Mechanistic Perspectives on Stereocontrol in Lewis Acid-Mediated Radical Polymerization: Lessons from Small-Molecule Synthesis

The regulation of stereochemistry in radical polymerization has thus far proved enormously challenging, being the subject of more than 50 years of intense and innovative research. The relatively recent development of living radical polymerization has revitalized interest in stereocontrol, as the lack of tacticity control is now the greatest obstacle to the synthesis of precisely controlled macromolecules by highly versatile and efficient radical-based processes. Lewis acids (LAs) have been employed in an attempt to overcome the relatively poor stereoselectivity of unmediated radical polymerization, although the level of stereocontrol achieved is highly variable and condition dependent. We examine the use of LAs in radical polymerization from a mechanistic perspective, considering the underlying mechanism of tacticity determination and the likely requirements for effective stereocontrol. Concurrently, we examine the use of LAs in synthetic radical transformations, where they can facilitate very high levels of diastereocontrol but similar variability and condition dependencies are also noted. This chapter outlines and assesses some of the key factors likely to underlie the success or failure LA-mediated isotactic control in radical polymerization.