Polymers Types, Classification, Properties

Polymers

Many things of everyday use are made of polymers. For example, fabric for clothing and furniture, plastic utensils, containers, toys, toothbrushes, synthetic rubber for automobile tyres, paints and varnishes, paper and pens.

• Large molecules (macromolecules) formed by a series of chemical reactions between small molecules are called polymers; the word is derived from the Greek polymers meaning “many parts” (poly = many, meros = parts).
• The small molecules that make up the giant polymer are called monomers, meaning “of one part”. The formation of large molecules by the linking together of small molecules is called polymerisation.
• The structural formulae of some common polymers, along with the monomers from which they are made, are given below.

• Polymers made up of a few monomers are called oligomers. An example is crystalline sulphur, a molecule of which has eight sulphur atoms arranged in a ring. As is obvious, the monomeric unit is a sulphur atom.
• – OH
• The properties of polymers are related to molecular mass, size and structure. The molecular mass of a polymer depends on the conditions of polymerisation.
• The length of the polymer chain depends upon the availability of monomer molecules in the reaction mixture. Thus, a polymer contains chains of varying lengths.
• Therefore the molecular mass of a polymer is expressed as an average.

Polymers Classification

Polymers May Be Classified In Several Ways.

On the basis of the type of chain

Polymers may be made up of three types of chains-linear, branched and crosslinked.

1. Linear-chain polymers:

High-density polymers with long straight chains are called linear-chain polymers. Examples are polyethene and polyvinyl chloride. The chains in such a polymer may be depicted as follows.

2. Branched-chain polymers:

Low-density polymers having linear chains with some branches are called branched-chain polymers. An example of such a polymer is polypropylene. The chains in a branched-chain polymer may be shown as follows.

3. Crosslinked polymers:

Crosslinked polymers contain strong covalent bonds between various linear chains. Examples of such polymers are bakelite and melamine. The chains in the polymer may be depicted. The monomers in a crosslinked polymer generally have two or three functional groups.

On The Basis Of Intermolecular Force

Polymers are known for their special mechanical properties such as toughness, elasticity and tensile strength. The degree of toughness, elasticity and so on depends upon the intermolecular forces in the large polymer molecules.

These forces may be van der Waals forces, forces due to hydrogen bonds, etc. Such forces are present in smaller molecules too but their effect is not so pronounced. The larger the molecule, the greater is the effect.

Polymers may be divided into four groups based on the strength of their intermolecular forces—

1. Thermoplastics,
2. Thermosetting Polymers,
3. Elastomers And
4. Fibres.

Thermoplastics

• As you know, a polymer may be linear, branched or crosslinked. The first two types of polymers are said to be thermoplastic as they can be moulded into different shapes, after being heated and then cooled.
• They soften on heating and harden on cooling. They also dissolve in the appropriate solvents.. These polymers possess an intermolecular force of attraction that is between those of elastomers and fibres.
• Common examples of thermoplastics are polythene, polystyrene and polyvinyl chloride.

Thermosetting polymers

• A crosslinked polymer, i.e., one in which the monomer chain is crosslinked to other adjacent chains to form a three-dimensional network, is a thermosetting polymer. It is less soluble in solvents than a thermoplastic is.
• It hardens on heating and can be moulded only once because it becomes rigid and heat cannot soften it for remoulding. Examples of thermosetting polymers are bakelite and urea-formaldehyde resins.

Elastomers

• Polymers with elastic qualities are known as elastomers. In these polymers, each chain is held together by weak van der Waals forces of attraction. Among the polymers, the binding force is the weakest in elastomers.
• These weak forces allow the polymer to be stretched. Elastomers also contain a few short chains of sulphur atoms, which serve as linkages between the polymer chains.
• The sulphur chains help align the polymer chains, so the material does not undergo a permanent change when stretched, but springs back to its original shape and size when the stress is removed.
• Vulcanised rubber is a common example of an elastomer. Other examples are buna-S, buna-N and neoprene.

Stretched vulcanised rubber retains its elasticity.

