The Influence of the Metal (Al, Cr, and Co) and the Substituents of the Porphyrin in Controlling the Reactions Involved in the Copolymerization of Propylene Oxide and Carbon Dioxide by Porphyrin Metal(III) Complexes. 1. Aluminum Chemistry

Abstract

The reactivities of aluminum(III) complexes LAlX, where L = 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TFPP), and 2,3,7,8,12,13,17,18-octaethylporphyirn (OEP) and X = Cl or OEt, have been studied with respect to their ability to homopolymerize propylene oxide (PO) and copolymerize PO and CO₂ to yield polypropylene oxide (PPO) and polypropylene carbonate (PPC), respectively, with and without the presence of a cocatalyst, namely, 4-dimethylaminopyridine (DMAP) or a PPN⁺ salt where the anion is Cl⁻ or N₃⁻. In the presence of a cocatalyst (0.5 equiv), the TFPP complex is the most active in copolymerization to yield PPC, with the latter being effective even at 10 bar CO₂. An increase in the PPN⁺X⁻/[Al] ratio decreases the rate of PPC formation and favors the formation of propylene carbonate, (PC). Studies of the polymers formed in reactions involving Al-alkoxide initiators and PPN⁺ salts by mass spectrometry indicate that one chain is grown per Al center. These results are compared with earlier studies where the reactions display first order kinetics in the metal complex.