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Polymers

Polymers are referred to as macromolecules, which are formed by joining or repeating structural units on a large scale. The simple and reactive molecules from which their repeating structural units are derived are called monomers. Polymers are the backbone of modern civilization. The use of polymers on the manufacture of plastic buckets, cups and saucers, toys, packaging bags, synthetic clothing materials, and machine parts, automobile tyres gears and seals, and electrical insulation materials has completely revolutionised daily life as well as the industrial scenario. In fact, polymers are the backbone of four major chemical industries, viz., plastics elastomers, fibers and paints and varnishes dome of the important aspects of the chemistry of polymers will be discussed in this unit.

Download pdf of NCERT Solutions for Class chemistry Chapter 15 Polymers

Download pdf of NCERT Examplar with Solutions for Class chemistry Chapter 15 Polymers

Exercise 1

  • Q1

    What are polymers?

    Ans:

     POLYMERS or Macromolecules are very high molar mass compounds consisting of several structural units inter-connected by covalent bonds. Polymer is obtained from Greek words POLY means MANY & MEROS means PARTS or UNITS.The structural units which combine to form polymers are called MONOMERS. For eg

     

    Polymers

     

     


    Q2

    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 straight chains. They can be depicted as:

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

     

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

    For e.g., low density polythene (LDP), 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. Examples of such polymers include bakelite and melmac.


    Q3

    Write the names of monomers of the following polymers:

    Ans:

    (i) Hexamethylenediamine

    [H2N  -  (CH2)6  -  NH2]

    and adipic acid

    [HOOC  -  (CH2)4  -  COOH]

     

    (ii)

    (iii) Tetrafluroethene

    (CF2  =  CF2)


    Q4

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

    Ans:

    Addition polymers:

     

    A polymer formed by direct addition of repeated monomers without the elimination of by product molecules is called addition polymers. Polyvinyl chloride, polythene

     

    Condensation polymers:

     

    A polymer formed by the condensation of 2 or more than 2 monomers with the elimination of simple molecules like water,ammonia is called condensation polymer. Terylene, bakelite 


    Q5

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

    Ans:

    Buna – N

    Buna - N is a copolymer of 1, 3−butadiene and acrylonitrile. It is also called Nitrile rubber(NBR).It is obtained by polymerization of 1,3-butadiene & acrylonitrile in the presence of sodium .It is used for making oil seels,manufacturing of tank linings,protective gloves etc.

     

    Buna − S

    Buna - S is a copolymer of 1, 3−butadiene and styrene. It is obtained by polymerization of 1,3-Butadiene & styrene in the ratio 3:1 in the presence of sodium.

    In Buna –S, Bu stands for butadiene,na for sodium & S for styrene.It is also called SBR(Styrene Butadiene Rubber).It has slightly less tensile strength than natural rubber. It is used for making automobile tyres,rubber soles,belts,hoses etc.


    Q6

    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:

    Polymers can  be classified on the basis of molecular forces like vander waals forces, hydrogen bonds & dipole dipole interactions.They are classified as :

    1 elastomers- they have elastic character & have weak intermolecular forces.Eg natural rubber

    2 Fibres- thay have strong hydrogen bond or dipole dipole interactions between polymers chain.Eg nylon 66

    3 Thermoplastics- they have intermolecular forces intermediate between elastomers & fibres.Eg polythene

    4 Thermosetting polymers- they undergoes permanent change on heating.Eg bakelite

    (i) Buna − S < polythene < Nylon 6, 6

    (ii) Neoprene < polyvinyl chloride < Nylon 6


    Q7

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

    Ans:

    Addition polymers:

    Polyvinyl chloride, polythene

    Condensation polymers:

    Terylene, bakelite


Exercise 2

  • Q1

    Explain the terms polymer and monomer.

    Ans:

    Polymers are 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 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.


    Q2

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

    Ans:

    Natural polymers are polymers that are found in nature. They are formed by plants and animals. Examples include protein, cellulose, starch, etc.

    Synthetic polymers are polymers made by human beings. Examples include plastic (polythene), synthetic fibres (nylon 6, 6), synthetic rubbers (Buna - S).


    Q3

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

    Ans:
     

    Homopolymer 

    Copolymer 

    The polymers that are formed by the polymerization of a single monomer are known as homopolymers. In other words, the repeating units of homopolymers are derived only from one monomer. For example, polythene is a homopolymer of ethene.

    The polymers whose repeating units are derived from two types of monomers are known as copolymers. For example, Buna - S is a copolymer of 1, 3-butadiene and styrene.


    Q4

    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 and propene is one and that of 1, 3-butadiene and adipic acid is two.


    Q5

    Define the term polymerisation.

    Ans:

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


    Q6

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

    Ans:

    ( NH — CHR — CO )n is a homopolymer because it is obtained from a single monomer unit, NH2 - CHR - COOH.


    Q7

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

    Ans:

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

    (i) Elastomers

    (ii) Fibres

    (iii) Thermoplastic polymers

    (iv) Thermosetting polymers


    Q8

    How can you differentiate between addition and condensation polymerisation?

    Ans:

    Addition polymerization is the process of repeated addition of monomers, possessing double or triple bonds to form polymers. For example, polythene is formed by addition polymerization of ethene.

    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.


    Q9

    Explain the term copolymerisation and give two examples.

    Ans:

    The process of forming polymers from two or more different monomeric units is called copolymerization. Multiple units of each monomer are present in a copolymer. The process of forming polymer Buna-S from 1, 3-butadiene and styrene is an example of copolymerization

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


    Q11

    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 and hardened on heating. Hence, they can be modified again and again. Examples include polythene, polystyrene.

