[Chemistry Class Notes] on Preparation of Polythene, Teflon and Polyacrylonitrile Pdf for Exam

We use many polymers in our daily life such as polythene is used in packaging and carry bags, coatings of non-stick utensils are composed of Teflon and polyacrylonitrile we use as a substitute for wool. The importance of polymers is increasing day by day. Its various uses have completely revolutionized our daily life as well as the industrial scenario. 

 

Infact, polymers are the foundation of four major industries namely plastics, elastomers, fibres, paints and varnishes. Polythene, Teflon and polyacrylonitrile are polymers formed by the process of polymerization. Polymers are generally very large molecules with high molecular mass, so these are also called macromolecules. 

 

The term ‘Polymer’ is made up of two Greek words – Poly (Polus) and mer (meros) poly means many and mer means unit or part. Polymers can be defined as those macromolecules which are composed of many repetitive units or parts. These repeated units are called monomers. The process of formation of polymers from respective monomers is called polymerization. 

 

Polymers can be of mainly three types – Natural polymers, Semi-Synthetic polymers and Synthetic polymers. Natural polymers such as starch, cellulose and proteins etc. are obtained from plants and animals. Semi-Synthetic polymers such as rayon are obtained by using natural polymers but by synthetic methods. Synthetic polymers are man-made polymers such as polyethene, Teflon and polyacrylonitrile etc. 

 

The process of polymerization can be divided into mainly two types –

 

Polythene, Teflon and Polyacrylonitrile are formed by addition or chain-growth polymerization. So, in this article, we will discuss the mechanism of addition or chain-growth polymerization in detail with preparation methods of polythene, Teflon and polyacrylonitrile. 

 

What is Addition Polymerization? 

The polymerization in which the molecules of the same monomer or different monomers add together on a large scale to form a polymer is called addition polymerization. The polymers obtained by addition polymerization are called addition polymers. If in the polymerization process only a single type of monomeric unit is used, then the formed polymer is called a homopolymer. 

 

For example, polythene, Teflon and polyacrylonitrile are homopolymers. While those addition polymers which are formed by polymerization of two different monomer units are called copolymers. In addition – polymerization, unsaturated compounds such as alkenes, alkadienes etc. are used as monomer units. This type of polymerization leads to chain growth through the formation of free radicals or ionic species. That’s why it is also called chain-growth polymerization. 

 

Examples of Addition polymerization – Formation of homopolymer – Polyolefins –

n RCH=CH2 ? [RCH-CH2]n

Formation of Copolymer – Polyethylene glycol 

HOCH2CH2OH + n C2H4O ? HO(CH2CH2O)n+1H

 

Mechanism of Addition Polymerization

As discussed, addition polymerization takes place by chain growth and chain-growth takes place by the formation of free radicals and ionic species. Most commonly addition polymerization is governed by free radical formation. 

 

Addition polymers namely Polythene, Teflon and polyacrylonitrile are formed by a free radical mechanism of addition polymerization. So, we are describing here the free radical mechanism of addition polymerization.

 

Free Radical Mechanism 

By free radical mechanism, many alkenes and their derivatives are polymerized in presence of catalysts such as benzoyl peroxide, acetyl peroxide, t-butyl peroxide etc. 

 

The free radical mechanism is governed by following three steps –

• Chain initiation step 

• Chain propagation step 

• Chain terminating step 

 

We are explaining the free radical mechanism by using an example of the formation of polyethene. Polyethene is formed by the addition polymerization of ethene (alkene) in presence of heat or exposure to light and catalyst benzoyl peroxide by a free radical mechanism. Consolidated reaction is given below –

• Chain Initiation Step – The addition polymerization process by free radical mechanism starts with the formation of phenyl free radical by benzoyl peroxide. Phenyl radical reacts with ethene and the pi-bond of ethene breaks thus leading to the formation of new and larger free radicals which initiates the chain reactions. This step is called the chain initiation step. 

 

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• Chain Propagation Step – Now this large and new free radical formed in the initiation step, propagates the chain reaction ahead. It reacts with another molecule of ethene and forms a larger free radical which again with another molecule of ethene and forms a bigger sized free radical. Thus, reaction is carried forward. This step is called the chain propagation step. 

 

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• Chain Termination Step – At the end, at some stage, the free radical formed in the propagation step reacts with another radical to form the polymerized product. As the chain reaction terminates in this step by the formation of a polymer product. so, it is called the chain termination step. For termination of the long-chain free radicals can combine in many ways to produce polythene. One mode of its chain termination is given below –

 

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Preparation of Polythene 

Polythene is polymers of monomer unit ethene. These are linear or slightly branched long-chain molecules formed by a free radical mechanism. These are thermoplastic polymers. Polyethylene, also known as polythene, is a lightweight, flexible synthetic plastic derived from the polymerization of ethylene. 

 

Polyethylene is a type of polyolefin resin found in the family of primary polyolefin resins. It is the most commonly used plastic on the planet, with uses ranging from clear food wrap and shopping bags to detergent bottles and vehicle fuel tanks. It can also be sliced or spun into synthetic fibres, or given rubber-like elasticity.

