Ruthenium complexes, ROMP

Synthesis of block copolymers of norbornene derivatives, with different side groups, has been reported via ROMP [101]. Initially, exo-N-bulyl-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboximide was polymerized in acetone at room temperature with a ruthenium initiator (Scheme 40). The conversion of the reaction was quantitative. Subsequent addition of norbornene derivative carrying a ruthenium complex led to the formation of block copolymers in 85% yield. Due to the presence of ruthenium SEC experiments could not be performed. Therefore, it was not possible to determine the molecular weight... 

Fig. 11. Gibson and coworkers synthesis of a polymer displaying the nucleobase thymine using ROMP initiated by Grubbs ruthenium complex... 

For the synthesis of carbohydrate-substituted block copolymers, it might be expected that the addition of acid to the polymerization reactions would result in a rate increase. Indeed, the ROMP of saccharide-modified monomers, when conducted in the presence of para-toluene sulfonic acid under emulsion conditions, successfully yielded block copolymers [52]. A key to the success of these reactions was the isolation of the initiated species, which resulted in its separation from the dissociated phosphine. The initiated ruthenium complex was isolated by starting the polymerization in acidic organic solution, from which the reactive species precipitated. The solvent was removed, and the reactive species was washed with additional degassed solvent. The polymerization was completed under emulsion conditions (in water and DTAB), and additional blocks were generated by the sequential addition of the different monomers. This method of polymerization was successful for both the mannose/galactose polymer and for the mannose polymer with the intervening diol sequence (Fig. 16A,B). 

Allenylidene-Ruthenium Complexes in RCM, Enyne Metathesis and ROMP... 

Allenylidene-ruthenium complex Ib readily promotes the ROMP of norbornene, much faster than the precursor RuCl2(PCy3)(p-cymene) [39] (Table 8.1, entry 1). The ROMP of cyclooctene requires heating at 80 °C (5 min), however a pre-activation of the catalyst allows the polymerization to take place at room temperature. The activation consists, for example, in a preliminary heating at 80 °C or UV irradiation of the catalyst before addition of the cyclic aikene, conditions under which rearrangement into indenylidene and arene displacement take place [39] (Table 8.1, entries 2,3). The arene-free allenylidene complexes, the neutral RuCl2(=C=C=CPh2)... 

Kinetic studies of diallyltosylamide RCM reaction monitored by NMR and UV/VIS spectroscopy showed that thermal activation of the catalyst precursors la and Ib (25-80 °C) led to the in situ formation of a new species which could not be identified but appeared to be the active catalytic species [52]. Attempts to identify this thermally generated species were made in parallel by protonation of the catalysts I. Indeed, the protonation of allenylidene-ruthenium complex la by HBF4 revealed a significant increase in catalyst activity in the RCM reaction [31,32]. The influence of the addition of triflic acid to catalyst Ib in the ROMP of cyclooctene at room temperature (Table 8.2, entries 1,3) was even more dramatic. For a cyclooctene/ruthenium ratio of 1000 the TOF of ROMP with Ib was 1 min and with Ib and Sequiv. of TfOH it reached 950min [33]. 

Applications of Isolated Indenylidene-Ruthenium Complexes in ROMP... 

Two observations initiated a strong motivation for the preparation of indenylidene-ruthenium complexes via activation of propargyl alcohols and the synthesis of allenylidene-ruthenium intermediates. The first results from the synthesis of the first indenylidene complexes VIII and IX without observation of the expected allenylidene intermediate [42-44] (Schemes 8.7 and 8.8), and the initial evidence that the well-defined complex IX was an efficient catalyst for alkene metathesis reactions [43-44]. The second observation concerned the direct evidence that the well-defined stable allenylidene ruthenium(arene) complex Ib rearranged intramo-lecularly into the indenylidene-ruthenium complex XV via an acid-promoted process [22, 23] (Scheme 8.11) and that the in situ prepared [33] or isolated [34] derivatives XV behaved as efficient catalysts for ROMP and RCM reactions. 

Ruthenium complexes have been described that are active both in the ROMP reaction and in a subsequent hydrogenation step (30). These catalysts have the pyrimidin moiety incorporated, for example, (l,3-diisopropyltetrahydropyrimidin-2-ylidene) (ethoxy-methylene) (tricyclohexylphosphine) ruthenium dichloride. 

The metallacycle ruthenium complex 39, bearing a tethered NHC-alkyli-dene unit, catalyzes the formation of cyclic polymers by ROMP of COD (Eq. 28). Polymer formation is believed to proceed through a transient macrocyclic complex in which both ends of the growing polymer chain remain attached to the ruthenium center. Subsequent intramolecular chain transfer releases cyclic polymer [90,91]. 

Other methods developed by Grubbs136 for ruthenium complexes which can act as catalysts for ring opening metathesis polymerization (ROMP) and metathesis of conjugated and cumulated alkenes include the following ... 

