Polymerization metallacyclobutane

The stoichiometry of this conversion is in accordance with a carbene starting structure. An alternating alkyUdene/metallacyclobutane mechanism [102, 131-133], which has precedent in the ethylene polymerization catalyzed by a Ta(III) neopentilydene complex [134], has been proposed where the chro-miiun alkyUdenes undergo [2-1-2] cycloaddition to give chromacyclobutane intermediates (mechanism III in Scheme 7). 

Confirmation that the polymerizations proceed via metallacyclic intermediates was obtained by studying the ROMP of functionalized 7-oxanorbornadienes. These polymerize slower than their norbornene analogs, allowing NMR identification of the metallacyclobutane resonances and subsequent monitoring of ring opening to the first insertion product. In addition, the X-ray crystallographic structure of complex (212) has been reported.533... 

Metallacyclobutanes have been proposed as intermediates in a number of catalytic reactions, and model studies with isolated transition metallacyclobutanes have played a large part in demonstrating the plausibility of the proposed mechanisms. Since the mechanisms of heterogeneously catalysed reactions are especially difficult to determine by direct study, model studies are particularly valuable. This article describes results which may be relevant to the mechanisms of isomerization of alkanes over metallic platinum by the bond shift process and of the oligomerization or polymerization of alkenes. 

These results at least demonstrate that ethylene can be polymerized by an alkylidene hydride catalyst, probably by forming a metallacyclobutane hydride intermediate. The extent to which this is relevant to the more classical Ziegler-Natta polymerization systems (27) is unknown. Recent results in lutetium chemistry (28), where alkylidene hydride complexes are thought to be unlikely, provide strong evidence for the classical mechanism. 

Ring-opening metathesis polymerization of cycloolefins,93-99 a reaction of significant practical importance (see Section 12.3), is catalyzed by a number of well-defined transition-metal complexes. Alkylidene and metallacyclobutane... 

With a titanacyclobutane complex as initiator the rate of polymerization is independent of [M] and therefore governed by the rate of opening of the titanacyclobutane chain carrier432. The comparative stability of the metallacyclobutane bearing an endo substituent is a feature which is also found in the ROMP of other substituted norbomenes see Section Vm.C.12. In contrast, the rate of ROMP of 235 induced by ReCl5/Me4Sn (1/1.5) is first-order in both catalyst and monomer and here the addition of monomer to the metal carbene complex must be rate-determining562. 

V(IV) and V(V), with imido, amido, N-donor ligands, 5, 20 Alkylidiene-metallacyclobutane pathway, as alkene polymerization competitor, 1, 136 Alkylidyne-alkynes, in Ru-Os mixed-metal clusters, 6, 1080... 

As a complement to the stable metallacyclobutane catalysts, a series of stable alkylidene catalysts have been prepared and shown to be active living polymerization catalysts. The complex W(CHt-Bu)(NAr)(Ot-Bu)2 [47,48] (Ar = 2,6-diisopro-pylphenyl) (35) was reacted with 50-200 equivalents of norbornene in toluene at 25 °C, followed by end-capping with benzaldehyde, yielding polymers in which the major component has a molecular weight proportional to the number of equivalents of norbornene consumed, with dispersities of approximately 1.05,... 

The mechanism of ADMET polymerization (Scheme 5) contains intermediates similar to those found in ROMP chemistry in that both polymerizations contain, inclusively, various metallacyclobutane/carbene species." Although ROMP propagates exclusively via trisubstituted metallacycles, whereas ADMET requires disubstituted metallacycles, the major difference is that ADMET step chemistry is an equilibrium process driven by condensation and ROMP chain chemistry propagates irreversibly owing to the high reactivity of the carbene with strained cycloalkenes. Therefore ROMP is much faster than ADMET simply because competing equihbria, absent during ROMP, decrease the net productive rate in ADMET chemistry. 

