Hexadienes ADMET polymerization

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... 

Polybutadiene containing an unprecedented high trans stereochemistry can be synthesized via the ADMET polymerization of 1,5-hexadiene. " The extension to other purely hydrocarbon-based homopolymers has shown that (aside from steric factors that influence the formation of the metallacycle in the mechanism) the polymerization method is both trivial and broad in scope. 

The stereochemistry of the C=C bond in the polymer chains that result from ADMET of dienes of the type H2C=CH-(CH2) -CH=CH2 tends to be mostly trans in contrast to the result from ROMP of simple cycloalkenes, where trans C=C bond content may not be the predominant stereochemistry. For example, ADMET polymerization of 1,5-hexadiene gave a linear polymer with a trans C=C bond content of over 70% (catalyzed by Schrock catalyst 35), which is close to the value expected on the basis of thermodynamics.59 Earlier (equation 11.21), we saw that a similar polyalkenamer results from ROMP of methylcyclobutene (catalyzed by (CO)5W=CPh2) this time the stereochemistry of the C=C bond was 93% cis.60... 

The next approach for obtaining higher molecular weight polymers was to explore acyclic diene metathesis (ADMET) polymerizations.The aim was to achieve higher molecular weight flame-resistant polymers. We modeled the reaction using aliphatic diene monomers such as 1,5-hexadiene and 1,9-deca-diene under test conditions to optimize conditions before making BPC-derived products. At this point, we decided to functionalize the BPC with an olefin. 

Several early attempts at ADMET polymerization were made with classical olefin metathesis catalysts [57-59]. The first successful attempt was the ADMET polymerizations of 1,9-decadiene and 1,5-hexadiene with the WClg/EtAlf l,. catalyst mixture [60]. As mentioned in the introduction, the active catalytic entities in these reactions are ill-defined and not spectroscopically identifiable. Ethylene was trapped from the reaction mixture and identified. In addition to the expected ADMET polymers, intractable materials were observed, which were presumed to be the result of vinyl polymerization of the diene to produce crosslinked polymer. Addition to double bonds is a common side reaction promoted by classical olefin metathesis catalysts. Indeed, reaction of styrene with this catalyst mixture and even wifh WCl, alone led to polystyrene. Years later, classical catalysts were revisited in fhe context of producing tin-containing ADMET polymers wifh tungsten phenoxide catalysts [61], Alkyl tin reagents have long been known to act as co-catalysts in classical metathesis catalyst mixtures, and in this case the tin-containing monomer acted as monomer and cocatalyst [62]. Monomers with less than three methylene spacers between the olefin and tin atoms did not polymerize (Scheme 6.14). 

The kinetics of the ADMET reaction is not amenable to study by many traditional means, as these polymerizations are mostly conducted in bulk. The most effective way to measure the kinetics of the polymerization is to monitor the volume of evolved ethylene. This technique has been used to probe the difference in activity between [Mo] 2 and [Ru]l for ADMET polymerization of 1,9-decadiene [37]. At 26 °C in bulk monomer, [Mo] 2 promotes ADMET polymerization of 1,9-decadiene at a rate approximately 24 times that of [Ru]l. Additionally, [Mo] 2 polymerizes 1,5-hexadiene 1.7 times faster than 1,9-decadiene, while [Ru]l only cyclodimerizes 1,5-hexadiene to 1,5-cyclooctadiene. Monomers with coordinating functionality, specifically ethers and sulfides, were also investigated. Predictably, these monomers did not undergo polymerization as rapidly as hydrocarbon monomers however, this difference was dramatically more pronounced with [Ru]l than with [Mo]2. In fact, the dialkenyl sulfide monomers that were studied completely shut down the polymerization with [Ru]l, whereas the catalytic activity of [Mo]2 was only slightly lowered. This reduction in polymerization rate is most likely due to coordination of the heteroatom to the vacant coordination site of [Ru] 1, following phosphine dissociation. This reversible coordination of heteroatoms to the ruthenium complex likely occurs both intramolecularly and intermolecularly. Conversely, the steric bulk of the ligands in [Mo] 2 makes it less likely to intramolecularly form a coordinate complex, despite molybdenum being far more electrophilic than ruthenium. 

