerization of ethene.
$\begin{array}{l} \underset{E t h e n e}{n C H_{2} = C H_{2}} \to_{t r a c e s o f O_{2}}^{350 - 570 K, 1000 - 2000 a t a t m} \underset{P o l y t h e n e}{(- C H_{2} - C H_{2} -)_{n}} \end{array}$
Condensation polymerization is the process of formation of polymers by repeated condensation reactions between two different bi-functional or tri-functional monomers. A small molecule such as water or hydrochloric acid is eliminated in each condensation. For example, nylon-6,6 is formed by condensation polymerization of hexamethylenediamine and adipic acid.

9. Explain the term copolymerization and give two examples.

Ans. The process of forming polymers from two or more different monomeric units is called copolymerization. The copolymers contain multiple units of each monomer in the chain. Polymerization of 1, 3-butadiene and styrene is an example of copolymerization, where Buna-S is formed.

Nylon-6, 6 is also a copolymer formed by hexamethylenediamine and adipic acid.

10. Write the free radical mechanism for the polymerisation of ethene.

Ans. Polymerization of ethene to polythene consists of heating or exposing to light a mixture of ethene with a small amount of benzoyl peroxide as the initiator.
The reaction involved in this process is given below:

Chain initiation: Organic peroxides undergoes homolytic fission to form free radicals.
Chain Propagation: Free radical adds to an alkene (ethene) molecule to form a new radical.
Chain termination: The chain reaction comes to terminate when two free radical chains combine.

Chapter End Question

11. Define thermoplastics and thermosetting polymers with two examples of each.

Ans. Thermoplastic polymers are linear (slightly branched) long chain polymers, which can be repeatedly softened on heating and hardened on cooling. Hence, they can be modified again and again. For example: polythene, polystyrene etc.
Thermosetting polymers are those polymers which on heating undergoes excessive cross linking and become hard. These polymers cannot be softened again on heating. For example bakelite, Urea-formaldehyde resin, melamine-formaldehyde resin.

12. Write the monomers used for getting the following polymers.
(i) Polyvinyl chloride
(ii) Teflon
(iii) Bakelite

Ans. (i) Vinyl chloride (CH2=CHCl)
(ii) Tetrafluoroethene (CF2=CF2)
(iii) Formaldehyde (HCHO) and phenol (C6H5OH)

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

Ans. One common initiator used in free radical addition polymerization is benzoyl peroxide. Its structure is given below.

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

Ans. Natural rubber is a linear cis-1,4-polyisoprene in which the double bonds are present between C2 and C3 of the isoprene units.

These cis- configuration about double bonds do not allow the polymer chains to come closer for effective interactions therefore this intermolecular force of interactions between the various strands of isoprene unit are quite weak. As a result, various strands in natural rubber are arranged randomly. Hence, it shows elasticity.

15. Discuss the main purpose of vulcanisation of rubber.

Ans. Natural rubber though useful has some problems associated with its use. These limitations are discussed below:

Natural rubber is quite soft and sticky at room temperature. At elevated temperatures (> 335 K), it becomes even softer. At low temperatures (< 283 K), it becomes brittle. Thus, to maintain its elasticity, natural rubber is generally used in the temperature range of 283 K -335 K.
It has the capacity to absorb large amounts of water.
It has low tensile strength and low resistance to abrasion.
It is soluble in non-polar solvents.
It is easily attacked by oxidizing agents.

Vulcanization of natural rubber is done to improve upon all these properties. In this process, a mixture of raw rubber with 3-5% sulphur and appropriate additive is heated at a temperature range between 373 K and 415 K. The function of sulphur is to introduce sulphur brides or cross links between polymer chains thereby imparting more tensile strength, high elasticity over a large range of temperature and has low water adsorption tendency. It is resistant to the action of organic solvents and oxidising agents and is less plastic and non-sticky rubber.

16. What are the monomeric repeating units of Nylon-6 and Nylon-6, 6?

Ans. The monomeric repeating unit of nylon -6 is [NH-(CH2)5-CO], which is derived from Caprolactam. The monomeric repeating unit of nylon-6,6 is [NH-(CH2)6-NH-CO-(CH2)4-CO], which is derived from hexamethylene diamine [H2N-(CH2)6-NH2] and adipic acid [HOOC-(CH2)4-COOH].

17. Write the names and structures of the monomers of the following polymers:
(i) Buna-S
(ii) Buna-N
(iii) Dacron
(iv) Neoprene

Ans. The names and structures of the monomers are:

Polymer		Monomer	Structure of monomer
i	Buna-S	1, 3-butadiene	CH2 = CH - CH = CH2
i	Buna-S	Styrene	C6H5CH=CH2
ii	Buna-N	1, 3-butadiene	CH2 = CH - CH = CH2
ii	Buna-N	Acrylonitrile	CH2 = CH - CN
iii	Neoprene	Chloroprene	$C H_{2} = \overset{C l}{\overset{\|}{C}} - C H = C H_{2}$
iv	Dacron	Ethylene glycol	HOH2C - CH2OH
iv	Dacron	Terephthalic acid

18. Identify the monomer in the following polymeric structures.
(i)
(ii)
Ans. (i) The monomers of the given polymeric structure are decanoic acid (Sebacic acid) [HOOC-(CH2)8-COOH]and hexamethylene diamine [H2N(CH2)6NH2].

(ii) The monomers of the given polymeric structure are
2,4,6-triamino-1,3,5-triazine Formaldehyde

19. How is dacron obtained from ethylene glycol and terephthalic acid?

Ans. Dacron is prepared by condensation polymerization of ethylene glycol and terephthalic acid on heating at 425-475K in the presence of catalyst containing a mixture of zinc acetate and antimony trioxide.

20. What is a biodegradable polymer? Give an example of a biodegradable aliphatic polyester.

Ans. The polymers which are degraded by biological species like micro-organisms within a suitable period of time so that the polymers and their degraded products do not cause any serious effects on the environment are called biodegradable polymers. For example,
Poly- $β$ -hydroxybutyrate-CO- $β$ - hydroxyvalerate (PHBV) is a biodegradable aliphatic polyester.