Metal catalyzed copolymerization processes involving carbon oxides as substrates

Abstract

Studies concerning two different copolymerization processes are detailed in this dissertation: propylene oxide/CO2 coupling to afford poly(propylene carbonate) and Nbutylaziridine/ CO coupling to afford poly-??-butylalanoid. The copolymerization of propylene oxide and CO2 to form the industrially useful poly(propylene carbonate) has been investigated employing chromium(salen)N3 complexes as catalysts. Unfortunately the reaction could not be studied in real time via in situ infrared spectroscopy, thereby obtaining detailed kinetic data, because of the copolymer-limited solubility in most solvents. Studies employing batch reactor runs concentrating on varying the cocatalyst, the equivalents of cocatalysts, and the steric and electronic structure of the catalyst through modification of the salen ligand were undertaken. It was discovered that the optimal catalyst for copolymer selectivity vs. the monomeric propylene carbonate was one that contained a salen ligand with an electron withdrawing phenylene backbone and electron donating tert-butyl groups in the phenolate rings. This catalyst was used to investigate the effect of altering the nature of the cocatalyst and its concentration. The coupling of carbon monoxide and aziridines has been shown to be selective for comonomer-alternating enchainment in the presence of PhCH2C(O)Co(CO)4 to afford poly-??-peptoids. The mechanistic aspects of the reaction of CO and Nbutylaziridine by means of in situ infrared spectroscopy employing CH3C(O)Co(CO)3L (L = PPh3 and P(o-tolyl)3) as precatalysts was investigated. It was found the PPh3 precatalyst exists in solution under catalytic conditions as an equilibrium mixture of CH3C(O)Co(CO)3PPh3 and CH3C(O)Co(CO)4, and affords both poly-??-butylalanoid and the corresponding lactam as a side-product. By way of contrast, the P(o-tolyl)3 precatalyst which possesses the sterically bulky and labile phosphine ligand, affords only the acyl cobalt tetracarbonyl species in solution during catalysis with the selective production of the copolymer. Kinetic studies conducted with CH3C(O)Co(CO)3P(otolyl) 3 showed the coupling reaction to have a first order dependence on catalyst, a first order dependence on N-butylaziridine, and only a slight dependence on the concentration of CO over the pressure range 17-69 bar. The working mechanistic model for the copolymerization reaction involves first aziridine insertion into the cobalt-acyl bond, rate determining ring opening by the cobaltate species, followed by the migratory CO insertion.