Polymers

 1. Polymers: Polymers are the high molecular mass macromolecules, formed by the combination of large number of simple molecules called monomers. The process by which monomers are converted into polymers is called polymerisation.

2. Classification of Polymers

(a) Classification based on sources:

(i) Natural polymers: Polymers found in nature, mostly in plants and animals are called natural polymers, e.g., proteins, natural rubber, etc.

(ii) Semisynthetic polymers: Polymers which are obtained by making some modifications in natural polymers by artificial means, e.g., nitrocellulose, cellulose acetate, etc.

(iii) Synthetic polymers: These are man-made polymers prepared in the laboratory, e.g., polythene, teflon, nylon, etc.

(b) Classification based on structure of polymers:

(i) Linear polymers: These polymers consist of long and straight chains, e.g., high density polythene, polyvinyl chloride, nylon, etc.

(ii) Branched chain polymers: These polymers contain linear chains having some branches, e.g., low density polythene, glycogen, etc.

(iii) Cross-linked or Network Polymers: In this type of polymers, the initially formed linear polymer chains are joined together to form three dimensional network structure. Due to presence of cross links these polymers are also called cross-linked polymers, e.g., bakelite, melamine, etc.

(c) Classification based on mode of polymerisation:

(i) Addition polymers: The addition polymers are formed by the repeated addition of same or different monomer molecules. The monomers used are unsaturated compounds, e.g., alkenes, alkadienes and their derivatives. Polythene is an example of addition polymer.

(ii) Condensation polymers: The condensation polymers are formed by the repeated condensation reaction between different bifunctional or trifunctional monomer units usually with elimination of small molecules such as water, alcohol, hydrogen chloride, etc. Nylon-6, nylon-6, 6 and terylene are some examples.

(d) Classification based on molecular forces:

(i) Elastomers: These are the polymers having the weakest intermolecular forces of attraction between the polymer chains. The weak forces permit the polymer to be stretched. A few ‘cross links’ 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. Elastomers thus possess an elastic character, e.g., buna-S, buna-N, neoprene, etc.

(ii) Fibres: These are the polymers which have the strongest intermolecular forces such as hydrogen bonds or dipole–dipole interactions. These polymers can be used for making fibre as their molecules are long and thread-like. Nylon-6, 6 and terylene are some common fibres.

(iii) Thermoplastics: These polymers possess intermolecular forces of attraction intermediate between elastomers and fibres. These are linear or slightly branched chain polymers capable of repeatedly softening on heating and hardening on cooling, e.g., polythene, polyproplene, polystyrene, polyvinyl chloride, etc.

(iv) Thermosetting polymers: These polymers are cross-linked or heavily branched molecules, which on heating undergo extensive cross-linking in moulds and again become infusible. These polymers cannot be reshaped, e.g., bakelite, urea–formaldehyde resins, etc.

Table 15.1: Differences between Thermoplastic and Thermosetting Polymers

S.No.	Thermoplastic Polymers	Thermosetting Polymers
(i)	These polymers soften on heating and harden on cooling.	On heating they undergo excessive cross linking and become hard.
(ii)	These polymers can be remoulded, recast and reshaped.	These polymers cannot be remoulded or reshaped.
(iii)	These are less brittle and soluble in some organic solvents.	These are more brittle and insoluble in organic solvents.
(iv)	These are formed by addition polymerisation.	These are formed by condensation polymerisation.
(v)	These polymers have usually linear structures.	These polymers have three dimensional cross-linked structures.
(vi)	Examples: Polyethylene, PVC, teflon, nylon, etc.	Examples: Bakelite, urea–formaldehyde resin, terylene, etc.

3. Types of polymerisation reactions

There are two broad types of polymerisation reactions:

(a) Addition polymerisation.

(b) Condensation polymerisation.

(a) Addition polymerisation: This type of polymerisation involves successive addition of monomer units to the growing chain carrying a reactive intermediate such as a free radical or anion.

Depending upon the nature of the reactive species involved addition polymerisation occurs by the following three mechanisms:

(i) Free radical polymerisation,

(ii) Cationic polymerisation,

(iii) Anionic polymerisation

Free radical polymerisation: A variety of alkenes or dienes and their derivatives are polymerised in the presence of a free radical generating initiator like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. 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 sequence of steps may be depicted as follows:

Chain initiation steps: Benzoyl peroxide undergo homolytic fission to form free radicals.

C6∙H5+CH2==CH2→C6H5—CH2—C∙H2

Chain propagating step:

C6H5—CH2—C∙H2+CH2==CH2→C6H5—CH2—CH2—CH2—C∙H2↓C6H5(CH2—CH2)nCH2—C∙H2

Chain termination step: The chain reaction stops when two free radical chains combine.

