METHODS OF POLYMERIZATION
Polymerization reaction may take place in any of the three states of matterm attersolid, liquid, gas. The solid state reactions are usually slow, they are not considered practical for this reason. The gas-phase reactions reactions do take place but normally they require maintenance of high temperatures, a condition in which whic h almost all high molecular weight polymers become unstable. Hence almost all commercial process of polymerization are liquid phase reactions. LIQUID-PHASE REACTIONS: → Homogenous type
Bulk Polymerization/Mass Polymerization Solution Polymerization
→Heterogeneous type
Emulsion Polymerization Suspension Polymerization/Pearl/Bead Polymerization
The homogenous or heterogeneous terms refers only to initial conditions of the system and are not to the subsequent conditions. For example, Polymerization of vinyl chloride by mass polymerization method is considered a homogenous type of reaction, because initially the system is homogenous. But the polymer being incompatible to the monomer phase, the system subsequently becomes heterogeneous. Hence the concept of homogeneity or heterogeneity is based on only the initial conditions of the system. MASS POLYMERIZATION
In this type of polymerization the monomer is taken in liquid form. The initiators, chain transfer agent- used to control the molecular weight are dissolved in monomer. Now the system becomes homogenous. The reaction mass is heated or exposed to a radiation source for the purpose of initiating initi ating the polymerization reaction and is kept under agitation for proper heat and mass transfer. As the polymerization proceeds the viscosity of the medium increases and mixing becomes becomes difficult leading to products with very ver y broad molecular weight distribution. The reaction is purely exothermic. Batch reactors are used for polymerization. The
phenomenon in which the the molecular weight of a substance increase increase tremendously is known as “Autoaccleration” Advantages Advantages
High purity. High molecular weight and molecular weight distribution.
No solvent contamination. Good flow characteristics. Longer polymerization time for high molecular weight polymers (nearly 1016 hrs.) Product can be used directly and no isolation is required due to its resistance towards solvents.
Disadvantages Disadvantages
High viscosity. Stirring becomes difficult after 30-40% conversion. As said above the mol.wt increases enormously which leads to autoaccleration. Due to gel effect/transdorff effect R p is very high. Heat removal is difficult. Hot spots may form and explosion may m ay possible. Removal of last traces of unreacted monomer is difficult.
Applications Applications
Casting PMMA sheets. Used in free radical polymerization of MMA or styrene to get moulding powders.
SOLUTION POLYMERIZATION
Monomer is dissolved in suitable sui table solvent along with chain transfer agents and initiators (free radical initiator). Catalyst may be ionic or co-ordination catalyst which can either dissolved or suspended. The presence of inert solvents medium helps to control viscosity, increase incr ease and promote proper heat transfer. Process may be continuous/batch wise operation. The product obtained is in liquid phase. For most cases water is taken as solvent.
Advantages Advantages
Heat transfer easy due to presence of solvent
Stirring is easy because the solution is low viscous Mass transfer is easy The product obtained is in liquid form and can be directly used for applications such as adhesives
Disadvantages Disadvantages
High mol.wt polymers cannot be manufactured by this process because addition of solvent lowers monomer concentration which lowers rate of polymerization Complete removal of solvent is more difficult Polymer obtained is Impure Requires drying to be used as solid
Applications Applications
Widely used for coating and adhesives. Industrial production of PAN by free radical polymerization. Polyisobutylene by cationic polymerization use this technique.
