A principal paradigm of catalysis involving coordination complexes is that high selectivities require rigid coordination environments. Flexible catalytic sites adapt easier to unfavourable monomer coordination and lead to reduced differentiation of transition states and thus reduced (stereo-)control. This is undoubtedly true if stereocontrol is obtained from a chiral catalytic site, where fast catalytic site racemication is highly detrimental, but typically also if the growing polymer chain provides the chiral information.<br/><br/>An exemption from this are “catalytic-site mediated chain-end control” mechanisms, in which a chiral catalytic site is responsible for stereocontrol, but the chiral information is obtained from growing polymer chain. Here, fast catalytic-site isomerisation is a requirement, not a hindrance to stereocontrol. We present mechanistic investigations on diiminopyrrolide copper complexes, which show that these complexes polymerize <i>rac</i>-lactide following a “catalytic-site mediated chain-end control” mechanism, as well as an example how stereocontrol was introduced into unselective zinc catalysts by facilitating catalytic-site epimerization.

Increasing stereocontrol in lactide polymerization with flexible catalysts

2019 Fall National Meeting

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