A. Chain Polymerization
• The polymerization in which polymer is formed through the rearrangement of chemical bond of unsaturated organic compound by chain reaction mechanism is known as chain polymerization. In chain polymerization polymer is formed by the major three steps such as initiation, propagation and termination. In this case reaction is occurred very rapidly within a few second. During polymerization small molecules such as H2O, HCl, NH3 etc. are not eliminated and any kind of bi-product is not produced. Monomers that undergoing in chain polymerization most of them are in vinyl compound category so chain polymerization is also called vinyl polymerization.Monomers used in chain polymerization
a) Free radical
• Free radical is an unpaired electron which initiate the polymerization. When energy such as heat, light or catalyst is applied to the initiators, then they are homolytically decomposed and produced free radical. If initiator is expressed by R:R,Then free radical is produced as follows:-
Initiators are thermally unstable chemical compounds which produce free-radical for the initiation of polymerization. When energy such as heat, light, catalyst or radiations are applied to the initiators, then they are homolytically decomposed and produce free radical which initiate the polymerization. If initiator is expreesed by R:R, Then free radical will be produced as follows:-
Most of the low molecular weight compound having azo group, hydro per oxides, per acides and per esters group are widely used as initiator.
Examples of initiator:
i) Azo-bis-diphenyl methane
ii) Benzoyl per oxide
iv) Per benzoic acid
v) Per acetic acid
STEPS OF FREE RADICAL POLYMERIZATION:
1) 1. INITIATION:
• Free radical polymerization is occurred between monomer and initiator. When energy such as heat/light/catalyst is applied to the initiator, then it is homolytically decomposed and produce free radical. At first the free radical attacks the double bond of a monomer molecule and break down. Then it add to the free radical and produce active free radical. It is called initiation. It is also called birth of polymerization. If monomer is M and free radical is R. , then initiation will be as follows:
• The active free radical obtained from initiation step attacks the double bond of the next monomer molecule and break down. Then it add to the active free radical and produce a new active free radical. This process is continued to produce a large growing chain of a polymer .It is called propagation. It is also called growth of polymer.
There are several modes of addition in propagation of vinyl polymerization:
i) Heat to Heat
ii) Tail to Tail
iii) Heat to Tail
iv) Tail to Heat
• In propagation step polymer growing chain will be large. Once two large growing chain come to each other and polymerization will be stopped. It is called termination. It is also called death of polymer. In free radical polymerization termination is occurred by the following two ways:
a) Termination by coupling mechanism:
• In this case two large growing polymer chain come to each other and termination is occurred by the combination of activity of their free radical through the mutualisation to each other.
b) Termination by disproportionation
• In this case hydrogen atom of a growing polymer chain is transferred to other growing polymer chain. As a result unsaturated end- group is produced in the losing growing polymer chain. e.g.:-
• Inhibitor: The chemical substances which are used in killing the growth of polymer chain by combining with the active free radicals and forming either stable products or inactive free radical is called inhibitor. Hydroquinone, Nytrobenzene, Dinitrobenzene and Benzothiazine are mostly used as inhibitors.
• Retarder: The material which slow down the growth of polymer chain is called retarder.It also decreases molecular weight of polymer. e.g. Dinitrobenzene.
Chain transfer agent: The compound which is used in chain transfer reaction is called C.T.A. If the C.T.A is present in large quantity, then the DP of produced polymer will be very low. Such polymer is called Telomer.
Examples of C.T.A are given bellow: CCl4, CBr4, H2 gas etc.
b1. Cat-ionic polymerization:
In this case a catalyst and a co-catalyst are used in polymerization. These together generate a carbonium ion which initiate the polymerization.
Acid compounds are used as catalysts such as H2SO4, CH3COOH, AlCl3, AlBr3, TiCl4, BF3COOH, BF3 etc.
Most common co-catalyst is H2O.As the above catalysts can not active willingly, so water is applied as a co-catalyst to active them.
Conditions of cat-ionic polymerization:
• Cat-ionic polymerization is occurred in the following conditions:
• i) In a medium of low electric constant.
• ii) In low temperature.
• iii) Using a co-catalyst (say water) with catalyst.
• Iv) Polymer formation occurs extremely fast.
Steps of cat-ionic polymerization:
• 1) INITIATION: At first catalyst reacts with co-catalyst and produce active catalyst. Then positive ion of active catalyst is transferred to monomer and generate a carbonium ion. It is called initiation.
• In this case polymerization is stopped through the losing positive charge of growing polymer chain. It is called termination. Termination is occurred by the following two ways:
• i) Termination is occurred by the rearrangement of cat-ion and an-ion, resulting unsaturated end group containing polymer is produced. As a result original complex acid is formed. Reaction is given bellow:
• ii) Termination is occurred by the addition of cat-ion and an-ion in such a way, so that original catalyst is released.
b2. An-ionic polymerization:
• In this case carbon ion acts as a chain carrier.
• Mostly used monomers in an-ionic polymerization are as follows:
Iniators/Catalysts widely used in an-ionic polymerization:
• Organic alkali compounds-àSodium nephthaline
• Alkali metal amine à NaNH2/ KNH2
• Alkoxides àRONa
• Grignard reagent àRMgX etc.
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