Controlled Polymerization of ϵ-Caprolactone using Aluminum and Zinc Complexes with Iminophosphorane Ligands

Abstract

Several novel aluminum complexes [Al(R1)2{o-O C(R)=C(H)P(Ph2)=N)C6H4(3,5-Me2 -pyrazolyl)} (R=Ph, R1=Me, 3; R=Ph, R1=Et, 4; R=tBu, R1=Me, 5) had been prepared. These aluminum complexes were structurally characterized by nuclear magnetic resonance spectroscopy (1H, 13C and 31P) and elemental analyses. The aluminum complex 3 was determined by X-ray diffraction. The catalytic activities of the aluminum complexes towards the ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL) were investigated. Catalysis of two known zinc complexes with the same ligands, [Zn(Et) {o-(OC(R)=C(H)P(Ph2)=N)C6H4(3,5-Me2-pyrazolyl)}] (R=Ph, 6; R=tBu, 7), in the ROP of ϵ-CL was also determined. The catalytic results show that these complexes are effective catalysts in the ROP of ϵ-CL. Kinetics of the polymerization was studied. These polymerizations reaction are consistent with the first-order kinetic equation. The molecular weight of the polycaprolactone (PCL) is linear with the monomer conversion, and the molecular weight distribution is narrow. These kinetic studies confirm that the catalytic process is controllable.