On the importance of xanthate substituents in the MADIX process

Abstract : The structural effect of miscellaneous xanthates RS(C=S)OZ' on the level of control of free radical polymerizations was investigated. Such polymerizations referred as to the MADIX process involve the use of xanthates as reversible chain transfer agents (CTAs). In the case of S-alkyl-O-ethyl xanthates, RS(C=S)OEt, the process involves slow degenerative transfer of xanthate end-groups between polymer chains. This leads to styrene and acrylate-derived polymers with predetermined molar masses at high conversion and polydispersity index (PDI) between 1.5 and 2. In this series, the more substituted and stabilized the R leaving group, the better the control over molar masses as a function of the monomer conversion. Excellent control can be achieved in the polymerization of vinyl esters, acrylic acid and acrylamide. The reactivity of the C=S double bond of xanthates can be dramatically increased towards polystyryl and polyacrylyl radicals by incorporating electron-withdrawing substituents in the activating Z=OZ' group of S-(1-ethoxycarbonyl)ethyl-O- alkyl xanthates, C2H5OCO(CH3)CH-S(C=S)OZ'. This allows the preparation of polymers with controlled molar masses and PDI close to unity.