LA-mediated isotactic control

Another crucial requirement of LA-mediated isotactic control is that the LA remains in the terminal-penultimate position during propagation that is, that the terminal-penultimate binding position is the most reactive in the polymerization. However, chelation can occur not only at the terminal-penultimate position but at any other point along the polymer chain, including the penultimate-antepenultimate and/or the terminal-incoming... 

Considering the proposed mechanism of LA-mediated isotactic control (Scheme 4), the reported synthetic radical transformations provide some usefiil insights into its potential modes of failure (Scheme 5). We reexamine these potential modes of failure while concurrendy drawing on synthetic hterature where relevant. We should clarify that our goal is not simply to highhght the factors that likely cause LA and condition dependencies but... 

Undoubtedly, one of the most challenging limitations of LA-mediated isotactic control is the requirement for the position-specific coordination of the terminal and penultimate groups of the polymer terminus by LA. This requirement is fairly polymerization specific and has only limited synthetic parallel, as generally substrates for small-molecule transformations are deliberately selected so as to eliminate (or at least minimize) the potential formation of alternative chelates. Indeed, the vast majority of successful LA-mediated chelation control has been reported for substrates that only possess two donor groups and hence only one potential site for chelation. In contrast, in a typical radical polymerization there are a multitude of different sites of chelation, not only at the polymer terminus but at any point along the polymer chain. While this aspect of selectivity is only pertinent in a few of the reported synthetic transformations, these few studies do offer some very useful insights that highlight issues that may arise for polymerization processes. 

The interaction between modestly strong LAs and relatively weak Lewis bases afibrds excellent stereocontrol in many synthetic radical transformations. While weak coordinative interactions may limit the effectiveness of weak LAs for isotactic control, it is unlikely to be a limiting factor in many of the LA-mediated radical polymerizations investigated to date particularly those that employ strong LAs (such as rare earth metal triflates). 

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