UV irradiation of latent sulfur-chelated ruthenium olefin metathesis precatalysts produced a cis-dichloro-transdichloro photoisomerization. The enhanced activity of the trans isomer, when aromatic substituents were attached to the sulfur chelating atom, induced several metathesis reactions, including ROMP, at room temperature.
The synthesis of novel sulfur-chelated ruthenium benzylidenes afforded latent catalysts with a wider range of activities and new isomeric forms. A ruthenium complex with a tridentate ligand displayed latency for even one of the most reactive ROMP monomers, dicyclopentadiene, while a room temperature latent trifluoromethyl-substituted thioether derivative was shown to be the most active sulfur-chelated precatalyst to date in several metathesis reactions at higher temperatures. These new complexes widen the spectrum of activity for this family of catalysts, enabling several practical applications and enhancing the understanding for the mechanisms of activation in strongly chelated ruthenium alkylidenes.
Mechanochromic functionality
realized through force-responsive
molecules (i.e., mechanophores) has great potential for spatially
localized damage warning in polymers. However, in structural plastics,
for which damage warning is most critical, this approach has had minimal
success because brittle failure typically precedes detectable color
change. Herein, we report on the room-temperature mechanochromic activation
of spiropyran in high T
g bisphenol A polycarbonate.
The mechanochromic functionality was introduced by polymerization
of dihydroxyspiropyran as a comonomer while retaining the excellent
thermomechanical properties of the polycarbonate. The mechanochromic
behavior is thoroughly evaluated in response to changes in stress,
deformation, and time, providing new insights regarding how loading
history controls stress accumulation in polymer chains. In addition,
a new method to incorporate mechanochromic functionality in structures
without dispersing costly mechanophores in the bulk is demonstrated
by using a mechanochromic laminate. The room-temperature mechanochromic
activation in a structural polymer combined with the new and efficient
preparation and processing methods bring us closer to the application
of mechanochromic smart materials.
Olefin metathesis catalysed by ruthenium has emerged at the frontier of modern synthetic chemistry. The desire to enhance catalyst stability, gain control over the catalytic process and deepen the understanding of the mechanisms of metathesis has yielded a class of latent ruthenium precatalysts of delayed initiation and with switchable activity. One of the main methodologies developed for this purpose has been the introduction of tethered carbene ligands. Herein we track the evolution of ruthenium based metathesis catalysts bearing chelated alkylidenes, from the early oxygen Hoveyda type benzylidenes to the latent sulphur containing complexes.
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