Chemistry Reference
In-Depth Information
These complexes display varying degrees of stability, but the general trends are
that solutions of the complexes are not rapidly decomposed by alcohols or water.
Solutions of these complexes may be prepared on the bench-top in reagent-grade
solvents and will slowly decompose over a matter of hours by oxidation to a green
material. The solid complexes are stable over a period of weeks in air and indefi-
nitely in a dry-box. The PCy
3
complexes are more stable than the PPh
3
complexes.
Benzylidene 6 is stable indefinitely in air in the solid state and in inert solution at
temperatures up to 60
C [77, 78, 80].
6.5.2.2
Use in ADMET
The metathesis activities of these complexes also vary throughout the series. The
PPh
3
complexes, such as
3
, are active only for ROMP of strained cyclic olefins.
However, the PCy
3
analogues are active towards acyclic olefins, and thus promote
ADMET [84]. The importance of the identity of the carbene is illustrated by the
difference between complexes
5
and
6
, in that
5
initiates very slowly, and typically
only a small amount (
10%) of the vinyl carbene is converted into the alkylidene.
Thus, complex
5
is a poor ADMET catalyst at normal catalyst loading (0.1 to 1.0
mol%). Complex
6
, however, undergoes quantitative initiation of the initial benzy-
lidene and is an active ADMET catalyst, producing polymers with
M
n
of 25 to
35 kDa. This complex has been used to polymerize monomers containing ke-
tones, alcohols, esters [49, 53], ethers [35], silyl chlorides, siloxanes [85], amides
[55, 56], and carboxylic acids. However, thioethers are not tolerated [35]. The devel-
opment of complex
6
greatly expanded the scope of ADMET by allowing polymer-
ization of these functionalized dienes and by easing the requirement of absolute
dryness of monomers required for polymerization with
1
or
2
. Some desirable
ADMET monomers are non-distillable liquids, and ADMET with
6
proceeds nor-
mally after simply heating the purified monomer under vacuum in the liquid
state overnight to remove oxygen [53].
The kinetics of ADMET with complex
6
were compared to those of complex
2
by measuring the volume of ethylene liberated from ADMET reactions over time
[35]. Obtaining an approximate second order rate constant from the DP versus
time curves, it was found that molybdenum complex
2
polymerizes 1,9-decadiene
24 times faster than ruthenium complex
6
(Tab. 6.1).
The polymerization of ether and thioether monomers was also studied, and it
was found that the rate of polymerization was a great deal slower with the func-
tionalized monomers. The number of methylene units between the olefin and the
heteroatom greatly affected the rates observed, giving credence to the chelation ef-
fect shown in Fig. 6.1. In addition, catalyst
2
polymerizes 1,5-hexadiene, whereas
catalyst
6
mainly cyclizes the metathesis dimer to cyclo-1,5-octadiene. At this
point there is no clear explanation for this result, and, furthermore, the reason
that the COD generated did not undergo ROMP in these reactions is unclear. The
data from these experiments clearly shows that Lewis basic functionality retards
the rate of metathesis with complex
6
more than with complex
2
, although
6
is
clearly the more functional group-tolerant complex overall [35].
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