EMULSION POLYMERIZATION
This technique suits well for addition polymerization. In this type apart from dispersed particles and dispersion media me dia several other components are added. Usually the dispersion media is water. The monomer is dispersed in water in the form of small droplets of diameter 1µm. These droplets droplets have the natural tendency tendency to coalesce and gradually become larger and forms immiscible layer. In order to discourage this tendency emulsifying agents such as soap s oap like materials are added. Apart from that surface tension regulators, regulators, buffers, protective colloids are added to prevent pre-mature coalescence and precipitation of droplets of monomer as well as those of partially polymerized polymer by preventing adhesion of sticky drops. Mechanism involved in Emulsion Polymerization
Emulsifying agent → Sodium or potassium salts of organic fatty acids (soap) and sulphates
Emulsifying agent are spermatozoa like representation includes inorganic head (hydrophilic in nature) and organic tail (hydrophobic in nature). The emulsifying agents are substances with very low solubility in medium and therefore as its concentration is increased, incr eased, the molecularly dispersed substances begins to form molecular aggregates known as as micelles which exists either in spherical form or thin layer. In micelles because of their hydrophilic and hydrophobic characteristics, the molecules orient themselves so as to have the tails towards centre and head towards water phase. They have average dimensions of order 5060Å. The concentration of emulsifying agent at which ion like molecules begin to form micelle is called critical micelle concentration (CMC) which changes changes the rate of change of physical properties
When monomer is added which is organic in nature prefers the interior of the micelle because of organic environment. envir onment. Micelle has the tendency to contract because of surface tension. Hence the migration of monomer from monomer droplets to the interiors of micelle continues till equilibrium is reached between swelling and contracting tendencies. When initiators are decomposed in water phase it bounces around around the solution to create radicals. radicals. At this point the surface area presented by monomer-swollen micelle is much greater than monomer droplets so the probability for radicals to enter the monomer-swollen micelle is very high. When the radicals enter the micelle it initiates the reaction. When polymerization takes place within micelle it lowers the concentration of monomer monomer so that a gradient is established between monomer which in uninitiated and
initiated one. Both diffuse. Thus the uninitiated monomer swollen micelles m icelles gradually disappear losing their monomer content. When a second s econd radical find its way into the micelle, for the conditions prevailing within the micelle the rate of termination is much greater than the rate of propagation. Hence chain terminates eventually and remains until a third radical enters, initiating the growth of second chain and so on. Advantages Advantages
Disadvantages Disadvantages
No chain transfer problem and heat Because of several additives added to transfer problem. The products obtained the system the product obtained in less in the form of latex are directly usable pure when compared to bulk polymerization Very high mol.wt can be obtained in To recover the polymer in the solid form very less time nearly 1-2 hours, high considerable technology is needed average chain lengths are achievable Suitable for ionic/radical condensation Use of very large quantities of water lowers the yield per reactor volume Applications Applications
Most widely used industrial technique and monomers such as vinyl chloride, butadiene, chloroprene, vinyl acetate, acrylates are polymerized by this technique. Suitable for producing very high mol.wt products.
SUSPENSION POLYMERIZATION
This method is employed to overcome some of the disadvantages of emulsion polymerization, for Example the tendency of small polymer particles to adsorb impurities. The size of monomer dispersed is greater compared to emulsion polymerization. Here the monomer monomer is insoluble in dispersion medium. Initiator, suspension stabilizer are dispersed in water or inert organic liquids provided the monomer and polymer is insoluble in it. The mechanism is similar to mass and solution polymerization. Unlike in emulsion em ulsion polymerization where close, intimate contact between the two phases is established by emulsifying emulsi fying agent, each bead or pearl of the monomer in suspension polymerization acts as minute, m inute, discrete mass polymerization system. A protective colloid which is inert with
respect to the polymerization reaction and which increase the viscosity of the continuous phase is added so that it would hydro-dynamically hinder the coalescence of monomer droplets. A protective colloid such as Magnesium carbonate is insoluble is the solution and collects at the droplet-water interface by surface tension and prevents the coalescence of drops upon collision. Advantages Advantages
Disadvantages Disadvantages
No chain transfer problem
Only water insoluble monomers are used No mass transfer problem Emulsifier is needed to wash the suspending agents No heat transfer problem Purity is less when compared to polymer that obtained by mass polymerization Stirring is easy Yield is low when compared to mass polymerization The product obtained is in granular form Requires long time for very high which is convenient to handle, isolation conversions is easy Applications Applications
Expandable polystyrene beads, styrene-divinylbenzene copolymer beads used in preparation of ion exchange resins are produced. PVA beads are produced by this technique using free radical initiators.
REFERENCES
1. 2. 3. 4.
Polymer Science - By Gowariker, V.R & Others Principles of polymerization - By George Odian Textbook of Polymer technology – I By R.Sinha Textbook of Polymer Science – By By F.W. Billmeyer
