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Methods of Polymerisation- Chemistry


Posted Date: 13-Nov-2009  Last Updated:   Category: Education    
Author: Member Level: Gold    Points: 15






Polymer: A polymer is a macromolecule (giant molecule of high molecular mass) built-up by the linking together of a large number of simple molecules.
Example: Polythene is formed by linking a large number of ethene molecules together.

Monomer: A monomer is a simple molecule having two or more bonding sites through which each can link to other monomers to form a polymer chain. Monomers are often called building blocks of a polymer chain.

Functionality: The total number of functional groups or bonding sites present in a monomer molecule is called the functionality of the monomer. For a substance to act as a monomer, it must have at least two reactive sites or bonding sites.


Polymerization: The chemical process by which the monomers (low molecular weight) are converted into polymers (high molecular weight) is called polymerization. All polymerization reactions need suitable initiators.

Degree of polymerization (DP): The number of repeating units (n) in a polymer chain is called degree of polymerization. There may be hundreds or thousands or tens of thousands or more monomer molecules linked together in a polymer molecule.

Where ‘n’ is the degree of polymerization.
The DP can be used to calculate the molecular weight of a polymer.

Molecular weight of polymer = DP × molecular weight of repeat unit.
If the DP of polyethylene is 1,000, then Mol. wt = DP × Mol. wt. of ethylene
= 1,000 × 28
= 28,000
High polymers: The polymers with high degree polymerization (10,000 to 1,00,000) and high molecular masses (10,000 to 10,00,000) are called high polymers.

CLASSIFICATION OF POLYMERS:
Polymers can be classified in several ways – based on their origin, structure, methods of preparation, response to heat and crystallinity.

(a) Natural and Synthetic polymers (based on origin): The polymers which are obtained from natural sources such as plants and animals are called natural polymers.
Eg. Cotton, Wool, Silk, Starch, Cellulose, Proteins, Natural rubber, Leather.

The polymers which are synthesized from simple molecules are called synthetic polymers. Eg. Polyethylene, Polystyrene, Teflon, PVC, Plexiglass etc.

(b) Homopolymers, Copolymers and Linear, Branched and Crosslinked polymers (based on structure):
A polymer may consist of identical monomers or monomers of different chemical structure and accordindly they are, called homopolymers and copolymers respectively.
-M-M-M-M-M-M-M-M- Homopolymer (Eg. Polyethylene, PVC etc.)
-M1-M2-M1-M2-M1-M2- Copolymer (eg. Nylon 66, Polycarbonates etc.)
Based on the way in which repeat units are linked, they are named as Linear, Branched and Cross linked polymers.

A Linear polymer is one in which each monomer unit is linked only to two neighbouring units.
-M-M-M-M-M-M-M-M- Linear homopolymer (eg. Polyethylene).
-M1-M2-M1-M2-M1-M2-M1- Linear copolymer (eg. Nylon 66).
Linear copolymers in which a block of one repeat unit is followed by a block of another repeat unit consecutively is called a block copolymers.
-M1-M1-M1-M1-M2-M2-M2-M2-
Where M1 and M2 are acrylic acid and methylmethacrylate respectively.

A branched chain polymer is one in which long linear chain may branched out forming a branched structure.


Branched copolymers with one kind of monomers in their main chain and another kind of monomers in their side chain are called graft copolymers.


Cross linked polymer is formed when linear molecules under certain conditions are linked to neighbouring ones in such a way that it results in the formation of a three dimensional structure of unlimited size.

(c) Stereoregular polymers or Tacticity (based position of substituent groups in the polymer chain): Depending on the position and regularity of the repeating substituent groups, three different arrangements can be visualized, namely, isotactic, syndiotactic and atactic.
(i) The head –to-tail configuration, in which the substituent groups are all on the same side of the chain (either above or below), is called isotactic polymer.

(ii) If the arrangement of substituent groups are in alternating fashion (above and below), it is called syndiotactic polymer e.g., gutta percha.

