Chemistry Reference
In-Depth Information
5.2
Transition Metal Alkylidenes
[28]
5.2.1
Group IVA and VA Transition Metal-Based Initiators
Though this topic focuses on late transition metals, the few well-defined Group
IVA- or VA-based systems that exist shall be outlined briefly. Among the first tran-
sition metal alkylidenes that were observed in ROMP was the bimetallic com-
pound (Cp
2
TiCH
2
ClAlMe
2
) [29, 30]. Upon reaction with norborn-2-ene (bicyclo[2.2.1]
hept-2-ene, NBE) at ambient
temperature it forms the metallacycle shown in
Fig. 5.1 which opens at 65
C under ring-opening [31].
This system was the first well-documented example of a truly “living” [32-36]
ROMP. In analogy, Cp
2
TiMe
2
,Cp
2
TiMe
3
,Cp
2
TiMe
2
Cl, and Cp
2
Ti(CH
2
SiMe
3
)
2
cat-
alyze the ROMP of NBE. The rate-determining step was proposed to be the loss
of CH
4
, ClCH
3
, and SiMe
4
, respectively [37]. Other Ti-alkylidenes exist, yet are not
of any significance to polymer chemistry [38].
Nb and Ta form a number of well defined and stable metal carbenes such as
Cl
2
CpTa(=CHCMe
3
) and Cl
2
CpTa(=CHCMe
3
)PMe
3
[6, 39, 40]. In particular
Ta(=CHCMe
3
)(O-2,6-i-Pr
2
-C
6
H
3
)
3
(THF) and Ta(=CHCMe
3
)(TIPT)
3
(TIPT=2,4,6-i-
Pr
3
-C
6
H
2
-S-) are good initiators for living ROMP [41, 42]. Ta[CMeCMeCHCMe
3
]
(DIPP)
3
(DIPP=2,6-i-Pr
2
-C
6
H
4
-O), Fig. 5.2) has been reported to allow the living
polymerization of 2-butyne up to a degree of polymerization (DP) of 200 [43]. So
Fig. 5.1
Reaction of NBE with Cp
2
TiCH
2
ClAl(CH
3
)
2
.
Fig. 5.2
Structure of Ta[CMeCMeCHCMe
3
](DIPP)
3
(Ar
=2,6-i-Pr
2
-C
6
H
3
).
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