• Monomers may be polymerized by the following methods

(1) polymerization in homogeneous systems

(2) polymerization in heterogeneous systems

Polymerization in Homogeneous systems:• The homogeneous polymerization techniques involve pure monomer or

homogeneous solutions of monomer and polymer in a solvent.

These techniques can be divided into two methods:

(i) the bulk and

(ii) the solution polymerizations

Bulk polymerization

• Bulk polymerization is the simplest technique and produces the highest-purity

polymers

• Only monomer, a monomer-soluble initiator are used

• This method helps easy polymer recovery and minimum contamination of product

• The viscosity of the mixture is low initially to allow ready mixing, heat transfer, and

bubble elimination, this method is used for the preparation of polyethene,

polystyrene, etc.

Disadvantages:

• Reaction medium becomes increasingly viscous as reaction goes to higher

conversion,making stirring, heat removal and processing more difficult

• It leads to uneven polymerization and loss of monomer, Free-radical polymerizations are

typically highly exothermic

• An increase temperature will increase the polymerization rate; generate heat dissipation

and a tendency to develop of localized “hot spots”

• Near the end of polymerization, the viscosity is very high and difficult to control the rate

as the heat is “trapped” inside

• It leads to the auto acceleration process in which the propagation rate is very higher than

that of termination rate, this method is seldom used in commercial manufacture

Solution polymerization:

• This method is used to solve the problems associated with the bulk polymerization

because the solvent is employed to lower the viscosity of the reaction, thus help in the

heat transfer and reduce auto acceleration

• It requires the correct selection of the solvents. Both the initiator and monomer be

soluble in each other and that the solvent are suitable for boiling points, regarding the

solvent-removal steps

• It is often used to produce copolymers, this method is used for the preparation of

polyvinyl acetate, poly (acrylic acid), and polyacrylamide

Advantages:

(i) Solvent has low viscosity, reaction mixture can be stirred

(ii) Solvent acts as a diluent and aids in removal of heat of polymerization

(iii) Solvent reduces viscosity, making processing easier

(iv)Thermal control is easier than in the bulk and

(v) “Cheap” materials for the reactors (stainless steel or glass lined)

Disadvantages:

(i) Reduce monomer concentration which results in decreasing the rate of the reaction and the

degree of polymerization

(ii) Mobility is reduced and this can affect termination events, so the rate of reaction is increased

(iii) Solvent may terminate the growing polymer chain, leading to low molecular weight polymers

(iv) Difficult to remove solvent from final form, causing degradation of bulk properties

(v) Clean up the product with a non solvent or evaporation of solvent

(vi) Small production per reactor volume

(vii) Not suitable for dry polymers

Polymerization in heterogeneous systems

• Polymerization occurs in disperse phase as large particles in water or

occasionally in another non-solvent (suspension polymerisation), or

dispersed as fine particles

• The last-named process is usually known as emulsion polymerisation

Suspension (Bead or Pearl) polymerisation

• Monomer, initiator (must soluble in monomer) and polymer must be insoluble in

the suspension media such as water i.e., the reaction mixture is suspended as droplets

in an inert medium

• Suspension polymerization consists of an aqueous system with monomer as a

dispersed phase and results in polymer as a dispersed solid phase

• This method is used for the preparation of polystyrene, polyvinyl chloride, polyvinyl

acetate, etc

• A reactor fitted with a mechanical agitator is charged with a water insoluble monomer and

initiator

• Droplets of monomer (containing the initiator) are formed, as the polymerization proceeds,

the viscosity of dispersed phase increases and they become sticky

• Aggregation of these sticky droplets is prevented by the addition of a dispersing agent

(protective colloid, e.g., water-soluble colloid such as gum acacia)

• Near the end of polymerization, the particles are hardened, are the bead or pearl shaped

polymers recovered by filtration, and followed by washing step

Advantages:

(i) Polymerisation to high conversion

(ii) Low viscosity due to the suspension

(iii) Easy heat removal due to the high heat capacity of water

(iv) Excellent heat transfer because of the presence of the solvent

(v) Solvent cost and recovery operation are cheap

(vi) Polymerization yields finely divided, stable latexes and dispersions to be used

directly in coatings, paints, and adhesives

Disadvantages

(i) Contamination by the presence of suspension and other additives low polymer

purity

(ii) Must separate and purify polymer, or accept contaminatedproduct

(iii) Reactor cost may higher than the solution cost

Emulsion polymerisation:

• An emulsion polymerization consists of

(a)Water (solvent or as the heat-transfer agent),

(b) Monomer,

(c) Initiator (is soluble in water and insoluble in the monomer),

(d) A surfactant or emulsifier (such as sodium salt of long-chain fatty acid)

• This method is used for the preparation of polyvinyl acetate, polychloroprene,

butadiene/styrene/acrylonitrile copolymers, etc

• A typical recipe for emulsion polymerization consists of water, monomer, fatty acid soap

(emulsifying agent), and water soluble initiator

• When a small amount of soap is added to water, the soap ionizes and the ions move around

freely, the soap anion consists of a long oil-soluble portion (R) terminated at one end by the

water-soluble portion, so emulsifier molecules arrange themselves into colloidal particles

called micelles

• In water containing a insoluble monomer molecule, the soap anion molecules orient

themselves at the water–monomer interfaces with the hydrophilic ends facing the water, while

the hydrophobic ends face the monomer phase

• When the water-soluble initiator undergoes thermal decomposition to form the water-

soluble radicals react with monomer dissolved in interior of the micelle

• Emulsion polymerization takes place almost exclusively in the micelles

• As polymerization proceeds, the active micelles consume the monomers within the micelle,

monomer depletion within the micelle is replenished first from the aqueous phase and

subsequently from the monomer droplets

• The active micelles grow in size with polymer formation, to preserve their stability; these

growing polymer particles absorb the soap of the parent micelles

Advantages:

(i) Overcomes many environmental problems: “solvent” is water

(ii) If final desired product is polymer is washed with water to remove the soap phase by

coagulation