ALUMINUM ALKOXIDES - A FAMILY OF VERSATILE INITIATORS FOR THE RING-OPENING POLYMERIZATION OF LACTONES AND LACTIDES

Aluminium alkoxides, such as Al(O(i)Pr)3 or Et(3-p)Al(O(CH2)2X)p with 1 less-than-or-equal-to p less-than-or-equal-to 3, are very effective in initiating the polymerization of lactones, e.g. epsilon-caprolactone and delta-valerolactone, and lactides (D,L- or L,L-isomers). The ring-opening polymerization proceeds through a "coordination-insertion" mechanism that involves the selective rupture of the acyl-oxygen bond of the monomer and its insertion into an Al-O bond of the initiator. Polymerization is typically "living" and allows block copolyesters with perfectly controlled molecular weight and composition to be prepared. Aluminium alkoxides carrying functional alkoxy groups (X = -Br, -CH2-NEt2, -CH2-CH=CH2, -CH2-OC(O)-C(Me)=CH2...) provide asymmetric telechelic polyesters (end-groups being -X and -OH, respectively) and very interesting polyester macromonomers. Coupling the asymmetric telechelic chains via the hydroxyl end-group - or better its Al alkoxide precursor - is a straightforward way to the symmetric telechelic polymer bearing the X functional group.