ca01 only detailed ToC 1..24

98 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
Scheme 8 Alkene Metathesis
R 1 R 2
+
R 1
R 2
+ H 2C CH 2
2.6.1.5.1 Variation 1:
Ring-Opening Metathesis Polymerization (ROMP)
The living polymerization of strained cyclic alkenes such as norbornenes and substituted
norbornadienes is catalyzed by molybdenum and tungsten carbene complexes (Scheme
9). [35] The living nature of this process allows control of the chain length and the preparation
of block copolymers, which may also have a high level of tacticity. [37] A Wittig-like
capping reaction with aldehydes can be used to cleave the metal fragment off the living
polymer. Essentially monodisperse products 22 with x up to 500 have been obtained from
norbornene by using di-tert-butoxo[(2,6-diisopropylphenyl)imido](2,2-dimethylpropylidene)tungsten(VI)
as a ROMP initiator. [38] The catalyst does not attack the less reactive
double bonds in the polymeric product. The use of 7,8-bis(trifluoromethyl)tricyclo[4.2.2.0
2,5 ]deca-3,7,9-triene as monomer gives oligomers from which polyenes with up
to 15 conjugated double bonds have been obtained via a retro-Diels–Alder ejection of an
arene upon thermolysis. [39] Other substrates suitable for the ROMP process are substituted
cyclooctatetraenes, leading to functionalized and soluble polyacetylenes of low polydispersities.
[40]
Scheme 9 Ring-Opening Metathesis Polymerization of Norbornene [35]
[W] CHBu t + x
[W] = W(Ot-Bu) 2( NC6H3-2,6-iPr2)
−80 o C
[W] CHBut x
PhCHO
PhHC CHBut x
22
1,4-Dihydro-1,4-methanonaphthalene, Homopolymer (x = 100); Typical Procedure: [41]
A soln of 1,4-dihydro-1,4-methanonaphthalene (291 mg, 2.05 mmol) in toluene (3.0 mL)
was added dropwise to a rapidly stirred soln of [Mo(=CHt-Bu)(Ot-Bu) 2(=NC 6H 3-2,6-iPr 2)]
(10 mg, 0.020 mmol) in toluene (3.0 mL) and the soln was stirred for 20 min. The polymerization
was quenched by addition of pivalaldehyde (25 ìL). After 20 min, the soln was
added to hexane (250 mL), and the precipitated polymer was isolated by centrifugation,
washed with hexane, and placed under vacuum overnight; yield: 280 mg (94%).
2.6.1.5.2 Variation 2:
Alkyne Polymerization
Terminal alkynes have yielded polyenes 23 with up to 150 conjugated double bonds under
molybdenum–carbene catalysis (Scheme 10). [42] The nature (in particular the size) of
the ancillary ligands is important in controlling the selective Æ-addition. Acetylene itself
produces insoluble and air-sensitive polymers that are difficult to characterize. Cyclopolymerization
of dipropargyl derivatives such as 24 have been shown to yield polyenes of
low polydispersity containing only six-membered rings, following a selective â-addition