Fibres

• Synthetic fibres are thread forming semicrystalline solids which possess high tensile strength and high modulus of elasticity.
• In order to have a high tensile strength, the chains of atoms in a polymer should be able to attract one another, but not so strongly that the plastic cannot be initially extended to form the fibres.
• Ordinary covalent bonds would be too strong. Hydrogen bonds, with a strength of about one-tenth that of ordinary covalent bonds, link the chains in the desired manner.
• Examples of fibres are polyamides [nylon 6, 6, nylon 6 and polyesters (Terylene)].

On The Basis Of Their Sources

On the basis of their sources, polymers are classified as natural, semi-synthetic and synthetic.

Natural polymers

• Plants produce an enormous number of molecules, which vary in size, shape and function. Some of them, such as cellulose, starch and rubber, are large polymers.
• The polymer cellulose is a constituent of cotton, wood and the cell walls of plants. It is a condensation polymer whose monomer unit is the molecule β-glucose.

• Another natural polymer is starch. It is a constituent of many plants, including potatoes, wheat, rye, oats, corn and rice. The a-glucose molecule is the monomer unit for this polymer.
• This molecule differs from that of B-glucose only in respect of the relative position of the OH group bonded to Cl. In a-glucose, this -OH group is pointed towards the bottom of the ring; in γ-glucose, it is pointed towards the top.

The structures of cellulose and starch

Another naturally occurring polymer is rubber. The name rubber was given to this substance when it was found to rub out pencil marks. Rubber is formed from the monomer isoprene, C₅H₈ and is also called cis-polyisoprene.

Where n = 11, 000 to 20,000

The structure may be represented as

The cis structure of natural rubber is vital to its elasticity.

Gutta-percha is a naturally occurring isomer of rubber in which all the -CH₂-CH₂-groups are trans.

• Poly trans isoprene (the-CH₂-CH₂– groups are trans)
• The trans compound is hard and brittle.
• Rubber is extracted from trees. It occurs as latex (a suspension of rubber particles in water) that oozes from rubber trees when their trunks are slit.
• On treatment with 1% acetic acid, the rubber particles are precipitated from the latex as a gummy mass. The gummy mass is not only elastic and water-repellent but also very sticky, especially when warm.
• In 1839, Charles Goodyear discovered that heating latex with sulphur produces a material (vulcanised rubber) that is no longer sticky, but still elastic and water-repellent.
• Vulcanised rubber contains short chains of sulphur atoms which bind the polymer chains of natural rubber. It is an elastomer, and its structure may be represented.

The composition of a car tyre:

Semisynthetic polymers

• Cellulose was the first polymer to be chemically modified to new substances useful to human beings. When made to react with acetic anhydride in acetic acid using a little sulphuric acid as a catalyst, cellulose is converted into its acetate.
• When a solution of cellulose triacetate is forced through small holes into a solution of dilute acetic acid, the water precipitates it in the form of a continuous thread that can be used to weave fabrics.
• The use of cellulose in a variety of other products is based on similar modifications of structure. For example, the hydroxyl groups in cellulose are converted into alkoxide anions by a base. These anions add to CS₂ to give compounds known as xanthate esters.
• The basic solution of cellulose xanthate salts can be forced out of spinners into dilute H₂SO₄ to form rayon threads. If thin slits are used, sheets of cellophane are formed. Both rayon and cellophane are essentially cellulose in a transformed physical state.

Synthetic polymers

These are polymers synthesised in the laboratory or in manufacturing units and may be obtained by two processes polymerisation or chain-growth polymerisation and condensation polymerisation or step-growth polymerisation.

Addition polymerisation:

• Addition polymerisation generally occurs between molecules containing one or more double bonds. No small molecules are liberated during this process.
• A very important group of olefinic compounds that undergoes addition polymerisation is of the type CH₂=CH-Y, where Y may be H, X, COOR, CN, etc.

⇒\(n \mathrm{CH}_2=\mathrm{CH}-\mathrm{Y} \longrightarrow\left(\mathrm{CH}_2-\mathrm{CH}-\mathrm{Y}\right)_n\)

Polymerisation Mechanism:

Alkenes and their derivatives polymerise by the free-radical mechanism in the presence of organic peroxides such as benzoyl peroxide, acetyl peroxide and t-butyl peroxide.

