Catalysis in Acyclic Diene Metathesis (ADMET) Polymerization - Late Transition Metal Polymerization Catalysis - page 197

Proof of the reversibility of ADMET and trans -metathesis. Scheme 6.7

6.2.5
General ADMET Mechanism

Examining the mechanism of olefin metathesis uncovers a number of other is-
sues associated with this catalysis (Scheme 6.8). The first point is that there are
two distinctly different metal carbenes involved in the catalytic cycle; the metal
methylidene and the metal alkylidene. The metal alkylidene is the metal carbene
at the end of a polymer chain. Since the size of the polymer chain to which the
metal is appended could affect reactivity, it could be stated that there are a great
number of metal alkylidenes, corresponding to monomer, dimer, oligomers, and
polymers of various lengths. It has been postulated that the size of the polymer
chain affects the reactivity of the metal alkylidene to which it is attached due to
increased steric shielding of the metal as the molecular weight increases,
although this has not been proven experimentally in the context of ADMET [36].
Chain extension can only occur by the connection of two terminal olefins and is
accompanied by the release of a molecule of ethylene, thus each chain extension
event must start with the alkylidene and generate the methylidene (paths C and D
in Scheme 6.8). Reaction of the methylidene with a terminal olefin regenerates
the alkylidene and liberates the ethylene molecule (path A and B). Thus both the
methylidene and the alkylidene are present in an ADMET reaction. The relative
amounts of the methylidene and alkylidene in an ADMET reaction will depend on
the rates of reaction of all steps in the cycle, principally the rates of reaction of
the methylidene and alkylidene with a terminal olefin, and is thus a function of
the particular catalyst used. Obviously the rates of both reactions should be as

Scheme 6.8 Generic catalytic cycle for productive ADMET.