Tungsten complexes, ROMP

To date, low volumes of materials have been produced commercially from norbomene and cyclo-octene. Numerous products are expected to result from the materitd produced by the ROMP of dicyclopentadiene in a RIM (reaction injection molding) process. In a RIM process, two streams of a monomer are mixed in the mold where it is polymerized to the final part. In this case, one of the monomer streams contains a tungsten complex while the second contains an alkyl aluminum activator. When the two streams of dicyclopentadiene are mixed, the metathesis catalyst is formed and the monomer is ROMP polymerized (equation 12). 

Scheme 5.2 Synthesis of a ROMP-active cyclometallated aryl-oxy(chloro)neopentylidene tungsten complex [50].

Scheme 2. Sjmthesis of a ROMP-Active Cyclometidated Arylojgf(chloro)neopentylidene Tungsten Complex...

The successful PROMP of cycloolefins, like 2-norbornene and dicyclopentadiene, with simple dialkyl-tungsten complexes by irradiation with UV- or visible light leads to the in situ formation of metal-carbenes by a Ha-abstraction reaction [20]. Metal carbenes have proven to be the active ROMP-initiators [21] and are supposed to be intermediates in the polymerization of substituted acetylenes as well [22]. Nevertheless, to the best of our knowledge, a one component catalytic system for the photopolymerization of mono- and disubstituted acetylenes has not yet been reported before our work [23]. 

In the case of other Group 6 metals, the polymerization of olefins has attracted little attention. Some molybdenum(VI) and tungsten(VI) complexes containing bulky imido- and alkoxo-ligands have been mainly used for metathesis reactions and the ring-opening metathesis polymerization (ROMP) of norbornene or related olefins [266-268]. Tris(butadiene) complexes of molybdenum ) and tungsten(O) are air-stable and sublimable above 100°C [269,270]. At elevated temperature, they showed catalytic activity for the polymerization of ethylene [271]. 

A variety of catalytically active five-coordinate tungsten oxo and imido alkylidene complexes also have been prepared that contain some donor amine or pyridine linked either to the imido ligand or to a phenyl ligand bound to the metal (A, Scheme 2) [101-105]. Such species show metathesis activity (e.g., ROMP of norbornene),but there does not appear to be any proof that the integrity of the initiator is maintained. 

For the ROMP of the 5-alkylthiocyclooctenes (115), with R = Et, Bu, Hex, c-Hex, t-Bu, initiated by 12, the most reactive monomers are those with branched alkyl substituents on the sulphur atom for R = t-Bu, reaction is 95% complete in about 10 min. The variations in rate are likely to be connected with the strength of coordination of the sulphur atoms in the monomer and/or the propagating species to the tungsten centre. Coordination of the monomer to the metal centre through the sulphur atom will be impeded when R is t-Bu or c-Hex, allowing a higher equilibrium concentration of the precursor complex that leads to addition of monomer. For R = Bu the rate of polymerization is proportional to both monomer and initiator concentrations189,364. 

The proposed idea that metal alkyhdene complexes are be able to catalyze olefin metathesis was confirmed in 1980 [8] and consolidated in 1986 by Schrock with the development of the first well-characterized, highly active, neutral tungsten (Cl, Fig. 3) [9] and molybdenum (C2) [10] alkylidene complexes. These complexes were able to catalyze both the metathesis of different olefins and the ROMP of functionalized norbomene to polynorbomene with low polydispersities [11]. Moreover, these catalysts were used by Wagener and coworkers to perform the first quantitative ADMET polymerization [12] and copolymerization [13] of 1,5-hexadiene and 1,9-decadiene. However, the low stability of these catalysts in... 

The living polymerization prindples developed using the titanacene metall-acyclobutane catalysts have been extended to ROMP reactions catalyzed by other transition-metal complexes. These new systems include tantalacyclobutane complexes and preformed tungsten(VI) carbene complexes. 

The initial observation of a metal carbene that reacted with an alkene to give a metallacyclobutane complex was reported by Osborn and coworkers for the reaction shown in equation (10). This reaction was observed by NMR spectroscopy at low temperature (—70°C). When this reaction mixture was allowed to warm to higher temperature, polynorbornene was produced in high yield. Shortly after this discovery, the titanocene complex (4) was shown to be an efficient catalyst for the synthesis of monodisperse polynorbornenes. These discoveries, along with the synthesis of a new family of tungsten (5a), molybdenum (5b), and rhenium (6) catalysts,shown in Figure 1, have opened a new era of ROMP chemistry in which the polymer synthesis is guided by the selection of a catalyst... 

Another clear example of an equilibrium metathesis reaction is the ROMP of cyclopentene initiated by the tungsten analogue of 2. The reaction may be totally reversed by placing the system under vacuum thereby stripping off the monomer units from the living metal carbene complexes (Schrock 1989). 

Many stable tungstacyclobutane complexes are known, but few will initiate the metathesis of internal olefins or ROMP of cyclic olefins. Yet many will undergo exchange reactions with ethene or terminal olefins by a mechanism which must involve dissociation to a tungsten carbene complex. A great deal can therefore be learnt about the olefin metathesis mechanism from a study of such reactions. The following is a short summary. 

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