Chemistry Types of Polymerisation Reactions : Addition Polymerisation
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Topics Covered :

● Types of Polymerisation Reactions
● Addition Polymerisation or Chain Growth Polymerisation
● Free Radical Mechanism
● Preparation of Some Important Addition Polymers

Types of Polymerisation Reactions :

There are two broad types of polymerisation reactions, i.e., the addition or chain growth polymerisation and condensation or step growth polymerisation.

Addition Polymerisation or Chain Growth Polymerisation :

`=>` In this type of polymerisation, the molecules of the same monomer or diferent monomers add together on a large scale to form a polymer.

`=>` The monomers used are unsaturated compounds, e.g., alkenes, alkadienes and their derivatives.

`=>` This mode of polymerisation leading to an increase in chain length or chain growth can take place through the formation of either free radicals or ionic species.

`=>` The free radical governed addition or chain growth polymerisation is the most common mode.

Free radical mechanism :

`=>` A variety of alkenes or dienes and their derivatives are polymerised in the presence of a free radical generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc.

`color{red}("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 process starts with the addition of phenyl free radical formed by the peroxide to the ethene double bond thus generating a new and larger free radical. This step is called chain initiating step.

● As this radical reacts with another molecule of ethene, another bigger sized radical is formed. The repetition of this sequence with new and bigger radicals carries the reaction forward and the step is termed as chain propagating step.

● At some stage the product radical thus formed reacts with another radical to form the polymerised product. This step is called the chain terminating step.

● The sequence of steps may be depicted as shown in fig.

Preparation of some important addition polymers :

(a) `color{green}("Polythene ")` :There are two types of polythene as given below :

(i) `color{green}("Low Density Polythene ")` : It is obtained by the polymerisation of ethene under high pressure of `1000` to `2000` atmospheres at a temperature of `350 K` to `570 K` in the presence of traces of dioxygen or a peroxide initiator (catalyst).

● The low density polythene (LDP) obtained through the free radical addition and `color{red}(H)`-atom abstraction has highly branched structure.

● Low density polythene is chemically inert and tough but flexible and a poor conductor of electricity.

`color{green}("Uses ")` : In the insulation of electricity carrying wires and manufacture of squeeze bottles, toys and flexible pipes.

(ii) `color{green}("High Density Polythene ")` : It is formed when addition polymerisation of ethene takes place in a hydrocarbon solvent in the presence of a catalyst such as triethylaluminium and titanium tetrachloride (Ziegler-Natta catalyst) at a temperature of `333 K` to `343 K` and under a pressure of `6-7` atmospheres.

● High density polythene (HDP) thus produced, consists of linear molecules and has a high density due to close packing.

● It is also chemically inert and more tougher and harder.

`color{green}("Uses ")` : It is used for manufacturing buckets, dustbins, bottles, pipes, etc.

(b) `color{green}("[Polytetrafluoroethene (Teflon) ]")` : Teflon is manufactured by heating tetrafluoroethene with a free radical or persulphate catalyst at high pressures.

● It is chemically inert and resistant to attack by corrosive reagents.

● Teflon coatings undergo decomposition at temperatures above `300°C`.

`color{green}("Uses ")` : It is used in making oil seals and gaskets and also used for non-stick surface coated utensils.

`color{red}(undersettext(Tetra fluoroethene)(n CF_2 = CF_2) undersettext(High pressure) overset(Catalyst)→ undersettext(Teflon)(-[- CF_2-CF_2 - ]_n))`

(c) `color{green}("Polyacrylonitrile ")` : The addition polymerisation of acrylonitrile in presence of a peroxide catalyst leads to the formation of polyacrylonitrile.

`color{red}(undersettext(Acrylonitrile )(n CH_2 = CHCN) undersettext(Polymerisation) oversettext(Peroxide catalyst)→ undersettext(Polyacrylonitrile)(-[-CH_2- overset(overset(CN)(|))CH-]_n))`

● Acrylic fibres have good resistance to stains, chemicals, insects and fungi.

`color{green}("Uses ")` : Polyacrylonitrile is used as a substitute for wool in making commercial fibres as orlon or acrilan.