Unit 3- Class 4- High Polymers (Mechanism of addition polymerization)

Mechanism of addition polymerization:  Free radical mechanism:

The polymerization of ethylene monomer by free radical mechanism proceeds in three distinct stages: 1. Initiation 2. Propagation 3. Termination.

Initiation: It involves two reactions. The first is the production of free radicals usually by the homolytic dissociation of an initiator such as dibenzoyl peroxide to yield a pair of radicals. A free radical is an atomic or molecular species having an odd or unpaired electron. They are highly active species.
                                Heat                              heat                         . 
       (C6H5COO)2--------->2C6H5COO.--------->2CO2 + 2C6H5 (R.)


The second part of initiation involves the addition of this radical to the first monomer molecule to produce the chain initiating species. 
                                                                                               . 
            R. + CH2=CH2-------->R-CH2-CH2    


Propagation: In the propagation, the radical attacks another monomer to produce yet another free radical and the process continues until termination occurs.


 R-CH2-CH2 +   CH2=CH2-------------> R-CH2-CH2-CH2-CH2  
                                                                                 |
 
                                                                                 |    n CH2=CH2  

                                                                                  |       . 
                                                               R-(CH2)n-CH2-CH2      
                                                                      In general


Termination: At some point, the propagating polymer chain stops growing and terminates. Termination is by 1. Coupling or combination i.e. a. the two growing chain may react with each other.
                                  .          .   
        R-(CH2)n-CH2-CH2 +  CH2-CH2-(CH2)n -R

                                   |

                                   |

                R-(CH2)n- CH2-CH2-CH2-CH2-(CH2)n-R

2. Coupling of growing chain with initiator free radical.
                                         .
               R-(CH2)n-CH2-CH2 + R.-------------->R-(CH2)n-CH2-CH2-R      

3. Or by disproportionation in which a hydrogen atom of one radical center is transferred to another radical center. This results in the formation of two polymer molecules one saturated and another unsaturated.
                                               .                                  .
                     R-(CH2)n-CH2-CH2 + R-(CH2)n-CH2-CH2 ------------------------------------->R-(CH2)n-CH2-CH3 +  R-(CH2)n-CH=CH2   
                                                                      Dead polymer