 

There are two types of polythene –

 

Low-Density Polythene – It 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. The rea
ction is given below –

 

The abbreviated form of low-density polythene is LDP. It is a highly branched structure. It is obtained by free-radical addition and hydrogen atom abstraction. LDP have a straight-chain structure with some branches as shown below –

 

Uses of Low-Density Polythene – Low-density polythene is inert in nature, tough but flexible and a poor conductor of electricity. Due to these properties, it is very useful in many fields. It is used in electrical wires, toys, pipes, and the manufacturing of squeeze bottles. 

 

High-Density Polythene – It is prepared by polymerization of ethene under low pressure (6 – 7atm) in presence of triethyl aluminium and titanium tetrachloride (Ziegler – Natta catalyst) initiator or catalyst at a temperature of 333K – 343K. The reaction is given below –

 

[n CH_2 = CH_2 ] [:xrightarrow[6 – 7 atm, catalyst]{333K – 343K}: ][[CH_2 – CH_2]_n] HDP

 

The abbreviated form of high-density polythene is HDP. It consists of linear molecules and possesses high density due to the close packing of compact structure. These are also called linear polymers. Its linear structure is given below –

 

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Uses of High-Density Polythene – These are also chemically inert but tougher and harder than low–density polythene. Due to these properties, it is used in manufacturing buckets, dustbins, bottles, and pipes etc. 

 

Uses of Polythene

• Polythene with a lower density can deform without breaking and provides superior elongation, stretching up to six times its original length before breaking.

• It’s a versatile plastic that can be moulded and extruded into a variety of shapes, including bottles, sheets, and pipes.

• Because of its clear and crystalline nature, it is utilized for plastic bags and stretch films; its life expectancy outside is reported to be 25 years, but it degrades when exposed to ultraviolet radiation.

• Polythene has higher chemical resistance and can withstand a wide variety of chemicals.

 

Preparation of Teflon 

Teflon is a synthetic fluoropolymer of tetrafluoroethylene which is also called polytetrafluoroethylene. It is a well-known brand name of PTFE (polytetrafluoroethylene) by Chemours. It is a thermoplastic polymer. It maintains high strength, toughness and self–lubrication at low temperatures down to 5K. It possesses fairly high heat resistance. Its melting point is 327℃. It is almost chemically inert and highly insoluble in most solvents. It is resistant to attack by corrosive reagents. 

 

At temperatures exceeding 194 K (79 °C; 110 °F), its mechanical characteristics decline gradually. PTFE is primarily composed of carbon-fluorine bonds, and it derives its qualities solely from the bonds formed. The only compounds that can change its property are alkali metals and the most highly reactive fluorinating agents.

 

It possesses the third-lowest coefficient of friction of any known solid substance, ranging from 0.05 to 0.10. It possesses one of the most effective dielectric characteristics.

 

It is manufactured by heating tetrafluoroethene with peroxide or ammonium persulfate catalyst at high pressures. Reaction is given below –

 

[n F_2C = CF_2 rightarrow -(F_2C-CF_2)_n -]

 

Uses of Teflon

It has numerous uses in various fields. Few uses of Teflon are listed below –

• 50% of its production is consumed in making insulation wiring for aerospace and computer application. 

• It is used in plain bearings, gears, side plates, seals, brushings etc. 

• It is used in permanent magnets.

• It is widely used in the production of carbon fibre composites and fibreglass composites. 

• It is used in non – stick utensils.

• With a pressure-sensitive adhesive backing, it is commonly used as a film interface patch for sports and medical applications.

• It’s usually used to coat in non-stick frying pans since it can withstand high temperatures. It is employed in medical syntheses, tests, and many other drugs. It is installed in one of the high friction locations of footwear, insoles, and ankle-foot orthosis.

 

Preparation of Polyacrylonitrile 

The abbreviation of polyacrylonitrile is PAN. It is also known as polyvinyl cyanide and Creslan 61. It is a polymer of acrylonitrile. It is a semi-crystalline synthetic polymer. It is a thermoplastic polymer. Although it melts above 300℃. It is insoluble in water. It degrades before boiling and melting. 

 

The repeat unit of PAN has a molecular weight of 53.06 g/cm3 and an amorphous density of 1.184 g/cm3. Because PAN discolours when exposed to temperatures above 175 degrees Celsius, it demonstrates that it has excellent thermal stability. It also protects products from insect attack because Acrylic is never attacked by insects, moths, or mildew.

 

It is manufactured by addition polymerization of acrylonitrile in presence of a catalyst such as a peroxide. The reaction is given below –

 

[n H_2C = CHCN rightarrow  −(H_2C−CHCN)_n− ]

 

Uses of Polyacrylonitrile

It has various uses few of them are listed below –

• It is used as a substitute for wool.

• It is used to manufacture knitted clothing such as socks and sweaters because it is an excellent fibre at a particular temperature; it is also used as a fibre in reinforced concrete, boat sails, and other applications.

• It is used for military purposes. 

• It is used in aircraft, bicycles, tents, rockets and components to provide insulation. 

• It is the principal material of fireproof clothes. 

• It is used as an electrolyte separator in batteries.  

 

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