• For example, ethylene polymerises under high pressure (1000 atm) at an elevated temperature (473 K). The reaction is initiated by a free radical (catalyst) produced by organic peroxides, for example benzoyl peroxide, acetyl peroxide, and t-butyl peroxide.
• The polymerisation of ethylene, initiated by dibenzoyl peroxide, is a radical chain reaction. The peroxide is first cleaved homolytically to give two benzoate radicals, which produce the phenyl radical. The phenyl radical adds to the alkene to give an unstable primary carbon radical.
• This step is called the chain-initiating step. The primary carbon radical adds to another molecule of the alkene, and so on. This step is termed as the chain-propagating step. Finally, the chain is terminated by combination with another radical (the chain-terminating step).

The following steps are involved.

1. Chain Initiation:

2. Chain Propagation:

⇒ \(\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \dot{\mathrm{C}} \mathrm{H}_2+\mathrm{CH}_2=\mathrm{CH}_2 \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \dot{\mathrm{C}} \mathrm{H}_2 \rightarrow \mathrm{C}_6 \mathrm{H}_5\left(\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_2-\dot{\mathrm{C}} \mathrm{H}_2\right.\)

3. Chain Termination:

Anionic and cationic polymerisations also take place, but they are not as common as the free radical processes. Ionic polymerisations are usually very fast and exothermic.

Example 1. Give the mechanism for the formation of a segment of polyvinyl chloride containing three units of vinyl chloride initiated by HO.

Solution:

Mechanism for the formation of a segment of polyvinyl chloride containing three units of vinyl chloride initiated by HO

Addition polymers:

Polymers synthesised by addition polymerisation are called addition polymers. A polymer made of only one type of monomer is known as a homopolymer. A polymer made of more than one type of monomer is known as a copolymer.

Polyethylene or polyethene: There are two varieties of polythene-

1. Low-density polyethene (Ldp) And
2. High-density polythene (Hdp).

1. Low-density polythene:

Low-density polythene is produced at high temperatures (473 K) and high pressure (≈ 1000 to 2000 atmospheres) using oxygen or peroxide as the initiator (catalyst). Under these circumstances, free radicals attack the chain at random positions, thus causing irregular branching.

• The polythene with irregular branching is less dense and more flexible since the molecules are generally far apart and their arrangement is not so precisely ordered. this material is used for making squeeze bottles toys and flexible pipes , among other things

⇒ \(\underset{\text { Ethylene }}{n\left(\mathrm{CH}_2=\mathrm{CH}_2\right)} \underset{473 \mathrm{~K}}{\stackrel{\mathrm{O}_2}{\longrightarrow}}+\underset{\text { Polythene }}{\left.\mathrm{CH}_2-\mathrm{CH}_2-\right)_n}\)

2. High-density polythene:

High-density polyethene is produced at low temperatures (333 K To 343K) and comparatively low pressure(6-7atmospheres) in the presence of a catalyst (C₂H₂)₃ Al/TiCl₄ (Ziegler-Natta catalyst).

• This catalyst yields almost exclusively linear polythene. The molecules in linear polythene are strongly attracted to one another by van der Waals forces, yielding a tough, high-density compound due to close packing, which is useful in making toys, bottles and buckets.

Polythenes formed under different pressures and catalytic conditions have different molecular structures and hence different physical properties.

Polypropylene:

Polypropylene was prepared for the first time by the Italian chemist Natta in 1960, an achievement for which he was awarded the Nobel Prize in 1963.

He prepared polypropylene by dissolving propylene in the inert solvent heptane containing triethylaluminium and titanium chloride catalyst at a high temperature (373 K) under a pressure of 10 atm.

Polypropene is a substitute for polythene. It is lighter and stronger than the latter. Its softening point is relatively high, and it is used for making hard fibres because it has a high tensile strength.

These fibres are used for making carpets and ropes. Polypropene is also used to make bottles, glasses, pens, toys, electrical goods and pipes.

Polystyrene:

Polystyrene On polymerisation in the presence of dibenzoyl peroxide (catalyst), styrene yields polystyrene.

Styrene itself is prepared from benzene by the following method.

Polystyrene is a transparent polymer and is used in manufacturing food containers, bottles, plastic cups, combs, toys and television cabinets.

