Gary D. Jaycox

Stimuli-responsive polymers. 4. Photo- and thermo-regulated chiroptical behavior in azobenzene-modified polymers fitted with main chain spirobiindane turns and chiral binaphthyl bends

Abstract A series of azobenzene-modified polyamides fitted with main chain spirobiindane turns and chiral binaphthyl bends was prepared from the solution polycondensation of trans-azobenzene-4,4′-dicarbonyl chloride with appropriate diamine monomers. When evaluated in their all trans-azobenzene configurations, these materials exhibited a good mix of physical properties suitable for high performance applications. Photoinduced trans→cis isomerization reactions were effected by irradiating polymer solutions with near UV light. Reverse cis→trans isomerization of the backbone azobenzene segments was triggered by either photochemical or thermal means and was monitored by optical absorbance spectroscopy. Thermally induced cis→trans reorganization within each polymer followed the first-order rate law. Activation energies calculated for this process in DMAC all fell near 21–23xa0kcalxa0mol−1 and were not strongly correlated to backbone content. Polymers containing axially asymmetric S-(−)- or R-(+)-2,2′-binaphthyl main chain linkages exhibited thermo- and photo-responsive chiroptical behavior when evaluated in dilute THF solutions. Specifically, the trans-azobenzene-modified materials were all characterized by CD spectra showing intense molar ellipticities within the 300–400xa0nm spectral window. Specific rotation magnitudes determined for the trans-polymers at the sodium D-line ranged into the hundreds of degrees and were dependent on the extent of binaphthyl loading along the polyamide backbone. The irradiation of the polymer samples to drive the trans→cis isomerization reaction resulted in an immediate chiroptical response, with CD band intensities and optical rotations significantly diminished. These effects were fully reversible and were attributed to the presence of putative one-handed helical conformations in the trans-azobenzene-modified polymers that were severely disrupted following the trans→cis isomerization reaction.

Azobenzene modified poly(aryl ether ketone amide)s. 2. Photo- and thermo-responsive behaviour in dilute solution‡

Abstract A number of azobenzene modified poly(aryl ether ketone amide)s with differing backbone geometries wereevaluated for their photo- and thermo-regulated behaviour in dilute solution. Photoinduced trans → cis isomerization reactions were carried out by irradiating the polymer samples with ultraviolet light at wavelengths between 370 and 400 nm. Photostationary state compositions achieved under these conditions typically consisted of about 70% of the higher energy cis isomer distributed along the polymer main chain. Reverse cis → trans isomerization of the backbone azobenzene moieties was triggered by either photochemical or thermal means and was monitored by optical absorbance and 1 H n.m.r. spectroscopies. Thermally induced cis → trans return in each of the polymers obeyed the first-order rate law. Activation energies calculated for the ‘dark’ isomerization reaction fell near 21 kcal mol −1 for each of the polymer samples evaluated. These values were not dependent on the overall structure or molecular weight of the polymer backbone and were nearly identical to those determined for several lower molecular weight model compounds. Calculated half-lives for the isomerization of cis -azobenzene linkages buried in the polymer backbone ranged from 1 day near room temperature to about 1 h at the 60°C isotherm. Data gleaned from SEC experiments suggested that polymers endowed with conformationally restricted geometries underwent a two-fold reduction in hydrodynamic radius in response to ultraviolet light exposure. Photo-contractions in more flexible polymer samples appeared to be less dramatic, consistent with molecular modelling and dilute solution viscosity measurements.

Azobenzene modified poly(aryl ether ketone amide)s. 1. Synthesis and physical properties

Abstract A series of azobenzene modified poly(aryl ether ketone amide)s was prepared by the low temperaturepolycondensation of trans-azobenzene-4,4′-dicarbonyl chloride with bis-1,4-(3-aminophenoxy-4′-benzoyl)-benzene and other aromatic diamines containing ether and keto groups. The polymers were amorphous, possessed glass transition temperatures between 167 and 218°C and displayed good thermal stabilities under nitrogen and air up to about 400°C. They dissolved readily in a number of organic solvents giving stable solutions. By employing conventional solvent casting techniques, mechanically robust polymer films were obtained with excellent levels of optical clarity. The poly(aryl ether ketone amide)s described here appear to be well suited for photochemical studies in both the solid state and in solution.

Stimuli-responsive polymers. III. Poly(aryl ether ketone amide)s with reversible trans ↔ cis-4,4'-azobenzene and fixed 2,2'-binaphthyl kinking elements in the main chain

The low-temperature polycondensation of trans-azobenzene-4,4′-dicarbonyl chloride with (S)-(−)-1,1′-binaphthyl-2,2′-diamine and/or 1,4-bis(3-aminophenoxy-4′-benzoyl)benzene afforded a new series of poly(aryl ether ketone amide)s with both fixed and photoinducible kinking elements positioned randomly along the main chain. In their lower energy, trans-azobenzene configurations, the orange, film-forming materials were amorphous, highly tractable, and thermally stable under air or nitrogen up to about 420°C. Variants endowed with higher loadings of the bent binaphthyl monomer were soluble in a variety of organic solvent media including THF and acetone. The introduction of cis-azobenzene backbone kinks into these materials was carried out by irradiating the polymer solutions with near-UV light. Up to 70% of the azobenzene moieties in these polymers were capable of assuming the higher energy cis-configuration, thus greatly increasing the number of bent or kinked sites positioned along each polymer backbone. In solution, reverse cis → trans isomerization reactions were triggered thermally and were quantitatively tracked by both optical absorbance and 1H NMR spectroscopies. Activation parameters calculated for cis → trans reorganization of the polymer backbone were not dependent upon the chemical composition or molecular weight of the polymers but did exhibit a small dependence upon the nature of the solvent medium used to conduct the isomerization experiment.

Helical conformations of isotactic polyacetaldehyde and other isotactic polytrihaloacetaldehydes

Minimum potential energy helical conformations for a family of four isotactic polyacetaldehydes have been determined. Our results indicate that all of the polymers form irrational helices. Comparisons have been made with the reported structures for two of these stereoregular polymers based on earlier X-ray diffraction data. c-Axis values associated with the pitch of the helix for polyacetaldehyde and for polytrichloroacetaldehyde (polychloral) were experimentally measured to be 0.48 and 0.51 nm, respectively. Our calculated conformations afforded values for a helix pitch of 0.47 and 0.52 nm, respectively, which derive from a 3.9/1 helix for polyacetaldehyde and a 3.7/1 helix for polychloral. The structure for polytribromoacetaldehyde (polybromal) was predicted to be similar to that for polychloral. For polytrifluoroacetaldehyde (polyfluoral) and polyacetaldehyde, a number of helical conformations with similar energies were found. All of these conformations could be related to the polychloral helical structure.