Bifunctional Squaramides as Organocatalysts for Lactide Polymerization: Catalytic Performance and Comparison with Monofunctional Analogues

Amine-functionalized squaramides 1 and 2 were prepared and shown to be suitable polymerization organocatalysts for the controlled ring-opening polymerization (ROP) of l-lactide (lLA) in the presence of an alcohol source such as BnOH (which acts as an initiator) to afford chain-length-controlled and narrow-dispersion poly(l-lactide) (PLLA) under mild reaction conditions. The ROP experimental and polymer analysis data are consistent with the action of 1 and 2 as bifunctional hydrogen-bonding (HB) catalysts that are able to activate both the lactide monomer and initiator BnOH thanks to their dual HB acceptor and donor properties. As a comparison, aminosquara-mide 3, a direct analogue of 1 but a weaker HB donor because of the absence of electron-withdrawing NH substituents, displays little lactide ROP activity, which highlights the key role of monomer activation through HB in the present systems. Unlike aminosquaramides 1 and 2, related monofunctional squaramides 4 and 5 are inactive in l-LA ROP in the presence of BnOH, but the addition of NEt3, as an external HB acceptor, allows the ROP to proceed with the production of well-defined PLLA. A cooperative dual activation with an activated monomer/activated chain-end mechanism is most likely operative in the lactide ROP mediated by 1 and 2 in the presence of BnOH.