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Objectives Polymers - National Council of …

Do you think that daily life would have been easier andcolourful without the discovery and varied applicationsof Polymers ? The use of Polymers in the manufactureof plastic buckets, cups and saucers, children s toys,packaging bags, synthetic clothing materials, automobiletyres, gears and seals, electrical insulating materials andmachine parts has completely revolutionised the dailylife as well as the industrial scenario. Indeed, thepolymers are the backbone of four major industries , elastomers, fibres and paints and word polymer is coined from two Greek words:poly means many and mer means unit or part. Theterm polymer is defined as very large molecules havinghigh molecular mass (103-107u). These are also referredto as macromolecules, which are formed by joining ofrepeating structural units on a large scale. The repeatingstructural units are derived from some simple andreactive molecules known as monomers and are linkedto each other by covalent bonds.

427 Polymers C:\Chemistry-12\Unit-15.pmd 28.02.07 Polymers can also be classified on the basis of mode of polymerisation into two sub groups.

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Transcription of Objectives Polymers - National Council of …

1 Do you think that daily life would have been easier andcolourful without the discovery and varied applicationsof Polymers ? The use of Polymers in the manufactureof plastic buckets, cups and saucers, children s toys,packaging bags, synthetic clothing materials, automobiletyres, gears and seals, electrical insulating materials andmachine parts has completely revolutionised the dailylife as well as the industrial scenario. Indeed, thepolymers are the backbone of four major industries , elastomers, fibres and paints and word polymer is coined from two Greek words:poly means many and mer means unit or part. Theterm polymer is defined as very large molecules havinghigh molecular mass (103-107u). These are also referredto as macromolecules, which are formed by joining ofrepeating structural units on a large scale. The repeatingstructural units are derived from some simple andreactive molecules known as monomers and are linkedto each other by covalent bonds.

2 This process offormation of Polymers from respective monomers iscalled polymerisation. The transformation of ethene topolythene and interaction of hexamethylene diamine andadipic acid leading to the formation of Nylon 6, 6 areexamples of two different types of studying this Unit, you will beable to explain the terms - monomer, polymer and polymerisation andappreciate their importance; distinguish between variousclasses of Polymers and differenttypes of polymerisation processes; appreciate the formation ofpolymers from mono- and bi-functional monomer molecules; describe the preparation of someimportant synthetic Polymers andtheir properties; appreciate the importance ofpolymers in daily Copolymerisation has been used by nature in polypeptides whichmay contain as many as 20 different amino acids. Chemists are stillfar behind.

3 UnitUnitUnitUnitUnit15 PolymersPolymersPolymersPolymersPolymers 15 PolymersPolymersPolymersPolymersPolymers NCERTnot to be republished426 ChemistryC:\Chemistry-12\ are several ways of classification of Polymers based on somespecial considerations. The following are some of the commonclassifications of Polymers :Under this type of classification, there are three sub polymersThese Polymers are found in plants and animals. Examples areproteins, cellulose, starch, some resins and Semi-synthetic polymersCellulose derivatives as cellulose acetate (rayon) and cellulose nitrate,etc. are the usual examples of this sub Synthetic polymersA variety of synthetic Polymers as plastic (polythene), synthetic fibres(nylon 6,6) and synthetic rubbers (Buna - S) are examples of man-made Polymers extensively used in daily life as well as in are three different types based on the structure of the Linear polymersThese Polymers consist of long and straight chains.

4 The examplesare high density polythene, polyvinyl chloride, etc. These arerepresented as:2. Branched chain polymersThese Polymers contain linear chains having some branches, ,low density polythene. These are depicted as follows:3. Cross linked or Network polymersThese are usually formed from bi-functional and tri-functionalmonomers and contain strong covalent bonds between variouslinear polymer chains, bakelite, melamine, etc. These polymersare depicted as Polymersof Polymersof Polymersof Polymersof Basedon Basedon Structureof Polymers NCERTnot to be republished427 PolymersC:\Chemistry-12\ can also be classified on the basis of mode of polymerisationinto two sub Addition polymersThe addition Polymers are formed by the repeated addition ofmonomer molecules possessing double or triple bonds, , theformation of polythene from ethene and polypropene from , the addition Polymers formed by the polymerisation of asingle monomeric species are known as homopolymers, , Polymers made by addition polymerisation from two differentmonomers are termed as copolymers, , Buna-S, Buna-N, Condensation polymersThe condensation Polymers are formed by repeated condensationreaction between two different bi-functional or tri-functionalmonomeric units.