Teflon:

Teflon is prepared by heating tetrafluoroethene under high pressure in the presence of ammonium peroxy sulphate.

Teflon is incombustible and is not affected by acids or alkalis. It is used for making insulating material, bearings and nonstick utensils.

Polyvinyl chloride (PVC):

Polyvinyl chloride is obtained by heating vinyl chloride in the presence of benzoyl peroxide in an inert solvent.

Vinyl chloride itself is obtained by the reaction of ethyne (acetylene) with HCl in the presence of HgCl₂ catalyst.

PVC is used in making pipes, plastic syringes, hard plastic bottles, raincoats, shoes, curtains and garden hoses.

Polyacrylonitrile (orlon):

Orlon is prepared by the polymerisation of acrylonitrile (vinyl cyanide) in the presence of FeSO₂/H₂O₂

Orlon is water-resistant and is used in making carpets and blankets.

Polymethyl methacrylate (PMMA):

It is prepared by the polymerisation of methyl methacrylate in the presence of benzoyl peroxide (catalyst).

It is tough and transparent, and is popularly known as plexiglass. It finds use in the manufacture of aeroplane windows, contact lenses, automobile tail lights and in plastic surgery.

Neoprene:

Neoprene, a synthetic rubber, is a polymer of chloroprene (2-chloro-1, 3-butadiene).

Chloroprene:

Chloroprene itself is prepared from ethyne (acetylene).

Neoprene is more resistant than natural rubber to oils and solvents. It is tougher and wears better than rubber. It is used mostly in applications where its toughness and resistance to oil and grease are important, such as in gaskets, sealing rings, and engine mountings. It is also used in making automobile tyres.

Example 2: Draw the structures of the monomers used to synthesise the following polymers.

Solution:

Buna-N-rubber:

 The polymerisation of butadiene in the presence of sodium gives a polymer known as buna-N- rubber. It was the first synthetic rubber to be manufactured. However, it is not very useful.

All the addition polymers described above are homopolymers—they contain the same type of monomer units.

Copolymerisation

• If two or more monomers polymerise to give a single polymer containing different subunits, the product is a copolymer and the process is called copolymerisation.
• The copolymer can be made by addition polymerisation (chain-growth polymerisation) and condensation polymerisation (step-growth polymerisation).
• A copolymer can have useful properties that are different from and often superior to those of a homopolymer. Buna-S-rubber is an example of a copolymer.

Buna-S-rubber:

 Buna-S-rubber is a copolymer prepared by the polymerisation of three moles of butadiene and one mole of styrene. This polymer is tough and possesses properties close to those of natural rubber.

The double bonds in the chain allow this polymer to undergo vulcanisation like a natural rubber polymer chain.

Buna-S-rubber is manufactured on a large scale and used to make automobile and truck tyres. A pure form this polymer is used as a replacement for the latex in chewing gum.

All synthetic rubbers can be vulcanised and can be stretched to twice their length. Once the external force is removed, they return to their initial size and shape.

Condensation polymers:

Condensation polymers are formed when bifunctional monomer molecules are linked. This happens when the monomer molecules react, and a small molecule such as that of water, HCl or alcohol is released. Polyesters and polyamides are examples of condensation polymers.

Polyester:

A typical polyester is prepared by heating a alcohol (ethylene glycol) and a diacid (terephthalic acid) in the presence of a catalyst.

Terephthalic acid (containing two carboxylic acid groups) and ethylene glycol (containing two alcohol groups) can react at both ends.

The reaction of one carboxylic acid group of terephthalic acid with one alcohol group of ethylene glycol initially produces an ester molecule with an acid group at one end and an alcohol group at the other.

Subsequently, the remaining acid group can react with another alcohol group, and the alcohol group can react with another acid molecule. The process continues until an extremely large polymer molecule, known as a polyester, is produced with a molecular weight in the range of 10,000-20,000.

This condensation polymerisation is also called step-growth polymerisation since each step produces a distinct functionalised species.

Polyester can be spun into a fibre from the melt. The fibre is used in making textile fibres marketed under the names Dacron and Terylene. The blending of polyester with cotton provides a fabric with high durability and anticrease properties.

