Transition metal carbenes : controlled ring-opening metathesis polymerizations and metathetical reactions with acyclic olefins

Citation

Wu, Zhe (1994) Transition metal carbenes : controlled ring-opening metathesis polymerizations and metathetical reactions with acyclic olefins. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xx2p-3468. https://resolver.caltech.edu/CaltechETD:etd-08162006-082323

Abstract

Although strained cyclic olefins undergo facile ring-opening metathesis polymerization (ROMP) catalyzed by transition metal alkylidene or carbene complexes, there are few examples of living ROMP systems because the rate of the polymerization is much faster than that of the initiation. The phosphines reversibly bind to both the initial and the propagating tungsten or molybdenum alkylidene complexes, thus altering the rates of both initiation and the propagation. Due to the steric bulk difference of the propagating species and the initiating complexes, phosphines bind more strongly to the propagating alkylidene than to the initial alkylidene, which reduces the rate of propagation much more than that of the initiation and results in a living system. The kinetics and mechanism for both initiation and propagation have been studied. By using this living system, linear and near monodispersed polybutadiene and polyethylene can be synthesized. Block copolymers of both practical and theoretical interests can be prepared.

Metathesis of functionalized olefins using a molybdenum alkylidene complex was conducted. The metathesis of functionalized olefins is controlled by a fine balance between the coordination of olefins and that of the heteroatoms. The formation of chelated alkylidene complexes during the metathesis of alkyl-3-alkenoate slows down the rate of productive metathesis. The formation of a stable chelated metallacyclobutane complex during the metathesis of N,N-dimethyl alkenamide prevent its productive metathesis.

Reactions of the late transition metal ruthenium carbene complexes with strained acyclic olefins and functionalized olefins are explored. Several [alpha]-hetero substituted carbene complexes and active ruthenium carbenes are synthesized through the metathesis of functionalized olefins and strained acyclic olefins, respectively. The mechanism for the rearrangement of alkoxy substituted ruthenium carbenes is determined by deuterium and 13C labeling studies.

Perfect rubber (cis-1,4-polyisoprene) is synthesized by ROMP of 1-methylcyclobutene using a molybdenum alkylidene complex. New living polymerization systems of both bicyclo[3.2.0]heptene and bicyclo[4.2.0]octene are discovered using a ruthenium carbene complex. Narrow dispersed and regioselective alternating copolymers are synthesized through the ROMP of 3-methylcyclobutene and 3,3-dimethylcyclobutene.

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