ADMET with ruthenium catalysts

The ruthenium catalyst system, 14, shown in Fig. 3, also carries out ADMET condensation chemistry, albeit with higher concentrations being required to achieve reasonable reaction rates [32]. The possibility of intramolecular compl-exation with this catalyst influences the polymerization reaction, but nonetheless, ruthenium catalysis has proved to be a valuable contributor to overall condensation metathesis chemistry. Equally significant, these catalysts are tolerant to the presence of alcohol functionality [33] and are relatively easy to synthesize. For these reasons, ruthenium catalysis continues to be important in both ADMET and ring closing metathesis chemistry. 

These data in total have led to the revising of the ADMET mechanism with ruthenium-based catalysts to include the very important aspect of phosphine dissociation (Scheme 6.27). In this mechanism k, k4, ky, and kg are assumed to be very fast, such that the dissociative olefin-phosphine exchange is die rate-determining process [100]. 

Ferulic acid, a phenolic acid that can be found in rapeseed cake, has been used in the synthesis of monomers for ADMET homo- and copolymerization with fatty acid-based a,co-dienes [139]. Homopolymerizations were performed in the presence of several ruthenium-based olefin metathesis catalysts (1 mol% and 80°C), although only C5, the Zhan catalyst, and catalyst M5i of the company Umicore were able to produce oligomers with Tgs around 7°C. The comonomers were prepared by epoxidation of methyl oleate and erucate followed by simultaneous ring opening and transesterification with allyl alcohol. Best results for the copolymerizations were obtained with the erucic acid-derived monomer, reaching a crystalline polymer (Tm — 24.9°C) with molecular weight over 13 kDa. 

The same acidic chloroaluminate ionic liquids have been used as solvent for tungsten aryl oxide complexes for the metathesis of alkenes [24]. Slightly acidic chloroaluminates also dissolve the [Cl2W=NPh(PMe3)3] complex which catalyze ethene oligomerization without the addition of co-catalysts [25]. In a similar way, Ni-catalyzed 1-butene dimerization into linear octenes was carried out in acidic chloroaluminates buffered with small amount of weak bases [26]. Neutral chloroaluminates (l-ethyl-3-methylimidazolium chloride/AlCl3 = 1) were employed to immobilize ruthenium carbene complexes for biphasic ADMET (acyclic diene metathesis) polymerization of an acyclic diene ester [27]. 

Schrock catalysts 22 are highly active, effective catalysts, not only for ROMP and ADMET, but also other useful types of olefin-metathesis reactions. However, due to their relatively high electrophilicity, they do not tolerate polar functions well. Nonetheless, they have been effectively employed in complex organic synthetic applications, and have been modified to effect enantioselective reactions with high selectivity. Ruthenium-based Grubbs catalysts were developed to increase the range of functional groups in which metathesis could be performed. 

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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