Microstructural control of L-Lactide/ε-caprolactone copolymers using amidinate-based aluminium catalysts

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 aluminium complexes supported by scorpionate-derived ligands have been prepared. Thus, dialkyl derivatives [AlR 2 (κ 2 -NN’)] (κ 2 -NN’ = pbpamd, R = Me 1 , Et 2 ; tbpamd, R = Et 3 ) were easily obtained by reaction of AlR 3 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 aluminium dimers [Al(R)(μ-OR)(κ 2 -NN’)] 2 (κ 2 -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 ε -caprolactone under different procedures. Among them, alkyl/alkoxide 5 exhibited the highest activity and showed 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 T m 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.