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Polythene, Teflon and Polyacrylonitrile are synthetic polymers formed by addition or chain-growth polymerization. Polymers, also known as macromolecules, are giant molecules having very high molecular masses. There are primarily three types of polymers namely Natural polymers, Semi-Synthetic polymers and Synthetic polymers. Natural polymers are obtained from plants and animals such as starch, cellulose and proteins. Semi-Synthetic polymers are obtained using natural polymers but by synthetic methods such as rayon. Synthetic polymers are man-made polymers, for instance, polythene, Teflon and polyacrylonitrile. These polymers find a variety of applications in daily life.
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Key Terms: Polymers, Polymerisation, Addition Polymerisation, Condensation Polymerisation, Teflon, Polythene, Polyacrylonitrile, Acrylonitrile, Polymer Chains
What is a Polymer?
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A polymer can be defined as a large molecule of very high molecular mass, that is formed through the repeated combination of a very large number of one or more types of small molecules called monomers.
Although a polymer molecule is very large, it is a single chain-like molecule. The repeating units in this molecule are derived from monomers and are linked together by covalent bonds. The process by which monomer molecules combine together to form a polymer is called polymerization.
Polymerization
Classification of Polymers
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Polymers can be classified in several ways. Some important ones are as follows:
- Classification on the basis of the source of origin
- Classification on the basis of mode of Polymerization
On the Basis of the Source of Origin
Depending upon the source of origin, polymers can broadly be classified into the following two classes:
- Natural Polymers: Natural polymers are classified as those polymers that occur in nature. Such polymers have their origin in plants and animals. For example, starch, cellulose, proteins, nucleic acids, natural rubber, etc., are natural polymers and occur in plants and animals.
- Synthetic Polymers: Synthetic polymers are man-made polymers and find a variety of applications in our day-to-day life. Most synthetic polymers are long-chain organic molecules containing thousands of monomer units. Some important synthetic polymers are polyethene, polyacrylonitrile, Teflon, polystyrene, nylon, etc.
On the Basis of Mode of Polymerisation
As pointed out earlier, polymers are formed as a result of polymerisation. Polymerisation, i.e., the chemical combination of a large number of monomer molecules, proceeds either through the condensation process or through the addition process. Therefore, polymerisation may be of two types:
- Addition polymerisation
- Condensation polymerisation
Polythene, Teflon and Polyacrylonitrile are formed through addition or chain-growth polymerization. The preparation methods of polythene, teflon and polyacrylonitrile are elaborated below in detail.
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Addition Polymerisation
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When a large number of monomer molecules add up together to form the polymer chain, the process is called addition polymerisation. The polymers formed by addition polymerisation are called addition polymers. If only a single type of monomeric unit is used in the polymerization process, then the formed polymer is called a homopolymer.
Some examples of addition polymerisation and addition polymers are polythene, teflon, polypropylene, etc.
Preparation of Polythene
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Polythene, also known as polythene, is one of the most common polymers and is used in making a variety of household articles. They are linear or slightly branched long-chain molecules formed by a free radical mechanism. Polyethene is a lightweight, flexible synthetic plastic derived from the polymerization of ethylene.
Polythene
Preparation
It is prepared by the addition polymerisation of ethylene. The following two methods are used for the manufacture of polythene.
- Polymerisation at High Pressure: In this method, ethylene is polymerised at high pressure, of the order of 1000-5000 atmospheres and at 520K in the presence of traces of oxygen. The polythene thus obtained possesses low density and is therefore called low-density polythene or high-pressure polythene.
- Polymerisation at Low Pressure: Ethylene can be polymerised at 6-7 atmosphere pressure at 333-343K in presence of Triethyl aluminium and titanium tetrachloride (Ziegler- Natta catalyst). The polymer so obtained is termed high-density polythene.
Preparation of Polythene
Properties of Polythene
- Low-density polythene is highly branched. Typical low-density polythene may have 50 short branches and one long branch. The branching in the polythene molecule restricts it to pack close together, thus, leading to a low density.
