Catalyst activity and functional group tolerance

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. 

Other ruthenium catalysts have also been studied. In one study, ((IPrH2) (PCy3)(Cl)2Ru=CHPh) promoted ADMET faster than [Ru]2, although it too promoted olefin isomerization [28b]. The initiation rate of [Ru]3 was reported to be 1/30 that of [Ru]l, but the propagation rate was found to be four times faster [16]. The activity of [Ru]4 has even been shown to surpass that of Schrock s molybdenum-based catalysts [39]. 

Polyethylene (PE) is arguably the most important commercially produced polymer. Modeling PE copolymers has proved to be one of the most common apph-cations of ADMET polymerization, because the hydrogenation of an unsaturated ADMET polymer results in the equivalent of a sequence-specific copolymer of ethylene and another monomer. However, in order to provide an accurate model of PE copolymers produced by other methods, the hydrogenation reaction must be quantitative. 

Several methods of hydrogenation have been developed that are quantitative within the limits of detection of H and NMR, and IR spectroscopy. Catalytic hydrogenation with hydrogen gas and an appropriate metal-based catalyst is the most common approach. Wilkinson s catalyst RhCl(PPh)3 [44] has proved particularly effective [45], and works with both soluble and insoluble polyolefins. Pd/C is also capable of quantitatively hydrogenating ADMET polymers, although repeated treatments may be necessary [46]. 

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Recent developments in ADMET polymerization and its use in materials preparation have been presented. Due to the mild nature of the polymerization and the ease of monomer synthesis, ADMET polymers have been incorporated into various materials and functionaUzed hydrocarbon polymers. Modeling industrial polymers has proven successful, and continues to be appUed in order to study polyethylene structure-property relationships. Ethylene copolymers have also been modeled with a wide range of comonomer contents and absolutely no branching. Increased metathesis catalyst activity and functional group tolerance has allowed polymer chemists to incorporate amino acids, peptides, and various chiral materials into metathesis polymers. Sihcon incorporation into hydrocarbon-based polymers has been achieved, and work continues toward the application of latent reactive ADMET polymers in low-temperature resistant coatings. 

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