Reverse orders of reactivities in the polymerization of cyclic esters using N(2)O(2) aluminium alkoxide complexes.

Three aluminium alkoxide complexes containing N(2)O(2) bis(phenoxy)-amine ligands were synthesized from reactions of the corresponding N(2)O(2) ligands with Al(O(i)Pr)(3) in toluene. Different amine side chains of the ligands included pyridine (1), CH(2)NMe(2) (2), and CH(2)NEt(2) (3). The related chloro aluminium analog (4) was prepared from a reaction between AlCl(3) and the potassium salt of the N(2)O(2) ligand having CH(2)NMe(2) side chain. X-Ray crystallography reveals that complexes 3 and 4 have a monomeric five-coordinate aluminium center. Complexes 1-3 catalyzed the polymerization of epsilon-caprolactone (epsilon-CL) at 70 degrees C in toluene with the relative reactivities of 1<2< 3. In contrary, only complex 1 was active for the polymerization of lactide under the same polymerization conditions. (1)H NMR spectroscopy shows that treatment of 2 with 1 equivalent of lactide afforded the ring-opened product L(2)Al-OCH(Me)C(O)OCH(Me)C(O)O(i)Pr. Electronic effects are believed to be responsible for the observed trend in the epsilon-CL polymerization rates. On the other hand, steric hindrance at the amine side chain is the main contributor to the observed rates of lactide polymerization.