(iii) If the arrangement of substituent groups are at random around the main chain, it is called atactic polymer e.g., polypropylene.

(d) Amorphous and crystalline polymers (based on crystallinity):
Many polymers are amorphous and have a tendency to get transformed into ordered structures when their melts are cooled. Polymers possess several crystalline and amorphous zones. The crystalline zones consist of an ordered arrangement of molecules. The polymers possessing a large number of crystalline zones (high degree of crystallinity) are referred to as crystalline polymers. Eg. fibres.

(e) Addition and condensation polymers (based on methods of formation):
Addition polymers are those formed by a process of self-addition to give polymers whose molecular masses are integral multiples of molecular mass of the monomer. Example – polyethylene, polypropylene etc.
Condensation polymers are formed by condensation reaction i.e., reaction between two or more monomer molecules with elimination of simple molecules like water, ammonia and hydrogen chloride etc. Example- phenol-formaldehyde resins and synthetic fibres such as polyesters and polyamides.

TYPES OF POLYMERIZATION

Polymerization occurs basically through two different modes. They are addition (chain growth) polymerization and condensation (step growth) polymerization.
(i) Addition polymerization: Addition polymerization is brought by linking together simple unsaturated molecules such as alkenes (e.g. ethylene) or substituted alkenes (e.g. propylene).

Addition polymerization is initiated using small amounts of substances called initiators. Water soluble potassium persulphate and dibenzoyl peroxide which is soluble in organic solvents have been used as initiators since early days of polymer chemistry. Zieglar-Natta catalyst [combination of TiCl4 and (C2H5)3Al] is also used. A characteristic feature of the catalyst is that it imparts regular structure to the polymer.

(ii) Condensation polymerization: Condensation polymerization takes place by linking together of different monomer molecules and is accompanied by the elimination of small molecules like H2O, NH3 and HCl. Thus resulting material is a copolymer. e.g. Nylon is made by the condensation of adipic acid with hexamethylene diamine.

Terylene (polyester) is obtained by condensing terephthalic acid with ethylene glycol.

Free radical mechanism of addition polymerization (taking ethylene as example):
The free radical mechanism of addition polymerization is illustrated in the synthesis of polyethylene from ethylene using dibenzoyl peroxide (C2H5COO)2 as the initiator.

(i) Generation of free radicals: Initiation of chain reaction is induced by initiators which undergo thermal or photochemical decomposition generating highly reactive species referred to as free radicals.

(ii) Initiation: The free radicals generated initiate the chain process by attacking the unsaturated monomer at the double bonds generating new free radicals.

(iii) Propagation: The new free radicals attack monomer molecules further in quick succession leading to chain propagation.

or in general,

(iv) Termination: At some stage the chain propagation is terminated when the free radicals mutually combine either by coupling (combination) of two free radicals or by disproportionation. In coupling a polymer molecule of longer chain length is formed where as disproportionation results in two polymer molecules of shorter chain lengths, one of them being unsaturated. The products obtained after the termination are some times referred to as the dead polymers.

METHODS OF POLYMERIZATION (TECHNIQUES OF POLYMERIZATION):

(i) Bulk or mass polymerization:

In bulk or mass polymerization, only the monomer and the initiator are involved. The monomer taken in the liquid state and a very small amount of the initiator is dissolved in the monomer so as to constitute a homogeneous phase. Initiation of polymerization is effected by either heating or exposing to radiation. The reaction is exothermic and as it proceeds the reaction mixture becomes viscous resulting in polymer molecules of a wide range of molecular masses. The heat liberated is dissipated by constantly stirring the mixture. However, as the reaction proceeds stirring becomes difficult as the product becomes more and more viscous. Uncontrolled temperature rise may lead to discolouration, thermal degradation and branching or cross linking. In view of these disadvantages bulk polymerization is not employed on a commercial scale. However, this technique is used in making polystyrene, polymethymethacrylate and PVC since the technique is simple and pure polymer is obtained.

