Unit 15 New 18.10.2018 - NCERT

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 The process offormation of polymers from respective monomers iscalled 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 . Polymersof Polymersof Polymersof Polymersof PolymersThere are several ways of classification of polymers basedon some special considerations. One of the commonclassifications of polymers is based on source from whichpolymer is this type of classification, there are three polymersThese polymers are found in plants and are proteins, cellulose, starch, some resinsand polymersCellulose derivatives as cellulose acetate (rayon) and cellulosenitrate, etc.

3 Are the usual examples of this sub polymersA variety of synthetic polymers as plastic (polythene), syntheticfibres (nylon 6,6) and synthetic rubbers (Buna - S) are examplesof man-made polymers extensively used in daily life as well asin can also be classified on the basis of their structure, molecularforces or modes of 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. Themonomers used are unsaturated compounds, , alkenes, alkadienesand their derivatives. This mode of polymerisation leads to an increase inchain length and chain growth can take place through the formation ofeither free radicals or ionic species. However, the free radical governedaddition or chain growth polymerisation is the most common radical mechanismA variety of alkenes or dienes and their derivatives are polymerised inthe presence of a free radical generating initiator(catalyst) likebenzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc.

4 For example,the polymerisation of ethene to polythene consists of heating orexposing to light a mixture of ethene with a small amount of benzoylperoxide initiator. The process starts with the addition of phenyl freeradical formed by the peroxide to the ethene double bond thusgenerating a new and larger free radical. This step is called chaininitiating step. As this radical reacts with another molecule of ethene,another bigger sized radical is formed. The repetition of this sequencewith new and bigger radicals carries the reaction forward and the stepis termed as chain propagating step. Ultimately, at some stage theproduct radical thus formed reacts with another radical to form thepolymerised product. This step is called the chain terminating sequence of steps involved in the formation of polythene aredepicted as follows:Chain initiation ofTypes ofTypes ofTypes ofTypes are polymers ?Intext QuestionsIntext QuestionsIntext QuestionsIntext QuestionsIntext Questions2022-23435 PolymersChain propagating stepChain terminating stepFor termination of the long chain, these free radicals can combinein different ways to form polythene.

5 One mode of termination ofchain is shown as under:The addition polymers formed by the polymerisation of a singlemonomeric species are known as homopolymers, for examplepolythene discussed above is a polymers made by addition polymerisation from two differentmonomers are termed as copolymers. Buna-S, which is formed bypolymerisation of buta 1, 3 diene and styrene is an example ofcopolymer formed by addition ImportantAddition Polymers(a)PolythenePolythenes are linear or slightly branched long chain are capable of repeatedly softening on heating andhardening on cooling and are thus thermoplastic 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 2000atmospheres at a temperature of 350 K to 570 K in thepresence of traces of dioxygen or a peroxide initiator(catalyst). The low density polythene (LDP) is obtainedthrough the free radical addition and H-atom has highly branched structure.

6 These polymers havestraight chain structure with some branches asshown density polythene is chemically inert and tough butflexible and a poor conductor of electricity. Hence, it is used2022-23436 Chemistryin the insulation of electricity carrying wires and manufactureof squeeze 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 as shown below and has a high density due toclose packing. Such polymers are also called linear density polymers are also chemically inert and moretough and hard. It is used for manufacturing buckets,dustbins, bottles, pipes, etc.(b)Polytetrafluoroethene (Teflon)Teflon is manufactured by heating tetrafluoroethene with a freeradical or persulphate catalyst at high pressures.

7 It is chemicallyinert and resistant to attack by corrosive reagents. It is used inmaking oil seals and gaskets and also used for non stick surfacecoated utensils.(c)PolyacrylonitrileThe addition polymerisation of acrylonitrile in presence of aperoxide catalyst leads to the formation of is used as a substitute for wool in makingcommercial fibres as orlon or a homopolymer or a copolymer?It is a homopolymer and the monomer from which it is obtainedis styrene C6H5CH = type of polymerisation generally involves a repetitivecondensation reaction between two bi-functional or trifunctionalmono-meric units. These polycondensation reactions may result inthe loss of some simple molecules as water, alcohol, hydrogenchloride, etc., and lead to the formation of high molecular masscondensation these reactions, the product of each step is again a bi-functionalspecies and the sequence of condensation goes on. Since, each stepproduces a distinct functionalised species and is independent of eachother, this process is also called as step growth formation of terylene or dacron by the interaction of ethyleneglycol and terephthalic acid is an example of this typeof polymerisation.

8 (a) PolyamidesThese polymers possessing amide linkages are importantexamplesof synthetic fibres and are termed as nylons. Thegeneral method of preparation consists of the condensationpolymerisation of diamines with dicarboxylic acids orcondensation of amino acids or their (i)Nylon 6,6: It is prepared by the condensationpolymerisation of hexamethylenediamine with adipic acidunder high pressure and at high 6, 6 is fibre forming solid. It possess high tensilestrength. This characteristic can be attributed to the strongintermolecular forces like hydrogen bonding. These strongforces also lead to close packing of chains and thus impartcrystalline 6, 6 is used in making sheets, bristles for brushesand in textile industry.(ii)Nylon 6: It is obtained by heating caprolactum with waterat a high Step GrowthPolymerisationnCH+ nCOOH2 HOH COHHOOC2 Ethylene glycol(Ethane-1, 2 - diol)Terephthalic acid(Benzene-1,4 - dicarboxylic acid)Terylene or dacronCCnOOOCH CH22 O ImportantCondensationPolymers2022-23438 ChemistryNylon 6 is used for the manufacture of tyre cords, fabricsand ropes.

9 (b) PolyestersThese are the polycondensation products of dicarboxylicacids and diols. Dacron or terylene is the best known exampleof polyesters. It is manufactured by heating a mixture of ethyleneglycol and terephthalic acid at 420 to 460 K in the presence ofzinc acetate-antimony trioxide catalyst as per the reaction givenearlier. Dacron fibre (terylene) is crease resistant and is usedin blending with cotton and wool fibres and also as glassreinforcing materials in safety helmets, on heating with formaldehyde undergoes cross linkingto form an infusible solid mass called bakelite. It isthermosetting polymer which cannot be reused or , bakelite is formed by cross linking of linear chains of thepolymer novolac. Bakelite is used for making combs, phonographrecords, electrical switches and handles of various utensils.(c)Phenol formaldehyde polymer (Bakelite and relatedpolymers)Phenol formaldehyde polymers are the oldest syntheticpolymers.

10 These are obtained by the condensation reaction ofphenol with formaldehyde in the presence of either an acid or abase catalyst. The reaction starts with the initial formation ofo-and/or p-hydroxymethylphenol derivatives, which furtherreact with phenol to form compounds having rings joined toeach other through CH2 groups. The initial product could be alinear product Novolac used in (d) Melamine formaldehyde polymerMelamine formaldehyde polymer is formed by the condensationpolymerisation of melamine and formaldehyde. It is used in the manufacture of unbreakable C2CH2CH2CH2~~~~~~~~H C2CH2CH2CH2 OHOHOH~~~~OHCH2~~~~ the names of monomers of the following polymers: the following as addition and condensation polymers: Terylene, Bakelite,Polythene, QuestionsIntext QuestionsIntext QuestionsIntext QuestionsIntext QuestionsCopolymerisation is a polymerisation reaction in which a mixture ofmore than one monomeric species is allowed to polymerise and forma copolymer.