ADMET with molybdenum catalysts

Representative data illustrating the influence of Lewis base functional groups in the ADMET reaction are shown in Table 1. When molybdenum catalysts are used to polymerize ether or thioether dienes, little change in reaction rate is observed as compared with the standard, 1,9-decadiene, which possesses no heteroatoms in its structure. When a sulfur atom is three carbons atoms away from the alkene site, the reaction rate is reduced approximately one order of magnitude otherwise, the kinetics are all essentially unaffected [20a]. 

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]. 

A series of methyl-substituted polymers, with varying numbers of methylene units between the terminal olefin and the branch point [58], demonstrated that, when there are at least two methylene units separating the olefin and branch point, there is little effect on the catalysis of the reaction. A series of poly(l,4-alkylenephenylene)s have also been prepared by ADMET with Schrock s molybdenum, Grubbs first-generation, and classical catalysts [59]. This series demonstrated that hydrocarbon dienes containing aromatic groups are readily polymerizable by ADMET, even when there is only one methylene unit between the olefin and the aromatic group. 

Ethers were the first functionalized dienes to be polymerized successfully by ADMET [74]. It was found, however, that at least three methylene spacers between the oxygen atom and the olefin were required for successful polymerization with Schrock s catalysts. The resultant polymers were cross-linkable by both chemical and photochemical means [75]. Both [Mo]2 and [W]l were capable of producing the polymer, but polymerization with the molybdenum catalyst proceeded at a rate roughly 10 times faster than that with the tungsten catalyst [76]. ADMET polymerization of diallyl ether was attempted with both [Mo]2 and [Ru]l, resulting in a low molecular weight polymer. The major reaction product for both catalysts is 2,5-dihydrofuran, the result of RCM (Figure 13.6). Divinyl ether was not metathesis-active with any of Schrock s catalysts. 

A series of polycarbonates has been synthesized via ADMET with Schrock s molybdenum catalyst [90]. A diene containing a bisphenol-A unit was polymerized to a polymer with an of 1.5 x lO gmol and a PDI of 1.9 (Figure 13.11). 

In 2000, the Nobel Prize in Chemistry was awarded for the discovery and development of conducting polymers, and the conversion of polyacetylene to a conducting material was integral in this effort [151]. Oligomeric polyacetylene has been produced via ADMET polymerization of 2,4-hexadiene with both Schrock s tungsten and molybdenum catalysts [63a]. Similarly, oligomers were also formed by ADMET polymerization of 2,4,6-octatriene, although no polymer resulted from the reaction of either 1,3-butadiene or 1,3,5-hexadiene. 

These limitations were overcome with the introduction of the well-defined, single-component tungsten and molybdenum (14) alkylidenes in 1990. (Fig. 8.4).7 Schrock s discoveiy revolutionized the metathesis field and vastly increased die utility of this reaction. The Schrock alkylidenes are particularly reactive species, have no side reactions, and are quite effective as polymerization catalysts for both ROMP and ADMET. Due to the oxophilicity of molybdenum, these alkylidenes are moisture and air sensitive, so all reactions using these catalysts must be performed under anaerobic conditions, requiring Schlenk and/or glovebox techniques. 

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... 

Scheme 5 suggests that every step of the ADMET polymerization cycle is in equilibrium and that, by shifting the relative concentrations of the condensate and polymer, depolymerization would result. In fact it has been shown that various unsaturated polymers can be depolymerized with excess ethylene, as well as substituted ethylenes. These depolymerizations can be done either with the tungsten or the molybdenum versions of Schrock s catalyst. 

The photoluminescence efficiencies of the synthesized m,/7-phenylenevinylenes reached pretty high levels (up to 52%), but there are still some improvements to be made in connection with solubility. It will be necessary to use only substituted comonomers in order to achieve higher molecular weights and an increased processability of the polymers. As ADMET polymerizations of ortho-alkoxy substituted divinylbenzenes are very slow due to coordination of the oxygen at the molybdenum atom of the catalyst and the substituent should be alkyl in o- or m-positions. 

Nonetheless, ADMET is a versatile technique that allows the incorporation of a wide variety of functional groups into the resultant polymers. Scheme 1.9 shows the catalytic cycle of ADMET, controlled by the metathesis catalyst, which can be either ruthenium- [76, 77] or molybdenum-based [78, 79]. While the kinetics are controlled by the catalyst (there is no reaction in its absence), it still follows the kinetic picture described in Section 1.3.2. This is because the catalyst is removed from the chain end after each successful alkene metathesis reaction (i.e., coupling) and the olefin with which it subsequently reacts is statistically random. 

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