Polymers

Polymer is a macromolecule formed by repeating unit of several simple molecules called monomer.The process of linking of monomers (repeating unit) is called as polymerization. Monomer is a simple molecule which undergo self addition to form long chain called polymer.Monomers are building blocks of polymers.A molecule is said to be monomer ,if it has at least two bonding sites in the form of double bond or triple bond or should contain groups like OH , COOH ,NCO, NH2 etc.

Polymerisation: the process of conversion of substance of low molecular mass(monmers) into substance of high molecular mass with or without elimination of by products like HCl ,H2O, NH3 etc.Polmerisation reaction usually takes place in presence of initator.Degree of polymerization:The number of repeating unit present in a polymer is called degree of polymerization.Here n is the degree of polymerization .When the value of n is very large (in range of 100 or 1000)the polymers are said to be high polymers.When the value of n is less than 10 , the polymers are called as oligopolymers.Functionality :The total number of bonding sites or functional groups present in the monomer is called as functionality of monomer.For Ex: Ethylene , methylmethaacrylate,adipic acid ,are bi functional. Phenol and glycerol are trifunctional.TYPES OF POLYMERISATIONThere are two types of polymerization.1)Addition Polymerisation 2)Condensation Polymerisation1)Addition Polymerisation:The process of addition of monomer to each other by covalent bonds without elimination of by products is called addition polymerization and the polymer is called addition polymer.For ex: Polyethylene,Polyvinylchloride(PVC), Polymethylmethaacrylate(PMMA) ,Teflon, Polypropylene

a)it is also called chain polymerization. b)Only alkene or vinyl compounds undergo addition polymerization. c)Polymer initiates by an initiator like free radical d) The composition of the polymer is same as monomer. e)Addition polymerization occurs rapidly f)No eleimination of byproducts. 2)Condensation polymerization: The process of condensation of two or more monomersv(similar or different) with elimination of by products lke water , HCl ,NH3. Alcohol ,etc is called condensation polymerisaton and the polymer is called condensation polymer.For ex: Nylon 6,6 , Expoxy resin , Polyesters.

1)It is called as step-growth polymer 2)Elimination of by products occurs c) Reaction catalysed by acid or base d)The polymer chain built up is slow and stepwise. e)The composition of polymer is different from monomer. f) Polymerization proceeds through inter molecular condensation.

Mechanism of polymerization of Vinyl Chloride by Free radical method (6MARKS)Polymerisation of vinyl chloride is an example of addition polymerization and takes place by free radical mechanism.It involves four steps.1)Generation of free radical 2)Chain Initiation Step 3)Chain Propogation step 4)Chain Termination step.1)Generation of free radical In this step homolytic dissociation of dibenzoyl chloride (initator) takes place.This results in generation of pair of free radical. Free radicals are highly active species with unpaired (odd) electron.

2)Chain initiTION STEP: It involves addition of free radical to monomer, which leads to formation of chain initiating species(radical).

3)Chain propogation step: The radical formed in above step ,attacks another monomer to give new radical. This process continiuos and growth of polymer chain occurs.

4)Chain termination: The propgating chain stops growing and terminates.Termination occurs by two methods a) Coupling b)Disproportionation.a)Coupling: Combination of two long chain radical ocuurs .

b)Disproportionation. Here transfer of hydrogen atom of one polymer chain to another chain ocuurs. It leads to formation of two dead polymer chains ,out of which one is saturated and other is unsaturated

Molecular weight of polymersPolymer chains contain chains of different length , so it is not possible assign specific molecular mass. Hence average molecular mass is taken . The average molecular mass is expressed in two waysa) Number average molecular mass: it is given by Mnb) Weight average molecular massNumber average molecular mass:It is represented by Mn, If a sample is having N1 molecules with molecular mass M1N2 molecules wit molecular mass M2Ni molecules with molecular mass MiThe number average molecular mass , Mn = n1M1+n2M2++niMi . . n 1+n2++niN represents number of each species M represents molecular mass of each species

Number average molecular mass:It is represented by Mw, If a sample is having w1 weight of molecules with molecular mass M1w2 weight molecules with molecular mass M2wi weight molecules with molecular mass Mi

The weight average molecular mass , Mw = w1M1+w2M2++wiMi . . w 1+ w2++wi

We know that , W=n M Mw = n1M12+ n2M22 ++niMi2 . . n1M1+n2M2++niMi

N represents number of each species M represents molecular mass of each speciesThe ratio of Mw/Mn is either equal to or greater than 1. { Mw/Mn 1} .The ratio is called poldispersity index , if the ratio is equal to one the polymer is said to be mono dispersed . If the ratio is greater than one the polymer is said to be polydispersed. Polydispersity is the measure of distribution of molecules of different sizes.

ELASTOMERSAre the polymers which elongates on stretching and regain their original shape on leaving.Ex :Rubber is an example of elastomerA linear polymer can be converted into rubber if it meets following characteristics1)It should posses glass transition temperature less than working temperature of rubber.2)Possibility of cross linking should be there to get required rigidity.3)Long chains should be geometrically irregular , so that coiling can takes place on thermal agitation.Synthesis , properties and applications of silicon rubberSilicon rubber is an example of inorganic polymerIt is chain of alternating silicon and oxygen atoms.Each silicon atom carries two methyl groups.Synthesis: Silicon rubber is synthesized by condensation polymerization of dimthyl silanol.The dimethyl silanol is prepared by hydrolysis of dimethyl dichlorosilane.Dimethyl silanol is unstable and immediately undergoes intermolecular condensation to give silicone