Mechanistic studies of the metal catalyzed formation of polycarbonates and their thermoplastic elastomers

摘要

Studies concerning the formation of industrially useful polycarbonates are thefocus of this dissertation. Of particular importance is the biodegradable polymer,poly(trimethylene carbonate) which has a wide range of medical applications. Theproduction of polycarbonates can be achieved by the ring-opening polymerization ofcyclic carbonate, or the copolymerization of carbon dioxide and oxiranes or oxetanes.For the production of polycarbonates from these monomers, Schiff base metalcomplexes have been designed, synthesized, and optimized as catalysts. Detailed kineticand mechanistic studies have been performed for the ring-opening polymerization ofcyclic carbonates, as well as the copolymerization of carbon dioxide and oxiranes oroxetane. In addition, the copolymerization of cyclic carbonates and cyclic esters tomodify the mechanical and biodegradable properties of materials used for medicaldevices has been studied using biocompatible metal complexes. In the process for ring-opening polymerizations of trimethylene carbonate orlactides, Schiff base metal complexes (metal = Ca(II), Mg(II) and Zn(II)) have beenshown to be very effective catalysts to produce high molecular weight polymers withnarrow polydispersities. Kinetic studies demonstrated the polymerization reactions toproceed via a mechanism first order in [monomer], [catalyst], and [cocatalyst] if anexternal cocatalyst is applied, and to involve ring-opening by way of acyl-oxygen bondcleavage. The activation parameters (?H?, ?S? and ?G?) were determined for ringopeningpolymerization of trimethylene carbonate, ring-opening polymerization oflactides, and copolymerization of trimethylene carbonate and lactide.In the process for copolymerization of carbon dioxide and oxetane, metal salenderivatives of Cr(III) and Al(III) along with cocatalyst such as n-Bu4NX or PPNX (PPN= bis(triphenylphosphine)iminium, and X = Br, Cl and N3) have been shown to beeffective catalysts to provide poly(trimethylene carbonate) with only trace amount ofether linkages. The formation of copolymer is proposed not to proceed via theintermediacy of trimethylene carbonate, which was observed as a minor product of thecoupling reaction. To support this conclusion, ring-opening polymerization oftrimethylene carbonate has been performed and kinetic parameters have been comparedwith those from the copolymerization of carbon dioxide and oxetane.