Initiators, ROMP

Most ruthenium-initiated ROMP studies have been performed using (233) and strained cyclo-olefinic monomers such as norbornene688 and cyclobutenes,689 although several reports on the polymerization of 8-membered rings have also appeared.690-692 A wide range of functionalities are tolerated, including ethers, esters, amines, amides, alcohols, carboxylic acids, and ketones. 

The alkyl-substituted titanium carbene complex 18 reacts with norbornene 24 to form a new titanacycle 25, which can be employed for the ROMP of 24 (Scheme 14.13). The titanacycle generated by the reaction of the Tebbe reagent with 24 is also used as an initiator for the same polymerization [23]. These preformed titanacyclobutanes also initiate ROMP of various other strained olefin monomers [24],... 

Cyclopentene yields mixtures of ROMP and double-bond polymerization with some Ti and V initiators. ROMP occurs exclusively with molybdenum and tungsten initiators, as well as Re, Nb, and Ta initiators. The relative amounts of cis and trans structures vary with the initiator and temperature [Dall Asta et al., 1962 Pampus and Lehnert, 1974]. Metallocene initiators polymerize cyclopentene through the double bond, but the polymer structure consists of cis 1,3-placement (Coates, 2000 Kaminsky, 2001 Kelly et al., 1997]. 

Scheme 6 Oxygen-chelated propagating species observed during 22b-initiated ROMP. ...

Soon after the discovery of the alkene metathesis reaction, reports indicated that complexes of ruthenium, osmium, and iridium could initiate ROMP. Michelotti et al. initially reported the polymerization of norbornene and its derivatives catalyzed by the hydrates of RuC13, OsCl3, and IrCl3 in refluxing ethanol (Eq. 1) [22, 23],... 

Recycled solutions of 2 initiated ROMP as quickly as the recycled Ru(III) solutions, closer examination of which revealed NMR resonances identical to those of the alkene protons in 2 [25]. It was therefore suggested that a key step in the initiation process using Ru(III) was the in situ formation of a Ru(II)-alkene complex [27]. Current evidence supports the disproportionation of the Ru(III) species to form Ru(II) and Ru(IV) species, followed by formation of a Ru(II)-alkene complex [25]. The equilibrium constant for disproportionation is small, accounting for the poor initiation efficiency of the Ru(III) systems [30]. An alternative, the disproportionation of an equilibrium amount of Ru(III)-alkene complex to a Ru(II)-alkene complex and a Ru(IV) species, is unlikely since Ru(III)-alkene complexes are generally unstable. Formation of a ruthenium alkylidene, the requi-... 

The insight derived from the investigation of ill-defmed ruthenium ROMP initiators was successfully applied to the development of Ru(II) alkylidenes 8 and 9 [15-18], In contrast to the classical complexes, these well-defined alkylidenes initiated ROMP quickley and quantitatively, reacted readily with acyclic alkenes, and could be used to initiate living polymerizations in organic solvents. 

In contrast to the classical catalysts, alkylidenes 10 and 11 initiated ROMP quickly and quantitatively [49, 50], A propagating alkylidene species was directly observed by NMR during the polymerization of most monomers, although the catalyst often decomposed before polymerization was complete. In the ROMP of water-soluble monomers 12 and 13 initiated by 10, for example, conversions ranging from 45 to 80% were usually observed (Eq. 9) [50],... 

The use of diazoalkanes instead of 3,3-diphenylcyclopropene provides a series of air-stable alkylidene complexes, Ru(=CHR)(Cl)2(PPh3)2 and Ru(=CHR) (Cl)2(PCy3)2 (R = Me, Et, Ph, p-CgHtCl), which are very efficient catalysts for the ROMP of norbomene and substituted cyclobutenes. Thus, for the Ru(=CHPh)(Cl)2(PPh3)2-initiated ROMP of norbomene, kj/kp = 9 (see Table 11.4). Reaction of Ru(=CHPh)(Cl)2(PCy3)2 with excess ethene gives Ru(==CH2)(Cl)2(PCy3)2, the first methylidene complex which has been isolated and shown to be an active metathesis catalyst (Schwab 1995, 1996). 

A solution of W[=C(OMe)Ph](CO)5 in CCI4 becomes more active for the ROMP of cycloocta-1,5-diene when irradiated with UV (Kolesnikov 1981). There is evidence that the W[=C(OMe)Ph](CO)5/Av-initiated ROMP of norbomene... 

In fact, it was reported that DCPD is a poison for ruthenium-initiated ROMP Tanielian, C., Kiennemann, A. and Osparpucu, T. (1979) Can. J. Chem. 57, 2022. 

The group of Nomura has explored the end-functionalization of molybdenum carbene initiated ROMP polymers extensively. Terminal monool [81, 82] or diols [83] were prepared via ROMP and used to synthesize different copolymer architectures. Pyridine and bi-pyridine ligands were successfully introduced to the polymer chain end in order to complex to ruthenium carbenes. Polymeric recyclable hydrogen transfer reduction catalysts were prepared in this manner [84, 85]. Notestein et al. [86] used a polymeric mono-aldehyde to functionally terminate a living ROMP initiated with a molybdenum catalyst to prepare diblock copolymers during the end-capping step. 

One of the newest developments in ROMP-based monohths for bioseparations is boronate affinity columns synthesized via Schrock-initiated ROMP of NBE and DMN-H6 with the porogenic solvents w-hexane and 1,2-dichloroethane. Functionalization was achieved via post-grafting of a boronate-containing monomer. These columns were used to separate cis-diol-based biomolecules, such as adenosine [116]. 

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