Leucohydroxide

The triphenylmethane leucohydroxide residues were incorporated into the pendant groups by copolymerizing thd vinyl derivative (72, R=CH=CH2, X=OH) with 7V,Af-dimethylacrylamide [20]. Upon ultraviolet irradiation (X > 270 nm), the solution became deep green, and at the same time its reduced viscosity increased from 0.55 to 1.6 dl/g, as depicted in Fig. 6. After the light was shut, the viscosity returned to the initial value with a half-life of 3.1 min. The close correlation between the viscosity change and the absorption intensity at 620 nm implies that the electrostatic repulsion was responsible for the expansion of the polymer conformation. 

Fig. 6. Changes of (A) absorption at 620 nm and (B) viscosity of poly(A,W-dimethylacrylamide) having pendant triphenylmeth-ane leucohydroxide groups (9.1 mol%) in methanol at 30 °C on exposure to ultraviolet light (A > 270 nm) [20]. Polymer concentration was 0.06 g/dl...

Polyacrylamide gels containing a small amount of triphenylmethane leucohydroxide or leucocyanide groups 21) were prepared by free radical copolymerization of di(iV,iV-dimethylaniline)-4-vinylphenylmethane leucohydroxide 12, X=OH, R=... 

Fig. 17. Photostimulated dilation and contraction of polyacrylamide gel having pendant triphenylmethane leucohydroxide groups (3.7 mol %) with light of wavelength longer than 270 nm at 25 C [43]. Wo is the weight before photoirradiation...

A wettability change was also observed when triphenylmethane leucohydroxide was introduced [47]. The contact angle with water of the surface of polystyrene having triphenylmethane leucohydroxide chromophores increased from 0.2 to 0.8. This increase in wettability is due to an enhanced hydrophilicity of the film surface brought about by ionic dissociation of the chromophores. 

Triphenylmethane leucohydroxide residues were introduced into the pendant groups by copolymerizing the vinyl derivative (2, X= OH,... 

The ratio of the specific viscosity during photoirradiation to that in the dark, Dp/Dd, increased with increasing content of triphenylmethane leucohydroxide residues in the pendant groups and reached a maximum of 3,3 at 0,18 mole fraction. Above the content,the ratio decreased, the decrease being due to the low solubility of the residues in methanol. 

The concept to adopt the electrostatic repulsive force as a driving force for a photostimulated expansion of the polymer chain is useful and widely applicable to other polymer systems. Polystyrene and polyacrylamide having pendant leucohydroxide and leuco-cyanide groups were found to change their solution viscosity in methylene chloride and in water, respectively. 

For substrates that are precursors to stable cations these photoheterolysis reactions occur in neutral media. For instance, leucohydroxides like 38 are photo-chromic and give increased pH values when irradiated in aqueous solutions, Eq. 

Kodzwa, M.G., Staben, M.E., and Rethwisch, D.G. (1999) Photoresponsive control of ion-exchange in leucohydroxide containing hydrogel membranes, J. Membr. Sci., 158,85-92. 

Scheme 5.2 Photogeneration of triphenylmethyl cations in poly(N,N-dimethylacrylamide) containing pendant triphenylmethane leucohydroxide groups.

In some cases it appears that direct photodissociation to ions can oc-cur. For example, irradiation of 4,4 -bis(dimethylamino)triphenylmeth-ane leucohydroxide (125, 4.8 x lO M) in aqueous micellar solutions led to the ejection of hydroxide ion and formation of 4,4 -bis(dimethylamino)tri-phenylmethyl cation (126, equation 12.79), which caused a pH change from 5.4 to 10.0. The pH returned to 5.4 over a period of 15 minutes as the hydroxide ion recombined with the carbocation. °... 

Isomerization from cis to trans and back of azo groups, however, is not the only mechanism that can affect photo-viscosity change in polymeric solutions. Thus, reversible solution viscosity changes were also observed [222] in solutions of poly(dimethylacrylamide) with pendant triphenylmethane leucohydroxide in methanol. This can be illustrated as follows ... 

The gel having triphenylmethane leucohydroxide groups swelled even in the dark when the aqueous solution became acidic. At pH of 4.0, the gel weight increased as much as 20 times in comparison with its weight at pH 8.0. To minimize the pH-sensitivity, the hydroxide was replaced by cyanide. Figure 14 shows the photoresponsive behavior of the gel... 

Triphenylmethane leucohydroxide can be introduced to the pendant groups by a polymer reaction as described in the following equation. 

A disk-shaped gel (10 mm in diameter and 2 mm in thickness) with 3.7 mol% triphenylmethane leucohydroxide residues showed photostimulated reversible dilation in water. Figure 27 shows that on UV irradiation ( > 270 nm), the gel swells and the weight increases by as much as three times its original weight in 1 h. The dilated gel contracts in the dark to its initial weight in 20 h. The cycles of expansion and contraction of the gel were... 

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