Acetic Acid and Homologues

A similar phencanenolr was obserred when acetic acid was replaced by propionic, acrylic and methacrylic acids However, with these initiators there was less pdymerisation than with AcOH, and more esterification. 

CH2CICOOH is somewhat stronger than acetic acid but remains within the family of w ak acids. Its pK in acetic add must close to 12.5 and in 1,2-dichloroethane it cxild around 17. Chloroacetic acid is appreciably dimerised in non polar solvents ) and its tendency towards homoconjugation with its anion is very pronounced ) (pKhbj = —2.94 in acetonitrile). 

Brown and Matheson found that this acid was too weak to induce the polymerisation of styrene, but that it could activate o-methylslyrene. This is yet another typical example of limiting situation where the critical parameter is the nucleophilicily of the mraiomer. That particular system will be discussed in the next secticxi tr ther with the bulk of Brown and Mathesai s work. 

Concomitant propagatirai by ionic and non-ionic active specks is not new. Perchloric acid and styrene give rise to a similar situation as will be seen later in this chapter. 

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