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. Infect, polymers are the foundation of four major industries namely plastics, elastomers, fibres, paints and varnishes. Polythene, Teflon and polyacrylonitrile are polymers and 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 polymer but by synthetic method. Synthetic polymers are man made polymers such polyethene, Teflon and polyacrylonitrile etc.
The process of polymerization can be divided into mainly two types –
Addition or Chain growth polymerization
Condensation or step growth polymerization
Polythene, Teflon and Polyacrylonitrile are formed by addition or chain growth polymerization. So, in this article we will discuss mechanism of addition or chain growth polymerization in detail with preparation methods of polythene, Teflon and polyacrylonitrile.
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 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
As discussed, addition polymerization takes place by chain growth and chain growth takes place by 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. Free radical mechanism is governed by following three steps –
Chain initiation step
Chain propagation step
Chain terminating step
We are explaining free radical mechanism by using an example of formation of polyethene. Polyethene is formed by addition polymerization of ethene (alkene) in presence of heat or exposing to light and catalyst benzoyl peroxide by free radical mechanism. Consolidated reaction is given below –
Chain initiation step – The addition polymerization process by free radical mechanism starts with formation of phenyl free radical by benzoyl peroxide. Phenyl radical reacts with ethene and pi- bond of ethene gets break thus leads to the formation of new and larger free radical which initiates the chain reactions. This step is called the chain initiation step.
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 if carried forward. This step is called the chain propagation step.
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 formation of a polymer product so it is called 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 –
Polythene are polymers of monomer unit ethene. These are linear or slightly branched long chain molecules formed by free radical mechanism. These are thermoplastic polymers.
There are two types of polythene –
Low density polyethene
High density polyethene
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 temperature of 350K – 570K. Reaction is given below –
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 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 electricity wires, toys, pipes, and manufacturing of squeeze bottles.
High density polythene – It is prepared by polymerization of ethene under low pressure (6 - 7atm) in presence of triethylaluminium and titanium tetrachloride (Ziegler – Natta catalyst) initiator or catalyst at temperature of 333K – 343K. Reaction is given below –
Abbreviated form of high - density polythene is HDP. It consists of linear molecules and possesses high density due to close packing or compact structure. These are also called linear polymers. Its linear structure is given below –
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.
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.
It is manufactured by heating tetrafluoroethene with peroxide or ammonium persulfate catalyst at high pressures. Reaction is given below –
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 production of carbon fiber composites and fiberglass composites.
It is used in non – stick utensils.
It is manufactured by addition polymerization of acrylonitrile in presence of a catalyst such as peroxide. Reaction is given below –
Uses of Polyacrylonitrile – It has various uses few of them are listed below –
It is used as a substitute of wool.
It is used for military purposes.
It is used in aircrafts, 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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