82 CYTOLOGICAL TECHNIQUE 



with anhydrous sodium sulphate. The temperature is then in- 

 creased to about 50° C and the hardening starts. 



Each molecule of benzoyl peroxide loses two of carbon 

 dioxide, and two phenyl radicles are set free. Each phenyl radicle 

 reacts with a monomer molecule, forming a covalent bond with 



H 

 the carbon of the C= group. The compound necessarily has an 



H 

 uneven number of electrons, but the odd electron now appears 



CH3 



H I 



HaCe-C-C- 



\0(CH,)3CH3 



A phenyl radicle combined with butyl methacrylate 

 Note the unpaired electron. 



in a new place, at the other end of the methacrylate. Meanwhile 

 the double bond has become single. The end of the molecule 

 provided with the odd electron has become as active as the 

 phenyl radicle was, and in exactly the same way. It combines 

 with another monomer, and an unpaired electron is thus pro- 

 duced at the extremity of the latter. So the process goes on, and a 

 high polymer is built up. The whole of it consists of methacrylate 

 monomers, except one extremity of the chain. 



This process of polymerization leads to the production of very 

 long molecules, but it does not continue indefinitely. The growing 

 end of the chain may come up against another growing end, or 

 against a free radicle; in either case combination occurs, and the 

 chain becomes 'dead'. 



The process that has been described would lead only to the 

 formation of molecules having the form of long, thin threads. 

 In fact, however, branching and net-formation occur. Branching 

 takes place in various ways in addition-polymers,"" but the 

 following is probably what happens in methacrylates.^^^ A 

 carbon atom, other than a terminal one, becomes active. The 

 atom is thought to be one of those in the butyl group. This 

 carbon atom lets go of one of its hydrogens, which departs with 



