Methods of polymerisation

Methods of polymerisation It is also called as Zeigler – Natta polymerisation.
Zeigler (1953) and Natta (1955) discovered that in the presence of a combination of transition metal halides like TCl4, ZnBr3 etc, with an organometallic compound like triethyl-aluminium or trimethyl-aluminium, stereospecific polymerisation can be carried out.
Combination of metal halides and organometallic compounds are called Zeigler Natta catalyst.

1. Methods of polymerisation By CH. SAI SRI RAMA CHANDRA MURTHY

2. Co-ordination polymerisation: It is also called as Zeigler – Natta polymerisation . Zeigler (1953) and Natta (1955) discovered that in the presence of a combination of transition metal halides like TCl 4 , ZnBr 3 etc , with an organometallic compound like triethyl - aluminium or trimethyl - aluminium, stereospecific polymerisation can be carried out. Combination of metal halides and organometallic compounds are called Zeigler Natta catalyst. The mechanism involves three steps, I nitiation P ropagation Termination

3. Initiation: The catalyst from monomer catalyst complexes by reacting with monomer molecule.    

4.   + n(                       Living polymer Propagation: The monomer catalyst complex reacts with fresh monomers resulting the chain growth as show below.

5.                     Termination: Termination is carried out with an active halogen compound. Dead polymer

6. Methods of Polymerization Emulsion polymerization : In this type of polymerization, the monomer is dispersed in a large amount of water as fine droplets ( to mm in size) which are then stabilized (emulsified) by the addition of soap or detergent or protective colloids. The initiators used are either water soluble or monomer soluble. Quite often, potassium persulphates (K 2 S 2 O 8 ) are used which give rise to the ion radical SO 4 •-. The ion radical interacts with the monomer in the emulsion or micelles-form and then polymerization takes place. It may be used in the emulsion form itself when required for use as adhesives, surface coatings or textile finishes. Or alternatively, the emulsion can be broken by adding an acid or electrolyte. Usually a 3% solution of Al 2 (SO 4 ) 3 is added to break the emulsion.  

7. Eg : Polystyrene is obtainable by this method using styrene (monomer), K2S2O8 (initiator), Na2HPO4 (buffer) and sodium lauryl sulphate (emulsifier), vinyl polymers like PVC, PVA, etc , are obtained by this method.

8. Advantages: 1. The rate of polymerization is high. 2. A very high molecular weight polymer is obtained due to the continuous supply of monomers. 3. Easy heat control. Disadvantages: 1. It is difficult to remove entrapped coagulants, emulsifiers, etc. 2. The polymer needs additional cleanup and purification. 3. It is economically costly.

9. b) Suspension polymerization: This method is an improvement over the emulsion polymerization method. It is also known as pearl polymerization or bead polymerization or granular polymerization. The monomer is dispersed as large droplets (0.1-1mm in size) in water and kept in dispersion by mechanical agitation. Water soluble cellulose derivatives and gelatin are used as stabilizers, to prevent the droplets from coalescing. The reaction mixture is agitated at a temperature of 50-60oC in a sealed vessel and flushed with nitrogen at high pressure. A monomer soluble initiator, like benzoyl peroxide for vinyl chloride, is used and the reaction is almost complete in about 8 hours (90% conversion). The unreacted monomer is recycled. The polymer can be obtained as fine spherical beads by configuration. It is then washed and dried.

10. Eg : PVC and PMMA can be obtained by this method. PMMA is obtainable from methyl methacrylate with acetyl dimethyl benzyl ammonium chloride as stabilizer and poly vinyl alcohol as a protective colloid. The polymer beads serve as ion exchangers. Advantages: 1. Higher purity product. 2. The viscosity build up of polymer is negligible. 3. Efficient thermal control. Disadvantages: 1. It is difficult to control particle size. 2. The method is applicable only for polymerization of water insoluble monomers.

11. Properties of Polymers The major structural feature of polymeric compounds is the presence of chain molecules in which a large number of atoms are combined successfully. They have two types of bonds, chemical and intermolecular, which greatly differ in energy and length. The atoms in the chain are joined to each other by strong chemical bonds about 1-1.5 Å in length. Much weaker intermolecular forces interact between the chains at distances of about 3-4 Å. Cross-linked or three-dimensional polymers have chemical bonds (cross-links) between their chains. The presence of large molecules and two types of bonds predominates all properties which are typical of polymers. The properties of polymers are dependent on many factors including inter and intra chain bonding, the nature of the back bone, processing events (polymerization), presence or absence of additives including other polymers, chain size, geometry and molecular weight.

12. The properties of polymers can be explained as follows Physical properties Mechanical properties Physical properties Crystallinity : The degree of structural order arrangement of polymeric molecules is known as crystallinity. The tendency of crystallization of a polymer depends upon the ease with which chains can be aligned in an orderly arrangement. The degree of crystallinity has considerable influence on properties of polymer like solubility, diffusion, hardness, toughness, density, transparency, etc.

13. Crystallization imparts a denser packing of molecules, thereby increases the intermolecular forces of attraction. This leads to higher boiling points, higher rigidity, strength and density when compared to amorphous polymers. A completely crystalline polymer is highly brittle in nature.

