On the difference of reactivities of various aggregated forms of aluminium triisopropoxide in initiating ring-opening polymerizations

The polymerization of ε-caprolactone (εCL) initiated with aluminium triisopropoxide (Al(OⁱPr)₃) trimer ( A ₃) and/or tetramer ( A ₄) was studied. The rate of A ₃ ⇔︁ A ₄ interconversions in the diluted (≤0,1 mol · L^-1) C₆D₆, C₆D₆/εCL, and THF/εCL solutions were found to be slow, when compared with the rate of propagation. Comparison of the ¹H NMR spectra of the initiators with those of the polymerization mixtures revealed that A ₃ is much more reactive than A ₄ in their reactions with εCL. From the initiator reacted with εCL all threeOⁱPr groups from Al(OⁱPr)₃ are transferred into the poly(εCL) as end groups. Kinetic studies of polymerization confirmed the large reactivity difference between A ₃ and A ₄.