EMBEDDING 81 



ethanol. It would then gradually harden by polymerization, if left 

 at room-temperature, but the process is too slow for practical 

 use. It is necessary to add an activator or so-called catalyst. The 

 latter name is inaccurate, for, as we shall see, the substance in 

 question actually forms part (though a very small part) of the 

 hardened material. 



The extremely active substance that promotes polymerization 

 is the free phenyl radicle, H5C6. It will be remembered that free 

 radicles differ from the vast majority of chemical compounds in 

 possessing an uneven number of electrons in the sum-total of 

 their constituent atoms. In the formulae shown here, it will be 



H 



• • • 



•. C . . C 



H : C '• • C : H H : C *• '* C : H 



H :C .• C:H H :C / ..C :H 



•• C** •• C 



• • •• 



H H 



Diagram of the structure Comparable diagram of 



of benzene, showing the the structure of the free 



valency-electrons phenyl radicle 



noticed that 8 electrons of the outer shell are associated with each 

 carbon atom in benzene, but only 7 with one of the carbon atoms 

 in the free phenyl radicle. The formula for the latter is conveni- 

 ently written HjCe* to remind the reader of the existence of the 

 unpaired electron. 



Benzoyl peroxide, (C6H5CO)202, is used to produce the free 

 phenyl radicle. This colourless, crystalline solid may be regarded 

 as hydrogen peroxide in which each of the hydrogen atoms has 

 been replaced by benzoyl, C6H5CO~. It is an explosive, and care 

 should be taken not to grind it with metal instruments; for 

 storage it is damped with water. The tissue is transferred from 

 the unmixed monomer to a weak, freshly prepared solution of 

 benzoyl peroxide in the monomer. It is left in this at room- 

 temperature until the peroxide has had time to penetrate the 

 tissue before active polymerization begins. Since the peroxide 

 was damped, it is necessary once more to dry the methacrylate 



G 



