Molybdenum-Based Initiators

less acidic alcohols show no reaction, while phenol derivatives result in the formation of dineopentyl derivatives. Finally, binuclear molybdenum alkylidenes are obtained by reaction of a Schrock carbene with a.cu-dienes such as divi-nylbenzene or with octatetraene [91]. 

Reactivity of Molybdenum-imido-alkoxy-alkylidene Complexes [92] 

Since little anti-form is present under equihbrium conditions (without irradiation) in Mo(NAr )(CHR)(OCMe(CF3)2)2, and syn- into anti-conversion is slow (ca. 10 s ), cis-polymers are proposed to form from the syn-species of a catalyst via olefin attack on the CNO-face of the initiator [94]. In a t-butoxide system, where interconversion is relatively fast (ca. 1 s ), it was proposed that the anti-form was the only propagating alkylidene species. This proposal was further supported by studies carried out by Feast and co-workers [100]. Using sterically hindered and therefore unreactive monomers such as 1,7,7-trimethylnorbornene, only the reaction of the anti-rotamer at a very slow, monomer concentration-independent rate was observed. Additionally, the calculated rate constant was essentially identical with the one for syn-anti onver- 

7-Qxanorbornene and 7-oxanorbomadiene-derivatives were found to form observable and even isolable molybdametallacycles. Thus, 7-oxa-2,3-(bistrifluoro-mefhyl)norbornadiene reacts wifh Mo(N-2,6-i-Pr2-C,H),)(CHCMe),)(OCMe),)2 to form a remarkably stable metaUacycle [106]. A similar tungstametallacycle was observed in fhe reaction of 2,3-bis(trifluoromethyl)norbornadiene with W(N-2,6-i-Pr2-CgHsKCHCMesKOCMeslz [98]. 

NBDF6 = 2,3-bis(trifluoromethyl)norbomadiene, DMMNBE = (+)-(2.S.3S)-2,3-bis(dimethoxy-methyl)- 

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The suspension polymerization of norbom-5-ene-2,3-dicarbox-ylic anhydride in dichloromethane using a molybdenum-based initiator results in living, linear polymer chains with the active initiator at the polymer chain end. The solubility of the poly(norbom-5-ene-2,3-dicarboxylic anhydride) is dependent on the chain length. 

While the decomposition products of molybdenum-based initiators may conveniently be removed as molybdates after any metathesis reaction by addition of aqueous base, ruthenium tends to form RUO2, a black insoluble product. The removal of decomposed Ru-sites from any product is best performed by the use of a water-soluble phosphine such as tris(hydroxymethyl)phosphine [182]. Alternatively, addition of triphenylphosphinoxide or DMSO followed by filtration over sihca was reported to be advantageous [183]. For purposes of completeness, the investigations of the actual catalytic cycle in the reaction of Cl2Ru(CHPh)(PCy3)2 with cyclic olefins carried out with substituted cyclobutenes shall be mentioned [184]. 

The synthesis of peptide derived monomers started with glycine derivative 7, which was first activated as an acid chloride and then reacted with an amino ester or dipeptide derivative to give the desired monomers (Scheme 1). In this way, five monomers derived from di- or tri-peptides were prepared. Each of the monomers was polymerized using molybdenum based initiators 1-3, giving a series of polymers for which representative characterizing data is given in Table 1. 

Their size was ideal for their convenient removal at the end of any reaction by means of filtration. Because of their favorable initiation properties, only molybdenum-based initiators were used, which were removed quantitatively in... 

The capability of ROMP for polymerizing even complex functional monomers was demonstrated by the fact that the cationic NHC precursor l,3-di(l-mesityl)-4- [(bicyclo[2.2.1]hept-2-ene-5-ylcarbonyl)oxy]methyl -4,5-dihydro-l//-imidazol-3-ium tetrafluoroborate was polymerized using both ruthenium- and molybdenum-based initiators. Thus, reaction of this monomer with RuCl2(PCy3)2(CHPh) in CH2CI2 at 45 °C resulted in complete consumption of the initiator and formation of an oligomer with a DP of 7. 

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]. 

AROM)-RCM- and -CM sequences initiated by chiral molybdenum-based catalysts [194] or, more recently, also by ruthenium-based [195] catalysts. 

