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Tuesday, 30 June 2015



Polymers are compounds of very high molecular masses formed by the combination of a large number of simple molecules called monomers. The process by which the simple molecules are converted into polymers is called polymerization.
nCH2═CH2 ---------- > -(CH2-CH2-) n
ethylene (monomer) polymerisation polyethylene (polymer)
The process by which the simple molecule converted into polymers is called polymerization.
Homopolymer and Copolymer
A polymer formed from one type of monomers is called homo polymer. A polymer formed from two or more different monomers is called co polymer or mixed polymer.
Polymers and macro molecules
All polymers are macro molecules but all molecules are not polymers. Polymers always consists of thousands of repeating monomer units but macro molecules is a giant molecule which may or may not contain monomer molecules.

A. Classification of Polymers on the Basis of Source.
1. Natural polymers— The polymers obtained from nature are called natural polymers. For example, starch, cellulose, natural rubber, proteins etc.
2. Synthetic polymer—The polymer which are prepared in the laboratories are called synthetic polymers. For example, polyethylene, PVC, nylon, Teflon eyc.
B. Classification of Polymers on the Basis of Structure.
1. Linear polymers—These are the polymers in which 
monomeric units are linked together to form linear chains. Example polyethylene, nylons, polyesters etc.
2. Branched chain polymersThese are the polymers in which monomers are joined to form long chain with side chains or branches of different lengths. Example glycogen, starch etc..
3.Cross-linked polymers—These are polymers in which monomer units are cross-linked together to form a three-dimensional network. Examples Bakelite, resin etc.
C. Classification of Polymers on the Basis of Molecular Forces
1. ElastomersThe polymers that have elastic character like rubber are called elastomers. The polymers chains are held together by weak intermolecular forces. Example natural rubber.
2. Fibers—These are the polymers which have strong intermolecular forces between the chains. These forces are either hydrogen bonds or dipole-dipole interaction. Example nylon-66, Dacron, silk etc.
3. Thermoplastics—These are the polymers which can be easily softened repeatedly when headed and hardened when cooled with little change in their properties. Examples polythene, polystyrene, Teflon etc.
4. Thermosetting polymers—These are the polymers which undergo permanent change on heating. Examples Bakelite, melamine formaldehyde etc.
D. Classification of Polymers on the Basis of mode of Synthesis
1.Addition polymers—A polymer formed by direct addition of repeated monomers without the elimination of by product molecules is called addition polymers. Examples the addition polymers polythene or polypropylene.
2.Condensation polymers—A polymers formed by the condensation of two or more monomers with the elimination of simple molecules like water, ammonia, alcohol etc. is called condensation polymers. Example nylon 66.
  1. Addition polymerization or chain growth polymerization.
This process involves a series of reaction each of which consumes a reactive particle and produced another similar particle resulting a chain reaction. So this is called chain growth polymerization.
It consists of three steps: chain initiation, chain propagation and chain termination
In the initiation step, the initiator (free radical, a cation, or anion) produces a reactive species. This species combines with a molecules of a monomer forming another reactive molecule (1) Initiator I*, (2) I* + M (monomer) = IM*.
In chain propagation this active molecule is added to another monomer forming another intermediate. (3) IM* + M = IMM*, (4) IMM* + M = IMMM*.
In chain termination step the reaction is terminated when the active end reacts
with a species which do not formed reactive species. IMM* + T I(M)nMT.
B. Condensation polymerization or step growth polymerization.
In this type the monomers contain two functional groups. In this process, no Initiator is needed and each step is the same type of chemical reaction. The polymer is formed in a step wise manner, it is called step growth polymer and the process is called step growth polymerization.
A + B (monomer) A-B (dimmer)
A-B + A A-B-A
Alternatively, A + B A-B
A-B + A-B A-B-A-B or (A-B)2
Polymers obtained from two or more different monomer units are called copolymers. Copolymerization of monomer mixtures often leads to the formation of polymers which have quite different properties than those corresponding homopolymer. Copolymers are obtained depending upon the distribution of monomer units in the chain. Two monomers can combine either regular fashion or random fashion.
Two other types of copolymers that can be prepared under certain conditions are –
  1. Block copolymers are those in which different blocks of identical monomer units alternate with each other. –(A-A-A-B-B-B-A-A-A-B-B-B-)n.
  2. Graft polymers are those in which homopolymer branches of one monomer units are grafted on the homopolymer chains of another unit as
- (A – A – A – A – A – A – A – A – A – A -)n
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Natural rubber is a polymer of isoprene (2-methyl buta-1,3-diene)
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CH2═C – CH ═CH2
In natural rubber, about 11,000 to 20,000 isoprene units are linked together in a chain like arrangement.
Vulcanisation of Rubber
The process of heating natural rubber with sulphur to improve its properties is called vulcanisation. During vulcanisation, sulphur cross links are formed which makes it hard, tough and greater tensile strength.When the rubber is starched, the chains straighten out to some extent. The chains cannot slip past each other because of the polysulphide bridges. Thus, rubber can be stretched only to a limited extent. When the tension is removed, the chains try to coil up again and the rubber resumes its original shape.
Synthetic Rubber
The synthetic rubber is obtained by polymerizing certain organic compounds which may have properties similar to rubber and some additional properties. Examples are neoprene, styrene butadiene rubber, thiokol, silicones, polyurethane rubber etc..
  1. Neoprene rubber
It is prepared by polymerization of chloroprene (2-chlorobuta-1, 3-diene)
nCH2═C - CH═CH2 --------- >[ -CH2 – C═CH – CH2 ]n
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Cl (cloroprene) Cl (Neoprene)
Neoprene is superior to natural rubber in its stability to aerial oxidation and its resistance to oils, gasoline and other solvents.
  1. Styrene Butadiene Rubber (SBR)
It is obtained by the polymerization of buta-1,3-diene and styrene in the ratio of 3:1 in the presence of sodium.
It is also called Buna-s, in which Bu stands for butadiene, Na for sodium and S stands for styrene. It has slightly less tensile strength than natural rubber.
  1. Thiokol
It is prepared by condensation of 1,2 dichloroethene with sodium polysulphide Na2S4 in the process of Mg(OH)2.
nCl-CH2-CH2-Cl + Na2S4 (-CH2-CH2-S-S-S-S-)n
It is also called polysulphide rubber. It is mechanically inferior to natural rubber but has exceptional resistance to mineral oils, solvents, oxygen and ozone.
There are two common types of condensation polymers:
 poly amides and polyesters.
Polyamides are formed by reacting two monomers: one having two carboxylic groups and other having two amino groups. 
For examples nylon-66 is obtained by condensation of two monomers.
Hexamethylenediamine H2N – (CH2)6 – NH2
Adipic acid HOOC – (CH2)4 – COOH
nH2N–(CH2)6-NH2 + nHOOC – (CH2)4 – COOH (-NH-(CH2)6-NH-CO-(CH2)4-CO-)n
In this polymerization reaction, -NH2 group of hexamethylenediamine combines with COOH group of adipic acid forming –NH-CO- linkage with the elimination of H2O molecules.
Polyesters are formed by the condensation of a monomer containing two carboxylic acids and a monomer containing two hydroxyl groups (-OH group).
Telyrene is a polyester prepared from:
Terephthalic acid HOOC-C6H6 COOH
Ethylene glycol HO-CH2-CH2-OH

