Circular dichroism polyacetylenes

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. 

In circular dichroism (CD) spectra of the polyacetylene thin films synthesized under the (fi-2)- and (S-2)-chiral nematic LCs, positive and negative Cotton effects are observed, respectively, in the region from 450 to 800 nm corresponding to tt — it transition of polyacetylene chain (Figure 3.8), despite the absence of chiroptical substituent in side chains. This indicates that the polyacetylene chain itself is helically screwed. It is evident that the above Cotton effect is not due to the chiral dopant [( )- or... 

FIGURE 3.8 Circular dichroism (CD) spectra of helical polyacetylene films. The polyacetylene films synthesized in the (R-2)- and (S-2)-chiral nematic LCs including (R)- and (S)-6,6 -PCH506-2,2 -Et-Binol are designated as PA by R-2 and PA by S-2, respectively. 

Optically active polymers show another properties namely thermosensitivity, e.g., main chains helical poly(iV-isopropylacrylamide) and thermosensitive part as side chain of poly(A -isopropylacrylamide) (PNlPAm). Such type of polymers synthetic method described elsewhere [137]. The polymer with optically active cores (helical polyacetylenes) and thermosensitive shells (PNlPAm) brashes self-assembled core/shell structured nanoparticles in aqueous solution. Another example of optically active polymer is poly[/V-(L)-(l-hydroxymethyl)-pro-pylmethacrylamide] (P(l-HMPMA)) of lower critical solution temperature and thermosensitivity. Circular dichroism and microcalorimetric measurements of the polymer showed the polymer chains in a state of relatively low hydration compared to that of by racemate synthesized monomers by free-radical reaction formed P(d,l-HMPMA). Thermosensitivity and structural effects were obtained by microscopic observation of aqueous solution of polymers and its hydrogels [138]. 

Only one paper has discussed the application of magnetic circular dichroism (MCD) to a study of the excited states of bi- and terthio-phenes. The MCD spectra of T1 and 1 showed a broad asymmetric B term absorption corresponding to the absorption band, and suggested So >Si transitions, as expected from theory. In contrast, quantum-mechanically forbidden Sq T transitions were detected by the same authors in the irradiation of several naturally occurring polyacetylenes (182). 

McLachlan, D., J.T. Arnason, B.R. Hollebone, and J. Lam Excited States of Phototoxic Polyacetylenes Elucidated by Magnetic Circular Dichroism. Photobiochem. Photobiophys. 9, 233 (1985). 

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