Techniques of Polymerization
Bulk Polymerization
• Bulk polymerization is carried out in the absence of any solvent or
dispersant and is thus the simplest in terms of formulation.
• No contaminants added
• Simple equipments
• The thermal conductivity of monomers and polymers is low, and as the
viscosity builds up, the ability for heat transfer via convection is
substantially diminished. If the heat energy cannot be dissipated,
temperature rises, and the reaction becomes highly exothermic.
• Occlusion of free radicals by polymer droplets can lead to
autoaccelaration.(Rp>Rt)
• Removal of unreacted monomer can be a problem. This is a large concern if
safe polymer is prepared from monomers which are toxic. The Federal Drug
Administration (FDA) puts limits on how much monomer can be present in a
polymer system used for food containment.
• Keep to low conversion,separate and recycle unreacted monomer
• Mainly used in casting formulations and low molecular-weight polymers for
use as adhesives, plasticizers,tackifiers and lubrication additives
https://www.youtube.com/watch?v=QIYYoN_eZJ4
A low-molar mass prepolymer is prepared and then main
polymerization is carried out in situ taking advantage of Trommsdorff
effect
Suspension(Pearl Polymerization)
• If the monomer is insoluble in water, bulk polymerization can be carried out
in suspended droplets, i.e., monomer is mechanically dispersed. The water
phase becomes the heat transfer medium. Since it is a continuous phase,
viscosity changes very little as the monomer converts to polymer, so the
heat transfer is very good.
• For suspension polymerization, there are two phases, water and organic, and
the starting point may be 10 parts of the former, and 1 part of the latter.
• In this system, the monomer must be either 1) insoluble in water or 2) only
slightly soluble in water, so that when it polymerizes it becomes insoluble in
water.
Usually the initiator is organic soluble
• Monomers are kept in suspension by continuous agitation and the use of
stabilizers. Poly(vinyl alcohol) dissolved in the aqueous phase is a typical
suspending agent(stabilizer).Others are starch, gelatin, calcium phosphate
• If the process is carefully controlled, polymer is obtained as granular
beads.
• The polymeric particles tend to aggregate
https://www.youtube.com/watch?v=X4rDrNLd18M&t=3s
• Size 0.01-0.5cm
Solution Polymerization
• Efficient heat transfer.
• Solvents must be chosen carefully to prevent chain
transfer reactions.
• It is difficult to remove the solvent completely. This
requires a distillation, and that costs an appreciable
amount of money.
• Cost of solvent added
• Cause of pollution.
• Precipitation reaction-when the product of solution
polymerization is insoluble in the solvent
https://www.youtube.com/watch?v=nlPgiRhuYGg&t=1s
Emulsion
An emulsion is a mixture of two immiscible (unblendable) substances. One substance
(the dispersed phase) is dispersed in the other (the continuous phase).
Emulsions are unstable and thus do not form spontaneously. Energy input through
shaking, stirring, homogenizers, or spray processes are needed to form an emulsion.
Over time, emulsions tend to revert to the stable state of oil separated from water.
Surface active substances (surfactants) can increase the kinetic stability of emulsions
greatly so that, once formed, the emulsion does not change significantly over years of
storage.
Micelles Are Formed By The Surfactants
Nonpolar tail Oil
Polar head
Water
Microemulsion Reverse Microemulsion
Emulsion Polymerization
• A monomer is dispersed or emulsified in a solution of surfactant and water
forming relatively large droplets of monomer in water.
• Excess surfactant creates micelles in the water.
• Small amounts of monomer diffuse through the water to the micelle.
• A water-soluble initiator is introduced into the water phase where it reacts
with monomer in the micelles.
• The total surface area of the micelles is much greater than the total
surface area of the fewer, larger monomer droplets; therefore the initiator
typically reacts in the micelle and not the monomer droplet.
• Monomer in the micelle quickly polymerizes and the growing chain
terminates.
• More monomer from the droplets diffuses to the growing particle, where
more initiators will eventually react.
• Eventually the free monomer droplets disappear.The final product is a
dispersion of polymer particles in water.
• Size 0.01-0.15µm
Termination occurs by radical combination when a new radical diffuses into the
micelle, therefore high molecular weight polymers are obtained.
Generally redox initiator is used
Mainly used for the synthesis of water based paints, adhesives and tacky
polymers.
Disadvantage- the surfactant is a soap and it contaminates the
polymer.Washing and drying required
https://www.youtube.com/watch?v=e_TgUSioQFE&t=137s
Emulsion for the synthesis of nanoparticles
CH2 CH CH2 CH
COOH CO
γ radiation, 30min
NH
CH2
NH
CO
Size : 50-80 nm
CH2 CH
Poly(acrylic acid) nanoparticles (PAANP)
Reverse microemulsion of Span80
and Tween80 in hexane
The two differences between emulsion and suspension
polymerization are:
1) that a suspension polymerization is a mechanical process, and must
have a stabilizing agent until the droplets are far apart,
2) the emulsion polymerization is a chemical process which requires a
surfactant to make the monomer "emulsify."
Features of Free Radical Polymerization
• A high molar mass polymer is formed immediately when the
reaction begins, and the average chain length shows little
variation throughout the course of polymerization
• The monomer concentration decreases steadily throughout
the reaction
• Only the active center can react with the monomer and add
units onto the chain one after the other
• Long reaction times increase the polymer yield but not the
molar mass
• An increase in temperature increases the rate of the reaction
but decreases the molar mass
Billmeyer : 126-130, 132
https://www.youtube.com/watch?v=6G_rsIpU5kw&t=162s