In addition to the design of the solubility properties, the reactivity of organome-tallic species toward CO2 [13] (and many other potential supercritical reaction media) must be considered as important criteria for the choice of the catalyst. For example, the bisallyl ruthenium complex shown in Table 1 cannot be utilized as a precursor for ring-opening metathesis polymerization (ROMP) in SCCO2, because the insertion of CO2 into the Ru-allyl bond prevents the initiation mechanism [14]. Metal-mediated oxygen transfer to form CO and phosphine oxide was found to lead to deactivation of the [Ni(cod)2]/PMe3 (cod = 1,5-m-cycloocta-diene) catalyst system [15]. On the other hand, the reactivity of CO2 with metal... 

Poly(l,4-butadiene) segments prepared by the ruthenium-mediated ROMP of 1,5-cyclooctadiene can be incorporated into the ABA-type block copolymers with styrene (B-106) and MMA (B-107).397 The synthetic method is based on the copper-catalyzed radical polymerizations of styrene and MMA from the telechelic poly(butadiene) obtained by a bifunctional chain-transfer agent such as bis(allyl chloride) or bis-(2-bromopropionate) during the ROMP process. A more direct route to similar block copolymers is based on the use of a ruthenium carbene complex with a C—Br bond such as Ru-13 as described above.67 The complex induced simultaneous or tandem block copolymerizations of MMA and 1,5-cyclooctadiene to give B-108, which can be hydrogenated into B-109, in one pot, catalyzed by the ruthenium residue from Ru-13. 

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). 

Karlen and co-workers have described a photoinitiated ROMP (PROMP) system in water/ethanol mixtures using a variety of cationic ruthenium complexes with photolabile ligands [32, 33]. For example, the irradiation of [Ru(CH3CN)6](tos)2 or [(C6H6)2Ru9](tos)2 leads to partially and fully solvated Ru(II) species which initiate the ROMP of highly strained alkenes, presumably in the manner outlined below (Eq. 4). 

Ruthenium complexes, prepared by heating RU-CI3 with eyeloocta-1,5-diene in ethanol, also initiate the ROMP of norbomene, and the activity correlates with the strength of an IR band (1900-2100 cm ) attributed to Ru—H bonds (Laverty 1976b). Complexes of the type 29-32 are also effective for the polymerization of norbomene, but not of cyclopentene (Hiraki 1971 Porri 1974). 

Examples are listed in Table 8.7 for various numbers of bonds (x) between the double bonds. For the compounds with x = 6, the formation of the 7-membered ring is the preferred reaction. For x >6, the polymer is the favoured product. For x = 4 there is a remarkable variation in behaviour with the catalyst no reaction is observed with the molybdenum carbene catalyst, but with the rhodium complex there is 86% conversion of substrate in 72 h to products consisting of about 5% of cyclic dimer , 4% of cyclic trimer and 91% of linear oligomers (M = 1815). In the early stages of reaction the products are mainly the cyclic species but these undergo ROMP once their equilibrium concentration has been exceeded. With the ruthenium complex as initiator the kinetics of ROMP are less favourable and the products after 72 h consist of 25% cyclic dimer, 17% cyclic trimer and 58 % of linear oligomers (Marciniec 1995a). 

The original ruthenium carbene complexes prepared by Grubbs, Ru-1 and Ru-2, see eqn. (15), have served very well to effect numerous RCM and ROMP reactions. Nevertheless, the race is on to find even better ruthenium complexes that are either more stable or will catalyze metathesis of an even wider range of substrates. Amongst these are 11-14. 

In apparent contradiction with our results, however, no catalyst decomposition was observed over the course of two tandem ROMP-ATRP polymerisations (Scheme 10) mediated by a difunctional ruthenium complex 9 incorporating both a ROMP and an ATRP initiator [27]. Furthermore, halogen exchange has not been reported. In the preparation of these PMMA-PBD diblock copolymers, ROMP and ATRP occurred simultaneously provided that excess PCy3 be added to the reaction mixture. With this in mind, the stability of catalyst 9 over the course of the polymerisations may be related to the observation that additional PCy extends the longevity of the catalyst [25]. 

The monomer is made by the Diels-Alder reaction of dicyclopentadiene with ethylene. The catalyst for ROMP is ruthenium chloride in butanol. The process is relatively simple as the two liquids (ruthenium chloride in butanol and norbornene) are directly mixed in the extruder, in air. The norbornene to ruthenium ratio is very high (c.a. 25000) and the conversion reaches 50%. As the process operates in air, a small amount of norbornene is oxidized into epoxynorbornane (the epoxide to ruthenium ratio is c.a. 5) which can accelerate the polymerization. Indeed, mechanistic studies have shown that the catalytic reaction passes through a ruthenium hydride (formed by substitution of chlorine by butoxy ligands and further p-H abstraction) or through a ruthenium oxametallacyclobutane (formed by reaction of the ruthenium complex with epoxynorbornane) [33]. 

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