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Description
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The development of copolymers derived from L-lactide (L-LA) and ε-caprolactone (CL) represents a sustainable strategy in the design of next-generation biodegradable materials, alternative to conventional non-degradable industrial polymers. In this work, novel dialkyl and alkyl/alkoxide aluminum complexes supported by scorpionate-derived ligands have been prepared. Thus, dialkyl derivatives [AlR2(κ2-NN’)] (κ2-NN’ = pbpamd, R = Me 1, Et 2; tbpamd, R = Et 3) were easily obtained by reaction of AlR3 with the corresponding protio ligand. Complexes 1 and 2 adopt a chelating coordination mode, while complex 3 can display a CH– 3a or NH– 3b tautomer, depending on the experimental conditions. Further exposure of 1 and 3a to dry air afforded two mixed alkyl-alkoxide aluminum dimers [Al(R)(μ-OR)(κ2-NN’)]2 (κ2-NN’ = pbpamd, R = Me 4; tbpamd, R = Et 5) by oxygen insertion on the Al–C bond. 2D DOSY NMR experiments evidenced the different dimerization tendencies of 4 and 5. X-Ray crystal structure analysis of 2, 4 and 5 confirmed unambiguously the different ligand arrangements proposed. Interestingly, complexes 1, 3a, 3b, 4, and 5 were initially evaluated as initiators for the ring-opening copolymerisation (ROCoP) of L-lactide and -caprolactone under different procedures. Among them, alkyl/alkoxide 4 and 5 exhibited the highest activity, with 5 showing moderate random preference in the poly(L-lactide)-co-poly(ε-caprolactone) copolymers, via a “one-pot/one-step” method. Alternatively, dialkyls 1–3 promoted blockier architectures, facilitating preferential L-LA insertion, under these conditions. More importantly, initiators 1 and 5 successfully promoted diblock poly(L-lactide)-b-poly(ε-caprolactone) materials, via a “one-pot/two-step” procedure. Expectedly, copolymers composition directly impacted over material thermal properties. Thus, DSC analysis of mild gradient-type architectures revealed lower Tm values than blockier microstructures, whereas diblock compositions exhibited two endotherms in the range of PCL and PLLA homopolymers, respectively. Significant impact of monomer incorporation in the PLA-co-PCL copolymers was also revealed in the TGA profiles; as PLLA block increases, the degradation onset temperature clearly decreases. (2026-05-21)
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