(ii) Solution polymerization:

In solution polymerization the medium chosen is an inert solvent in which both the monomer and the polymer dissolve or in which the monomer dissolves and the polymer precipitated. The free radical initiator is dissolved in the solvent and the solution is heated in a reaction vessel with constant agitation. The solvent used will reduce the viscosity of the reaction mixture and hence better heat transfer can be achieved. The disadvantages are (a) the polymer formed will not be pure and has to be isolated by chemical techniques and (b) high molecular mass polymers are not obtained. Polyacrylonitrile and polyisbutylene are obtained by this technique.

(iii) Suspension (Pearl) polymerization:
In this method polymerization occurs in heterogeneous system. The water insoluble monomer is suspended in water as tiny droplets (0.1 to 1mm) by continuous agitation. These droplets are prevented from coagulation by the use of small amounts of water soluble polymers such as polyvinyl alcohol or colloids. Initiators used are also soluble in monomer droplets. The reaction mixture is continuously stirred so that reaction progresses within each droplet to form polymer. The product formed being water insoluble separates in the form of beautiful spherical ‘pearls’ or ‘beads’. They are isolated by easy filtration followed by washing.
The advantages of this technique are:
(a) The viscosity build up of polymer is negligible.
(b) Isolation of product is easy as it needs only filtration and washing.
(c) Molecular mass distribution is uniform.
(d) Isolated products need no further purification.
The disadvantages of this technique are:
(a)The method is applicable only for polymerization of water insoluble monomers.
(b)It is difficult to control particle size.
The method is used for commercial production of polyvinyl chloride, polyvinyl acetate, styrene-divinyl benzene (ion exchanger), etc.

(iv) Emulsion polymerization:

In this method, the monomers are dispersed as fine droplets (about 10-5 to 10-6 mm) in a large amount of water and then emulsified (or stabilized) by the addition of a surfactants (soap or detergents i.e. R-COONa, R-SO3Na). Then water soluble initiators such as persulphates and hydrogen peroxide are added. After adding the initiator, the system is kept agitated in the absence of oxygen at 700C.
Emulsifier (surfactant) contains a hydrophilic (water loving) polar end group (head) and a hydrophobic (water hating) non-polar end group (tail). At low concentration the soap or detergent dissolves completely. When the concentration of surfactant exceeds critical micelle concentration (CMC), the molecules form aggregates called micelles (aggregation of 50-100 molecules). Each micelle contains non-polar tail of emulsifier molecules inside and polar head outwards. The monomer molecules dissolve in the hydrocarbon center of the micelles. The free radicals are generated in the aqueous phase.

The free radicals diffuse into the micelles centre through the aqueous phase, penetrate into the micelle and initiate the polymerization in each micelle. The polymerization growth inside the micelle causes its swelling. The monomers consumed inside the micelle are replenished by continuous diffusion of fresh monomers from aqueous phase. The polymer growth continues until the growing chain is encountered by radical and gets terminated. The polymer formed is in the well stabilized latex. It is isolated either by coagulation using electrolytes or by freezing.

Advantages of emulsion polymerization are:

(a)The rate of polymerization is high.
(b)Polymers with higher molar masses are formed.
(c)No viscosity build up and hence agitation is easy.
(d)Thermal control and control over polymer molar mass is possible.

Disadvantages are:

(a)The polymer needs additional clean up and purification.
(b)It is difficult to remove entrapped coagulants, emulsifiers, etc.
This method is widely used to prepare polymers like PVC, Polyvinyl acetate, PMMA, Neoprene, etc.

GLASS TRANSITION TEMPERATURE (Tg.)