It is obtained by the condensation of these two monomers as:
In this polymerization reaction –OH group of one combines with –COOH group of other forming –O-CO- linkage, which is repeated.

Molecular Mass of Polymers
A synthetic polymer contains a number of species of varying chain lengths. Since each species has a different molecular mass and a given sample of a polymer contains a such species, therefore, the polymer as a whole has an average molecular mass.
Types of average molecular mass-
  1. Number average molecular mass (Mn)-
If N1, N2, N3…. are the number of macromolecules with molecular masses M1, M2, M3… respectively then the number average molecular mass of the polymer is given by
Mn = N1M1 + N2M2 + N3M3 +…. = ∑ NiMi
N1 + N2 + N3…. ∑ Ni
Where Ni is the number of macromolecules of ith type with molecular mass Mi.
  1. Weight average molecular mass(Mw)-
If m1, m2, m3…. are the masses of macromolecules with molecular masses M1, M2, M3….. respectively, then the weight average molecular mass of the polymer is given by
Mw = m1M1 + m2M2 + m3M3 + …. = ∑ miMi
m1 + m2 + m3 ∑ mi
But mi = NiMi where Ni is the number of macromolecules of ith type with
molecular mass Mi.
Mw = ∑ NiMi ×Mi = ∑ NiMi2
∑NiMi ∑ NiMi
Poly Dispersed Index (PDI)the ratio of weight average molecular mass and number average molecular mass is called poly dispersed index (PDI).
PDI = Mw .
PDI is used to determine the homogeneity of a polymer. On the basis of PDI, polymer have been classified in two categories.
  1. Monodisperse polymers- Polymers whose molecules have same or narrow range of molecular masses are called monodispersed polymers.
  2. Polydispersed polymers – polymers whose molecules have a wide range of molecular masses are called polydisperse polymers.

Biopolymers and Biodegradable Polymers
The polyamides contain the –CO-NH-, called peptide bond and the polymers are also called polypeptides. Proteins are naturally occurring polypeptides. The structures of polypeptides are made up of about 20 amino acids. A protein may contain many or all of them. That amino acid differs in the R-group.
Biodegradable polymers are the polymers which are degraded by micro-organisms within a suitable period so that biodegradable. Polymers and their degraded products do not cause any serious affects on the environment.

Chemistry for class 12 ...........
enzyme-catalysis regulation

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