Class 12 - Chemistry - Polymers

Question 15.1.

Explain the terms polymer and monomer.

Answer:

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.

 

 

Question 15.2.

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

 Answer:

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).

 

 

Question 15.3.

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

Answer:

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 ethane.

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.

 

 

 

Question 16.4.

How do you explain the functionality of a monomer?

Answer:

Functionality of a monomer is the number of binding sites in a molecule.

For example the functionality of ethene, Propene, styrene, acrylonitrile is one and that of 1-3 butadiene, adipic acid;

terephthalic acid hexamethylenediamine is two.

 

 

Question 16.5.

Define the term polymerisation.

 

 

Answer:

Polymerization is the process in which large number of repeating structural units derived from one or more monomers (simple molecules)

join together in a regular fashion through covalent bonds to form a high molecular mass macromolecule (polymer).

 

 

Question 15.6.

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

Answer:

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

 

 

Question 15.7.

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

Answer:

On the basis of magnitude of intermolecular forces polymers are classified into following four sub-groups:

 (i) Elastomers

 (ii) Fibres

(iii) Thermoplastic polymers

(iv) Thermosetting polymers

 

 

Question 15.8.

How can you differentiate between addition and condensation polymerisation?

Answer:

Addition Polymer:-

The addition polymers are formed by the repeated addition of monomer molecules possessing double or triple bonds,

e.g., the formation of polythene from ethene and polypropene from propene.

However, the addition polymers formed by the polymerisation of a single monomeric species are known as homopolymers, e.g., polythene.

nCH2=CH2  à - (CH2-CH2)-n Homopolymer

 Ethene            Polythene

Condensation Polymer:-

The condensation polymers are formed by repeated condensation reaction between two different bi-functional or tri-functional monomeric units.

In these polymerisation reactions, the elimination of small molecules such as water, alcohol, hydrogen chloride, etc. take place.

The examples are terylene (dacron), nylon 6, 6, nylon 6, etc.

For example, nylon 6, 6 is formed by the condensation of hexamethylene diamine with adipic acid.

nH2N (CH2)6NH2 + nHOOC (CH2)4COOH --> - [NH (CH2)6NHCO (CH2)4CO]-n + H2O

                                                                         Nylon (6, 6)

 

 

 

Question 15.9.

Explain the term copolymerisation and give two examples?

Answer:

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

nCH2=CH2-CH=CH2 + nC6H5CH=CH2        -->    - (CH2-CH=CH-C6H5-CH2-CH-CH2)-n

 1, 3- butadiene            styrene      copolymerization      Buna-S

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

nH2N (CH2)6NH2 + nHOOC (CH2)4COOH à - [NH (CH2)6NHCO (CH2)4CO]-n + H2O

                                                                         Nylon (6, 6)

 

 

Question 15.10.

Write the free radical mechanism for the polymerisation of ethene.

Answer:

Polymerization of ethene to polythene consists of heating or exposing to light a mixture of ethene with a

small amount of benzoyal peroxide as the initiator.

The reaction involved in this process is given below:

The sequence of steps may be depicted as follows:

Chain initiation steps

Chain propagating step

Chain terminating step

 For example:-

 

 

 

The polymerisation of Ethene to polythene consists of heating or exposing to light a mixture of Ethene with a small amount of benzoyl peroxide initiator.

The process starts with the addition of phenyl free radical formed by the peroxide to the Ethene double bond thus generating a new and larger free radical.

This step is called chain initiating step.

Class_12_Chemistry_Polymers_Chain_Initiating_Step

As this radical reacts with another molecule of Ethene, another bigger sized radical is formed.

The repetition of this sequence with new and bigger radicals carries the reaction forward and the step is termed as chain propagating step.

Class_12_Chemistry_Polymers_Chain_Propagating_Step

Ultimately, at some stage the product radical thus formed reacts with another radical to form the polymerised product.

This step is called the chain terminating step.

 Class_12_Chemistry_Polymers_Chain_Terminating_Step

 

Question 15.11.

Define thermoplastics and thermosetting polymers with two examples of each?

