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This chapter deals with polymers, their types and structures. Polymers can be defined as very large molecules having high molecular mass ($10^3-10^7$u), obtained by union of large number of simple molecules called monomers. Monomers are repeating structural units derived from some simple and reactive molecules linked to each other by covalent bonds. We shall then define an important term, Polymerisation as the process of formation of polymers from respective monomers. We shall then classify polymers based on various criteria like: a) Source b) Structure c) Mode of Polymerisation d) Molecular forces between them.

Next, we shall study the types of polymerization reactions: Addition and Condensation polymerization reactions. In the presence of an organic peroxide initiator, the alkenes and their derivatives undergo Addition polymerisation or chain growth polymerisation through a free radical mechanism. We will look at formation of Polythene, Teflon and Polyacrylonitrile by addition polymerisation. Condensation polymerisation or step growth polymerisation reactions are shown by the interaction of bi – or poly functional monomers containing – $NH_2$, – OH and – COOH groups. This type of polymerisation proceeds through the elimination of certain simple molecules as $H_2O$ and $CH_3OH$. We shall look at the preparation of Polyamides like Nylon, Polyesters, Phenol – formaldehyde polymers and Melanamine as a consequence of condensation polymerization.

Further, we shall learn about an important phenomenon called, Copylymerization as polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise to form a copolymer.

Then, we shall study about rubbers. Rubber is a natural polymer which possesses elastic properties manufactured from rubber latex obtained from the bark of rubber tree. We shall study Vulcanisation as the process by which Natural rubbers can be made tougher. We will also study in brief about, Synthetic rubbers. Synthetic rubbers are usually obtained by copolymerisation of alkene and 1, 3 butadiene derivatives.

Then, we will study about some Biodegradable polymers to mitigate the harmful effects of accumulation of non-biodegradable polymeric solid waste materials.

Lastly, we shall study about polymers of commercial importance. Their monomer unit, structure and use shall be dealt with.

15.0 Introduction:

Polymers are used in the manufacture of plastic buckets, cups and saucers, children’s toys, packaging bags, synthetic clothing materials, automobile tyres, gears and seals, electrical insulating materials and machine parts.

Thus, polymers are the backbone of four major industries: plastics, elastomers, fibres and paints and varnishes.

The word ‘polymer’ is coined from two Greek words: “poly” means many and “mer” means unit or part.

Definition box: Polymer or Macromolecules: Polymers can be defined as very large molecules having high molecular mass $(10^3-10^7$u), obtained by union of large number of simple molecules called monomers. Monomers: The repeating structural units derived from some simple and reactive molecules are known as monomers. The monomers are linked to each other by covalent bonds. Polymerisation: The process of formation of polymers from respective monomers is called polymerisation.

Questions for the section:

1. Define:

a) Polymers

b) Monomers

2. Define Polymerization. Give two examples for polymerisation reactions.

15.1 Classification of Polymers

The following are some of the common classifications of polymers:

15.1.1 Classification Based on Source

1. Natural polymers
These polymers are found in plants and animals. Examples include: proteins, cellulose, starch, resins and rubber.

2. Semi-synthetic polymers
These are obtained by chemical modification of natural polymers. Examples include: Cellulose derivatives like: Cellulose acetate (rayon) and Cellulose nitrate.

3. Synthetic polymers
These are manmade polymers extensively used in daily life as well as in industries. For example: Synthetic polymers like: a) plastics in the form of polythene, b) synthetic fibres like nylon 6,6 and c) synthetic rubbers like Buna – S.

15.1.2 Classification Based on Structure of Polymers

1. Linear polymers

These polymers consist of long and straight chains. Examples include: high density polythene, polyvinyl chloride. These are represented as:

2. Branched chain polymers
These polymers contain linear chains having some branches. For example: low density polythene. These are depicted as follows:

3. Cross linked or Network polymers

These are usually made of bi-functional and tri-functional monomers which contain strong covalent bonds between various linear polymer chains. Examples include: Bakelite and melamine. These polymers are depicted as follows:

15.1.3 Classification Based on Mode of Polymerisation

1. Addition polymers

Addition polymers are formed by the repeated addition of monomer molecules linked by double or triple bonds. Examples include: a) formation of polythene from ethene b) polypropene from propene.

However, the addition polymers formed by the polymerisation of a single monomeric species are known as homopolymers. For example: polythene.

The polymers made by addition polymerisation of two different monomers are termed as copolymers. Examples include: Buna-S and Buna-N.

2. Condensation polymers

• 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 and hydrogen chloride take place.
• Examples include: terylene (dacron), nylon 6, 6 and nylon 6.

