Ligand and initiator effect in late and early transition metal catalyzed living radical polymerizations

Date of Completion

January 2005


Chemistry, Inorganic|Chemistry, Polymer|Engineering, Materials Science




This thesis was concerned with the development of novel catalysts and initiators for living radical polymerization (LRP). The effect of metal, ligand and initiator was studied in the LRP of styrene using transition metal porphyrins as well as a series of 18 Ti complexes with O, N as well as substituted cyclopentadienyl ligands in conjunction with epoxide and halide initiators. ^ The nature of the metal center was first explored in a series of tetraphenyl porphyrins containing Cu, Ni, Pb, Pd, Pt, Zn, and Ag. Styrene polymerization in the presence of these catalysts showed a linear dependence of molecular weight (Mn) on conversion but also gave broad polydisperisties (PDI). This behavior was attributed to a copolymerization of the porphyrin with styrene. Subsequently, the Cp2TiCl-catalyzed radical ring opening (RRO) of epoxides was introduced as a novel initiating reaction for LRP mediated by the unprecedented reversible end-capping of the propagating chain by Ti(III). These results were supported by a combination of NMR, GPC and reinitiation experiments and generated polymers with very narrow PDI (<1.2) and functional OH chain ends. Ti(III) was also shown to be compatible with activated and unactivated halides yielding the first halide initiated LRP which does not follow an atom transfer polymerization mechanism. ^ Further studies of the stereoelectronic ligand effects in RRO and LRP ranked these catalysts as: L = metallocene (η5-RCp: R = H ∼ Et ∼ iPr ∼ tBu > Ind >> Cp*) >> O-ligands (alkoxides > bisketonates) >> N ligands (hydrotris(pyrazol-1-ylborato) > phthalocyanine), while X = Cl > Br >> F ∼ Me ∼ CO and LnTiCl m > Ln-1TiClm+1. Additional experimentation examined the effect of reaction conditions and optimum results were obtained with [St]/[epoxide]/[Ti]/[Zn] = 50/1/4/8 at 60 °C--90 °C in dioxane using a 1,4-butandiol diglycidyl ether. ^ Finally, the first example of a dual concurrent polymerization of styrene and ϵ-caprolactone was demonstrated using a single catalyst/initiator system (Ti/epoxide) and new criteria for quantifying the livingness of the process were outlined. ^