Acyclic diene metathesis (ADMET) is a step-growth polycondensation reaction for the polymerization of o -dienes 729 The process is catalyzed by the same metal alkylidene initiators used for ROMP, and is driven by the removal of ethylene from the system (Scheme 13). Both molybdenum and ruthenium-based initiators have been used to prepare a variety of materials including functionalized polyethy-... 

Kuroda and Tarui [498] developed a spectrophotometric method for molybdenum based on the fact that MoVI catalyses the reduction of ferric iron by divalent tin ions. The plot of initial reaction rate constant versus molybdenum concentration is rectilinear in the range 0.01-0.3 mg/1 molybdenum. Several elements interfere, namely, titanium, rhenium, palladium, platinum, gold, arsenic, selenium, and tellurium. 

Molybdenum-based systems of general formifla Mo(N-ArO(CHCMe2R) (ORO2 (Ar = 2,6-Me2-C6H3, 2,6-zPr2 - CeHs R = Me, Ph R = CMes, CMe(CF3)2), usually referred to as Schrock initiators, represent highly active initiators, particiflarly when used in combination with electronegatively substituted alkoxides (Fig. 1). 

Molybdenum-based catalysts are highly active initiators, however, monomers with functionalities with acid hydrogen, such as alcohols, acids, or thiols jeopardize the activity. In contrast, ruthenium-based systems exhibit a higher stability towards these functionalities (19). An example for a molybdenum-based catalyst is (20) MoOCl2(t-BuO)2, where t-BuO is the tert-butyl oxide radical. The complex can be prepared by reacting M0OCI4 with potassium tert-butoxide, i.e., the potassium salt of terf-butanol. 

Schrock, Gibson et al. [52d] found that styrene and 1,3-pentadiene could be used as chain transfer reagents for the living ring-opening olefin metathesis polymerization of norbornene with molybdenum based catalyst 35a. Renewed norbornene addition to a polymerization mixture containing initiator 35a and 30 equivalents of styrene resulted in the formation of polynorbomene with a low polydispersity and a molecular weight controlled by the number of norbornene equivalents in each of the individual monomer solutions, Eq. (38). This method allows a more efficient use of the catalyst. 

Initiators can be subdivided into ill-defined and well-defined initator-systems. The latter ones dominate in this review. Ill-defined initiators like WCl4(OAr)2-PbEt4 (Ar=2,6-diphenylphenyl) have only been used in isolated examples [21]. Preferred initiators comprise two classes molybdenum-based systems and ruthenium compounds. Most work summarized in the following sections is based on molybdenum aUcylidenes Mo(NAr )(CHCMe2R)(OR )2 (Ar =2,6-i-PrC6H3 R=Me, Ph R =t-But, CMejlCF,), CMe(CF3)2) 1-5 (Fig. 2). 

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... 

A large accumulation of Mo NMR data for Molybdenum (0) compounds is now presented. Some of these data have been previously reviewed but are included here to ensure a more complete data set. The chemical shift data are discussed in terms of ligand types, based initially on the donor atom... 

The modeling of molybdenum-based classical catalysts supported on alumina was improved by the use of calculations with periodic boundary conditions [10, 49, 52], which better represent the alumina surface [67]. It became possible to describe the relative stabilities of the different surface sites, including the effect of temperature and water pressure. The more stable (110) and (100) alumina surfaces were considered, and the investigation focused on the structure of the potential initial molybdenum-oxide monomeric and dimeric species, as well as the corresponding methylidene species and their reactivity with ethene [10,49]. 

Other ruthenium catalysts have also been studied. In one study, ((IPrH2) (PCy3)(Cl)2Ru=CHPh) promoted ADMET faster than [Ru]2, although it too promoted olefin isomerization [28b]. The initiation rate of [Ru]3 was reported to be 1/30 that of [Ru]l, but the propagation rate was found to be four times faster [16]. The activity of [Ru]4 has even been shown to surpass that of Schrock s molybdenum-based catalysts [39]. 

Table 1 Examples of well-defined molybdenum-based metathesis initiators. 

ROMP or norbomene esters bearing mesogenic side groups yields side-chain liquid-crystalline polymers [440,441]. Norbornenes which are functionalized in the 5- or in the 5- and 6-position with ester groups have been polymerized with both molybdenum and ruthenium-based initiators. 

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