Polymerisation & type of Polymerisation

Polymerisation

The polymerisation techniques used in industry are different for condensation and addition polymerization this is basically due to the differences in mechanisms in the two types of polymerisation.

Condensation polymerization

 This type of polymerisation can be conducted by either high temperature or by low temperature method. Most of the industries are using high temperature method. In this method, the polymerisation is most frequently conducted in molten monomers in an autoclave or reactor at temperatures above 200°C.Alongwith the' monomers, sometimes, inert gases to avoid side reactions, blocking agent for viscosity control, and delustering agents are used.

 Several hours are required to complete the polymerisation reaction. As the reaction progresses, viscosity increases. After the reaction, the system is evacuated to remove the low molecular weight product. 

Further, the whole mass is pushed out very quickly and quenched in water, followed by cutting, washing and drying. In general, monomers with low reactivity require high temperature polymerisation. If the monomer reactivity is high or if it is modified for high reactivity, then the polymerisation reaction can be conducted at lower

temperature. For example, Nylon 6,6 can be produced by reaction with adipic acid and hexa methylène diamine at temperatures above 250°C.But if instead of adipic acid, its acid chloride is used for, reaction, then the polymerisation can be done at low temperature i.e., below 40°C.0

Addition polymerization

 There are several methods of conducting åddition polymerisation of vinyl compounds or any other double bonded compounds. The methods vary only in the physical state of dispersions. Depending upon the state of dispersions, the different methods are: (a) Gas phase,

(b) Bulk

(c) Solid phase

(d) Solution

(e) Emulsion and

(f) Suspension polymerisation

Gas phase Polymerisation

This method is carried out with the monomer in the gaseous state. Polymer formation begins on the walls of the reactor or on the surface of already formed polymer. This method is used to produce polyethylene.

Bulk Polymerisation

This method is carried out in the liquid monomer at definite conditions like temperature, pressure etc. If the resulting polymer is soluble in monomer,
the viscosity of the medium gradually increases in the course of polymerisation. Then the polymer is a monolithic block of polymer. If the polymer is insoluble in monomer, the polymer is obtained as a powder or as a porous material. Polystyrene and poly(methyl methacrylate) are produced by bulk polymerisation method.

Solid-phase Polymerisation

This polymerisation method proceeds at temperatures below the melting point of the monomer. Only selected monomers can be polymerised at temperatures below their melting point. The polymerisation can be initiated by the action of light, irradiation with x-rays or gamma rays or by any other high energy particles. This method has not become popular.

Solution Polymerisation .

In this method, the reaction medium is a suitable solvent, where monomers, initiators and modifiers are added. Polymer may be formed in two ways.

(i) If the polymer like the monomer is soluble in the solvent, then the polymerisation results in a solution of polymer, called a lacquer. The lacquer may be used directly for further processing or application. Alternately, the polymer may be precipitated.

(ii) If the polymer is insoluble in the solvent, then the polymer precipitates and is separated from the solvent. This method is used to produce high density polyethylene, polypropylene and their copolymers.

Emulsion Polymerisation

 In this method, the liquid monomer is not soluble in the reaction medium or dispersion medium. Here, the medium is usually water and the monomer is dispersed in the medium (water) to form an emulsion. Initiators, emulsifiers and other additives are added to the medium. The reaction mass remains in the form of an emulsion till the polymerisation is over.

 During polymerisation, the monomer emulsion (monomer droplets) is slowly transferred into polymer emulsions (polymer droplets).The polymer droplets are smaller than that of the monomer droplets. Polyacrylonitrile, poly(vinyl acetate), poly(vinyl chloride) and poly(methyl methacrylate) are produced by emulsion polymerisation method.

Suspension Polymerisation

This method is similar in principle to emulsion polymerisation. But here the droplets are larger. Also, the polymerisation proceeds with the formation of large granules in the suspension of polymer in water