Glass transition temperature is the temperature at which a polymer abruptly transforms from the glassy (hard) to the rubbery state (soft). This transition corresponds to the beginning of a chain reaction and is attributed to the easier molecular rotation about single bonds at Tg and beyond. Below Tg, polymer chain motion virtually ceases and the polymer is said to be frozen in position. Thus the polymers become associated with hardness, stiffness, brittleness and transparency – properties which are associated with inorganic glasses. The rubbery state is also referred to as viscoelastic state. On further heating the polymer melts and reaches a molten or viscofluid state.

In the glassy state, there is neither segmental nor molecular motion. On heating beyond Tg, the polymer passes from glassy state into rubbery state. In rubbery state, there are only segmental motions while molecular mobility is forbidden. On further heating much above Tg, both segmental as well as molecular motion become possible and the polymer flows like a viscous liquid. This temperature is usually called a flow temperature since any polymer is only a mixture and has no sharp melting point.

Factors influencing Tg. Value:
1. Flexibility: A free rotational motion of the polymer chain imparts flexibility to the polymer. Linear polymer chains made of C-C, C-O and C-N single bonds have a higher degree of freedom of rotation. Presence of inherently rigid structures in the polymer chain such as aromatic or cyclic structure or bulky side groups on the backbone of C-atoms hinder the freedom of rotation thus lowering the chain flexibility and increase in Tg.

2. Crystallinity: The Tg value of a polymer largely depends on the degree of crystallinity. Higher the crystallinity, larger is the Tg value of a polymer. In crystalline polymer, the linear or stereoregular chains are lined up parallel to each other and are held by strong cohesive forces. This leads to a high Tg value of the polymer.
3. Branching and cross linking: A small amount of branching will tend to lower Tg.. Increase in chain ends in branched chain polymers increase the free volume thus decreasing the Tg. On the other hand, a high density of branching brings the polymer chains closer, lowers the free volume thus reducing the chain mobility and resulting in an increase in Tg.

4. Molecular mass: Generally Tg of a polymer increases with molar mass upto a particular value and beyond that there is no change.

5. Stereoregularity of the polymer: A syndiotactic polymer has a higher Tg. than atactic polymer which in turn has higher Tg than its isotactic stereoisomer.

6. Presence of plasticizers: Addition of plasticizers reduces the Tg value; for example, addition of diisooctyl phthalate to PVC reduces its Tg from 800C to below room temperature.

Significance of glass transition temperature:
(i)Tg can be used to evaluate the flexibility of a polymer and predict its response to mechanical stress.
(ii)Many polymers show an abrupt change in their physical properties at their glass transition temperature. Coefficient of thermal expansion, heat capacity, refractive index, mechanical damping, modulus of elasticity and electrical properties at Tg determine the usefulness of a polymer over a temperature range.
(iii)Polymeric materials are subjected to different processing operations such as moulding, calendring and extraction. Knowledge of Tg is useful in choosing appropriate temperature for such processing operations.

STRUCTURE AND PROPERTY RELATIONSHIP:

Polymers possess a wide range of properties. They may be elastic or rigid, hard or soft, transparent or opaque. They may have the strength of steel but very light in weight. They may soften on heating and the melt may set to a hard mass on cooling. These properties may vary from one type of polymer to another and even among the polymers of the same type, there may be differences in properties. The fundamental properties which influence the structure-property relationship are molecular mass, polarity, crystallinity, molecular cohesion, and the nature of polymeric chains and stereochemistry of the molecule.

(i) Strength: Generally, polymers of low molecular weight are quite soft and gummy. On the other hand high molecular weight polymers are tougher and more heat resistant. High molecular masses of polymers account for their high softening temperature and tensile strength. Cross linked ones are the stronger than their linear and less branched ones. Tensile and impact strengths increase with molecular mass up to a certain point and then become constant . The melt viscosity of the polymer initially shows a gradual increase with the molecular mass and a steep increase at higher molecular masses. Low melt viscosity and high tensile and impact strengths are desirable properties for a polymer to be commercially useful.


