The influence of the negative neighboring group has been
quantified kinetically using
hydrocarbon monomers containing oxygen and sulfur. Rate constants
were determined for the ADMET
polymerization of bis(4-pentenyl) ether (3),
bis(5-hexenyl) ether (4), bis(4-pentenyl)
sulfide (5), and bis(5-hexenyl) sulfide (6) using both Schrock's catalyst
Mo(CHCMe2Ph)(N-2,6-C6H3-i-Pr2)(OCMe(CF3)2)2
[Mo] and Grubbs' phenyl version of the ruthenium
metathesis catalyst
RuCl2(CHPh)(PCy3)2
[Ru]. Both
catalysts experience the effect of functional group presence.
Further, a pure hydrocarbon version of the
negative neighboring group effect has also been demonstrated using the
monomer 1,5-hexadiene. In
this case, [Mo] catalysis produces linear
1,4-polybutadiene in a reaction that exhibits a typical
ADMET
rate constant and activation energy. On the other hand,
[Ru] catalysis leads principally to the
formation
of cyclic compounds rather than linear polymers. This change in
mechanism for [Ru] catalysis can be
attributed to multiple factors including the dissociation of phosphine
ligands, thermodynamic parameters,
and π complexation with the adjacent olefin in the monomer, a
phenomenon similar to heteroatom
nonbonded electron complexation.
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