2C6H5(CH2—CH2)nCH2—C∙H2→C6H5(CH2—CH2)nCH2—CH2—CH2(CH2—CH2)nC6H5Polythene

(b) Condensation polymerisation: It occurs through a series of independent reactions (or steps). Each step involves the condensation between two bifunctional monomer units with elimination of simple molecule such as water, alcohol, etc., and leads to the formation of the polymer. Since the polymer is formed in a stepwise manner, the process is called step growth polymerisation, e.g., Nylon-6,6, dacron, bakelite, etc.

Table 15.2: Differences between Addition and Condensation Polymerisation

S.No.	Addition Polymerisation	Condensation Polymerisation
(i)	They are formed by adding monomers to a growing polymer chain without loss of any molecules.	Monomers combine together with the loss of small molecules like H2O, NH3, CO2, CH3OH, etc.
(ii)	It involves chain reaction.	It does not involve chain reaction.
(iii)	They are formed from unsaturated compounds.	Monomers have di or polyfunctional groups.
(iv)	Examples: Polythene, polypropene, PVC, teflon, etc.	Examples: Nylon-6,6, nylon-6, terylene, glyptal, bakelite, etc.

4. Preparation of Some Important Addition Polymers

(a) Polythene

(i) Low density polythene (LDP): It is used in the insulation of electricity carrying wires, and manufacture of squeeze bottles, toys and flexible pipes.

(ii) High density polythene (HDP): It is used for manufacturing buckets, dustbins, pipes, bottles, etc.

(b) Polypropene:

Uses: Manufacture of toys, ropes, pipes, carpet fibres, etc.

(c) Polystyrene:

Uses: As insulator, wrapping material, manufacture of toys, radio and television cabinets.

(d) Polyhaloolefins: These polymers are derived from halogen substituted olefins.

(i) Tetrafluoroethene (Teflon):

Uses: Making oil seals and gaskets, coating utensils to make them non-sticky.

(ii) Polyvinyl chloride (PVC):

Uses: Manufacture of rain coats, water pipes, electrical insulation, hand bags, vinyl flooring.

(e) Polyacrylates: These polymers are obtained from the ester of acrylic acid (CH2==CH—COOH).

(i) Polyacrylonitrile (PAN) or orlon or acrylane:

Uses: As a substitute for wool in the manufacture of commercial fibres such as orlon which is used for making clothes, carpets and blankets.

(ii) Polymethylmethacrylate (PMMA):

Uses: Manufacture of transparent objects such as aircraft windows, plastic jewellery, lenses, domes and sky lights.

5. Some Important Condensation Polymers

(a) Polyamides: Polymers possessing amide linkages (—CONH—) are called polyamides.

(i) Nylon-6, 6:

Uses: In making sheets, bristles for brushes and in textile industry.

(ii) Nylon-6:

Uses: Manufacture of tyre cords, fabrics and ropes.

(b) Polyesters: Polymers possessing ester linkages (—C||O—O—) are called polyesters and are prepared by the condensation polymerisation of dicarboxylic acids with diols.

(i) Dacron or terylene:

Uses: Manufacture of wash and wear fabrics, tyre cords, sails and seat belts.

(ii) Glyptal:

Uses: Manufacture of paints, lacquer and building materials.

(c) Phenol–formaldehyde polymer (Bakelite and related polymers): These are obtained by condensation reaction of phenol with formaldehyde in the presence of either an acid or base catalyst. The initial product could be a linear product, novolac.

Novolac on heating with formaldehyde undergoes cross linking to form bakelite.

Uses: Novolac is used in paints.

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

(d) Melamine–formaldehyde polymer: Melamine–formaldehyde polymer is formed by condensation polymerisation of melamine and formaldehyde.

Uses: It is used in the manufacture of unbreakable crockery.

6. Copolymerisation: When two or more different monomers are allowed to polymerise together, the product formed is called a copolymer and the process is called copolymerisation. Copolymers have properties different from homopolymers.

Uses: Buna-S is used for the manufacture of autotyres, floor tiles, footwear components, cable insulation, etc.

7. Rubber

(a) Natural rubber: Natural rubber may be considered as a linear polymer of isoprene (a-methyl-1, 3-butadiene) and is also called as cis-1, 4-polyisoprene. The cis-polyisoprene consists of various chains held together by weak van der Waals interactions and has a coiled structure. Thus, it can be stretched like a spring and exhibits elastic properties.

(b) Vulcanisation: Vulcanisation is the heating of natural rubber with sulphur and an appropriate additive to improve its physical properties. On vulcanisation, sulphur forms cross-links at the reactive sites of the double bond and thus rubber gets stiffened.

