Polymerizations metathetical

Furthermore, various 3-substituted cyclobutenes were polymerized metathetically [181]. 1-Methylcyclobutene has been polymerized with a WQg [370,373], RuCla [42d] or Mo -2,6-i-Pr2-C6H3)(=CHCMe2R)(OCMe (CF3)3 2 (R = Me, Ph) [374,375] initiator. Many other functionalized derivatives were successfully ROM polymerized, too [22,145, 181,376,377]. 

Additionally, Table 14 shows selected examples of dimethylnorbornenes, which were polymerized metathetically. 

Metathesis is a catalyzed reaction that converts two olefin molecules into two different olefins. It is an important reaction for which many mechanistic approaches have been proposed by scientists working in the fields of homogenous catalysis and polymerization. One approach is the formation of a fluxional five-membered metallocycle. The intermediate can give back the starting material or the metathetic products via a concerted mechanism ... 

Further variations of the basic motif of 1 comprise carbene le in which the anionic ligands are trifluoroacetate rather than chloride (Fig. 2) [4d]. However, this specific compound shows a significant tendency to isomerize the double bonds of the substrates in addition to its metathetic activity. Moreover, water soluble catalysts have been developed that contain bulky aliphatic phosphine ligands with either a quarternary ammonium group (lf,g) or a sulfonate function (lh) [12]. They have been tested in the ring opening polymerization... 

Ring-Opening Metathetic Polymerization of Porphyrazines under the Conditions Described in the Texta... 

Both W(CO)5[C(C6Hs)2] and the analogous di-p-tolylmethylene complex have been used in model studies of the olefin metathesis reaction.2 3 In contrast to heteroatom-stabilized carbene complexes such as W(CO)s [C(OCH3)(C6Hs)], pentacarbonyl(diphenylmethylene)tungsten(0) reacts with alkenes to give cyclopropanes and 1,1-diphenylalkenes.2 The compound W(CO)5 [C(C6H5)2] is the best reported catalyst for the metathetical polymerization of 1-methylcyclo-butene.4... 

Nonconjugated dienes may undergo polymerization under metathesis conditions. Metathesis, in turn, can be used for structure determination of polyalkenylenes via metathetical degradation with a low-molecular-weight olefin.6... 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

As already mentioned, oxocarbenium salts are efficient initiators42,45 for the polymerization of oxolane. They can be prepared in situ by a metathetic reaction between the corresponding acyl halide and silver hexafluoroantimonate ... 

Dimidcai carbene complexes of tungsten have also been prepared and they have been used to catalyze the metathetical polymerization of alkenes and alkynes 143],... 

Independently of the interest of the CG complexes in polymerization catalysis, similar reactivity to that of metallocenes can be observed. For instance, the metathetical reaction of [(C5H4)SiMe2(Nt-Bu)]ZrCl2 with [Mg(C4H6)(thf)2] affords the Zr-butadiene complex (39)." ... 

We have chosen for our metathetical study in the case of unsaturated disilanes the first generation of Grubbs catalysts because of the reduced degree of possible isomerization of the products during the time of polymerization [7]. 

In contrast to other alkenyl-substituted organosilicon compounds, a metathetical conversion of divinylsilyl derivatives in the ADMET polymerization as well as the RCM process does not occur in the presence of well-defined W, Mo, and Ru carbene complexes because of stereoelectronic effects of the silyl group stimulating non-productive cleavage of the disilyhnetallacyclobutane intermediate [16],... 

Metal carbenes are also initiators for the metathetical polymerization of alkynes by, e. g., catalysts such as M0CI5 and WClg. Reaction of the metal carbene with an alkyne gives a metallacyclobutene intermediate, followed by opening of this intermediate metallacycle into a new metal carbene that in its turn can interact with another alkyne molecule, and so on eq. (13). Metathesis of acetylenes proceeds through reactions between an alkylidyne complex and an alkyne via a metallacyclobutadiene intermediate (eq. (14)). 

In the field of homogeneous catalysis the metathesis of unsaturated hydrocarbons offers many intriguing possibilities for producing important intermediates and end products from alkenes and alkynes. Through major advances in catalyst design in recent years, metathesis has become an important synthetic route to be considered in chemical laboratories whenever a special organic product has to be obtained. In fine chemistry metathesis is a very valuable reaction for the synthesis of natural compounds. Metathetical polymerization has been accepted by the industry as a viable means for producing polymers. 

Results of investigations related to the mechanistic aspects of ring-opening reactions of cyclo-olefins have been discussed by Dolgoplosk et al the individual stages of the process were considered. Formation of stable complexes of carbenes of the type RQCH= (Q = O, S, or Si) inhibit the chain process of metathetical polymerization of cyclo-olefins. ... 

Hydrolysis, redox, metathetical, and halide abstraction reactions are covered here. Some of these reactions lead to specific complexes with Ti-O, Ti-N, and Ti-C bonds which are described in subsequent sections. Comments on the applications of the mono-Cp trihalo titanium complexes as olefin polymerization pre-catalysts have been mentioned in Section 4.05.3.1.1 and some recent advances in this field are also considered here. (See Chapter 4.09 of this work.)... 

Ring strain apparently determines the competitive pathway of RCM or metathetic polymerization to be adopted in the case of 1,2-diallyloxycyclohexane. Ring formation from the trans-isomer is favored (60% yield), while the c/s-isomer gives only 20% yield of the more strained bicyclic product. 

Another type of polymerization promoted by arene complexes is based on the well-known olefin metathesis reaction. Olefin and alkyne metathetical polymerizations have been observed with catalytic amounts of Group VI arene metal carbonyls under refluxing conditions [37]. The same process takes place at ambient temperatures when electron-transfer-chain catalysis is invoked [37]. 

Silanes. Group IVB metallocenes catalyze the stepwise polymerization (dehydrocoupling) of primary silanes, yielding H—(SiHR) —H and n H2 (232). The mechanism appears to he strictly metathetical for such molecules as (Cp)(C5(CH3)5)Hf(X) (R = H, alkyl X = H, alkyl, chloride), wherein the entire process takes place on the neutral manifold (Scheme 11) (233). 

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