Thiourea binding with carboxylic acid promoted cationic ring-opening polymerization

Abstract

H-bonding organocatalysis using (thio)urea/amine achieved massive success. H-bonding binding with Brønsted acid catalysis, especially in polymerizations, was scarcely explored. Here we disclosed 1,3-bis-(3,5-bis(trifluoromethyl)phenyl)thiourea (TU) as hydrogen bond donor (HBD), combined with trifluoroacetic acid (TFA) as a Brønsted acid (BA), promoted efficient controlled/living ring-opening polymerization (ROP) of δ-valerolactone (δ-VL) and ε-caprolactone (ε-CL) at room temperature. The ROPs of δ-VL and ε-CL with benzyl alcohol (BnOH) as the initiator yielded poly(δ-valerolactone) (PVL, Mn, NMR = 1900–9900 g mol−1, Mw/Mn = 1.28–1.34) and poly(ε-caprolactone) (PCL, Mn, NMR = 2400–11,600 g mol−1, Mw/Mn = 1.19–1.35) with predicted molecular weights and narrow dispersities. A plausible mechanism of the ROP promoted by associations of TU with TFA through double hydrogen bonding was suggested, in which stabilizing of the carboxylate anion and enhancing of the Brønsted acidity made the catalytic ROPs by TU–TFA viable. The quantitative incorporation of the initiator into the polymer chains was demonstrated by 1H NMR and MALDI-ToF MS measurements. The controlled/living fashion of the polymerization was supported by the kinetics and chain extension experiments. Synthesis of well defined block copolymers of PVL-b-PCL and PCL-b-PVL verified again the ROPs were in controlled/living manner, and suggested the hydrogen bond donor binding with Brønsted acid catalysis to be a generally applicable method.