Nylon 6, 6 is formed by the condensation of hexamethylene diamine with adipic acid as depicted:

Example 15.1: $\mathrm{\left(-CH_2-CH(C_6H_5)-\right)_n}$ a homopolymer or a copolymer?

Solution: It is a homopolymer and the monomer from which it is obtained is styrene $C_6H_5CH = CH_2$.

15.1.4 Classification Based on Molecular Forces

• The application of a polymer in different fields depends on its unique mechanical property like: tensile strength, elasticity, toughness and so on.
• The mechanical properties depend on intermolecular forces like van der Waal’s forces and hydrogen bonds present in the polymer. These forces are also responsible for binding the polymer chains.

Now we shall classify polymers on the basis of magnitude of intermolecular forces present in them:

1. Elastomers

• These are rubber – like solids with elastic properties.
• In these elastomeric polymers, the polymer chains are held together by the weakest intermolecular forces. These weak binding forces permit the polymer to be stretched.
• A few ‘crosslinks’ are introduced in between the chains, which help the polymer to retract to its original position after the force is released as in vulcanised rubber.

Examples include: buna-S, buna-N and Neoprene.

2. Fibres

• Fibres are the thread-like solids which possess high tensile strength and high modulus of elasticity.
• These characteristics can be attributed to the strong intermolecular forces like hydrogen bonding.
• These strong forces also lead to close packing of chains which gives rise to crystalline structure.
• Examples include: polyamides (nylon 6, 6) and polyesters (terylene).

3. Thermoplastic polymers

• These are the linear or slightly branched long chain molecules which are capable of repeatedly softening on heating and hardening on cooling.
• These polymers possess intermolecular forces of attraction whose strength lies between that of elastomers and fibres.
• Some common thermoplastics are: polythene, polystyrene and polyvinyls.

4. Thermosetting polymers

• These polymers are cross linked or heavily branched molecules.
• On heating in moulds, they undergo extensive cross linking and become infusible.
• They cannot be reused.
• Some common examples are: Bakelite and urea-formaldelyde resins.

15.1.5 Classification Based on Growth Polymerisation

The addition and condensation polymers are also called chain growth polymers and step growth polymers respectively, depending on the type of polymerisation mechanism they undergo during their formation.

15.2 Types of Polymerisation Reactions

There are two broad types of polymerisation reactions: a) The addition or chain growth polymerisation b) condensation or step growth polymerisation.

15.2.1 Addition Polymerisation or Chain Growth Polymerisation

Description:

• In this type of polymerisation, the molecules of the same monomer or different monomers add together on a large scale to form a polymer.
• The monomers used are unsaturated compounds, like, alkenes, alkadienes and their derivatives.
• This mode of polymerisation leads to an increase in chain length or chain growth through the formation of either free radicals or ionic species.
• However, the free radical governed addition or chain growth polymerisation is the most common mode.

1. Free radical mechanism

• A variety of alkenes or dienes and their derivatives are polymerised in the presence of a free radical generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide and tert-butyl peroxide. 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 which generates a new and larger free radical. This step is called chain initiating step.
• As this radical reacts with another molecule of ethene, another bigger sized radical is formed. This repetition of the sequence in which new and bigger radicals are formed marks the progress of the reaction. Thus, this step is called, 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.

The sequence of steps may be depicted as follows:

For termination of the long chain, these free radicals can combine in different ways to form polythene. One mode of termination of chain is shown as under:

Preparation of some important addition polymers

(a) Polythene:

There are two types of polythene as given below:

(i) Low density polythene:

• It is obtained by the polymerisation of ethene under high pressure of 1000 to 2000 atmospheres at a temperature of 350 K to 570 K in the presence of traces of dioxygen or a peroxide initiator (catalyst).
• The low density polythene (LDP) obtained through the free radical addition and H-atom abstraction (is it subtraction? Same in shastri n ncert) has highly branched structure.
• Low density polythene is chemically inert and tough. However, it is flexible and a poor conductor of electricity. Hence, it is used in the insulation of electricity carrying wires and manufacture of squeeze bottles, toys and flexible pipes.

(ii) High density polythene:

• It is formed by addition polymerisation of ethane, in a hydrocarbon solvent, in the presence of a catalyst such as triethylaluminium and titanium tetrachloride (Ziegler-Natta catalyst) at a temperature of 333 K to 343 K and under a pressure of 6-7 atmospheres.
• The High density polythene (HDP) produced, consists of linear molecules and exhibits high density due to close packing.
• It is also chemically inert, tougher and harder.
• It is used for manufacturing buckets, dustbins, bottles and pipes.