ADMET polymerization has also been applied to 1,5-hexadiene, and polybutadiene (PBD) exclusively in the 1,4 mode was obtained [49]. Unlike PBD produced by ROMP, this ADMET polymer has a trans content of 75% [52]. With [W]l and [Mo]2,theAfn of these polymers was approximately 8.0 X lO gmol , witha poly-dispersity near 2.0. Attempts to polymerize 1,5-hexadiene with [Ru]l, however, resulted in oligomers of approximately 1.0 x lO gmoD, in addition to cyclics and unreacted monomer, even after extended reaction times [53]. This decrease in activity was attributed to stable intramolecular ii-complexation of the distal olefin of the 4-penten-l-ylidene complex to the metal center. Such coordination could obstruct bimolecular coordination of another diene to the metal, and thereby prevent further polymerization. The absence of this effect with Schrock s catalysts was explained in part by the steric congestion around the metal center of those catalysts and the lack of a labile ligand. A number of hydrocarbon dienes have been utilized in ADMET polymerizations [54, 55]. 

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. 

Copolymerization of conjugated and nonconjugated dienes has also been achieved with ADMET [63a]. Poly(acetylene-co-octenamer)s were produced by ADMET polymerization of mixtures of 2,4-hexadiene and 2,10-dodecadiene. NMR and UV spectra revealed that these polymers were blocky, with an average of 4-5 acetylene units per conjugated block. 

ADMET has been used to prepare unsaturated telechelic oligomers of PBD by reacting 1,5-hexadiene with an appropriate mono or difunctional olefin [176b, 178]. Alternatively, 1,5-hexadiene may undergo ADMET polymerization to form ADMET PBD and subsequently be reacted with an appropriate functionalized olefin. This reaction is believed to proceed through a cyclic intermediate of the PBD followed by CM of the cyclic polymer with the functionalized olefins (Figure 13.23) [179]. In some cases, the formation of telechelics is incomplete. 

Acyclic diene metathesis (ADMET) [75] is the process by which a transition metal catalyst leads to a stepwise condensation polymerization of diene monomers, characterized by loss of gaseous ethylene and the production of linear polyolefins containing regular unsaturations along the polymer backbone (Scheme 1.8). In fact, many of the polymeric structures accessible by ADMET can be made by alternate mechanisms (e.g., 1,4-polybutadiene made by ADMET polymerization of 1,6-hexadiene is more commonly made by the anionic polymerization of 1,4-butadiene). 

Further, it is shown in Figure 6 that this methyl steric effect can be observed even when the methyls are placed alpha to the metathesizing olefin itself. Thus, 3,4-dimethyl-1,5-hexadiene does not polymerize, whereas 3-methyl-1,5-hexadiene dimerizes in the 5 position. These steric interactions are quite subtle, and by examining the ADMET polymerization cycle shown earlier, it becomes evident that the formation of a metallacyclobutane ring and its stability is influenced by simple steric interaction. 

The most recent application of olefin metathesis to the synthesis of polyenes has been described by Tao and Wagener [105,117], They use a molybdenum alkylidene catalyst to carry out acyclic diene metathesis (ADMET) (Fig. 10-20) on either 2,4-hexadiene or 2,4,6-octatriene. The Wagener group had earlier demonstrated that, for a number of nonconjugated dienes [118-120], these polymerizations can be driven to high polymer by removal of the volatile product (e. g., 2-butene). To date, insolubility limits the extent of polymerization of unsaturated monomers to polyenes containing 10 to 20 double bonds. However, this route has some potential for the synthesis of new substituted polyacetylenes. Since most of the monomer unit is preformed before polymerization, it is possible that substitution patterns which cannot be incorporated into an alkyne or a cyclic olefin can be built into an ADMET monomer. 

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