EMBEDDING 79 



the next carbon atom from the carboxyl carbon. The aldehyde 

 corresponding to acryhc acid is acrolein, and from this exces- 

 sively pungent substance the acid derives its name. 



From our point of view, the important character of the acrylic 

 acids is their strong tendency to polymerize, and in doing so to 

 become solids possessing special characters. ^°^' ^^^ Polymeriza- 

 tion occurs by the addition of one molecule to another, the double 

 bond becoming single in the process. There is no condensation: 

 each repeated unit or 'segmer' consists of the same atoms as the 

 monomer. The substance formed is thus an addition-polymer. 



The hydrogen of the carboxyl group of the acrylic acids can be 

 replaced by methyl, ethyl, or other alkyl groups, without loss of 

 the tendency of the molecule to polymerize. The esters thus 

 formed are of greater practical value than the simple monomers. 

 Reaction (in a roundabout way) with normal butyl alcohol, 

 HO(CH2)3CH3, gives «-butyl acrylate. This, when polymerized, 

 is a very soft, rubbery substance, not adapted to the purposes of 

 microtomy. If, however, a methyl group be substituted for the 

 hydrogen attached to the next carbon to that of the carboxyl 

 group, a marked change occurs in the physical consistency of the 



CH3 

 H H H I 



— C— C— — C— C— 



« i^O H \^^0 



\0(CH2)3CH3 \0(CH,)3CH3 



A segmer of n-butyl A segmer of n-butyl 



acrylate polymer methacrylate polymer 



polymer. We now have, in «-butyl methacrylate polymer, a 

 hard, colourless, transparent solid, admirably adapted to our 

 needs. All the methacrylates are harder than the corresponding 

 acrylates, but the length of the esterifying alcohol also affects the 

 hardness. The shorter the alcohol, on the whole, the harder the 

 polymer. Methyl methacrylate — better known under its commer- 

 cial name of perspex — is the hardest of all. It is often used as an 

 embedding-medium for electron-microscopy, usually with the 

 admixture of butyl methacrylate. The latter by itself, however, 



