Squaramide and amine binary H-bond organocatalysis in polymerizations of cyclic carbonates, lactones, and lactides

Multiple combinations of six squaramides (Sq) and eight amines as the co-catalysts were tried in ring-opening polymerizations (ROPs) of cyclic carbonates, lactones, and lactides. Sq and sparteine co-catalysts enabled ROPs of trimethylene carbonate (TMC) to poly(trimethylene carbonates) (PTMCs) with benzyl alcohol (BnOH) as the initiator. The polymerization proceeded in 3 to 16 h without decarboxylation to afford polycarbonates with precise molecular weights (M_n,NMR = 1.95 to 10.1 kg mol^-1) and narrow polydispersity indices (D= 1.12-1.17). ¹H NMR, SEC and MALDI-ToF MS measurements of the obtained PTMCs clearly indicated the quantitative incorporation of the initiator at the chain end. Kinetics and chain extension experiments demonstrated the controlled/living nature for the ROP of TMC using Sq and sparteine. NMR titration experiments confirmed that the polymerization proceeded in a H-bonding dual activation mechanism. In addition, 1,3-propanediol, pentaerythritol, propargyl alcohol, furfuryl alcohol and N-(2-hydroxyethyl)maleimide were used as functional initiators leading to production of α,ω-dihydroxy telechelic, star-shaped, and clickable end-functionalized polycarbonates. Homopolymers of valerolactone (VL), caprolactone (CL), and lactide (LA) and diblock copolymers PVL-b-PTMC and PCL-b-PTMC were successfully synthesized by using Sq and 1,8-diazabicyclo[5.4.0]undec-7-ene binary co-catalysts with BnOH as the initiator in dichloromethane at room temperature. Block copolymers PTMC-b-PVL, PTMC-b-PCL, and PTMC-b-PLA were successfully obtained by using the binary catalysts. Squaramides combined with amine co-catalysts are a generally applicable polymerization tool.