    Thermosetting polymers are cross-linked or heavily branched polymers which get hardened during the molding process. These plastics cannot be softened again on heating. Examples of thermosetting plastics include bakelite, urea-formaldehyde resins.


    Q12

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

    Ans:

    1) Polyvinyl chloride (PVC) –the monomeric unit present is vinyl chloride (CH2 =CH –Cl) PVC is used in the manufacture of rain coats,curtain cloths, hand bags, toys, artificial flooring,gramophone records & as a good insulating material in wires & other electrical goods.

     

    2) Teflon (PTFE)- The monomeric unit present is tetrafluoroethene (n F2C = CF2). Teflon is used as a material resistant to heat & chemical attack.It is also used for making gaskets, pump packings, valves, oil seals,non lubricated bearings.

     

    3) Bakelite- the monomeric unit present is formaldehyde (HCHO) & phenol (C6H5OH).Bakelite is used for making glue for binding laminated wooden planks & in varnishes,combs, fountain pens, electrical switches.


    Q13

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

    Ans:

    For polymerization reaction, an initiator is required. The initiator may be an cation,anion or free radical.So the type of polymerization in which free radical is used as an initiator is known as free radical addition polymerization. The common free radical initiator used is benzoyl chloride.

    The structure of benzoyl chloride is as follows:


    Q14

    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 double bonds are present between C2 & C3 of isoprene units.The cis configuration about double bonds does not allows the chains to come closer for effective intermolecular attraction due to weak intermolecular attractions.Therefore,the natural rubber has coiled structure & shows elasticity & is non crystalline.

     

    On the other hand, all trans configurations occur in gutta-percha(synthetic rubber) which is trans-polyisoprene.These zig-zag chains pack more closely in gutta-percha & hence it is highly crystalline & non elastic & is more hard & brittle than rubber.

     


    Q15

    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:

    1. 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.

    2. It has the capacity to absorb large amounts of water.

    3. It has low tensile strength and low resistance to abrasion.

    4. It is soluble in non-polar solvents.

    5. 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 sulphur and appropriate additive is heated at a temperature range between 373 K and 415 K.

    This is a slow process,therefore some additives like zinc oxide etc are used to accelerate the proess. During this process, sulphur cross links are formed which makes rubber hard, tough with greater tensile strength .The vulcanized rubber has excellent elasticity,low water absorption,resistance to oxidation & organic solvents.


    Q16 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 and adipic acid.


    Q17

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

    (i) Buna-S  (ii) Buna-N

    (iii) Dacron  (iv) Neoprene

    Ans:

     

    Polymer

    Monomer

    Structure of monomer

    i

    Buna-S

    1, 3-butadiene

    CH2 = CH – CH = CH2

    Styrene

    C6H5CH = CH2

    ii

    Buna-N

    1, 3-butadiene

    CH2 = CH – CH = CH2

    Acrylonitrile

    CH2 = CH – CN

    iii

    Neoprene

    Chloroprene

    iv

    Dacron

    Ethylene glycol

    HOH2C –CH2OH

    Terephthalic acid

    1) Buna-S(styrene butadiene rubber)- It is obtained by the polymerization of Buta-1,3-diene & styrene in the ratio of 3:1 in the presence of sodium.It is used for making automobile tyres & footwear.

     

    2) Buna –N (Nitrile rubber)- It is obtained by polymerization of 1,3-butadiene & acrylonitrile in the presence of a peroxide catalyst.It is used for making oil seals,manufacture of hoses & tank linings.

     

    3) Dacron – It is obtained by the polymerization of ethylene glycol & terphthalic acid.

     

    4) Neoprene –It is prepared by the free radical polymerization of chloroprene.It is used as insulator, for making belts, gaskets,hoses etc.


    Q18

    Identify the monomer in the following polymeric structures.

    (i)

    (ii)

    Ans:

    (i) The monomers of the given polymeric structure are decanoic acid [HOOC –(CH2)8 – COOH] and hexamethylene diamine [H2N(CH2)6 NH2 ].

     

    (ii) The given structure is the polymerization product of N2-methyl-1,3,5-triazine-2,4,6-triamine & formaldehyde (HCHO)


    Q19

    How is dacron obtained from ethylene glycol and terephthalic acid?

    Ans:

    Dacron(terylene) is obtained by the polymerization of ethylene glycol & terephthalic acid at 420 to 460 K in the presence of zinc acetate-antimony trioxide catalyst

    The reaction is as follows:

    It is used in blending with cotton or wool fibres , as glass reinforcing materials in safety helmets, for making magnetic recording tapes.


    Q20

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

    Ans:

    A polymer that can be decomposed by bacteria is called a biodegradable polymer.

    The biodegradable polymer are the polymers which are degraded by the micro-organism within a suitable period so that biodegradable polymers & their degraded products do not cause any serious affects on the environment. They degrade by enzymatic hydrolysis & oxidation.The decomposition reactions involves hydrolysis(either enzymatically induced or by non –enzymatic mechanism ) to non- toxic small molecules which can be metabolized by or excreted from the body.

     

    The common examples of aliphatic biodegradable polymers are polyglycolic acid(PGA), Polyhydroxy butyrate (PHB), Polyhydroxy butyrates-co-beta hydroxyl valerate( PHBV), Polycaprolactone(pcl), Nylon-2-nylon-6.

     

    These polymers are used mainly for medical goods such as surgical sutures, tissues in growth materials, for controlled drug release,plasma substitutes etc.They are also used in agriculture materials,such as films,seed coatings,fast food wrappers,personal hygiene products etc.

    Poly-β-hydroxybutyrate-CO-β- hydroxyvalerate (PHBV) is a biodegradable aliphatic polyester.


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