Polymerization alkylidene complexes

In this chapter I will cover only well-defined or well-characterized compounds. Results will be included that have appeared since reviews in 1991 on alkylidene and metalacyclobutane complexes [41] and in 1993 on ring-opening metathesis polymerization [30], but an overview of prior results that are especially relevant to olefin metathesis in particular will also be included. (An excellent and comprehensive text also has been published recently [1].) The terms well-defined or well-characterized originally were meant to imply that the alkylidene complex is isolable and is essentially identical to that in a catalytic reaction except for the identity of the alkylidene. These terms have been watered down from time to time in the literature, even to the point where they are used to describe a catalyst that is formed from a well-characterized transition metal precursor complex, but whose identity actually is not known. In this article I... 

Mo and W alkylidene complexes 4, the so-called Schrock carbenes, have explosively evolved the polymerization chemistry of substituted acetylenes. Although the preparation of these catalysts is relatively difficult because of their low stability, in other words, high reactivity, they elegantly act as living polymerization catalysts for substituted... 

Vanadium precursors tend to promote addition polymerization of olefins on activation with aluminum co-catalysts. Nomura and coworkers recently reported the first example of catalytic olefin metathesis by a vanadium alkylidene complex.The well-defined, thermally stable V(v) alkylidene complex 7 effected ROMP of NBE at 80 °C. Cettain alkylvanadium complexes, including V(CH2Ph)2(=NAr )(OAr ) (Ar = 2,6-Pr 2C6H3 Ar = 2,6-Me2C6H3) and... 

Although the alkylidene complexes initiate the ROMP of functionalized norbomenes and 7-oxanorbomenes in aqueous solution quickly and completely in the absence of acid, the propagating species in these reactions often decompose before the polymerization reaction is complete. For example, in the ROMP of the water-soluble... 

The proposed idea that metal alkyhdene complexes are be able to catalyze olefin metathesis was confirmed in 1980 [8] and consolidated in 1986 by Schrock with the development of the first well-characterized, highly active, neutral tungsten (Cl, Fig. 3) [9] and molybdenum (C2) [10] alkylidene complexes. These complexes were able to catalyze both the metathesis of different olefins and the ROMP of functionalized norbomene to polynorbomene with low polydispersities [11]. Moreover, these catalysts were used by Wagener and coworkers to perform the first quantitative ADMET polymerization [12] and copolymerization [13] of 1,5-hexadiene and 1,9-decadiene. However, the low stability of these catalysts in... 

Further synthetic utility can be introduced by changing the metal in the above alkylidene complexes from tungsten to molybdenum [52]. Molybdenum which is less oxophilic than tungsten will tolerate monomers containing mildly reactive functionalities such as esters [52a] and nitriles [52b] without appreciable catalyst deactivation during the polymerization reaction. For example, the polymerization of e rfo,endo-5,6-dicarbomethoxynorbornene (DCNBE) with Mo(CHtBu)(NAr) (OtBu)2 (Ar = 2,6-diisopropylphenyI) (35a) has been reported to give the cone-... 

Figure 3 Chiral molybdenum alkylidene complexes used in asymmetric ring closing metathesis polymerization...

Schrock has prepared alkylidene complexes of Mo and W whose activity is tuned so that they are unreactive to ordinary internal alkenes but actively catalyze polymerization of strained cyclic alkenes, such as norbornene. See R. R. Schrock, Acc. Chem. Res., 1990, 23, 158, and references therein. 

Alkyne Polymerization by Molybdenum-imido-alkoxy-alkylidene Complexes... 

The molybdenum catalyst 2 has been used extensively for ADMET polymerization. This complex is easier to handle than the tungsten analog and is more tolerant of functionality. This complex has allowed the synthesis of polymers containing esters, carbonates, ethers, sulfides, aromatic amines, boronates, dichlorosilanes, siloxanes, acetals, and conjugated carbon-carbon double bonds [38-45]. Aldehydes, ketones, and protic functionahty are not tolerated. The molybdenum alkylidene will react with aldehydes and ketones, but not esters, in a Wittig fashion [64]. 

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. 

Matyjaszewski et al. demonstrated that living ring opening metathesis polymerization (ROMP) could also be combined with ATRP to produce novel block copolymers [292]. ROMP of norbornene (NB) and dicyclopentadiene (CPD) were performed using an Mo-alkylidene complex, followed by reaction with p-(bro-momethyl) benzaldehyde to generate a benzyl bromide terminated polymer capable of being used as a macroinitiator for ATRP (Scheme 41). 

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. 

The main disadvantage of controlled radical polymerization is its low rate, being at least two orders of magnitude lower than that of the conventional one. Recently it was shown that the rate of ATRP catalyzed by rutheiuum cyclopentadienyl or alkylidene complexes may be increased by introduction of amines 19, 20). 

To date, two types of metallocene complexes have been used in the polymerization of olefins at CNT surfaces by PFT a ruthenium alkylidene complex anchored by two routes onto SWCNTs via a pyrene derivative spacer [94], and zirconium complexes of general formula [(R)2ZrCl2], R being a derivative of cyclopentadienyl or indenyl, anchored onto CNTs modified with MAO [97]. 

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