Electrostatic repulsion between charged radicals

Individual rate constants for homogeneous and heterophase DADMAC homopolymerizations have been calculated from the kinetic models above. These constants are summarized in Table 4. The overall activation energies, also given in Table 4, are relatively high. These values likely result from the electrostatic repulsion between the positively charged radicals and the monomer cations. 

Recent studies by Cole-Hamilton and coworkers have shown that a family of neutral heteropentalenes will quench [Ru(bipy)3] at rates which are typically an order of magnitude greater than those observed for MV this is attributed to a lack of electrostatic repulsion between the chromophore and the quencher. Further, the radical anion of a related compound (2,1,3-benzothiadiazole-4,7-dicarbonitrile), generated by direct photolysis in the presence of EDTA, has been successfully stabilized in CTAB micelles revealing the possibility of very large charge separation yields for the sensitized reaction, especially when sensitized by a multi-negative chromophore. 

Low yields, in the radical-initiated polymerization of MA, are attributed to suppression by the strong electrostatic repulsion between monomer and monomer radical in each step of the polymerization. Initiator radicals which have the same electropositive charge would have only a very small tendency to attack the MA monomer double bond. It is also suggested that the MA radical always has one readily replaceable hydrogen, resulting in the formation of a conjugated polymer system.This monomer characteristic also makes for difficulty in polymerizing MA. 

---