What is Polymerization ???

A reaction in which small molecules (monomers) combine together to form a big molecule(polymer) is

called polymerization .[1]

1. Classification of polymerization:

i. Step growth polymerization.[1]

ii. Chain growth polymerization.[1]

iii. Miscellaneous polymerization reactions.[3]

Different Monomers

polymerization

Polymer

fig.1

1.1 Step growth polymerization:This involves a random reaction of two molecules that may be any combination of a monomer or a longer chain

molecule. High molecular weight polymer is formed only near the end of polymerization when most of the

monomer has been depleted. [1]

It has two types.

i. Addition polymerization.

ii. Condensation polymerization.

iii. Ring opening polymerization.

1.1 (i) Addition Polymerization:

When a number of monomers undergo self-combination, resulting substance has a molecular mass many

times larger than the monomer, the type of polymerization is known as “ Addition polymerization”.

i. The empirical formula of addition polymer and the monomer is same.

ii. These reactions are catalyzed by peroxides or acids.

iii. Generally these reactions required a pressure of 100 atmosphere at 200oC.

iv. Examples: polyethylene, polypropylene, P.V.C(Poly Vinyl Chloride), Teflon. [5]

fig.1.1(i)

1.1 (ii) Condensation polymerization:

Condensation polymers are formed by the combination of monomers with the elimination of simple

molecules such as H2O or CH3OH.

There are two types of Condensation polymers:

1. Polyester.

2. Polyamide.

Examples: Nylon, terylene, Bakelite. [5]

fig.1.1(ii)

1.1 (iii) Ring opening polymerization:

The monomers containing ring structure can be polymerized by this method but monomers open up

their rings before getting polymerized. [1]

fig.1.1(iii)

1.2 Chain growth polymerization:

In this polymerization the only chain extension reaction is that of attachment of a monomer to an

“active” chain. The active end may be free radical or an ionic site (i.e., anion or cation). In contrast to

step growth polymerization, high molecular weight polymer is formed in the early stages of a chain

growth polymerization. [1]

i. Free radical polymerization.

ii. Ionic polymerization and co-polymerization.

iii. Co-ordination polymerization.

1.21 Free radical polymerization:

It involves three main steps.

i. Initiation of active monomer.

ii. Propagation of the active (free radical) chain by sequential addition of monomer.

iii. Termination of active chain to give final polymer product. [2]

fig.1.21(i)

ii. Propagation:

Additional monomer units are attached to initiated monomer species as

fig.1.21(ii)

iii. Termination:

It can occurs in three steps. Termination by

i. Chain combination.

ii. Disproportionation.

iii. Chain transfer.

i. Termination by combination:

By combining two active radical chains through covalent bond between them.

fig.1.21(iii)

ii. Termination by Disproportionation:

It gives two chains, one is fully saturated from its one end while other is unsaturated from its one end.

fig.1.21(iii)

iii. Termination by Chain transfer:

Here chain is terminated by hydrogen abstraction from an initiator, monomer or solvent molecule.

fig.1.21(iii)

1.22 (a) Ionic polymerization:

Ionic polymerization follow the same basic steps as free radical chain growth polymerization (i.e.,

initiation, propagation, termination); however, there are some differences. Either a carbanion (C-) or

carbonium (C+) ionic site can be formed in the initiation process. [2]

It follows two different mechanisms.

i. Cationic mechanism.

ii. Anionic mechanism.

i. Cationic mechanism:

This mechanism is accompanied by using an electron withdrawing group as initiator. Initiator

employs a true catalyst that is restored at the end of polymerization and does not become incorporated into

terminated polymer chain. Any strong Lewis acid like BF3 can be used as catalyst with a co-catalyst like

water which is a true proton source. [2]

ii. Anionic mechanism:

This mechanism is accompanied by using an electron donating group as initiator. Initiator may be any

strong nucleophile, including Grignard reagents and other organo-metallic compounds like n-butyl

lithium.[2]

1.22(b) Ionic co-polymerization:

As in the free radical polymerization two or monomers can be polymerized by an ionic mechanism.

For example , the commercial elastomer of polyisobutylene is a copolymer obtained by the cationic

copolymerization of isobutylene with 0.5% to 2% of isoprene. [2]

1.23 Co-ordination polymerization:

This polymerization involves formation of co-ordinate covalent bond. In the first step, monomer forms

a complex with organomettalic compound through a coordinate covalent bond. This bond acts as an active

site where other monomers get attached. As shown below: [1]

fig.1.23

1.3 Miscellaneous Methods of Polymerization:

The polymerization reaction may be carried out in the solid phase, liquid phase and gas phase. Most

of the commercial polymers are prepared in the liquid phase. Liquid phase polymerization may be subdivided

into four categories as: [3]

i. Bulk Polymerization.

ii. Solution Polymerization.

iii. Suspension Polymerization.

iv. Emulsion Polymerization.

i. Bulk Polymerization:This method involves heating the monomer or monomer mixtures without solvent with initiator in a

vessel. The monomer –initiator mixture then polymerizes to a solid shape fixed by the shape of

polymerization vessel. The main practical disadvantage is the removal of polymer from reactor and

dissipation of exothermicity of polymerization. [3]

ii. Solution polymerization:

