Polyamide with azobenzene

Polyamides with azobenzene groups in the backbone are among the earliest in which trans-cis isomerizable chromophores were used to regulate the polymer conformation [13, 14]. The intrinsic viscosity [q] of polyamide (6) in polar AJV-dimethyl-acetamide was found to decrease from 1.22 to 0.5 dl/g upon ultraviolet irradiation... 

Irie et al. [217] synthesized a number of polyamides with azobenzene groups in the backbone. All the polymers exhibit photo-viscosity effects. In solutimis in VA/ -dimethylacetamide, a 60% reduction in specific viscosity can be achieved by UV hght irradiation (410 > A > 350 nm). The initial viscosity is regained by storage in the dark at room temperature for 30 h. 

ABSTRACT. The concept of "crankshaft-like motion" in the backbones of chain molecules implies much slower conformational transitions than in small molecules if two energy barriers have to be surmounted simultaneously. Studies of the rates of hindered rotation around the amide bonds in solutions of piperazine polyamides by NMR and of the photochemical and dark isomerization of polyamides with azobenzene residues in the chain backbone revealed no difference between these rates in the polymers and their analogs. Polyoxyethylene with a dibenzylacetamide residue in the middle of the chain exhibited similar excimer emission as N,N -di(p-methylbenzyl)acetamide. These observations imply that only one energy barrier is surmounted in conformational transitions of polymer backbones. 

Concerning the polymers, Irie and collaborators prepared a number of polyamides with azobenzene groups in the backbone (e.g., 21). The viscosity of the polyamides in a polar solvent decreased upon irradiation and returned to the initial value in the dark. 

A polymer with photoisomerizable unsaturated linkage in the backbone is expected to change its conformation under photoirradiation. This concept was realized by using polyamide with azobenzene residues in the backbone of the polymer (1) [4]. Figure 2 shows the solution viscosity of the polymer in A,AT -dimethylacetamide in the dark as well as during irradiation with UV light (410 > X > 350 nm). The intrinsic viscosity during irradiation was 60% lower than the viscosity in the dark. The absence of a concentration effect... 

Irie, M., and Schnabel, W. Photoresponsive Polymers On the Dynamics of Cainformaiioiial Changes of Polyamides with Backbone Azobenzene Groups. Macromolecules 14, 1246 (1981). 

The conformation change of polyamide with pendant azobenzene grou (6) in aqueous solution also induced a change in pH [14]. Ultraviolet irradiation caused the pH of an aqueous solution of the polyamide to decrease and visible irradiation returned the pH to the initial value. 

Synthesis, under microwave irradiation conditions, of polyamides containing azobenzene units and hydantoin derivatives in the main chains has recently been proposed by Faghihi et al. [55]. Polycondensation of 4,4 -azodibenzoyl chloride with eight 5,5-disubstituted hydantoin moieties has been achieved in the presence of a small amount of o-cresol (Scheme 14.26). The polycondensations were performed in 8 min, in a domestic microwave oven, in a porcelain dish in which 1.0 mmol diacid chloride was mixed with an equimolar amount of diol in the presence of small amounts of o-cresol. The polymerization proceeded rapidly, compared with the bulk reactions under conventional conditions (8 min compared with 1 h), producing a series of polyamides in high yield and inherent viscosity between 0.35 to 0.60 dL g h... 

Irie, M. and Schnabel, W. (1981) Photoresponsive polymers - on the dynamics of conformational-changes of polyamides with backbone azobenzene groups. Macromolecules, 14, 1246-1249. 

It is interesting to note that the halflives of coil contraction of the copolymers decrease with the decrease in the azobenzene content and the values are slightly smaller than the halflives of coil expansion measured for polyamide having azobenzene groups in the main chain. 

Armstrong et al. [62] used a-cyclodextrin (R = H) and /8-cyclodextrin derivatives (R = hydroxypropyl) [63] as additives in the mobile phase for the separation of different diastereoisomers. Thus, they separated 5yn-azobenzene Rp = 0.09) from anti-azobenzene (Rp = 0.53) on polyamide or polyamide with fluorescent indicator adding a-cyclodextrin to the mobile phase (0.1 M) [62] or some steroid epimers and alkaloids on plates coated with chemically bonded octadecylsilane reversed-phase [62]. The used mobile phases obtained by adding hydroxypropyl-/8-cyclodextrin are presented in Table 8.9, beside the chromatographic results. 

Fig. 5. Changes in ( ) content of the trans azobenzene residues in polyamide (6) backbone and (O) viscosity of the polyamide in V,V-dimethylacetamide on alternate irradiation with ultraviolet (410 nm > A > 350 nm) and visible (A > 470 nm) light at 20 °C [14], Polymer concentration was 0.9 g/dl...

Polyamide (6) was irradiated with a single 20 ns flash (530 nm) in A. iV-dimethyl-ac tamide. The kinetics of the cis to trans isomerization of backbone azobenzene residues was followed by time resolved optical absorption, and the subsequent conformation change of the total polymer chain by time resolved light scattering. Before each laser e eriment, the polymer was brought to a compact conformation by eontiimous ultraviolet irradiation, and then the unfolding proce was traced by the laser flash photolysis method. 

The behavior of the polyamides described above could not be studied In bulk because the polymer Is crystalline and Is not transparent. However, with a methyl methacrylate copolymer carrying a small number of azobenzene residues in side chains... 

The experimental results revealed no significant difference in the rate of photoisomerization of azobenzene residues in the backbone of polyamides and in low molecular weight analogous azobenzene derivatives when both were studied in dilute solution.28 However, while the photochemical reactivity of the small species was relatively insensitive to the concentration of added polymer, the quantum yield for the photoisomerization of the azobenzene residues in the polymer backbone dropped precipitously with increasing concentration. In a glassy polymer film containing 8% DMSO plasticizer, the quantum yield for the isomerization of the polymer was reduced by a factor of 2500 while it was reduced only by a factor of 5 for the small molecule (Figure 3). 

Here the quantum yield of a small number of azobenzene residues built into the backbone of a polyamide was compared with the quantum yield of an analogous low molecular weight azobenzene derivative in a series of polyamide - DMSO mixtures. As expected, the quantum yield for the azobenzene residues in the polymer backbones was very small in systems containing a high polymer concentration, since here conformational transitions of the polymer chains would necessarily be cooperative, while the photoisomerization of the small molecule was relatively insensitive to the polymer concentration. However, in highly dilute solution there was absolutely no difference between the photoisomerization efficiency of azobenzene residues built into the polymer chains and the small azobenzene derivatives. This identity in the behavior of the polymer and its analog in dilute solution shows that the isomerization of azobenzenes in the chain backbone can be accomplished without a simultaneous hindered rotation in another portion of the chain. 

It is known that azobenzenes undergo cis-trans isomerization when exposed to UV light. The stable trans form is converted to the less stable cis form. Polymer 6a was used to study the effect of irradiation on the solution viscosity. Intrinsic viscosity measurements were done after exposing a NMP solution of the polymer to UV light. The intrinsic viscosity value decreased to 57 % of the original at the photostation state immediately after irradiation. A decrease of as much as 60 % with polyamides and 40 % for polyureas under similar conditions has been observed (16). 

Figure 3 Changes in the (a) content of the tra/i5-azobenzene residues in polyamide (1) backbone (b) viscosity of polyamide (1) in iVA -dimethylacetamide during alternate irradiation with UV (410 >, > 350 nm) and visible (X2 > 470 nm) light at 20 C. Concentration of the polymer is 0.9 g dL...

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