Well-Defined Transition Metal Catalysts for Metathesis Polymerization - Late Transition Metal Polymerization Catalysis - page 156

far this is the only well-defined catalyst-monomer combination involving internal
alkynes that represents a polymerization-active system.

5.2.2
Group VI Transition Metal-Based Initiators

5.2.2.1 Chromium-Based Initiators
Only few reports exist on the successful use of chromium carbenes for ROMP.
These few examples comprise the ROMP of 2,3-dihydrofuran catalyzed by
Cr(=CPh 2 )(CO) 5 [44] as well as the ROMP of 2,3,4,5-tetrahydrooxypin-2-yl acetate
[45].

5.2.2.2 Tungsten-Based Initiators
Tungsten-alkoxy-alkylidene Complexes
Following the reports of Casey et al. on tungsten(0) complexes [46, 47], the first
well-defined high oxidation state tungsten(VI) alkylidenes, W(=CR 2 )(OCH 2 - t -
Bu) 2 X 2 and W(=CR 2 )(OCH 2 - t -Bu) 3 X (X=Cl, Br, I; R= n -Bu, sec -Bu, t -Bu, Ph, -

cyclopentadienyl) were reported by J. A. Osborn and co-workers [48, 49]. Upon ad-
dition of GaBr 3 , W(=CH- t- Bu)(OCH 2 - t -Bu) 2 Cl 2 polymerizes a variety of substituted
NBEs. Using this compound, the living character of such a tungsten(VI)-based po-
lymerization system was observed for the first time via 1 H-NMR spectroscopy [50]
and further supported by the successful synthesis of block -copolymers. Addition-
ally, by employing low-temperature techniques, an intermediate (cationic) tungsta-
cyclobutane species [51] was observed (Scheme 5.1) [52-55]. In a consecutive
study, the equilibria that are relevant for ROMP have been studied in detail [56].
One of the first examples of Lewis-acid free initiators based on tungsten(VI),
W(O-2,6-i-Pr 2 -C 6 H 3 ) 2 Cl 2 (CH- t -Bu)(OR) 2 , (R=Et, i-Pr), which allowed the polymer-
ization of substituted NBEs, was reported by J.M. Basset et al. [57]. Additionally,
Lewis-acid free W(=C(CH 2 ) 4 )(OCH 2 - t -Bu) 2 Cl 2 was reported to effectively polymer-
ize substituted NBEs such as exo -norborn-5-ene-2,3-dicarboxylic anhydride [58, 59].

,

Scheme 5.1 Observation of the simultaneous presence of metal alkylidenes and
metallacyclobutanes via 1 H-NMR spectroscopy.