Addition Polymerisation or Chain Growth | Chemistry for JEE Main & Advanced PDF Download

(3) Addition polymerisation

(i) Polymers which are formed by addition reaction are knowns as addition polymers.

(ii) If monomer is ethylene or its derivative, then addition polymer is either linear polymer or branch-chain polymer.

Examples are : polystyrene, polytetrafluoroethylene, polyacrylonitrile etc.

(iii) If monomer is 1, 3-butadiene or 2-substituted-1, 3-butadiene, then polymer is always branched chain polymer.

(iv) Addition polymers retain all the atoms of the monomer units in the polymer.

(v) Addition polymerization takes place in three steps:

Initiation, chain propagation and chain termination.

(vi) Addition polymers are called as chain growth polymers.

Types of Addition Polymerization:

(A) Radical Polymerisation:

(i) Radical polymerisation takes place in the presence of radical initiators. The radical initiator may be any of the following :

(ii) Reaction intermediate of radical polymerization is a free radical.

(iii) Radical polymerization has more chance for those monomers whose free radicals are more stable.

Examples are:

(iv) Radical polymer has linear as well as branched chain structure. Most of the commercial addition polymers are vinyl polymers obtained from alkenes and their derivatives.  

This type of polymerization is performed by heating the monomer with only a very small amount of the initiator or by exposing the monomer to light. The general mode of radical polymerization of vinyl monomers is depicted below:

Chain initiation step:

Chain propagating step :

Chain terminating step :

In vinylic polymerization, various other reactions of free radicals with some other compounds present may compete with the parent addition chain reactions. One such reaction takes place with molecules that can react with the growing chain to interrupt the further growth. This leads to the lowering of the average molecular mass of the polymer. Such reagents are called as chain transfer agents and include CCl₄, CBr₄ etc.

For example, in the presence of CCl₄, styrene polymerises to form polystyrene of a lower average molecular mass which also contains some chlorine. What happens here is that growing polystyrene radical which normally would add on a monomer reacts with the chain transfer agent to end the original chain and produces a new radical. The latter initiates a new polymerization chain and thereby forms a new polymer as depicted below.

If the chain transfer agent a radical, which is highly unreactive, the reaction chain gets terminated such a compound thus inhibits or arrests polymerisation. Many amines, phenols, quinones etc. act as inhibitors. So, even traces of certain impurities, which can act as chain transfer agent or an inhibitor can interfere with the original polymerisation chain reaction. Hence, the monomers should be free from such inhibitors.

In case the alkene is a diene, the following kinds of polymerisation is possible:

(1) 1, 4-polymerisation-

When the polymerisation takes place at C₁ and C₄ of butadiene, an unbranched polymer is formed. This product is different from that formed from an alkene having a double bond, which at each of its carbons is substituted by different groups and hence can exist either as trans-polybutadiene or cis-polybutadiene or a mixture as shown below:

(2) 1, 2-polymerisation-

Alternatively, 1, 3-butadiene can undergo polymerization at C₁ and C₂ to yield the polymeric product, polyvinyl polythene.

The double bonds in these initial polymers can be linked by further treatment with chemicals to modify the properties of the polymers. These reactions form the basis of the formation of rubber.

(B) Cationic Polymerisation :

(i) Polymerisation which is initiated by an electrophile is known as cationic polymerisation.

(ii) Reaction intermediate of cationic polymerisation is a carbocation.

(iii) Carbocations can undergo rearrangement leading to the formation of a more stable carbocation.

(iv) The electrophile commonly used for initiation is BF₃.OEt₂.

(v) Monomers that are best able to undergo polymersation by a cationic mechanism are those with electron - donating substituents that can stabilise the carboncation. Some examples are:

(v) It is terminated by a base.

Thus, when the initiator is cationic in nature, it would generate a cationic intermediate on addition to the double bond for propagating the addition chain process and is termed as cationic addition polymerisation. The process is initiated by an acid. The stages of polymerisation are depicted below.

Chain initiation step :

Chain propagating step :

Chain terminating step :

Cationic polymerisation is facilitated in monomers containing electron - releasing groups. Thus, isobutylene undergoes cationic polymerisation easily as it has two electron releasing -CH₃ groups that will stabilize the intermediate carbocation.

(C) Anionic Polymerisation:

(i) Anionic polymerisation takes place in the presence of base or nucleopile, which is initiator in this polymerisation.

(ii) Reaction intermediate in propagation steps are carboanion.

(iii) The suitable initiator can be NaNH₂ or RLi.

