Unit 3- Class 3- High Polymers (polymerization)

 As said in our earlier class, today we will discuss about the polymerization process.

Polymerization: Is the process of conversion of low molecular weight substances into high molecular weight substances with or without the elimination of by products such as HCl, H2O, NH3 etc.

Types of polymerization:

1.    Addition (chain) polymerization: A polymerization reaction in which monomers containing one or more double bonds are linked to each other without the elimination of any by products, usually in the presence of initiators is called addition polymerization.

        Egs: 1. Formation of polythene.
   
             n CH2= CH2-------------->[- CH2 – CH2 -]n

The main features of addition (chain) polymerization are:
1. Only olefinic compounds can undergo addition polymerization.
2. No elimination of by products.
3. Double bond provides required bonding sites.
4. The addition of monomers takes place rapidly.
5. Linear polymers are produced.
6. The addition polymerization is brought about by free radical, ionic or co-ordination mechanism.
7. The molecular weight of the polymer is an integral multiple of the monomer.
8. The elemental composition of the polymer is same as that of monomer.


Condensation (step) polymerization: A polymerization reaction in which bi or poly functional monomers undergo intermolecular condensation with continuous elimination of by products such as H2O, HCl, NH3 etc. is called condensation or step polymerization.

Egs: 1. Formation of Nylon66
                   n  NH2-(CH2)6-NH2 + n  HOOC- (CH2)4- COOH -------------------------->
   Hexamethylene diamine                           Adipic acid


                          [-NH-(CH2)6-NH-CO-(CH2)4-CO-] n + 2n H2O
                                        Nylon66


2. Formation of polyester

     nOH-(CH2)2-OH + n HOOC-C6H4-COOH              [-O-(CH2)2-O-CO-C6H4-CO-] n               
      Ethylene glycol                   Terephthalic acid                   Polyester       +2nH2O


The main features of condensation polymerization are:

1. The monomers having two or more reactive functional groups can undergo condensation polymerization.   
2. There is elimination of by products.
3. Polymerization proceeds through intermolecular condensation.
4. The polymer chain build up is slow and stepwise.
5. Polymerization is catalysed by acids or alkali.
6. Linear or cross-linked polymers are produced.
7. The elemental composition of the polymer is different from that of the monomers.


in our next class we will study about the Mechanism of addition polymerization.