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Polymers is coined from two greek words : poly means many and mer means unit or part . The term polymer is defined as very large molecules having high molrculer mass . these are also referred to as high to as macromolecules , which are
Formed by joining of reapiting structural units on a large scale . The repeating structural units are derived from some simple and reactive molecules known as mono mers and are linked to each other by covalent bonds . As an example , a gaseous
Compound called butadiene , with a molecular weight of 54 , combines nearly 4000
Times and gives a polymer known as polybutadiene with about 200000 molecular
Weight.This process of formation of polymers from respective monomers is called polymerisation .
HISTORY OF POLYMERS
The polymer, called protein , got synthesized in nature from simple compounds like methane , ammonia and carbon dioxide . The life so started , then evolved through
The ages and form became 'human'. And as it did so, almost the whole human body
Came to built around the same polymer.
Meanwhile, polymers had also appeared in their other natural forms like wood,
Cotton, cellulose, starch which began to use . Even millions of years ago , mans basic
Needs happened to be the same as now - he had to drape himself , had to have a shelter ,woods clubs as weapons , & doing so , he had associated with natural polymers.
Polymers have obviously not been discovered not been discovered overnight. They
Came out of long and preserving studies by a host of motivated scientist whose
Work has enriched human life . Today, the overall insight into polymer science and technology is so deep that a material scientist can create an almost limitless range of
Most of the synthetic polymers are of a relatively recent origin. In fact , they appeared later than the radio and the airplane did, but look at the impact they have made on our life. Whether we are looking for fancy decoration articles, textiles, building materials or packing material, the polymers have brought to the fore a scintillating range of irresistibly choices.
In the fifteenth century, a famous discoverer, Christopher Columbus, noticed the native of South America playing a bouncing game with a solid mass collected from the exudate of a tree, that they called 'weeping wood'. While enjoying the game with the solid ball, little did they realize that what they were handling was to grow into a multibillion dollar ( or ruble or yen) rubber industry ! Years later, a renowned scientist, Joseph Priestly, showed that this solid mass could rub off pencil marks from paper and , hence, the material was called rubber.
CLASSIFICATION OF POLYMERS :
ON THE BASIS OF SOURCE-
1. NATURAL POLYMERS : These polymers are found in plants and animals .
Examples are proteins, cellulose, starch, resins and rubber.
2. SEMI - SYNTHETIC : Celluose derivatives as cellulose acetate and cellulose
Nitrate are the examples . They are chemically derived from natural polymers.
3.SYNTHETIC POLYMERS : They are man made polymers extensively used in
Daily life. e.g PVC , nylon & terylene.
Classification on the basis on the basis of force of attraction
These are rubber-like solid with elastic properties in these elastomeric polymer the polymer chains are held together by the weakest intermolecular forces.these weak binding forces permit the polmer to stretched.Afew crosslinks are introduced between the chains,which help the polymer to retract to its original position after the force is released as in vulcanized rubber.the eg are buna-s,buna-n,neoprene etc.
Fibres are the thread forming solid which possess high tensile strength and high modulus .strength eg are nylon 6,6,polyesters etc.
3- THERMOPLASTIC POLYMER
These are the linear and slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling.the eg are polythene,polystyrene,polyvinyl etc.
These polymer are cross linked or heavily breached molecules,which on heating undergo extensive cross linking in moulds and again become infusible.the eg are Bakelite,resins etc.
CLASSIFICATION BASED ON STRUCTURE OF POLYMERS
These polymer consist of long and straight chains.the eg are high density polythene,polyvinyl chloride etc.
2-BRANCHED CHAIN POLYMER
These polymer contain linear chains having some brached eg low density polythene.
3-CROSS LINKED POLYMER
These are usually formed from bi-functional and tri-functional monomer and contain strong covalent bonds between various linear polymer chains,eg Bakelite,melamile,etc.
CLASSIFICATION BASED ON MODE OF POLYMERISATION
The addition polymer are fprmed by the repeated addition of monomer molecules possessing double or triple bonds,eg are the formation of polythene from ethane,polypropene from propene.
The condensation polymers are formed by repeated condensation reaction between two different bi-fuctional and tri functional monomeric units.the elimination of small molecules of water,alcohol etc takes places.eg are nylon 6,6.
TYPES OF POLYMERISATION REACTION
1-ADDITION POLYMERISATION OR CHAIN GROWTH POLYMERISATION
Chain polymerization is characterized by a self -addition of the monomer molecles,to each other very rapidly through a chain reaction.no by product is formed ;the product has the same elemental composition as that of the monomer.the bifuntionality is provided by the double bond present in the monomer.compounds contaning double bonds can therefore undergo chain polymerization reaction.typicaleg are vinyl compounds,allyl compounds,olefins and dienes.since a majority of these monomers fall under the'vinyl' category.monomer concentration is decreases steadily with time.