Polyamides (nylons):

Polyamides Many nylons have been prepared and tried in the consumer market. Two of them  nylon 6, 6 and nylon 6-have been the most successful.

Nylon 6, 6 is prepared by the reaction of equimolecular quantities of hexamethylenediamine and adipic acid under high pressure and at high temperatures. The resultant melt is spun into fibre.

The polymer is made up of alternating —NH(CH₂)₆ NH- and -(C₂) 4 4O— units, each having six carbon atoms, and is called nylon 6, 6.

Nylon 6 is prepared by heating caprolactum with water at a high temperature.

This caprolactum monomer is a cyclic amide and the polymer does not have alternating units—each unit is the same containing six carbon atoms and is called nylon 6.

Caprolactum itself is synthesised from cyclohexanone in the following manner.

Nylons have been widely accepted as textile fibres because they are strong, have desirable elastic properties, and can be drawn into very fine fibres. They are used to prepare fishing nets, ropes and brushes, among other things.

Example 3: Identify the monomer units used to make the following polymers.

Solution:

Example 4:  What is the significance of the numbers 6, 6 and 6 in nylon 6, 6 and nylon 6?

Solution:

The significance of the numbers 6, 6 and 6 in nylon 6, 6 and nylon 6

• The numbers 6, 6 in the name of nylon 6, 6 refer to the six carbon atoms of hexamethylenediamine and the six carbon atoms of adipic acid.
• The number 6 in nylon 6 indicates that six carbon atoms are contributed by the reactant caprolactum.

Glyptal:

Glyptal is prepared by the reaction of ethylene glycol with phthalic acid. It is used to prepare paints and lacquers.

Bakelite or phenol-formaldehyde plastic:

Bakelite is obtained by condensing phenol with formaldehyde under either acidic or basic conditions. Under acidic conditions, polymerisation proceeds to give a three-dimensional network of phenol rings held together at the ortho- and para-positions by methylene groups.

Bakelite is a stiff material with little solubility in organic solvents and a high resistance to electricity and heat. It is used to make a variety of household objects and electrical fixtures. The polymer has the useful property of being thermosetting.

The reaction of phenol with formaldehyde also produces o-hydroxymethylphenol which further reacts with phenol to give a linear product-novolac-used in paints. On being heated with formaldehyde, novolac undergoes cross-linking to form bakelite.

Urea-formaldehyde plastic:

On being heated with formaldehyde in the presence of a dilute acid, urea gives urea-formaldehyde plastic. This plastic is colourless and does not fade in sunlight. It is used to make household materials and kitchenware.

Formica, used to cover the surfaces of furniture, cupboards, and so on, is also prepared from urea-formaldehyde plastic. In the form of an ion-exchange resin, urea-formaldehyde plastic is used to purify water.

Melamine-formaldehyde plastic:

Melmac, a polymer used in the manufacture of unbreakable kitchenware, is made by the condensation polymerisation of melamine and formaldehyde.

Melamine itself is produced by the trimerisation of cyanamide (NH₂)—CN).

Hard plastics are made softer by mixing them with a plasticizer. Di-isooctylphthalate is generally used as a plasticizer.

Biodegradable Polymers

Plastics are not very easily degraded and cause environmental problems such as soil pollution. In view of the current global concern for the environment, researchers have been attempting to come up with biodegradable polymers.

In recent years, synthetic polymers have been produced that have built-in susceptibility to bacteria or fungi. The functional groups of the polymers are similar to those of biopolymers.

Aliphatic polyesters and polyamides are important biodegradable polymers.

Poly (B-hydroxybutyrate-ß-hydroxy valerate), PHBV

It is a biodegradable copolymer obtained by the reaction of B-hydroxybutyric acid with β-hydroxyvaleric acid.

Nylon 2-nylon 6:

This a biodegradable copolymer obtained by the reaction of glycine with 6 aminohexanoic acid.

This polymer is made up of alternating -NH-CH₂)-C- and -NH(CH₂)₂)₅ C— units having two carbon atoms and six carbon atoms respectively, and is called nylon 2-nylon 6.

Polymers Multiple-Choice Questions

Question 1. Natural rubber is a polymer of

1. Ethylene
2. Benzene
3. Isoprene
4. None Of These

Answer: 3. Isoprene

Question 2. The polymerisation of which of the following leads to the formation of neoprene rubber?

