Tungsten-Based Initiators

One of the first examples of Lewis-acid free initiators based on tungsten(VI), W(O-2,6-i-Pr2-C6H3)2Cl2(CH-t-Bu)(OR)2, (R=Et, i-Pr), which allowed the polymerization of substituted NBEs, was reported by J.M. Basset et al. [57]. Additionally, Lewis-acid free W(=C(CH2)4)(OCH2-t-Bu)2Cl2 was reported to effectively polymerize substituted NBEs such as exo-norborn-5-ene-2,3-dicarboxylic anhydride [58, 59[. 

Van der Schaaf reported on the photoinduced ROMP of NBE and dicyclopentadiene (DCPD) using W(NPh)(OCMe(CF3)2)2(CH2SiMe3)2 and W(NPh)Cl(CH2SiMe3)3, respectively, as tungsten alkylidene precursors [71]. Unfortunately, the corresponding ROMP-active species were not isolated. Nevertheless, their existence was indirectly proven by reaction with an aldehyde and characterization of the resulting alkene. 

A diamido tungsten(VI)-based catalyst was reported by Boncella and co-workers [72]. It was prepared from the dilithio salt of N,N -bistrimethylsilyl-protected o-phe-nylenediamine and W(NPh)Cl4(OEt2). The conversion of the intermediate bisalkyl complex into the corresponding tungsten alkylidene requires the addition of an ex- 

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Counterion effects similar to those in ionic chain copolymerizations of alkenes (Secs. 6-4a-2, 6-4b-2) are present. Thus, copolymerizations of cyclopentene and norbomene with rhenium- and ruthenium-based initiators yield copolymers very rich in norbomene, while a more reactive (less discriminating) tungsten-based initiator yields a copolymer with comparable amounts of the two comonomers [Ivin, 1987]. Monomer reactivity ratios are also sensitive to solvent and temperature. Polymer conformational effects on reactivity have been observed in NCA copolymerizations where the particular polymer chain conformation, which is usually solvent-dependent, results in different interactions with each monomer [Imanishi, 1984]. 

If olehn metathesis is to be conducted in solution, solvents of low Lewis-basicity will generally give the best results (CH2CI2 > toluene > THF). As discussed above, metathesis is initiated by the formation of a jt-complex between the metal and the alkene. Hence, other nucleophiles will compete with the alkene for these coordination sites and in some systems even THF can lead to complete deactivation of the catalyst [786]. Tungsten-based catalysts which can even metathesize allyl thioethers have, however, been described [787]. 

Ring-opening metathesis polymerization (ROMP) of substituted bicyclo octa-dienes or paracyclophane-enes initiated by Gmbbs molybdenum, tungsten-based carbenes have been used to prepare PPV s [178—181]. The living character of ROMP has been exploited to prepare soluble well-defined precursors, which can be converted into XI. Yu and Turner have used ROMP of tetra octyloxy-substituted paracyclo-phanedienes initiated by reactive ruthenium-based carbenes to prepare monodisperse, soluble yellow fluorescent PPV with an alternating cis-trans microstructure and molecular weights as calculated [178] (Fig. 9.21). 

Cederqvist studied 17 different tool materials to friction stir weld 50 mm thick copper (Ref 33), and the first material evaluations were for use as the tool pin. Tungsten carbide-cobalt pins provided the initial welding parameter development, but tool life issues (due to large spindle eccentricities) made this tool material impractical for production. Likewise, eccentricity issues caused PCBN, alumino-silicate, and yttria-stabilized zirconium oxide pins to fail within the plunge or dwell sequence of the friction stir welds. A majority of e pins manufactured from refractory metals (four molybdenum-base and three tungsten-base) did not have dimensional... 

With experimental support for the metal-carbene-mediated mechanism of olehn metathesis, a number of groups initiated studies with isolated metal-carbene and metallacyclobutane complexes. Early work by Chauvin and Katz on the polymerization of strained olefins using Fischer-type carbenes demonstrated the success of such an approach [56], The introduction of high oxidation state alkylidene complexes led to well-defined catalyst in which the propagating species could be observed and studied, such as the tungsten-based systems developed by Osborn, Schrock, and Basset [59,60], The best-studied and useful of these have been the Schrock arylimido alkylidene complexes, and we will return to these later in this chapter. 

Grubbs-type initiators are well-defined ruthenium aUcylidenes. Compared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based Grubbs catalysts is somewhat different. In terms of polymer structure, ROMP of norbom-2-enes and norbomadienes using ruthenium-based systems generally results in the formation of polymers that, in most cases, predominantly contain frany-vinylene units. Polymerizations initiated by Grubbs-type initiators are best terminated by the use of ethyl vinyl ether, yielding methylidene-terminated polymers. 

The Grubbs ruthenium-carbene complexes (2 and 3) exhibit high reactivity (albeit lower than the best molybdenum- and tungsten-based catalysts) in a variety of metathesis processes while showing a remarkable tolerance towards many different organic functionalities. These initiators are stable for weeks and reactions can be carried out in the presence of air and humidity or even in water. 

The synthesis of high-oxidation-state molybdenum alkylidenes was reported by Schrock in 1987 [121]. Due to their improved tolerance towards functional groups (table) their better reaction profile and their lower costs well-defined molybdenum based initiators are now preferred over the related systems containing tungsten [122]. 

Table 2 Examples of well-defined tungsten-based metathesis initiators. 

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