conditions. The chain growth mechanism involves the addition of the reactive end of the growing chain across the double bond of the monomer. The addition polymerisation can follow any of the following three mechanisms depending upon the reactive intermediate involved in the process. i.
Free radical polymerisation ii. Cationic polymerisation iii. Anionic polymerisation Free radical polymerisation When alkenes are heated with free radical initiator such as benzoyl peroxide, they undergo polymerisation reaction. For example styrene polymerises to polystyrene when it is heated to ionic with a peroxide initiator.
The mechanism involves the following steps. . initiation – formation of free radical benzoyl peroxide phenyl free radical + 2CO . Propagation step CH • phenyl free radical C H Styrene (monomer) C • C H Stabilised radical The stabilized radical attacks another monomer molecule to give an elongated radical C • + C H C H C H CH C H C H CH C H growing chain monomer CH ( ) n C H Polymer cahin • • Chain growth will continue with the successive addition of several thousands of monomer units.
XII U15 Chemistry in XII U15 Chemistry in - - - - Termination The above chain reaction can be stopped by stopping the supply of monomer or by coupling of two chains or reaction with an impurity such as oxygen. CH ( ) n C H • CH ( C H CH CH ) n C H . . Preparation of some important addition polymers .
Polythene It is an addition polymer of ethene. There are two types of polyethylene i) HDPE (High Density Polyethylene) ii) LDPE (Low Density polyethylene). LDPE It is formed by heating ethene at to o C under oxygen as a catalyst. The reaction follows free radical mechanism.
The peroxides formed from oxygen acts as a free radical initiator. n CH = CH o - C