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via the synthetic protocol shown in Scheme 5.5 [73]. It was reported stable up to
T
=80
C and turned out to be an efficient initiator for the polymerization of NBE
yielding poly-NBE with a
cis
-content of >90%.
Tungsten-oxo-alkoxy-alkylidene Complexes
A tungsten-oxo-alkylidene, W(O)CHCHCPh
2
(OCMe(CF
3
)
2
)
2
·P(OMe
3
), and its ana-
logue W(O)CHCHCPh
2
(OCMe(CF
3
)
2
)
2
·THF were reported to be active in the
ROMP of NBE [74]. The synthetic route to these initiators entails the reaction of
W(O)Cl
2
[P(OMe)
3
]
3
with LiOCMe(CF
3
)
2
and 2,2-diphenylcyclopropene (Scheme 5.6).
Synthesis of W(O)CHCHCPh
2
(OCMe(CF
3
)
2
)
2
· L (L=P(OMe
3
), THF).
Scheme 5.6
This reaction of 2,2-diphenylcyclopropene with a transition metal complex has
also been used for the synthesis of Ru-based alkylidenes [75]. Another ROMP-ac-
tive tungsten-oxo-complex, W(CH-
t
-Bu)(O)(PMe
3
)
2
(OAr)
2
, was obtained by reaction
of W(CH-
t
-Bu)(O)(PMe
3
)
2
Cl
2
with KO-2,6-Ph
2
-C
6
H
3
[76]. This complex was re-
ported to be active in the ROMP of 2,3-bis(trifluoromethyl)-NBD and 2,3-dicarbo-
methoxy-NBD, producing highly tactic (>95%) polymers with a high
cis
-contents
(>95%). Finally, Boncella and co-workers reported on the use of the tris(3,5-di-
methyl-1-pyrazolyl)borate (Tp
) ligand for the preparation of the tungsten oxoalkyli-
dene complex W(O)(CH-
t
-Bu)(Cl)(Tp
) which was found to be an active ROMP cat-
alyst for cyclooctadiene (COD) in the presence of aluminum chloride [77].
5.2.2.3
Molybdenum-Based Initiators
Molybdenum-imido-alkoxy-alkylidene Complexes
In view of the more than encouraging results obtained in ROMP with W-based
initiators, the development of the analogous Mo compounds was pushed forward.
This development was mainly driven by two general aspects of organomolybde-
num chemistry. On one hand, the analogous molybdenum complexes could be
prepared in a more convenient way in terms of starting compounds and stability
compared to the parent W systems. On the other hand, the corresponding Mo ini-
tiators turned out to be even more functionality tolerant than the corresponding
tungsten systems [78-80]. Thus, functional monomers containing olefins, ethers,
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