14. ii) Amorphous state: It is characterized by a completely random arrangement of molecules. The intermolecular forces between chains in amorphous polymers are less when compared with crystalline polymers. So amorphous polymer can be moulded into desired shape. Both thermosetting and thermoplastic polymers can exist in amorphous state. Polymer with long repeating unit or with low degree of symmetry cannot crystallize easily and therefore they generally form amorphous structures. Many polymers consist of crystallites embedded in an amorphous matrix. In such polymers the crystallites provide hardness, rigidity and heat resistance whereas, the amorphous matrix provides flexibility. In these inter-chain forces are weak.

15. Molecular weight: A non-polymeric simple compound possess a fixed molecular weight. Eg : Na 2 CO 3 possess 106 as molecular weight. When Polymerisation takes place, the growing polymer chains are terminated at different size of molecule, as a result the polymer molecule have different number of monomeric units hence different molecular weights. As a result the molecular of polymer should be expressed as average molecular weight. ( M ) For example: The following three polyethylene molecules have different molecular weights in a polymer sample.

16. The molecular weight of polymer sample is an average of the three (2800+14000+1400) 3   = 6066.7 100 5 00 5 Mol.wt . = 100*28 = 2800 Mol.wt . = 50*28 = 1400 Mol.wt . = 500*28 = 14,000

17. Mechanical properties Strength: The properties of high polymers are determined mainly by the magnitude and distribution of the attraction forces between the molecules, viz. a.) Primary or chemical bond forces and b.) Secondary or intermolecular forces In straight chain and branched-chain polymers, the individual chains are held together by weak intermolecular forces of attraction, the strength of which increases with the chain length or molecular weight. Such polymers exhibit mechanical strength only when chain length is greater than 150-200 atoms in a line. Polymers of low molecular weights are quite soft and gummy, but they become brittle at low temperatures. On the other hand, higher-chain polymers are tougher and more heat-resistant.

18. Thus, by controlling the chain length or molecular weight, it is possible to vary the physical properties of the polymer from soft and flexible to hard and horn-link substances. The intermolecular forces can be greatly increased by the presence of polar groups like carboxyl, hydroxyl, chlorine, fluorine and nitrile along the chain length. Eg : Nylons, Teflon, polyester, etc.

19. Strength can be estimated with the help of stress-strain test. As the applied force (i.e. stress) increases, the amount of stretch is a measure of strain. Typical stress-strain curves for different types of polymers are shown in figure. Strength of straight chain polymers also depends on slipping power of one molecule over the other. Shape of the polymer molecules greatly affects the resistance to slip and consequent deformation of a polymer. Eg : Structure of polyethylene: It is a simple and uniform molecule, so there is only limited restriction to move one molecule over another.

20. Structure of polyvinyl chloride: It has large lumps of chlorine atoms periodically along its chain. As a result, movement is much more restricted between the molecules. And also there are strong Vanderwaal‟s attractive forces. Hence PVC is a tough and strong polymer than polyethylene.

21. Structure of polystyrene: In cross linked polymer, all structural units are connected by strong covalent forces, resulting in a giant solid molecule, extending in three dimensions. Consequently, they are most strong and tough, because the movement of inter molecular chains is totally restricted.

22. ii ) Plastic deformation: Plastic deformation is found in materials known as thermoplastics, whose structure is deformed to plastic stage on application of heat or pressure or both. This property of such materials has been used during processing them into desired shape articles. Generally , the polymer molecules are held together by either weak intermolecular forces or strong covalent bonds, most of the polymers have both the forces. The linear polymers show the greatest plastic deformation at high temperature and pressure. At high temperature the Vanderwaal‟s forces are weaken in linear polymer, results “slippage” of polymeric molecules take place. Thus permanent deformation occurs. By applying both heat and pressure, the linear polymers readily take mould shape and on cooling the material becomes rigid in the moulded shape. The plasticity of the materials decreases with fall of temperature. So plasticity of thermoplastic polymers is reversible.

23. In cross linked polymers, the polymeric molecules are linked through strong covalent bonds which do not weaken under pressure and temperature. So “slippage” of polymers does not occur. Hence cross-linked polymers do not involve in deformation, so it is thermosetting plastic . iii ) Elastic character: Elastic character of a polymer is characterized by recoverance of original shape, after a deformation stress is released. The term “elastomer” is frequently applied to this type of polymeric material. The elastic deformation in elastomers arises from the fact that each rubber-like or elastomer consists of very long-chain molecules, having free-rotating groups, which assume, in the unstressed condition, a peculiar configuration of irregularly coiled and entangled snarls. In the normal unstretched state, these snarls are in a random arrangement and hence, accounting for amorphous state of the polymer .

24. When such a polymer is stretched, the snarls begin to disentangle (like a spring) and straighten out, whereby orientation of chains occurs. Such chain orientation result in crystallization, which in turn enhances the attraction forces between different chains, thereby causing „stiffening‟ of the material. However, when the strain is released, the stretched snarls return to their original arrangement.

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