Grubbs-Type Initiators

Via this route, theresultingcompoundsofthe generalformulaRuCl2(PR3)2(CH Ph), (R = Ph, Cy3)-which today are well known as the first-generation Grubbs catalyst-are accessible in high yields. Phosphane-exchange from the parent bis(triphenylphosphane) systems into the more reactive bis(tricyclohexylpho-sphanes) may be performed either consecutively or in situ. The reaction of 

A synthetic protocol consisting of the reaction of RuCl2(PPh3)2(CHR) with imidazol-2-ylidenes [50-52] was used for the generation of another type of ruthenium-based system. While bis(N-heterocyclic carbene) complexes displayed a comparably low reactivity in ROMP [18, 19], the corresponding mono-N-heterocyclic carbene-monophosphane complexes turned out to be highly active [18, 19, 53]. 

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NBE/DMN-H6 Chemical, Grubbs type initiator 2-Propanol/toluene 5 mm, 3 mm, and 200 pm I.D., biopolymer chromatography and SEC of synthetic polymers [39-42]... 

Ferroelectric liquid crystalline monomers (Fig. 21, XXXVII) bearing two terminal vinyl groups were polymerized directly from their smectic A" liquid crystal phase using a Grubbs-type initiator. 

Irrespective of the reaction conditions used (/. e. ultrasound, microwave, changing reaction times, temperature and solvents), the maximum turnover number (TON) that was achieved was 75. In principle, second generation Grubbs-type initiators immobilized on non-porous silica should behave similar to those immobilized on monolithic supports[16]. In fact, catalysts immobilized onto monolithic supports give similar maximum TONs (< 65) in the absence of any chain transfer agent (CTA). Ruthenium measurements by means of ICP-OES revealed quantitative retention of the original amount of ruthenium at the support within experimental error ( 5%), thus otfering access to metal free products. 

FIGURE 3.23 Selection of Grubbs-type initiators of first to third generation. 

FIGURE 3.24 ROMP of norborn-5-ene-2-methanol with a Grubbs-type initiator in... 

Grubbs-type initiators are well-defined ruthenium aUcylidenes. Compared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based Grubbs catalysts is somewhat different. In terms of polymer structure, ROMP of norbom-2-enes and norbomadienes using ruthenium-based systems generally results in the formation of polymers that, in most cases, predominantly contain frany-vinylene units. Polymerizations initiated by Grubbs-type initiators are best terminated by the use of ethyl vinyl ether, yielding methylidene-terminated polymers. 

Figure 19.1 Selected examples of Schrock- and Grubbs-type initiators. X = Cl, Br,...

Figure 19.6 Schiff-base-substituted Ru(ll) and second-generation Grubbs-type initiators.

Scheme 19.11 Mechanism of ROMP initiated by Grubbs-type initiators.

Figure 19.9 P,0-dichelating Grubbs-type initiators for the alternating ROMP of norborn-2-ene and cis-cyclooctene.

Figure 19.11 4-Poly(ethylene glycol)-substituted pyridine-derived Grubbs-type initiators.

Example 3.34 ROMP of norborn-5-ene-2-methano with a Grubbs-Type Initiator in Solution... 

In principle the most common types of polyelectolytes like poly acids or polyamins should be accessible via ROMP with the Grubbs-type initiators. It turned out that direct polymerisation is problematic as both functional groups are able to form complexes with the initiator. So we decided to protect the carboxylic acid with a tetrahydropyrane group. After the polymerisation this group was easily hydrolysed and the polyacid or the respective salt was easily accessible [10]. 

While Grubbs-type initiators may only be used for grafting-from experiments, Schrock-type initiators may be used for both methods, both Schrock-type and Grubbs-type initiators were found suitable for that purpose. Scheme 2 gives an overview over the entire reaction sequence. 

Figure 3 Selection of modified Grubbs-type initiators.

Despite some reports on a successful polymerization of terminal alkynes by Grubbs-type initiators, the efficient and living polymerization of these monomers is still the... 

In an alternative approach to supported Grubbs-type initiators (norbom-2-ene-5-yl-trichlorosilane or norbom-2-ene-5-yl-triethoxysilane) (both exo/endo-mixtures), surface-... 

Scheme 23 Grubbs-type initiator triggered synthesis of 0NDCA-/ -NBE copolymers.

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