- Low-density polythene is a tough, inert, and poor conductor of electricity. It is pliable over a wide range of temperatures.
- High-density polythene possesses linear molecules with low branching. These molecules can pack themselves easily. Therefore, this type of polythene possesses high density.
- High-density polythene is also chemically inert. It is more crystalline. Due to high crystallinity, it is stiffer, harder and possesses a greater tensile strength. The softening temperature (408 K) of high-density polythene is also higher than that of low-density polythene whose softening temperature is only 388K.
Uses of Polythene
- Polythene is used in the preparation of household wares, toys, pipes, bottles and containers.
- Owing to its clear and crystalline nature, polythene is used for plastic bags and stretch films.
- It has higher chemical resistance and can withstand a wide variety of chemicals.
Preparation of Teflon
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Teflon, also known as polytetrafluoroethylene (PTFE), is a synthetic fluoropolymer of tetrafluoroethylene. It is a thermoplastic polymer that maintains high strength, toughness and self–lubrication at low temperatures down to 5K. Teflon has a high heat resistance and is chemically inert and highly insoluble in most solvents. It has high resistance to attack by corrosive reagents.
Teflon
Preparation
It is prepared by the free radical addition polymerisation of tetrafluoroethylene under pressure in the presence of water and oxygen which act as free radical initiators.
Preparation of Teflon
Properties of Teflon
- Teflon is a highly crystalline linear polymer that has a high melting point. Its melting point is 600 K.
- Its softening point is also high but it starts decomposing at its softening point. Therefore, it is not a true thermoplastic.
- Teflon is known for its resistance to heat and to almost all chemicals. It is insoluble in most solvents.
- It is a good electrical insulator, also, it has an extremely low coefficient of friction.
Uses of Teflon
- It is an excellent material for making valves. and lining of parts exposed to the corrosive environment.
- Due to the low coefficient of friction, its surface is somewhat waxy and prevents substances to stick on it. Therefore, it is used in making non-stick kitchen utensils and in electric irons.
Preparation of Polyacrylonitrile
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The polymers that are obtained through the polymerisation of acrylic monomers are referred to as polyacrylates. One important polymer belonging to this class is Polyacrylonitrile. Polyacrylonitrile (Acrilan) is a polymer of acrylonitrile (vinyl cyanide). The abbreviation of polyacrylonitrile is PAN and is also known as polyvinyl cyanide and Creslan 61. It is a semi-crystalline synthetic and thermoplastic polymer.
Polyacrylonitrile
Preparation
It is prepared by the free radical addition polymerisation of acrylonitrile (vinyl cyanide) in presence of a catalyst such as a peroxide.
Preparation of Polyacrylonitrile
Properties of Polyacrylonitrile
Polyacrylonitrile is a highly crystalline polymer. Due to high crystallinity, it is hard, horny and high melting material.
Uses of Polyacrylonitrile
- Polyacrylonitrile is the major constituent of acrylic fibres, eg, Acrilan and orlon. Acrilan fibres are used for making clothes, carpets and blankets.
- It is also used in the preparation of other polymers to improve their quality.
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Things to Remember
- Polymers are chains or a network of molecules with high molecular masses which are formed from the polymerisation of thousands of small repeating units called monomers.
- Polymerization, is the process of joining up or linking small molecules to form a large chain or network of molecules called polymers. It can proceed in two ways- Addition polymerisation and condensation polymerisation.
- Addition polymerization is in which the molecules of the same monomer or different monomers add together on a large scale to form a polymer.
- Polythene is used to make packing material, moulding toys, household wares and plastic containers.
- Teflon is corrosion resistant and hence used in making the outer layer of kitchen cookware.
- Polyacrylonitrile is a polymer of acrylonitrile and is a semi-crystalline synthetic and thermoplastic polymer.
