Molecular Modeling of a Key Step In Metal-Mediated Controlled Free Radical Polymerization of Acid Monomers

Metal-mediated controlled free radical polymerization processes such as ATRP and SET-LRP offer a route to a variety of controlled structure polymers.  The practical utility of these processes has been somewhat limited by their limited ability to incorporate acid monomers.  It has been presumed that acid monomers interfere with the control mechanism by protonating key ligands on the metal.  However, it has been demonstrated recently that SET-LRP of methyl methacrylate (MMA) can proceed in an acid environment, with kinetics similar to those observed in the absence of acid.  Copolymerization of moderate amounts of methacrylic acid (MAA) with MMA could be achieved via SET-LRP, but the reaction kinetics were significantly slower than those for MMA homopolymerization in the presence of acid.  This suggests that it is the incorporation of acid monomer, not the presence of acid, which inhibits metal-mediated controlled free radical polymerization. 

The key activation step in metal-mediated controlled polymerization processes is assumed to be cleavage of a terminal C—X (X = halogen) bond to form a C�� radical.  The radical-tipped chain can add monomer and so is active.  The C—X –tipped chain is dormant.  Molecular modeling suggests that under some conditions, the dormant chain can undergo heterolytic cleavage of the C—X bond, forming a halide anion and a dormant/inactive polymeric cation.  For (meth)acrylate polymers, the polymeric cation is stabilized by a lactone-like ring.  If the penultimate monomer unit is a (meth)acrylic acid anion, the ring structure is particularly stable.  This finding has implications for the choice of polymerization conditions for metal-mediated controlled free radical polymerization of monomeric acids.