Answer:

Thermoplastic polymers are linear or slightly branched chained molecules. It can be repeatedly softened and hardened on heating.

Thus they can be modified again and again. Examples: - Polyethene and polystyrene.

These polymers have intermolecular forces of attraction intermediate between elastomers and fibres.

Some examples of common thermoplastics are polyethene, polystyrene, polyvinyls, etc.

Thermosetting Plastics are cross-linked and heavily branched molecules, which get hardened during the moulding process.

These polymers cannot be reused. For example: - Bakelite and urea-formaldehyde resin etc.

 

 

Question 15.12.

Write the monomers used for getting the following polymers.

(i) Polyvinyl chloride (ii) Teflon (iii) Bakelite

Answer:

  • 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.

(ii) 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.

 

(iii) 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.

 

 

Question 15.13.

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

Answer:

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:

 Class_12_Chemistry_Polymers_Structure_of_Benzoyl_Chloride

 

Question 15.14.

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

Answer:

Natural rubber is a linear polymer of isoprene (2-methyl-1, 3-butadiene) and is also called as cis - 1, 4 polyisoprenes.

Due to the cis configuration about the double bond, it is difficult to come closer for effective compactness

due to the weak intermolecular attraction (van der Waals).

Thus natural rubber has a coiled structure and it can be stretched like spring (show elastic nature).

Class_12_Chemistry_Polymers_Structure_of_Natural_Rubber_1

 

Natural Rubber

  

Question 15.15.

Discuss the main purpose of vulcanisation of rubber?

Answer:

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.
  2. 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.
  3. It has the capacity to absorb large amounts of water.
  4. It has low tensile strength and low resistance to abrasion.
  5. It is soluble in non-polar solvents.
  6. It is easily attacked by oxidizing agents.
  7. Vulcanization of natural rubber is done to improve upon all these properties. In this process, a mixture of raw rubber
  8. with sulphur and appropriate additive is heated at a temperature range between 373 K and 415 K.
  9. This is a slow process; therefore some additives like zinc oxide etc. are used to accelerate the process.
  10. During this process, sulphur cross links are formed which makes rubber hard, tough with greater tensile strength .
  11. The vulcanized rubber has excellent elasticity, low water absorption, resistance to oxidation & organic solvents.

 Class_12_Chemistry_Polymers_Structure_of_Natural_Rubber

 

Question 15.16.

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

Answer:

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.

 

 

Question 15.17.

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

(i) Buna-S (ii) Buna-N (iii) Dacron (iv) Neoprene

Answer:

 

Polymer

Monomer

Structure of Monomer

1.   Buna- S

1,3 butadiene

Styrene

CH2=CH-CH=CH2

C6H5CH=CH2

2.   Buna-N

1,3 butadiene

Acrylonitrile

CH2=CH-CH=CH2

CH2-CH-CN

3.   Neoprene

Chloroprene

 Class_12_Chemistry_Polymers_Structure_of_Chloroprene

4.   Dacron

Ethylene glycol

Terephthalic acid

HOH2C – CH2OH

Class_12_Chemistry_Polymers_Structure_of_TerephthalicAcid

 

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.

 

 

Question 15.18.

Identify the monomer in the following polymeric structures.

Class_12_Chemistry_Polymers_Structure_of_Monomer_1

Answer:

  • The monomers of the given polymeric structure are decanoic acid

[HOOC-(CH2)8-COOH)] And hexamethylene diamine [H2N (CH2)6NH2]

(ii)  The monomers of the given polymeric structure are

Class_12_Chemistry_Polymers_Structure_of_Monomer

And HCHO

 

 

Question 15.19.

How is dacron obtained from ethylene glycol and terephthalic acid?

Answer:

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:

 Class_12_Chemistry_Polymers_Structure_of_Terylene

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

 

 

Question 15.20.

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

Answer:

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

The biodegradable polymers 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 effects 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), and Polyhydroxy butyrates-co-beta hydroxyl valerate (PHBV), Polycaprolactone (pcl), Nylon-2-nylon-6.

Class_12_Chemistry_Polymers_Structure_of_PHBV

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