Phosphazene-catalyzed ring-opening polymerization of ε-caprolactone: influence of solvents and initiators

Phosphazene base (t-BuP2) catalysed metal-free ring-opening polymerization of ε-caprolactone (ε-CL) at room temperature with various protic initiators in different solvents was investigated. The polymerization proceeded, in most cases, in a controlled manner to afford poly(ε-caprolactone) with low dispersities and expected molecular weights. Kinetic studies showed that when a primary alcohol was used as an initiator the polymerization rate in different solvents followed the order of dichloromethane ≫ toluene > 1,4-dioxane ≈ tetrahydrofuran. Extremely fast polymerization of L-lactide (LLA), which was added as a second monomer, was observed in different solvents giving rise to poly(ε-caprolactone)-b-poly(L-lactide) diblock copolymers with neat PLLA blocks despite incomplete conversion of ε-CL. The dependence of polymerization rate on the concentrations of ε-CL and t-BuP2 was also revealed. In addition to primary alcohol, the feasibility of using other protic initiators, such as secondary alcohol (either on a small molecule or a polymer chain-end), (aliphatic/aromatic) amide, carboxylic acid, phenol and thiophenol, was also investigated. These studies provided important information for designing a metal-free route towards polyester-based (bio)materials.