Guanidine and Amidine Organocatalysts for Ring-Opening Polymerization of Cyclic Esters

Abstract

1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD), N-methyl-TBD (MTBD), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) are effective organocatalysts for the ring-opening polymerization (ROP) of cyclic esters such as lactide (LA), δ-valerolactone (VL), and ε-caprolactone (CL). TBD is shown to polymerize LA, VL, and CL in a fast and controlled manner, whereas MTBD and DBU polymerized LA and addition of a thiourea cocatalyst led to the ROP of VL and CL being achieved. Each of the catalysts produced polymers displaying high end group fidelity, good correlation between theoretical and observed molecular weight, and linear relationships between conversion and molecular weight. The enhanced activity of TBD relative to MTBD and DBU is attributed to its bifunctionality, enabling the simultaneous activation of both the cyclic ester monomer and the alcohol group of the initiator/propagating species. Temperature-dependent NMR studies generated individual association constants for MTBD with benzyl alcohol and thiourea with VL. In combination with temperature-dependent ROP of VL in the presence of benzyl alcohol, MTBD, and thiourea, these data have led to the derivation of the activation energy for the ROP (49 ± 3 kJ mol-1). The simplicity of the reaction conditions, the ready availability of the catalysts, the variety of polymerizable cyclic ester monomers, and the exquisite control over the polymerization are demonstrated.