(ii) Crystallinity: Polymers invariably contain both crystalline and amorphous regions. The degree of crystallinity of a polymer depends on its structure (linear, branched, with large pendent groups in polymer chain) and configuration (stereoregular or not). Crystalline region occur when linear polymer chains without branching and carrying no bulky groups, are orderly arranged parallel and close to each other. The chains of polymers are held together by secondary forces such as Vander waal, hydrogen bonding, polar interaction, etc. Such type of close packing imparts a high degree of crystallinity. The polymers having high degree of crystallinity exhibit high tensile strength, impact resistance, high density and sharp and high melting point.
The polymers such a HDPE, stereoregular isotactic and syndiotactic isomers of poly propylene, PVC are highly crystalline. Atactic PVC, PS, polypropylene in which bulky pendant groups arranged randomly on the polymer backbone are amorphous. LDP which has extensive branching is also amorphous.

(iii) Elasticity: Elasticity of a polymer material is mainly because of the uncoiling and recoiling of the molecular chains on the application of force. For a polymer to show elasticity the individual chains should not break on prolonged stretching. Breaking takes place when chain slip past each other and get separated. In rubbers this is avoided by molecular engineering such as (a) introducing cross linking at suitable molecular positions (b) avoiding bulky side groups such as aromatic and cyclic structures on the repeat units and (c) introducing more non-polar groups on the chain so that the chains do not separate on stretching. The structure should be amorphous so that the material has a glass transition temperature at which it is used. This can be brought about by introducing a plasticizer molecule in the polymer chain by copolymerization or compounding the rubber with a suitable plasticizer liquid.

(iv) Plastic deformation (rheology) of polymers: Some polymers, on the application of heat and pressure, initially become soft, flexible rubbery matter and undergo deformation. On further heating beyond melting point, they melt and flow. Such property of polymer is called plasticity. On cooling they return to their original state. This property of plastic deformation is used in moulding operations. Thermoplastics exhibit this property. Thermoplastics are linear, stereoregular polymers. The polymer chains are closely packed and held by secondary forces such as vander waal, hydrogen bonding and dipolar interaction. Such polymers when heated, the chains acquire sufficient energy and overcome these inter chain attractive forces. They attain molecular mobility and flow like viscous fluid.
Thermosettings do not exhibit plasticity. Moulded thermosettings have three dimensional structure. All monomer units are held together through strong primary covalent bonds throughout the structure. Primary covalent bonds are not easily broken by heat. On strong heating, charring occurs instead of deformation. Therefore thermosettins do not undergo reversible plastic deformation.

(v) Chemical resistance and nature of polymer chain: The chemical attack on polymers involves softening, swelling and loss of strength of material. The resistance to chemical attack of a polymer depends on several factors such as (a) the presence of polar or non-polar groups (b) the degree of crystallinity and molecular mass, and (c) degree of crosslinking.
(a) Presence of polar and non-polar groups: A polymer is more soluble in a solvent of similar chemical structure. Polymers having polar groups such as –OH or –COOH groups are usually attacked or dissolved by polar liquids such as water or alcohols. (e.g. resol resins swell in alcohols). Polymers with non-polar groups such as –CH3 and –C6H5 are not easily attacked by polar solvents but they easily swell and sometimes dissolve in non-polar solvents such as petrol, benzene and carbon tetrachloride. Polymers with large number of aromatic groups dissolve in aromatic solvents such as benzene. Strong acids and alkalis attack groups such as –NHCO- and –NHCOO- present in polyamides (nylon) and polyurethanes respectively. Polyolefins, PVC, ABS plastics, and polystyrene have excellent resistance to acids and alkalis.
(b) Degree of crystallinity and molecular mass: For a given polymer, the swelling character decreases with the increase in the molecular mass. For polymers having same chemical character, the chemical resistance increases with increase in the degree of crystallinity.
(c) Degree of cross linking: Greater the degree of cross linking lesser is the solubility. Linear polymers readily dissolve in organic solvents and readily melt on heating. On the other hand, branched chain and cross linked polymers have very little solubility and may undergo rupture when heated.


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