Isotactic Regulation in the Radical Polymerization of Calcium Methacrylate: Is Multiple Chelation the Key to Stereocontrol?

Abstract

Accurate quantum chemistry is used to explain the origins of isospecificity in radical polymerization of calcium methacrylate hydrate (CaMA). Distonic radical-cation interactions are shown to be crucial in determining the reactivity of different coordination structures. Cation coordination to the terminal and incoming monomer side chains reduces radical‐cation separation, enhancing the reactivity of these modes over the stereocontrolling terminal‐penultimate binding modes. This explains why Lewis acid‐mediated radical polymerization often fails to produce highly isotactic polymer for common monomers such as methyl methacrylate. However, theoretical calculations suggest that the poly(CaMA) terminus forms a chelated bridging scaffold in N,N‐dimethylformamide (DMF), which involves the terminal, penultimate and incoming monomer carboxylate groups. This scaffold simultaneously activates the incoming monomer toward propagation and regulates the relative orientation of the terminal and penultimate side chains. The bridging scaffold is disrupted in more polar solvents and/or if alternative nonchelating counter‐cations are employed, leading to loss of isotactic control. These results suggest that higher levels of isotactic control may be achievable if reaction conditions are optimized to favor bridging scaffold formation. The broader importance of these findings to stereocontrol in radical polymerization is also discussed. Show more