Olefin metathesis polymerization mechanism

His proposal involved a metal carbene and a metallocyclobutane intermediate and was the first proposed mechanism consistent with all experimental observations to date. Later, Grubbs and coworkers performed spectroscopic studies on reaction intermediates and confirmed the presence of the proposed metal carbene. These results, along with the isolation of various metal alkyli-dene complexes from reaction mixtures eventually led to the development of well-defined metal carbene-containing catalysts of tungsten and molybdenum [23-25] (Fig. 2). After decades of research on olefin metathesis polymerization, polymer chemists started to use these well-defined catalysts to create novel polymer structures, while the application of metathesis in small molecule chemistry was just beginning. These advances in the understanding of metathesis continued, but low catalyst stability greatly hindered extensive use of the reaction. 

In metathesis polymerization, the catalyticaUy active species is a stable metal-carbene bond that is formed between the metal and the alkene. Upon reaction with cycloalkane, a living moiety capable of chain growth is formed. The olefin metathesis reaction mechanism is shown in Scheme 3.18. 

As with other transition metal-catalyzed reactions (Ziegler-Natta polymerization of alkenes, olefin metathesis), the mechanism of the Heck reaction is complicated. In brief, the species that reacts with the aryl halide is I Pd, where L is a ligand such as tiiphenylphosphine. By a process known as oxidative addition, palladium inserts into the carbon-halogen bond of the aryl halide. 

The mechanism of the olefin metathesis polymerization involves (a) an initial 2+2 cyclo-addition of a metal carbene with the cyclic olefin. This leads to the formation of a four-membered metallacyclobutane. The next steps (b) (c) and (d) show the polymer propagation reactions (Fig. 2.30). 

For more information regarding this mechanism, see Ivin KJ,Mol JC (1997) Olefin Metathesis and Metathesis Polymerization. Academic Press, London... 

It is worth noting that 6,7-dihydro-2(3//)-oxepinone is an unusual lactone because it can be polymerized by two distinct mechanisms ROP of the cyclic esters by aluminum alkoxides, and the ring-opening metathesis polymerization (ROMP) of endocyclic olefins by the Schrock s catalyst (Fig. 28) [121]. 

The polymerization of cyclic, strained olefins by transition metal alkylidenes of general formula L M = CRR (L = ligand, R, R = H, alkyl, aryl) yields polymers formed via ring-opening that contain unsaturated double bonds within each repetitive unit. Since the mechanism is based on repetitive metathesis steps, this polymerization reaction is known as ring-opening metathesis polymerization (ROMP) (Scheme 1). 

According to this mechanism, olefin metathesis is a chain reaction with the carbene as chain carrier. It predicts complete randomization of alkylidene fragments from the first turnover. An important additional feature is that both metathesis and ringopening polymerization of cycloalkenes can be explained by this mechanism, which provides ready explanation of polymer molecular weight. 

Figure 3.38. Mechanism of olefin metathesis and strategies for the cleavage of alkenes from polymeric supports by olefin metathesis.

X. POLYMERIZATION OF ACETYLENES BY OLEFIN METATHESIS CATALYSTS A. Proof of Mechanism... 

A recent report by Mayr of slow polymerization of PhC=CH by (PMe3)2Cl2(PhC=CPh)W=CHPh fulfills expectations based on the classic Chauvin mechanism for olefin metathesis (78). The presence of a carbene and a vacant coordination site are prerequisites for metallocyclo-butene formation with free alkyne. Mayr has both the carbene and the alkyne initially present in the catalyst, but there is no evidence for direct involvement of the cis alkyne in the actual polymerization mechanism. 

It is possible to suggest a polymerization scheme which is compatible with the mechanism suggested for the olefin metathesis reaction. For conciseness the polymerization scheme shown at the top of the next page assumes the strict absence of any acyclic olefins which might participate in the process. 

Mechanism 8-10 Acid-Catalyzed Opening of Epoxides 362 8-14 Syn Dihydroxylation of Alkenes 364 8-15 Oxidative Cleavage of Alkenes 366 8-16 Polymerization of Alkenes 369 8-17 Olefin Metathesis 373... 

Olefin additions to bridging alkylidenes yield dimetallacyclopentanes . These reactions also provide a mechanism for olefin metathesis, a topic not discussed here. Although addition of an olefin to a metal carbone, a 2n + In addition, would be symmetry forbidden in organic chemistry, ab initio calculations " of the conversion of a metal carbene-alkene to a metallocyclobutane show it to be a barrierless reaction. Metal d orbitals relax the symmetry restrictions for the In + 2n addition. The mechanism of reaction (p) has not been widely considered for the olefin polymerization, but it may be relevant to olefin dimerization and oligomerization—reaction (s), for example ... 

Thus systems that also catalyze olefin metathesis (MoCl5/SnPh4) polymerize alkynes by a metalloacyclobutene mechanism [reaction (0)] °, whereas catalysts (Ti(0-n- 4119)4/AlEts) that polymerize olefins as well as alkynes follow an insertion mechanism [reaction (n)] for alkyne polymerization. 

There are three different mechanisms by which the cyclic olefin norbornene can be polymerized to reasonably high molecular weights ring-opening metathesis polymerization (or ROMP), vinyl addition copolymerization with acyclic olefins such as ethylene, and vinyl addition homopolymerization (see Fig. 4.2). Carbocationic and free-radical initiated polymerizations are ignored since they yield only low molecular weight oligomers [8]. 

This chapter will present some of the history of ADMET and olefin metathesis in general, although the emphasis will be on the mechanism and kinetics of ADMET polymerization. The general mechanism for olefin metathesis will be presented before any of the specific catalyst structures are introduced or discussed in order to provide the reader with a firm basis upon which to compare the various popularly used catalysts for ADMET polymerization. In addition, procedural information will be given at the end of the chapter to give the reader an idea of what is specifically involved in a typical ADMET polymerization. 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

Of particular interest is the fact that many olefin metathesis catalyst systems are of the Ziegler—Natta type. This raises the question of the relationship between the mechanism of olefin metathesis and that of Ziegler-Natta polymerization this aspect is discussed in Ch. 4. 

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