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(O i Pr) 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 ) C 6 D 6 , C 6 D 6 /εCL, and THF/εCL solutions were found to be slow, when compared with the rate of propagation. Comparison of the 1 H 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O i Pr groups from Al(O i Pr) 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 .