H-bonding binary organocatalysis promoted amine-initiated ring-opening polymerizations of lactide from polysarcosine to diblock copolymers

Abstract

Alcohol-initiated ring-opening polymerization (ROP) of cyclic esters for synthesizing polyesters was well established, but amine-initiated ROP of cyclic esters was rare. A challenge is slow initiation and fast propagation in the amine-initiated ROPs. To address the difficulties, an ionic H-bond donor (iHBD) and H-bond acceptor (HBA) binary organocatalysis in amine-initiated ROP of lactide (LA) was developed. Guanidinium hexahydro-2H-pyrimido[1,2-a] pyrimidin-1-ium [(HppH2)+] and tertiary amine sparteine (SP), cooperatively played the role of iHBD/HBA binary catalysts, efficiently promoted ROP of LA from amine initiators toward polylactide (PLA) and polysarcosine-block-polylactide diblock copolymer (PSar-b-PLA). Benzyl amine and N-methylbenzylamine initiated ROPs of LA under mild conditions produced PLAs with predictable molecular weights (Mn,NMR = 2.9–17.1 kg mol−1) and narrow dispersities (ÐM,SEC = 1.13–1.23). Macroinitiator PSar copolymerized with LA under the iHBD/HBA catalysis, producing PSar-b-PLAs with controlled molecular weights (Mn,NMR = 5.7–14.8 kg mol−1) and low dispersities (ÐM,SEC = 1.16–1.21). Kinetics and chain extension experiments confirmed that the amine-initiated ROP of LA in presence of (HppH2)+BF4−/SP was in controlled/living manner. 1H NMR, 13C NMR, SEC, and MALDI-ToF MS analyses indicated that the obtained PLA was exclusively initiated from the corresponding amine with amide linkage. An iHBD/HBA binary activation mechanism was proposed.