5 In these polymerisation reactions, the eliminationof small molecules such as water, alcohol, hydrogen chloride, place. The examples are terylene (dacron), nylon 6, 6, nylon 6,etc. For example, nylon 6, 6 is formed by the condensation ofhexamethylene diamine with adipic Basedon Mode ofPolymerisa-tionA large number of polymer applications in different fields depend ontheir unique mechanical properties like tensile strength, elasticity,toughness, etc. These mechanical properties are governed byintermolecular forces, , van der Waals forces and hydrogen bonds,present in the polymer . These forces also bind the polymer this category, the Polymers are classified into the following foursub groups on the basis of magnitude of intermolecular forces presentin ElastomersThese are rubber like solids with elastic properties. In onMolecularForcesIs a homopolymer or a copolymer?

6 It is a homopolymer and the monomer from which it is obtainedis styrene C6H5CH = NCERTnot to be republished428 ChemistryC:\Chemistry-12\ Polymers , the polymer chains are held together by theweakest intermolecular forces. These weak binding forces permit thepolymer to be stretched. A few crosslinks are introduced in betweenthe chains, which help the polymer to retract to its original positionafter the force is released as in vulcanised rubber. The examplesare buna-S, buna-N, neoprene, FibresFibres are the thread forming solids which possess high tensilestrength and high modulus. These characteristics can beattributed to the strong intermolecular forces like hydrogenbonding. These strong forces also lead to close packing of chainsand thus impart crystalline nature. The examples are polyamides(nylon 6, 6), polyesters (terylene), Thermoplastic polymersThese are the linear or slightly branched long chain moleculescapable of repeatedly softening on heating and hardening on Polymers possess intermolecular forces of attractionintermediate between elastomers and fibres.

7 Some commonthermoplastics are polythene, polystyrene, polyvinyls, Thermosetting polymersThese Polymers are cross linked or heavily branched molecules,which on heating undergo extensive cross linking in moulds andagain become infusible. These cannot be reused. Some commonexamples are bakelite, urea-formaldelyde resins, addition and condensation Polymers are nowadays also referred aschain growth Polymers and step growth Polymers depending on thetype of polymerisation mechanism they undergo during their Basedon are Polymers ? are Polymers classified on the basis of structure?Intext QuestionsIntext QuestionsIntext QuestionsIntext QuestionsIntext QuestionsThere are two broad types of polymerisation reactions, , the additionor chain growth polymerisation and condensation or step this type of polymerisation, the molecules of the same monomer ordiferent monomers add together on a large scale to form a polymer .

8 Themonomers used are unsaturated compounds, , alkenes, alkadienesand their derivatives. This mode of polymerisation leading to an increasein chain length or chain growth can take place through the formationof either free radicals or ionic species. However, the free radical governedaddition or chain growth polymerisation is the most common ofTypes ofTypes ofTypes ofTypes orChain GrowthPolymerisa-tion NCERTnot to be republished429 PolymersC:\Chemistry-12\ Free radical mechanismA variety of alkenes or dienes and their derivatives are polymerisedin the presence of a free radical generating initiator(catalyst) likebenzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. Forexample, the polymerisation of ethene to polythene consists ofheating or exposing to light a mixture of ethene with a small amountof benzoyl peroxide initiator. The process starts with the additionof phenyl free radical formed by the peroxide to the ethene doublebond thus generating a new and larger free radical.

9 This step iscalled chain initiating step. As this radical reacts with anothermolecule of ethene, another bigger sized radical is formed. Therepetition of this sequence with new and bigger radicals carries thereaction forward and the step is termed as chain propagating , at some stage the product radical thus formed reactswith another radical to form the polymerised product. This step iscalled the chain terminating step. The sequence of steps may bedepicted as follows:Chain initiation stepsChain propagating stepChain terminating stepFor termination of the long chain, these free radicals can combinein different ways to form polythene. One mode of termination ofchain is shown as under:2 Preparation of some important addition Polymers (a) PolytheneThere are two types of polythene as given below:(i)Low density polythene: It is obtained by the polymerisationof ethene under high pressure of 1000 to 2000 atmospheresat a temperature of 350 K to 570 K in the presence of tracesof dioxygen or a peroxide initiator (catalyst).

10 The low density NCERTnot to be republished430 ChemistryC:\Chemistry-12\ (LDP) obtained through the free radical additionand H-atom abstraction has highly branched density polythene is chemically inert and tough but flexibleand a poor conductor of electricity. Hence, it is used in theinsulation of electricity carrying wires and manufacture ofsqueeze bottles, toys and flexible pipes.(ii)High density polythene: It is formed when additionpolymerisation of ethene takes place in a hydrocarbon solventin the presence of a catalyst such as triethylaluminium andtitanium tetrachloride (Ziegler-Natta catalyst) at a temperatureof 333 K to 343 K and under a pressure of 6-7 density polythene (HDP) thus produced, consists of linearmolecules and has a high density due to close packing. It isalso chemically inert and more tough and hard. It is used formanufacturing buckets, dustbins, bottles, pipes, etc.


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