In this method the reaction is carried out in the presence of a solvent. The monomer dissolved in a

solvent. A large number of initiators can be used in this process. The solvent facilitates the contact of

monomer and initiator and assist dissipation of exothermic heat of reaction. The disadvantage of this

process is the chain transfer due to solvent so that low molecular polymers are formed. [3]

iii. Suspension polymerization:This polymerization involves the polymerization in an aqueous system with monomer as a disperse

phase. The monomer is dispersed in a water in small droplets generally 0.01 to 0.5 cm in diameter

maintained by vigorous stirring. This process is quite good for the free radical polymerization of vinyl

polymer. [3]

iv. Emulsion polymerization:

In case of emulsion polymerization reaction carried out in presence of a surfactant. The surfactant may

be cationic, anionic or nonionic. Some of the surfactants commonly used are cetyltrimethyl ammonium

bromide (CTABr),sodium lauryl sulphate (NaLS) and trion-X etc. The monomer is dispersed in water

containing the initiator and the surfactant. The surfactant forms the micelles. This technique is extensively

used for the free radical polymerization of vinyl monomers containing water soluble initiators. [3]

Method of synthesis of PMMA

1. Suspension polymerization Method:

Suspension polymerization sometimes called pearl or bead polymerization can be considered to be mini bulk polymerization carried with the monomers dispersed in small droplets, typically 10 to 1000 µm in diameter. Droplets are suspended in medium in which the monomer is usually insoluble (water), by means of carefully controlled agitation and with the help of dispersing agent or colloid. Dispersing agent is a water soluble polymer generally poly vinyl acetate (PVA) or methyl cellulose in the aqueous phase, but an oil soluble dispersing agent may be added to the monomer phase. Dispersing agent prevents the aggregation of partially polymerized monomer particles at the stage when they are quite sticky. The size of the particles is controlled by the rate of agitation as well as amount and nature of dispersing agent used. [4]

Chemicals: Methyl methacrylate, benzoyl peroxide, cetyl dimethyl benzyl ammonium chloride, polyvinyl alcohol (PVA), water (medium). [4]

Procedure: 0.1g of benzoyl peroxide (initiator) is dissolved in 10 g of methyl metacrylate (monomer) contained in a 250 ml round bottom flask. To this, 40 ml of distilled water having 0.002 g cetyl dimethyl benzyl ammonium chloride (or any other suitable quaternary ammonium salt type emulsifier) and 0.2 g of polyvinyl alcohol (protective colloid) are added. The contents of the flask are flushed with nitrogen gas and 0.2 g of PVA (protective colloid) are added. The constant temperature bath maintained at 75±1’C. A medium stirrer is fitted and the contents are stirred uniformly. It can be seen at this stage that the monomer is dispersed in water phase as tiny droplets. The size of the droplets can be adjusted by controlling the stirrer speed. The experiment is continued for 8 hrs. At the end of which, fine spherical beads of PMMA settles at the bottom of the flask. The beads are filtered and washed several times with warm water to free them from the emulsifier / suspending agent. [4]

fig.1s Synthesis of PMMA by suspension polymerization method

1. Suspension polymerization Method:

Only water soluble polymer can be polymerized by this technique. The monomer is suspended in water,

in the form of fine droplets, which are stabilized by using a suitable water soluble protective colloids,

surface active agents and by stirring. The size of the monomer droplets formed depends on the monomer to

water ratio, nature of stabilizing agents and also on the type and speed of agitation employed. The initiator

should be monomer soluble. Polystyrene can also be prepared by suspension method, using monomer

soluble initiator. Styrene monomer can be suspended by using water as media. [4]

Chemicals: Styrene, polyvinyl alcohol (PVA-suspending agent), benzoyl peroxide (initiator), medium

(water), 1% NaOH.[2]

Procedure:

About 30 ml of styrene monomer is freed from stabilizer by washing three times 10 ml of 1% NaOH

solution followed by three times washing (20 ml each) with distilled water. The monomer is then dries over

anhydrous CaCl2. A mixture of 50 ml of distilled water and 1 g of PVA is taken in a 100 ml round bottom

flask and placed on a hot plate at 80 ± 20C. To the washed 10 ml of styrene, 0.5 g of benzoyl peroxide is

added into the round bottom flask by means of dropping funnel over a period of 15 minutes with constant

stirring. The reaction mixture in the water bath is stirred or shaken continuously for 1 period of 5 hours . The

prepared polystyrene (PS) pearls or beads are filtered off, washed with 2% HCl and water, then dried. Note

down the appearance and yield of PS. [4]

Method of synthesis of Polystyrene

fig.2s Synthesis of PS by suspension polymerization method

1. Polymer Science by V R Gowariker, N V Viswanathan, Jayadev Sreedhar

2. Polymer science and technology by Joel R. Fried

3. Textbook of polymer science by Padma L.Nayak, S.Lenka

4. Polymer synthesis Techniques by

5. http://www.citycollegiate.com/organic1.htm

References