(iv) Those monomers undergo anionic polymerisation reaction whose anion is stable.

Example of monomers are:

(v) Anionic polymerisation always give linear polymer.

(iv) Anionic polymerisation is terminated by an acid.

The formation of polystyrene from styrene in the presence of potassium amide is an important example of this category of polymerization. The mode of anionic polymerization is depicted below:

Chain initiation step :

Chain propagating step :

Chain terminating step :

(D) Ziegler- Natta polymerisation :

(i) Addition polymerisation which takes place in the presence of Ziegler- Natta catalyst [(C₂H₅)₃Al and TiCl₄] is known as Ziegler- Natta polymerisation or co-ordination polymerisation.

(ii) Ziegler- Natta polymerisation always gives linear, stereo-regular polymers.

(iii) Ziegler- Natta catalyst revolutionised the field of polymer chemistry because they allow the synthesis of stronger and stiffer polymers (due to linear geometry) that have greater resistance to cracking and heat. High density polyethylene is prepared using a Ziegler- Natta catalyst.

CLASSIFICATION BASED ON INTERMOLECULAR FORCES (SECONDARY FORCES)

(i) Intermolecular forces present between polymeric chains are (a) Van der Waals forces (b)Hydrogen bonds and (c) Dipole - dipole attractions.

(ii) Mechanical properties such as tensile strength, elasticity, toughness etc. depend upon the secondary force present between the polymeric chains.

(iii) Magnitude of secondary forces depend upon the size of the molecule and the number of functional groups along the polymeric chains.

Magnitude of secondary forces is directly proportional to the length of the polymeric chain. On the basis of magnitude of secondary forces, polymers can be divided into the following five categories:

(1) Elastomers-

An elastomer is a plastic that stretches and then reverts back to its original shape. It is randomly oriented amorphous polymer. It must have some cross-links so that the chains do not slip over one another. Very weak Van der Waal's forces are present in between polymeric chains.

When elastomers are stretched, the random chains stretch out, but there are insufficient Van der Waal forces to maintain them in that configuration and position. When the stretching force is removed, they go back to their random shape. Elastomers have the ability to stretch out over ten times their normal length.

Important examples are vulcanized rubbers.

Note : Addition polymers obtained from butadiene and its derivatives are elastomers.

(2) Fibres-

Fibres are linear polymers in which the individual chains of a polymer are held together by hydrogen bonds and/or dipole-dipole attraction. In the fibres, the polymeric chains are highly ordered with respect to one another.

Due to strong intermolecular forces of attraction and highly ordered geometry, fibres have high tensile strength and least elasticity. They have crystalline character and have high melting points and low solubility. Examples are cellulose, nylon, terylene, wool, silk etc.

Note: (i) Condensation polymers formed from bifunctional monomers are fibres in character.

(ii) Addition polymers of alkene derivatives having strong-I group are fibres in character.

(3) Thermoplastic Polymers-

Thermoplastic polymers are polymers that have both ordered crystaline regions (the regions of the polymer in which the chains are highly ordered with respect to one another) and amorphous, non crystalline regions (the regions of the polymer in which the chains are randomly oriented).

The intermolecular forces of attraction are in between elastomers and fibres. There are no cross-links between the polymeric chains. Thermoplastic polymers are hard at room temperature, but when they are heated, the individual chains can slip past one another and the polymer become soft and viscous. This soft and viscous material become rigid on cooling. The process of heating softening and cooling can be repeated as many times as desired without any change in chemical composition and mechanical properties of the plastic. As a result, these plastics can be moulded into toys, buckets, telephone and television cases.

Some common examples are: polyethene polypropylene, polystyrene, polyvinylchloride, teflon etc.

Note : Addition polymers obtained from ethylene and ethylene derivatives are thermoplastic polymers.

(4) Thermosetting Polymers-

Polymers which become hard on heating are called thermosetting polymers. Thermosetting polymers can be heated only once when it permanently sets into a solid, which cannot be remelted by heating.

Thermosetting polymers are cross-linked polymers. Greater the degree of cross- linking that exist, the more rigid is the polymer. Cross-linking reduces the mobility of the polymer chains, causing them to be relatively brittle materials, the hardening on heating is due to the extensive cross-linking between different polymer chains to give a three dimensional network solid. Examples are : phenol formaldehyde resin, urea-formaldehyde resin, melamine- formaldehyde resin.

DIFFERENCE BETWEEN THERMOPLASTIC AND THERMOSETTING POLYMERS