Norbornenes addition 4- carbenes

Electrophilic metal carbene complexes such as (CO)5W=C(Ph)OMe generally exhibit poor activity as catalysts for metathesis polymerisation, and higher temperatures are required to bring about the polymerisation of high-strained cycloolefins such as norbornene or cyclobutene [84,85], However, their activity can be enhanced by the addition of a Lewis acid such as TiCL into the polymerisation system [86]. Electrophilic complexes such as (CO)5W=CPh2 also generally exhibit poor activity but they are more active than those mentioned above and enable the polymerisation of various cycloolefins [87,88],... 

As shown by H NMR, the activated catalyst mixture reacts with norbornene (or a series of methyl-substituted norbornenes) to be partially converted to a new carbene species [53]. From the ratio of product carbene and residual initiator carbene concentrations, it was estimated that the rate constant for propagation is at least 3 times that for initiation. The three species present in the equilibrium situation of Eq. (39) may all possess their own intrinsic activities, resulting in a more complex polymerization behavior. In addition, a substantial amount of secondary metathesis occurs, as shown by changes in both the cis content of the polymer and head/tail ratio of the substituted carbenes when the catalyst was left in solution (120 min at 20 °C) after consumption of the monomer. 

When conducting the ROMP of norbornene or cyclooctadiene in miniemulsions [82], two approaches were followed (i) addition of a catalyst solution to a miniemulsion of the monomer and (ii) addition of the monomer to a miniemulsion of Grubbs catalyst in water. With the first approach it was possible to synthesize stable latexes with a high conversion, whereas for the second approach particles of >400 nm were created, without coagulum, but with 100% conversion. Subsequently, a water-soluble ruthenium carbene complex [poly(ethylene oxide)-based catalyst] was prepared and used in the direct miniemulsion ROMP of norbornene [83], whereby particles of 200-250 nm were produced. The catalytic polymerization of norbornene in direct miniemulsion was also carried out in the presence of an oil-soluble catalyst generated in situ, or with a water-soluble catalyst [84] the reaction was faster when using the oil-soluble catalyst. Helical-substituted polyacetylene could be efficiently polymerized in direct miniemulsion to yield a latex with particles that ranged between 60 and 400 nm in size, and which displayed an intense circular dichroism [85] that increased as the particle size decreased. The films were prepared from dried miniemulsion latexes that had been mixed with poly(vinyl alcohol) (PVA) in order to conserve the optical activity. 

While investigating the ROMP of 2-norbornene (NBE) with the Ru-carbenes 4 and 6 with and without additional olefins as chain transfer agents (CTA) to control the molecular weight, we discovered that after a fast formation of soluble poly(NBE), subsequent polymer degradation (slower) takes place, if the catalyst is not removed from the reaction mixture. The molecular weight (Mn) decreases from initial 710,000 g/mol (after 1 h reaction time) to 37,000 g/mol (after 73 h reaction time) and the polydispersity (PDI) increases from 1.52 (1 h) to 3.81 (73 h) [18]. It seems therefore essential to kill... 

A novel observation of participation in a carbene reaction serves as our final example here. enrfo-5-Hydroxymethyl-2-norbornene (238), which Factor and Traylor found to add mercuric ion with participation, affords oxohomobrendene derivatives (239) under dihalocarbene addition conditions. The process can be envisioned to proceed as shown in Eq. (71). 

As noted above, irradiation of 2-norbornene (14) in pentane afforded, in addition to the carbene-derived products 17 and 18, a mixture of alkanes 20-22, which appear to be derived from the radical 19, formed by hydrogen atom abstraction from the solvent. Analogous behavior was observed for 1-octene (10), which afforded the reduction product octane (13), and cyclopentene (32), which afforded the radical-derived products 34 and 35. In each case, a mixture of branched decanes arising from coupling of solvent-derived pentyl radicals was also obtained. Hydrogen-atom abstraction has also been attributed to the nn excited state. ... 

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