Basic Chemistry of Polymerization:
The polymerization process can be divided into three categories based on reaction taking place. Those are
1 Condensation or Step polymerization
2 Addition or Chain polymerization
3 Copolymerization
4 Coordination or Ziegler-Natta polymerization
1 Condensation polymerization:
This is a process of forming the polymer from polyfunctional monomers of organic molecules such as water, HCL etc; This type of polymerization is also known as step-growth polymerization, the molecular weight of the polymer chain build up slowly and there is only one reaction mechanism for the formation of the polymer.
An example is the formation of polyamide, Nylon6:6
2 Addition polymerization:
This occurs when small molecules join under the action of catalyst, heat or radiation to form a linear polymer usually without elimination of small molecules. This can be of three types
i. Free radical addition polymerization
ii. Cationic addition polymerization
iii. Anionic addition polymerization
An example is the formation of polythene
3 Co-Polymerization:
In this type of polymerization, two types of monomer units will act to form a polymer. This is of
1. Random copolymerization
2. Block copolymerization
3. Graft copolymerization
-A-A-B-A-B-B-A-A-B-A-A
Block Co-Polymerization: Here the two-monomer units are arranged in small blocks.
-A-A-A-B-B-B-A-A-B-B-A-A-B-B-
Graft Co-Polymerization: To one homopolymer backbone another homopolymer chains are attached. It is as follows:
Graft polymer:
B-B-B B-B-B-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-B-B-B B-B-B
An example for copolymerization is styrene-butadiene rubber
4 Coordination or Ziegler-Natta polymerization:
Ziegler (1953) and Natta (1955) discovered that the presence of the combination of a transition metal halide (like TiCl4, ZrBr3, halides of V, Zr, Cr, Mo and W) with an organometallic compound
(like triethylaluminium, trimethylaluminium)stereospecific polymerization can be carried out. An example for coordination polymerization is polypropylene, polyethene, polyamide.
Forms of Polymers
Polymer materials can exit at least in three forms. They are:
1. Straight chain
2. Branched-chain
3. Cross-linked chain
1 Straight chain:
Where the monomer molecules are joined in a linear arrangement
2 Branched-chain:
This arises as a result of the secondary processor three groups or else produced for a special purpose. A special category in the branched polymer is GRAFT polymer. It is obtained when branch consists of chains composed of variety monomer species.
3 Cross-linked:
This occurs between two grafted chains or else monomer chains itself. Cross-linking is the bridging formed between polymer chains to form a three-dimensional network.
4. Polymerization techniques:
The polymerization process can be done by three methods. Those are:
1 Bulk polymerization
2 Solution polymerization
3 Suspension polymerization
4 Emulsion polymerization
1Bulk Polymerization:
In this technique, the monomer is taken in the liquid state and the initiator is dissolved in the monomer. The chain transfer agent whenever used to control the molecular weight is also dissolved in the monomer. The whole system is heated or exposed to a radiation source for initiation.
Disadvantages:
a) As Reaction Proceeds the Viscosity increases and due to which agitation of the prepolymer becomes difficult
b) Heat Removal is difficult.
Advantages:
The products obtained in bulk polymerization technique will have high purity and transparency.
2 Solution Polymerization:
It is the polymerization of one or more type of monomers in solution. Addition & Condensation reactions are carried out by this method. In this method, a monomer, catalyst & solvent are stirred together to form a solution that reacts without heating or cooling and without special handling. In solution polymerization the polymer formed may be either soluble or insoluble in the solvent.
Disadvantages:
a) The product of solution polymerization has to be separated from its solvent for its subsequent sale.
b) Complete removal of the solvent is difficult in some cases.
c) With certain monomer solution polymerization leads to low reaction weight due to which low molecular weight polymers are formed.
Advantages:
a) Heat transfer in solution polymerization is more as the viscosity is low.
b) Handling of solution polymerization is easy compared to other methods.
3 Suspension Polymerization :
It is the polymerization of one more type of monomers by dispersion in the form of droplets in an immiscible liquid. In fact this is a type of Bulk or Mass polymerization process in which small droplets of liquid monomer are polymerized to solid spherical particles. The dispersion of monomer in water is achieved by vigorous agitation in presence of suspending & dispersing agents. In order to disperse the monomer in the suspension system, it must be immiscible in the reaction medium.
Disadvantages:
a) The formation & maintenance of a uniform suspension of the monomer is difficult.
b) Proper Agitation & stabilizing agents should be used for maintaining the suspension.
Advantage:
a) In this, the heat evolved during exothermic polymerization reaction can be controlled due to the presence of water.
b) It is the most economical method as water is used as a suspension medium.
c) A very good quality of polymer product is obtained.
4 Emulsion Polymerization:
In this type of polymerization, one or more monomers are polymerized in Emulsion. Monomers are brought together to form an aqueous emulsion with the help of soaps, detergents & Emulsifying agents. The polymer or copolymer thus obtained is in the form of a latex which can be used as such or it is isolated by adding acids to form lumps of final product
Disadvantages:
Presence of ionic materials which are introduced into the polymers by the surface active agents & inorganic salts results in poor electric properties in the final product.
Advantages:
1. Because of the high rate of reaction high molecular weight polymers are formed.
2. This method is suitable for producing very soft polymers.
The process involves continuous mass polymerization of styrene and rubber in the presence of a diluent. Polymerization is a chemical reaction where styrene monomer in the presence of chemicals and catalyst is chemically reacted under controlled conditions of temperature, pressure, the concentration of composition and resident time to form a chain polymer of high molecular weight.
This mass polymerization is carried out in five continuous stirred tank reactors. In the first three reactors, pre-polymerization is carried out. The viscous polymer formed during the pre-polymerization stage is transferred to the fourth reactor and then to the fifth reactors which are in series to obtain 80-85% solids of the outlet of the sixth reactor.
After the polymer comes out of the reactor it is then passed to pre-heater and de-volatilizer where diluent and un-reacted styrenes are separated from the polymer. High boiling impurities are removed from de-voltalizer. Vapours stream in water is cooled in a vertical heat exchanger and collected in a tank at the bottom of the heat exchanger. De-voltalizer vapours free from high boiling fractions are condensed and are pumped to recycle monomer tanks.
From the de-voltalizer a high-pressure gear pump pumps the polymer solution through a screen changer to the underwater pelletising system pumps bottom de-volatilized polymer out. In the underwater pelletising system, strands of three mm diameter are formed, simultaneously cooled and cut in the presence of water and conveyed in cooling line. At the end of cooling line majority of water is removed from the pellets and the pellets are conveyed to the drier. In the drier are further dried and fall on the classifier where there are sieved and packed