Ethers ADMET polymerization

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

ADMET polymerization of dienes containing functional groups has been explored to some extent. It appears that diene ethers such as 36 are tolerant of Schrock s W-alkylidene catalysts when undergoing ADMET polymerization, but Grubbs first-generation catalyst is required to successfully polymerize diene alcohol 37,61 because the OH group is too Lewis basic for catalysis by W- and Mo-alkylidenes.62... 

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]. 

Examples of such reactions are shown in Table 8.5. Divinyl ether is unreactive but diallyl ether, when treated in bulk with 8, gives an equilibrium mixture of 63% 2,5-dihydrofuran and 37% of its -trans, ring-opened polymer, albeit of low MW (Fig. 8.3). The analogous tungsten complex is not a good initiator for the ADMET polymerization of diallyl ether. 

Substituted unsaturated pyrans prepared by RCM using 18 as catalyst can be immediately submitted to zirconium-catalyzed kinetic resolution of the racemic product at 70°C. This provides a new route to medicinally important agents containing 6-membered cyclic ethers. A one-pot synthesis can give 63% conversion with >99% enantiomeric purity (Morken 1994). Dienes of the type CH2=CH(CH2)30(CH2(H20) CH2)3CH=CH2 (m = 2-4) readily undergo ADMET polymerization in the presence of catalyst 8 (Qiao 1995). 

ADMET polymerization and depolymerization methods have been used in the synthesis of telechelic oligomers. The metathesis depolymerization of 1,4-polybutadiene is accomplished in the presence or absence of a monofunctional diene by using either of the catalysts presented in Figure 13. Telechelic oligomers with terminal alkene, ester, and silyl ether and imide functional groups may be... 

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. 

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. 

Monomers with multiple ether functionalities have also been polymerized via ADMET [76, 77]. Acetal-containing dienes have undergone ADMET polymerization with Grubbs [Ru] 1 catalyst to yield the polymer in quantitative yield [78,53]. Additionally, dienes containing primary, secondary, and tertiary alcohols have been shown to be amenable to ADMET polymerization (Figure 13.7) [79]. 

In a similar work, polyrotaxanes were synthesized via ADMET polymerization of a supramolecular monomer comprising a polymerizable ammonium salt and crown ether (Figure 13.25) [189]. 

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]. 

This has been referred to as the negative neighboring group effecf and has been proposed to be responsible for the slower kinetics of ADMET of ether dienes compared to hydrocarbon dienes [35]. Three carbons between the olefin and a carbon bearing coordinating functionahty are usually sufficient to allow polymerization, although there are exceptions to this trend [33]. Intense catalyst development efforts are producing catalysts that are more and more tolerant to functionality closer to the olefin. 

These ruthenium complexes react rapidly and quantitatively with ethyl vinyl ether to form a Fischer carbene that is only weakly metathesis active at elevated temperatures [86, 87]. This property can be employed to end-cap ROMP and ADMET polymers and to ensure that there are no polymeric ruthenium alkyhdenes present. Since ruthenium alkylidenes are relatively robust complexes they could survive workup procedures, although experimental evidence has yet to confirm this notion. Treatment of an ADMET polymer with ethyl vinyl ether gives the polymer well-defined terminal olefinic endgroups and should prevent backbiting metathesis upon dilution of the polymer (Scheme 6.22). 

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