Metal-insertion ring-opening polymerisation

The metal-insertion ROP owes its name to the propagation mechanism [Fig. 21.2]. After coordination of the metal-alkoxide with the carbonyl group of the lactone, the addition of the nucleophilic alkoxides takes place onto the electrophilic ester bond. Subsequently, an elimination reaction occurs via acyl-oxygen scission. The novel alkoxide will act as the newly generated propagating species. 

After the initial work on aluminium alkoxides, the metal-insertion ROP was extended to other metal ions including titanium [36-38], tin (11) [39-41], tin (IV) [42-44], zirconium [45, 46] and, more recently, rare-earth metals (Y, Er, Sm, Dy, Nd and La) [47-49]. 

Despite the fact that the metal ions enable to control the polymerisation, they also pose an important downside as the formed polymers will always contain metal residues. These metal ions might have detrimental health effects. Aluminium derivatives have already been linked to Alzheimer s disease, although this hypothesis is controversial [50], while tin octoate, despite being FDA approved, has already been shown to be cytotoxic. As tin levels often exceed 1000 ppm, the FDA has set a limit of 20 ppm for polyesters used for biomedical applications [51]. To circumvent this problem, attention has already been paid to less toxic catalysts including magnesium and calcium alkoxides. Conversely, attempts are being made to remove the catalyst residues from the polymer. 

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Ring-opening polymerisation initiated by metal alkoxides according to the insertion-coordination mechanism. 

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