(c) Synthetic rubber: Synthetic rubbers are either homopolymers of 1, 3-butadiene derivatives or copolymers of 1, 3-butadiene or its derivative with another unsaturated monomer.

Neoprene:

nCH2==C—|ClCH==CH2Chloroprene−→−−−O2orPeroxide[CH2—C==|ClCH—CH2]nNeoprene

Uses: In the manufacture of conveyor belts, gaskets and hoses.

8. Biodegradable Polymers

Polymers that can be broken into small segments by enzyme-catalysed reactions are called biodegradable polymers. The required enzymes are produced by microorganisms. Since carbon–carbon bonds of chain growth polymers are inert to enzyme catalysed reactions, these polymers are non-biodegradable. To overcome this, certain bonds in the chain are to be inserted so that it can be made biodegradable. One such method to make a polymer biodegradable is to insert hydrolysable ester group into the polymer.

Aliphatic polyester, are biodegradable polymers and many of them are important commercial biomaterial. Some important examples are given below.

(i) Poly-b-hydroxybutyrate-co-b-hydroxy valerate (PHBV): This is a copolymer of 3-hydroxy butanoic acid and 3-hydroxy pentanoic acid.

Uses: It is used in speciality packaging, orthopaedic devices and in controlled release of drugs.

(ii) Nylon-2-Nylon-6: It is an alternating polyamide copolymer of glycine (H2NCH2—COOH) and amino caproic acid (H2N—(CH2)5 COOH) and is biodegradable.

9. Addition Polymers at a Glance

S.No.	Polymer	Monomer	Uses
1.	Polythene	Ethene (CH2==CH2)	Electrical insulator, packing materials, film, bottles, etc.
2.	Polypropene	Propene (CH3—CH==CH2)	Storage battery tanks
3.	Polystyrene	Styrene (C6H5—CH==CH2)	In combs, plastic handles, toys
4.	Polyvinyl chloride (PVC)	CH2==CHCl Vinyl chloride	Pipes, raincoats, vinyl floorings
5.	Polytetrafluoro ethane (PTFE) or Teflon	CF2==CF2 Tetrafluoroethene	Non-stick kitchenwares, electrical insulator
6.	Polymono chlorotrifluoro ethene	F—C|Cl==CF2 Monochlorotrifluoro ethene	Non-stick kitchenwares
7.	Buna-S	1, 3-Butadiene and styrene	Automobile, tyres
8.	Buna-N	1,3-Butadiene and acrylonitrile	Used for storing oil and solvents
9.	Neoprene	2-Chloro-1, 3-butadiene (Chloroprene)	Insulation conveyor belt
10.	Polymethyl methacrylate (PMMA) (Perspex, Lucite or Acrylite)	CH3—C—||CH2COOCH3	Substitute of glass and decorative material
11.	Polyethyl acrylate	CH2==CH—COOC2H5 Ethyl prop-2-enoate	Lacquers, films, hose piping
12.	Polyacrylonitrile or orlon	CH2==CH—C≡≡N Vinyl cyanide	For making clothes, carpets and blankets

10. Natural Polymers at a Glance

S.No.	Polymer	Monomer	Class	Uses
1.	Cellulose	b-Glucose	Biopolymer	Occurs in cotton, cell wall
2.	Starch	a-Glucose	Biopolymer	Food material storage in plants
3.	Proteins	Amino acids	Biopolymer	Essential for growth
4.	Nucleic acid	Nucleotides	Biopolymer	Essential for life perpetuation
5.	Rayon (Artificial Silk)	b-Glucose	Processed cellulose	Fabrics, surgical dressings
6.	Natural rubber	cis-Isoprene (Cis-2-methyl-1, 3-butadiene)	Natural polymer, elastomer	Used for tyres after vulcanisation
7.	Gutta percha	trans-Isoprene	Natural polymer, elastomer	Rubber-like material

11. Condensation Polymers at a Glance

S.No	Polymer	Monomer	Uses
1.	Terylene (Dacron)	Terephthalic acid and ethylene glycol	Ropes, safety belts, tyre cord
2.	Glyptal (Alkyl Resin)	Phthalic acid and ethylene glycol	Binding material, paints and in preparation of mixed plastics
3.	Nylon-6 (Perlon)	Caprolactam (cyclic amide)	Fibre, plastic, tyre cords and ropes
4.	Nylon-6, 6	Adipic acid and hexamethylene diamine	Sheets, bristles for brushes and in textile industry
5.	Bakelite	Phenol and formaldehyde	Electric switches and switch boards
6.	Melamine-formaldehyde resin	Melamine and formaldehyde	Crockery
7.	Urea–formaldehyde resin	Urea and formaldehyde	Crockery and laminated sheets