Fact box: G. Natta of Imperia and Karl Ziegler of Germany were awarded the Nobel Prize for Chemistry in 1963 for the development of Ziegler-Natta catalyst.

(b) Polytetrafluoroethene (Teflon)

• Teflon is manufactured by heating tetrafluoroethene with a free radical (or persulphate catalyst) at high pressures.
• It is chemically inert and resistant to attack by corrosive reagents.
• It is used in making oil seals and gaskets and also used for non – stick surface coated utensils.

Note box: Teflon coatings undergo decomposition at temperatures above 300°C.

(c) Polyacrylonitrile

• The addition polymerisation of acrylonitrile in presence of a peroxide catalyst leads to the formation of polyacrylonitrile.

• Polyacrylonitrile is used as a substitute for wool in making commercial fibres as orlon or acrilan.
• Acrylic fibres have good resistance to stains, chemicals, insects and fungi.

15.2.2 Condensation Polymerisation or Step Growth polymerisation

Description:

• This type of polymerisation involves a repetitive condensation reaction between two bi-functional monomers.
• These polycondensation reactions may result in the loss of molecules like, water and alcohol leading to the formation of high molecular mass condensation polymers.
• In these reactions, the product formed in each step is a bi-functional species and the sequence of condensation goes on.
• Since, each step produces a distinct functionalised species which are independent of each other, this process is also called as step growth polymerisation.
• For Example: The formation of terylene or dacron by the interaction of ethylene glycol and terephthalic acid.

Some important condensation polymerisation reactions characterised by their linking units are described below:

1. Polyamides

These are polymers possessing amide linkages. They are important examples of synthetic fibres and are termed as nylons.

The general method of preparation consists of the condensation polymerisation of diamines with dicarboxylic acids and also of amino acids with their lactams as explained by the reactions below:

(a) Preparation of nylons

(i) Nylon 6,6: It is prepared by the condensation polymerisation of hexamethylenediamine with adipic acid under high pressure and at high temperature.

Uses: Nylon 6, 6 is used in making sheets, bristles for brushes and in textile industry.

(ii) Nylon 6: It is obtained by heating caprolactum with water at a high temperature.

Uses: Nylon 6 is used for the manufacture of tyre cords, fabrics and ropes.

2. Polyesters

• These are the poly-condensation products of dicarboxylic acids and diols.
• It is manufactured by heating a mixture of ethylene glycol and terephthalic acid at 420 to 460 K in the presence of zinc acetateantimony trioxide catalyst (refer section 15.2.2 for the reaction).
• Dacron or terylene is the best known example of polyesters.

Uses: Dacron fibre (terylene) is crease resistant and is used in blending with cotton and wool fibres and also as glass reinforcing materials in safety helmets.

3. Phenol – formaldehyde polymer (Bakelite and related polymers)

Phenol – formaldehyde polymers are obtained by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst.

• The reaction starts with the initial formation of o- and (or) p-hydroxymethylphenol (or either of these) derivatives, which further react with phenol to form compounds having rings joined to each other through –CH2 groups.
• The initial product could be a linear product like – Novolac used in paints.

• Novolac on heating with formaldehyde undergoes cross linking to form infusible solid mass called bakelite.

Uses: It is used for making combs, phonograph records, electrical switches and handles of various utensils.

4. Melamine – formaldehyde polymer

• Melamine formaldehyde polymer is formed by the condensation polymerisation of melamine and formaldehyde.

• It is used in the manufacture of unbreakable crockery.

15.2.3 Copolymerisation

Description:

• Copolymerisation is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise to form a copolymer.
• The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also.
• It contains multiple units of each monomer in the polymeric chain.
  For example, a mixture of 1, 3 – butadiene and styrene can form a copolymer.

15.2.4 Rubber

1. Natural rubber

• Rubber is a natural polymer which possesses elastic properties. It is also termed as elastomer.
• It is manufactured from rubber latex which is a colloidal dispersion of rubber in water. This latex is obtained from the bark of rubber tree.
• Natural rubber may be considered as a linear polymer of isoprene (2-methyl-1, 3-butadiene) and is also called as cis – 1, 4 – polyisoprene.

• The cis-polyisoprene molecule consists of various chains held together by weak van der Waal’s interactions and has a coiled structure. Hence, it can be stretched like a spring and exhibits elastic properties.

Vulcanisation of rubber:

Physical properties of rubber:

• Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K) and shows high water absorption capacity.
• It is soluble in non-polar solvents and is non-resistant to attack by oxidising agents.
• Vulcanisation is necessary to overcome these problems.

Definition box: Vulcanisation is the process of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between: 373 K to 415 K.

