A DFT study of the ring-opening polymerization mechanism of L-lactide and ε-caprolactone using aluminium salen-type initiators: Towards an understanding of their reactivities in homo- and copolymerization

The initiation and first propagation steps of the ring-opening polymerization (ROP) of (L)-lactide (L LA) and epsilon-caprolactone (epsilon-CL) mediated by aluminium salen-type initiators with formulae LAIOCH(3) (LH2 =N,N'-bis(salicylidene)-1,2-ethylenediamine (la) and N,N',-bis(salicylidene)-2,2-dimethy1-1,3propanediamine (lb) as well as their corresponding 3,5-dichloro-substituted counterparts, 2a and 2b, respectively) are investigated at the molecular level using density functional theory (DFT). The results show that, even though the apparent ROP mechanisms are similar for all systems studied, the activation barriers for the rate -limiting transition states are different and depend on the initiator. The reactivities of the initiators toward the ROP of the two monomers are predicted to increase in the order 1a < 2a < 1 b <2b, in good agreement with experimentally and theoretically observed trends for systems with different bridging units and electronic properties of the ligand substituents. However, this does not entail similar reactivities in all polymerization processes. An explanation for this, here the different reactivities observed for homo-and copolymerization (for the two types of monomers) is found in their different binding affinities with the propagating active species. Several experimentally observed features, such as e.g. reaction rates, in other related homopolymerization and random copolymerization systems with these two monomers can also be rationalized. (C) 2017 Elsevier B.V. All rights reserved.