| Summary: | <p>Switchable catalysis is a useful one-pot method to prepare block polyesters utilising a single catalyst exposed to a mixture of monomers. The catalyst is switched between lactone ring-opening polymerization (ROP) and epoxide/anhydride ring-opening copolymerization (ROCOP) by controlling its chain-end chemistry. Here, novel aluminium(<small>III</small>) (<strong>1</strong>) and cobalt(<small>III</small>) complexes (<strong>2</strong>), coordinated by <em>ortho</em>-vanillin derived salen ligands, show excellent switchable catalytic activity and selectivity for the preparation of poly(ε-decalactone-<em>block</em>-cyclohexene phthalate-<em>block</em>-ε-decalactone) [PDL-<em>b</em>-PCHPE-<em>b</em>-PDL]. Both complexes have competitive activities with a commercial chromium salen catalyst for epoxide/anhydride ROCOP (TOF<small><sub>Cr(III)</sub></small> = 1200 h<small><sup>−1</sup></small> <em>vs.</em> TOF<small><sub>Al(III)</sub></small> = 350 h<small><sup>−1</sup></small>, 1 mol% catalyst loading <em>vs.</em> anhydride, 100 °C) and are significantly more active than the commercial catalyst for lactone ROP (TOF<small><sub>cr(III)</sub></small> = 3 h<small><sup>−1</sup></small> <em>vs.</em> TOF<small><sub>Al(III)</sub></small> = 300 h<small><sup>−1</sup></small>; 0.5 mol% catalyst loading <em>vs.</em> lactone, <em>T</em> = 100 °C). The catalysts are tolerant to low loadings (0.1 mol% <em>vs.</em> anhydride, 0.05% <em>vs.</em> lactone) and produce high molar mass triblock polyesters (<em>M</em><small><sub>n</sub></small> = 6–57 kg mol<small><sup>−1</sup></small>). The efficient production of high molar mass polyesters allows for future optimization of the block polyester thermal–mechanical properties and applications.</p>
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