Aluminium complexes of phenoxy-azo ligands in the catalysis of rac-lactide polymerisation

Fourteen new phenoxy-azo aluminium complexes comprising two series, namely, dimethyl{phenoxy-azo}aluminium complexes1a-7aand monomethyl{phenoxy-azo}aluminium complexes1b-7b, weresuccessfully synthesised and characterised. The molecular structure of complex4a, determined usingX-ray diffraction analysis, displayed a distorted tetrahedral geometry. The1H NMR spectrum of complex5brevealedfluxional behaviour caused by isomeric transformation that occurs in the solution at roomtemperature. The activation parameters determined by lineshape analysis of variable-temperature1H NMRspectra in toluene-d8are as follows:Delta H double dagger= 70.05 +/- 1.19 kJ mol(1),Delta S double dagger= 21.78 +/- 3.58 J mol(-1)K(-1)and Delta G double dagger(298 K) = 63.56 +/- 0.11 kJ mol(-1). All aluminium complexes are active initiators for the ring-openingpolymerisation ofrac-lactide, and the polymerisations proceeded in a controlled manner and were living.In comparison, the catalytic activity of the dimethyl{phenoxy-azo}aluminium complexes was insignifi-cantly different from that of the corresponding monomethyl{phenoxy-azo}aluminium complexes. Thesteric factor of theortho-phenoxy substituent was observed to exert a decelerating effect on the catalyticrate. Kinetic investigations revealedfirst-order dependency on both monomer and initiator concen-trations. Comparative catalytic investigations conducted on phenoxy-azo aluminium and phenoxy-iminealuminium complexes revealed that the former complexes exhibited lower catalytic activity.