1. Chloroprene
2. Isoprene
3. 1,3-Butadiene
4. Acetylene

Answer: 1. Chloroprene

Question 3. Which of the following is a natural polymer?

1. Protein
2. Polythene
3. Buna-S
4. Bakelite

Answer: 1. Protein

Question 4. Which of the following contains ester linkages?

1. Terylene
2. Nylon
3. Teflon
4. Bakelite

Answer: 1. Terylene

Question 5. Which of the following is obtained by the condensation of adipic acid and hexamethylene diamine?

1. Rayon
2. Terylene
3. Nylon 6, 6
4. Carbon Fibre

Answer: 3. Nylon 6, 6

Question 6. Teflon, polystyrene and neoprene are

1. Copolymers
2. Condensation Polymers
3. Homopolymers
4. Monomers

Answer: 3. Homopolymers

Question 7. Which of the following contains an amide linkage?

1. Nylon 6, 6
2. Terylene
3. Teflon
4. Bakelite

Answer: 1. Nylon 6, 6

Question 8. Phenol is used in the formation of which of the following?

1. Bakelite
2. Polystyrene
3. Nylon
4. PVC

Answer: 1. Bakelite

Question 9. In the Ziegler method, which of the following catalysts is used in the formation of polythene?

1. Lithium Tetrachloride And Triphenylaluminium
2. Titanium Tetrachloride And Triethylaluminium
3. Titanium Oxide
4. Titanium Isoperoxide

Answer: 2. Titanium Tetrachloride And Triethylaluminium

Question 10. PMMA is a polymer of which of the following?

1. Methyl Methacrylate
2. Methyl acrylate
3. Ethyl acrylate
4. All Of These

Answer: 1. Methyl Methacrylate

Question 11. Orlon is a polymer of which of the following?

1. Tetrafluoroethylene
2. Acrylonitrile
3. Ethanoic acid
4. Benzene

Answer: 2. Acrylonitrile

Question 12. Natural rubber is a polymer of which of the following?

1. Trans-Isoprene
2. Cis-Isoprene
3. Co-Cis- And Trans Isoprene
4. None Of These

Answer: 2. Cis-Isoprene

Question 13. Which of the following is used in making nonstick cookware?

1. Polystyrene
2. Polytetrafluoroethene
3. Polythene
4. None Of These

Answer: 2. Polytetrafluoroethene

Question 14. Which of the following is an example of a copolymer?

1. Nylon 6
2. Nylon 6, 6
3. PMMA
4. Polythene

Answer: 2. Nylon 6, 6

Question 15. Which of the following is formed by condensation polymerisation?

1. Polythene
2. PVC
3. Teflon
4. Nylon 6, 6

Answer: 4. Nylon 6, 6

Question 16. Using which of the following can PVC be prepared?

1. CH₃CH=CH₂
2. C₆H5CH=CH₂
3. CH₂=CH-CI
4. CH₂ = CH

Answer: 3. CH₂=CH-CI

Question 17. Which of the following is a thermosetting polymer?

1. Nylon 6
2. Nylon 6, 6
3. Bakelite
4. SBR

Answer: 3. Bakelite

Question 18. In an elastomer, the intermolecular forces are

1. Nil
2. Weak
3. Strong
4. Very Strong

Answer: 2. Weak

Question 19. Which of the following is a biodegradable polymer?

1. Polythene
2. PVC
3. Bakelite
4. PHBV

Answer: 4. PHBV

Question 20. Which of the following is formed by condensation polymerisation?

1. Rayon
2. Nylon
3. Dacron
4. Artificial Silk

Answer: 1. Rayon and 4. Artificial Silk

Question 21. Which of the following is formed by condensation polymerisation?

1. Polyethylene
2. Bakelite
3. Melamine
4. Vulcanised Rubber

Answer: 2. Bakelite 3. Melamine and 4. Vulcanised Rubber

Question 22. Which of the following pairs of monomer molecules will form an addition polymer?

Answer: 1

Question 23. Which of the following pairs of monomer molecules will form a condensation polymer?

Answer: 3