• On performing vulcanisation, sulphur forms cross links at the reactive sites of double bonds making the rubber stiff. This property is made use, in the manufacture of tyre rubber, 5% of sulphur is used as a crosslinking agent.

The probable structures of vulcanised rubber molecules are depicted below:

2. Synthetic rubbers

• Synthetic rubber is a vulcanisable rubber-like-polymer, which is capable of getting stretched to twice its length.
• However, it returns to its original shape and size as soon as the external stretching force is released.
• Thus, synthetic rubbers are either homopolymers of 1, 3 – butadiene derivatives or copolymers of 1, 3 – butadiene or derivatives with another unsaturated monomer.

Preparation of Synthetic Rubbers

1. Neoprene

• Neoprene or polychloroprene is formed by the free radical polymerisation of chloroprene.

• It has superior resistance to vegetable and mineral oils.
• It is used for manufacturing conveyor belts, gaskets and hoses.

2. Buna – N

(We have already studied about Buna-S, in Section 15.1.3. Now we will study about Buna – N.)

• Buna –N is obtained by the copolymerisation of 1, 3 – butadiene and acrylonitrile in the presence of a peroxide catalyst.

• It is resistant to the action of petrol, lubricating oil and organic solvents.
• It is used in making oil seals and tank lining.

Questions from sections 15.1 and 15.2:

1. Classify polymers based on the following and explain them with examples:

a) Source

b) Structure

c) Mode of polymerization

d) Molecular forces

2. Explain Addition Polymerization (or Chain growth polymerization) and explain the various steps involved in free radical mechanism.

3. Explain the methods of preparation of:

a) Low density polythene

b) High density polythene

c) Polytetrafluoroethene

d) Polyacrylonitrile

4. Write a note on Condensation Polymerisation with the example of formation of terylene.

5. What is Nylon?

6. Explain the preparation of: a) Nylon 6,6 b) Nylon 6

7. What are polyesters? State their uses.

8. How is phenol- formaldehyde polymer obtained? Explain the formation of Bakelite.

9. Explain the formation of Melamine polymer.

10. What is Copolymerisation? Give the chemical equation for formation of butadiene styrene copolymer.

11. What is rubber? How is it manufactured?

12. Give the chemical formula of rubber and open chain structure of natural rubber.

13. Explain Vulcanization of rubber, stating the limiting properties of rubber.

14. Write a note on synthetic rubbers.

15. Explain with relevant equations: a) Preparation of Neoprene b) Buna— N. Also give two uses for each of these synthetic rubbers.

15.3 Molecular Mass of Polymers

Polymer properties are closely related to their molecular mass, size and structure.

• The growth of the polymer chain during their synthesis is dependent upon the availability of the monomers in the reaction mixture. Thus, the polymer sample contains chains of varying lengths and hence its molecular mass is always expressed as an average.
• The molecular mass of polymers can be determined by chemical and physical methods.

15.4 Biodegradable Polymers

A large number of polymers are quite resistant to the environmental degradation processes and thus, are responsible for the accumulation of polymeric solid waste materials.

• These solid wastes cause acute environmental problems and remain un-degraded for quite a long time.
• For the welfare of public and to mitigate problems created by the polymeric solid wastes, certain new biodegradable synthetic polymers have been designed and developed. These polymers contain functional groups similar to the functional groups present in biopolymers and are thus, biodegradable.
• Aliphatic polyesters are one of the important groups of biodegradable polymers.
  Some important examples are given below:

1. Poly β-hydroxybutyrate – co-β-hydroxy valerate (PHBV)

• It is obtained by the copolymerisation of 3-hydroxybutanoic acidand 3 – hydroxypentanoic acid.
• PHBV is used in: speciality packaging, orthopaedic devices and in controlled release of drugs.
• It undergoes bacterial degradation.

2. Nylon 2–nylon 6

• It is an alternating polyamide copolymer of glycine ($H_2N–CH_2–COOH$) and amino caproic acid [$H_2N (CH_2)_5 COOH$]
• It is biodegradable.

Structure of Nylon 2–nylon 6:

15.5 Polymers of Commercial Importance

Commercially important polymers along with their structures and uses are given below in Table 15.1;

Questions from sections 15.3, 15.4 and 15.5:

1. What does the growth of polymer chain depend upon?

2. Write a note on biodegradable polymers. Give two examples with their structures.

3. Give two examples for commercially important polymers. Specify their monomer unit, structure and uses.

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

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

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

7. Arrange the following polymers in increasing order of their intermolecular forces.

(i) Nylon 6,6, Buna-S, Polythene.

(ii) Nylon 6, Neoprene, Polyvinyl chloride.