PYQs
- Which one of the following is amorphous?... [VITEEE 2006]
- Nylon 66 is formed by… [UPSEE 2019]
- Which one of the following molecules will form a linear polymeric structure… [NEET 2000]
- Dacron is polymer of… [BITSAT 2005]
- Polyethylene is a resin obtained by polymerization of… [MU OET 2003]
- Bakelite is a product of the reaction between… [NEET 1992]
- Peptide bond is a key feature in… [NEET 1994]
- The biodegradable polymer is… [NEET 2019]
- Which of the following polymers is prepared by condensation polymerisation… [NEET 2007]
- Which one of the following is a chain growth polymer… [NEET 2004]
- Nylon 66 is formed by...[UPSEE 2019]
- Dacron is polymer of :….[BITSAT 2005]
- The polymer that is used as a substitute for wool in making commercial fibres is :...[NEET 2019]
- Terylene is a condensation polymer of ethylene glycol and….[NEET 1999]
- Polyethylene glycol method is used for….[NEET 2009]
- Natural rubber is a polymer of...[NEET 1999]
- Which of the following has an ester linkage?...[VITEEE 2014]
- Which one is classified as a condensation polymer?...[BITSAT 2019]
- In case of condensation of polymers….[JKCET 2008]
Sample Questions
Ques. What are polymers and polymerization? (3 Marks)
Ans. A polymer is a large molecule of very high molecular mass formed by the repeated combination of a very large number of one or more types of small molecules called monomers.
Polymerization is defined as the process by which monomer molecules combine together to form a polymer.
Ques. What are Polythene, Teflon and Polyacrylonitrile? (3 Marks)
Ans. Polythene, Teflon and polyacrylonitrile are synthetic (man-made) polymers formed by the process of polymerization. If a single type of monomeric unit is used in the process of polymerization, then the resultant polymer is known as a homopolymer. Polythene, Teflon and polyacrylonitrile are all examples of homopolymers.
Ques. Explain the uses of Teflon. (3 Marks)
Ans. Teflon is a very good electrical insulator and has an extremely low coefficient of friction. Due to its unique properties, it finds a wide range of applications. It is an excellent material for making valves and the lining of parts exposed to the corrosive environment. Due to the low coefficient of friction, its surface is somewhat waxy and prevents substances from being on it. Therefore, it is used in making non-stick kitchen utensils and in electric irons.
Ques. Define Plastic Deformation. (1 Mark)
Ans. When a thermoplastic is subjected to heat, it gets deformed. This property of thermoplastics is known as plastic deformation.
Ques. What are the types of Polymers? (3 Marks)
Ans. There are three types of polymers mainly-
- Natural Polymers: They are obtained from plants and animals such as starch, cellulose and proteins etc.
- Semi-Synthetic Polymers: They are obtained by using natural polymers but by synthetic methods such as rayon.
- Synthetic Polymers: They are man-made polymers such as polyethene, Teflon and polyacrylonitrile etc.
Ques. What are the two types of polythene? (3 Marks)
Ans. There are two main types of polythene–
- Low-density Polyethene: This kind of polythene is prepared by polymerization of ethene under high pressure (1000–2000 atm) in presence of traces of dioxygen or peroxide initiator or catalyst at a temperature of 350K – 570K.
- High-density Polyethene: This kind of polythene is prepared by polymerization of ethene under low pressure (6 - 7atm) in presence of titanium tetrachloride and triethyl aluminium and initiator or catalyst at a temperature of 333K – 343K.
Ques. Briefly discuss the main steps involved in the addition polymerisation reaction. (3 Marks)
Ans. The general mechanism of addition polymerisation involves the following three steps.
- Initiation: This step involves the formation of an active centre.
- Propagation: In this step, a monomer adds to the chain and each new monomer unit creates an active site for the next attachment.
- Termination: This step involves the removal of the active centre from the final active polymer unit.
Ques. What are the uses of Polyacrylonitrile? (3 Marks)
Ans. The uses of Polyacrylonitrile are as follows:
- Polyacrylonitrile is used as an alternative for wool.
- It is used as a fibre in reinforced concrete, and boat sails.
- It is mainly used in bicycles, tents, aircraft, rockets and components to offer insulation.
- It is an important material for fireproof clothes.
- It also acts as an electrolyte separator in batteries.
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