Towards Stereocontrol in Radical Polymerization

Abstract

Despite over 50 years of research, control of the stereochemistry in free-radical polymerization using simple and scalable methods remains elusive for most common monomers. An attractive strategy, which has received considerable attention, is the use of Lewis acids to coordinate and constrain the terminal and penultimate side-chains of the growing polymer radical. Unfortunately, the control achieved to date usually is modest at best. In this work, a combination of theory and experiment has been used to explore why current methods are not optimal, with a view to designing better control strategies. In the course of this work, the dramatic effects of Lewis acids on other radical processes, such as photoinitiation, were also explored, along with other aspects of control in radical polymerization. The main findings of this work are as follows. Theoretical calculations revealed that a key problem with current Lewis acid-based control agents is their binding selectivity. Even when they bind the terminal and penultimate side-chains selectively, this binding mode stabilizes and deactivates the propagating radical. Moreover, it was found that a non-stereocontrolling terminal-monomer binding mode catalyses propagation and dominates the reaction. This propagation catalysis, which is useful in its own right, was confirmed experimentally using pulsed laser polymerization. On this basis, a better control strategy, targeting simultaneous binding of the monomer, terminal and penultimate side-chains, was proposed. Indeed, a further theoretical examination of methacrylic acids salts established that the reasonably high isospecificity reported results from the formation of bridging scaffolds. The use of methacrylic salts as ionic auxiliaries is suggested as one potential route to isotactic polymer, and an improved method for the conversion of calcium methylate to poly(methyl methacrylate) was designed for this purpose. Among the other important findings of this work was the discovery of dramatic effects of Lewis acids on photoinitiators. While these effects hampered pulsed laser polymerization experiments with strong Lewis acids, strategies for avoiding them were devised. While the effects of Lewis acids on photoinitiation were shown to be very complex, the results presented raise the exciting prospect of a simple route to visible light photoinitiation. Lewis acid/ base interactions were also shown to have dramatic effects on the redox activity of surface tethered nitroxide radicals, work that lays a basis for ongoing research on the use of electric fields to control nitroxide mediated polymerization.