Chain reaction do not continue definitely,but in the nature of free radical or ionic centre.they are likely to react radily in ways that will destroy the reactivity,eg in radical polymerization reaction two growing molecules may combine to extinguish both radical cetre with formation of a chemical bond.
Chain polymersation consist of these three phases namely initiation,propagation and termination. bsationsationecause the free radicl root to chain polymerization is the most important in terms of versatility and in terms of tonnage of commercial polymer produced annually,this is the mechanism considered in first and in the most detailed.
Free radical polymerization
The initiaton of the polymer chain growth is brought about by free radicals produced by the decomposition of compounds called initiators . The term chain growth represents a process involving a continous and very rapid addition of the monomer units to form polymer molecules or polymer chains . as more and more monomer units are added ,
The length of the polymer chains increase continuously and the chains grow rapidly.
INITIATION : it involves the formation of a free- radical from a radical initiator such as benzoyl peroxide , azobisisobutyronitrile or other that can generate free radicals.
The radical so formed then adds to a molecules of the monomer , thus producing another free radical .
Propagation : the radical generated above adds to a monomer molecule forming another radical , and successive addition of monomer to these radicals results in the formation of long- chain radicals :
A noteworthy feature of the propagrtion step in vinyl polymerization is that the radicals attack the monomer at the methylene group , thus producing the more stable secondary
Radical , giving what is called a head to tail addition . Addition to methylene group is also favoured because it is less sterically hindered than the other carbon atom , and in many cases this process leads to the formation of radical which is stabilized by resonance.
Termination : it usually occures by radical coupling or dispropation reaction .it occasionally brought about by chain transfer reaction due the presence of an impurity in the monomer . a growing polymer polymer radical abstracts an atom from the impurity to terminate the original polymerization chain and produce a new radical caoable of initating a fresh polymerization chain . there are many agents which reduces the rate of polymerization by terminating reaction chains. Such substances are called inhibitors depending upon whether they completely prevent the production of higher polymer or wheter they capable of effectively reducing the rate of polymerization
PREPARATION OF ADDITION POLYMERS
LOW DENSITY POLYTHENE : it is obtained by the polymerization of the ethane under high pressure of 1000 to 2000 atmosphere at a high temperature of 350 K to570K in the presence of traces of dioxygen or a peroxide initator . the LDP obtained through the free radical addition and H-atom abstraction has highly branched structure . It is chemically inert and tough but flexible and a poor conductor of electricity.
2)HDP : it is formed when addition polymerization of ethane takes place in the hydrocarbon ssolvents in the presence of catalyst such as trienthylaluminium and titanium tetrachloride at the temperature of 333k to 343k and under a pressure of 6-7 atmosphere high density polythene thus produced,consist of linear molecules and has a high density due to close packing.it is also chemically inert and more tougher and harder.it is used for manufacturing bucket,dustbin,bottles,pipes etc.
Teflon is manufactured by heating tetrafluroethane with a free radical or persulphate catalyst at high pressures.it is chemically inert and resistant to attack by corrosive reagents.it is used in making oil seals and gaskets and also used for non stick surface coated utensils.
Polyacrylonitrile(PAN) also known as polyvinyl cyanide, has the following structure:
It is produced from acrylonitrile by radical polymerization technique using peroxide initiators. Acrylonitrile monomer can be obtained from acetaldehyde and hydrogen cyanide:
It can also be produced by ammonoxidation of propylene:
PAN is soluble in dimethyl formamide, dimethyl sulphoxide, adipo nitrile, and so on. It has a remarkable resistance to heat upto around 220 degree Celsius and exihibits very good mechanical properties. Polyacrylonitrile is used to produce what are known as PAN fibres. The copolymer of acrylonitrile with butadiene is a material of great industrial importance.
PRACTICAL METHODS OF CHAIN POLYMERISATION
These reaction are used to prepare a variety of high molar mass polymers in four forms namely bulk,solution,suspension,and emulsion methods
The monomer is taken in 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 itself the system is in homogeneous phase.bulk polymerization is quite simple and the product obtained has a high purity since,except the initiator and the chain transfer agent ,no other additive that could contaminated tha product is used.
The polymer obtained can also be used as such since no isolation from other components is involved.
Finally, the bulk polymerization technique is used in the free-radical polymerization of methyl metcacrylate or styrene to get transparent moulding powder and cast sheetings and also of vinyl chloride to get PVC resins.