The ring-opening metathesis polymerization (ROMP, cf. Section 2.3.3) of strained-ring cyclic alkenes has attracted considerable attention in recent years due to the discovery that well-characterized metallacyclobutane [24] and metal alkylidene [25] complexes catalyze the living polymerization of monomers such as norbomene. Unfortunately, these catalysts often sulfer deactivating side reactions... 

Equation 7.39 describes a transformation with first the C-H bond of cyclopropane adding to the Rh complex, followed by RE of H2, and then rearrangement to give the rhodacyclobutane.82 Metallacyclobutanes are thought to be intermediates in some alkene polymerization and metathesis reactions these compounds will appear again in Chapter 11. 

Two of these are the cycloaddition of the methyhdene with ethylene (path E, non-productive), reaction of the methylidene with an internal olefin such that the alkyl substituent on the metallacyclobutane is in the j9-position (path H, non-productive). The other two pathways are the cycloaddition of the alkylidene with an internal olefin to give the trisubstituted metallacyclobutane (path G, frans-metath-esis, non-productive) and the reaction of the alkylidene with a terminal olefin to give the a,a -disubstituted metallacyclobutane (path F), which can be looked at as a chain transfer-type event, albeit not in the sense of a chain polymerization. In this case, the alkylidene is shifted from the end of one chain to the end of another chain. So, assuming that all pathways have somewhat similar rates, the elimination of ethylene will drive the reaction to high polymer. In the case of ADMET, these additional mechanistic pathways do not prevent the polymerization reaction, since these additional pathways are either degenerate or represent processes that do not affect the overall molecular weight distribution of the polymer. 

Well-defined alkylidene metallacyclobutane catalysts exist for the polymerization of olefines (alkenes) and acetylenes(alkines). 

Complex 17 shows versatile reactivity in both catalytic and stoichiometric reactions. It is a catalyst for olefin metathesis, in which a metallacyclobutane is proposed to be a key intermediate [52]. Grubbs et al. successfully isolated titanacyclobutanes 18 by treatment of 17 with alkenes in the presence of a Lewis base such as dimethylaminopy-ridine [56,57]. When cyclic olefins such as norbornene are treated with a catalytic amount of 18, ring opening metathesis polymerization occurs [58]. 

The interaction of metallocarbene (or alkylidene) complexes with unsaturated compounds via the formation of a metallacyclobutane intermediate plays an important role in several stoichiometric and catalytic reactions, e.g., metathesis, cycloaddition and polymerization. 

Grubbs has investigated the stereochemistry of ROMP using titanocene—metallacyclobutane complexes. These complexes offer little control over olefin stereochemistry or tacticity in the polymerization of norbornene. ° However polymerization of rac-1-methylnorbornene yielded a polymer with 90—95% trans-olefins and a partially regioregular (head—tail) sequence of monomers (the tacticity of the polymer was not determined). Using a related CpzTi—metallacyclobutane complex, a polymer of anti-7-methyl-norbornene was formed that was 80% trans in configuration and was partially syndiotactic ([r] =... 

Cossee type. The mechanism comprised of olefin metathesis type elementary processes involving a metal carhene complex and a metallacyclobutane was established only recently after the development of the chemistry of metal-carbene complexes [129], Understanding of the novel kind of reaction mechanism opened a new horizon in polymerization. Various new types of polymerization of cyclic monomers have been realized by ROMP. An advantage of the process is that the processes are tolerant to polar reactants and solvents enabling ready incorporation of polar substituents into polymers. Another advantage is that the ROMP is living in nature and polymers of narrow molecular weight distributions are available by the method. 

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. 

The corresponding metallacyclobutane can be isolated when the Tebbe complex reacts with one equivalent of norbornene (Fig. 4.19) [62]. This metallacycle continues to react with excess norbornene to generate poly (norbornene), and then carbene transfer to acetone can be used to remove the propagating metal species and end-cap the polymer chain. Subsequent studies estabhshed that these systems are living polymerizations. The experimental proof for living character includes the observations that (i) the propagating species can be observed for extended periods, (ii) the propagating species remains active for extended periods, (iii)... 

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