In this the monomer is dissolved in a suitable inert solvent along with the chain transfer
Agent , whenever used . the free radical initator is also dissolved in the solvent medium ,
While the ionic and coordination catalysts; can either be dissolved or suspended . the
Presence of the inert solvent medium helps to control viscosity increase and promote
a proper heat transfer. The major disadvantages of the solution polymerization technique is that , whoever inert the selected solvent may be, chain transfer to the solvent cannot be completely ruled out, hence, it is difficult to get very high molecular weight products. The polymer form will also have to be isolated from the solution either by evaporation of the solvent or by precipitation in a non solvent, and removal of the final traces is extremely difficult.
3. SUSPENSION POLYMRISATION
Only water in soluble monomers can be polymerized by this technique. The monomer is suspended in water, in the from of fine droplets, which are stabilized and prevented from coalescing by using suitable water soluble protective collides, Surface active agents .The size of the monomer droplets formed depends on the monomer-to water ratio, the type and concentration of the stabilizing agents and also on the type and of agitation employed.
The initiators are monomer soluble. Since each monomer droplet is isolated and independent of the other droplets .Expandable polystyrene beads , styreneDyvinyl Benzene copolymer beads and polyvinyl acetate beads are produced by the suspension technique using free radical initiators .
4. EMULSION POLYMERISATION
As in the case of suspension polymerization, in emulsion polymerization too, the monomer is dispersed in the aqueous phase not as discrete droplets, but as a uniform emulsion. The emulsion is stabilized by surface active agents. Protective collides and also by certain buffers. The surface active agents can be anionic, cationic or non ionic. They serve the purpose of lowering the surface tension at the monomer water interface and facilitate emulsification of the monomer of the water . Owing to the low solubility, agents get fully dissolved or molecularly dispersed only at low concentrations.
In step polymerization ,the polymer build up proceeds through a reaction betseen fuctionalgroups of the monomers . the reaction takes place in a step- wise manner,
And the polymer build- up, therefore,slow. Although many known reactions with
Organic fuctional groups can be made use of in step polymerization, condensation
,addition,ring opening, amidation and ester- interchanging reactions are mostly accompainedby the elimination of small molecules. Whether the elemental composition
Of the polymer formed and that of the monomers are the same or not depends on whether
The reaction is accompanied by the elimination of small molecules or not.
This type of polymerization generally involves a repetitive condensation reactions between two bi-functional monomer. These poly condensation may result in the lose of some simple molecule as water, alcohol etc, and lead to the formation of high molecular mass condensation polymers.
In these reaction, the product of each step is again a bi-function species the sequence of condensation goes on each step produces a distinct functionalized species and is independent of each other, this process is also called as step growth polymerization.
The formation of terylene or decron by the interaction of ethylelen glycol and tera phtalic acid is an example of this type of polymerization.
Polyamides :these polymers possessing amide linkages are important e.g of synthetic fibers and are termed as nylons.the general method of preparation consists of the condensation polymerization of diamines with dicarboxylic acids and also of amino acids and their lactams.
a)Preparations of nylons
i) NYLON-6,6: it is prepared by the condensation polymerization hexamethyldiamine with adipic acid under high pressure and at high temperature.
it is widely used as a fibres for fabric,ropes ,tyre cords as well as molded products.
It is use in making sheets,bristles,for brushes and in textile industries.
It is made from caprolactam.the reaction is iniated by the hydrolysis of XXVI to aminocaproic acid whose amino group attacks the other molecule of XXVI and the continuaton of the process results in the formation of a polymer of a high molecular weight.
nylon-6 is a polymer of major importance in synthetic fibre industry.
It is used in making ropes.
It is used for the manufacture of tyres cords,fabrics and ropes.
These are the polycondensation products of dicarboxylic acids and diols.dacron or tereleneis the best known example of polyesters.it is manufactured by heating a mixture of ethylene gycol and terapthalic acid at 420 to 460K in the presence of zinc acetate antimony trioxide catalyst as per the reaction given earlier.dacron fibre(terylene) is crease resistant and is used in blending with cotton and wool fibres and also as glass reinforcing materials in safety helmets etc.
3).PHENOL-FORMALDEHYDE POLYMER (BAKELITE AND RELATED POLYMERS)
Phenol - formaldehyde polymers are the oldest synthetic polymers.these are obtained by the condensation of phenol in the presense of an acid or the base catalyst.the reaction starts with the initial formation of o-and/or p-hydroxymethyl phenol derivatives,which further react with phenol to form compounds having rings joined to each other through R groups.the initial product could be a linear product novolac used in paints. Novolac on heating with formalaldehyde undergoes cross linking to form an infusible solid mass called Bakelite .
Uses : 1) it is used for making combs , phonograph records , electrical switches.
2) it is used in various utensils and switch board.
MELAMINE - FORMALDEHYDE POLYMER
It is formed by the condensation polymerization of melamine and formaldehyde.
USES: 1)it is used in the manufacture of unbreakable crockery.