H. K. Hall

Why δ-Valerolactone Polymerizes and γ-Butyrolactone Does Not

gamma-Butyrolactone, unlike delta-valerolactone, does not polymerize despite a strain energy of approximately 8 kcal mol-1 which could be relieved by opening the s-cis lactone ester bond to an s-trans ester bond in the polymer. To explain this anomaly, we have applied quantum mechanical methods to study the thermochemistry involved in the ring-opening reactions of gamma-butyrolactone and delta-valerolactone, the conformational preferences of model molecules that mimic their corresponding homopolyesters, and the variation of enthalpy associated to the polymerizability of such two cyclic lactones. The overall results indicate that the lack of polymerizability of gamma-butyrolactone should be attributed to the low strain of the ring, which shows much less geometric distortion in the ester group than delta-valerolactone, and the notable stability of the coiled conformations found in model compounds of poly-4-hydroxybutyrate.

New polymeric material containing the tricyanovinylcarbazole group for photorefractive applications

We report on the synthesis and characterization of a new, completely polymeric material with a large electro‐optic coefficient and photoconductivity. A methacrylic ester polymer containing the tricyanovinylcarbazole group with an alkylene spacer was synthesized. With 49 mW laser power, the photoconductivity of the polymer is 9.8×10−10 Ω−1 cm−1 at an applied electric field of 1.4×106 V/cm. Its electro‐optic coefficient is 6.1 pm/V. The quantum efficiency of photocarrier generation and the photorefractive figure of merit were calculated. This electro‐optic polymer is photoconductive and has photocarrier trap sites without any added sensitizer or photocarrier transport reagent, making it a suitable candidate for photorefractive applications.

Chemical trapping experiments support a cation‐radical mechanism for the oxidative polymerization of aniline

The oxidative polymerization of aniline in aqueous acidic solution was carried out in the presence of a variety of organic compounds as potential traps for postulated intermediates. The polymerization was inhibited by hindered phenols and electron-rich alkenes, traps for cation-radicals. However, polyaniline was still obtained in the presence of electron-rich arenes, such as 1,3-dimethoxybenzene and 1,4-dimethoxybenzene, known as excellent receptors of nitrenium ions. Polymerization of N-phenyl-1,4-phenylenediamine was similarly carried out in the presence of potential traps. Polyaniline containing an N-phenyl group was obtained in the presence of 1,3-dimethoxybenzene and 1,4-dimethoxybenzene. Hindered phenols and 4-methoxystyrene only slightly inhibited polymerization of N-phenyl-1,4-phenylenediamine which most probably proceeded by way of the stable diarylamino radical. Copolymerization of aniline with 10 wt % of N-phenyl-1,4-phenylenediamine in the presence of these traps gave similar results to the polymerization of pure aniline. These results have led to the proposed cation-radical polymerization mechanism of aniline, in which the polymerization is a chain growth reaction through the combination of a polymeric cation-radical and an anilinium cation-radical. Step growth character is also present when a polymeric aminium cation-radical end combines with a diarylaminoended polymer. The copolymerization of N-phenyl-p-phenylenediamine can also occur by reaction of aniline cation-radical with a polyarylamine radical. The nitrenium mechanism was further rejected by the fact that attempted polymerization of N-phenylhydroxylamine, which forms authentic nitrenium ions in acid, failed to give polymer.

Dual-grating formation through photorefractivity and photoisomerization in azo-dye-doped polymers

Gratings formed by photorefractive and photoisomerization processes in an azo-dye-doped polymer, poly(vinyl carbazole):trinitrofluorenone/Disperse Red I, are investigated by polarization- and field-dependent four-wave mixing experiments. High diffraction efficiencies and long storage times are observed for the photorefractive gratings. Both types of grating are erasable, and we can select them by choosing the polarization of the reading beam. The effect of photoisomerization on photorefractivity is discussed. Our results indicate that azo-dye-doped polymers are promising candidates for reversible optical storage applications.

A contemporary survey of covalent connectivity and complexity. The divergent synthesis of poly(thioether) dendrimers. Amplified, genealogically directed synthesis leading to the de gennes dense packed state

A new poly(thioether) dendrimer (d-PTE) family is synthesized using preformed branch cell reagents (BCR) in a “genealogically directed syntheses” (GDS) strategy. Bicyclic orthoester functionality is introduced into a branch cell reagent (BCR) to temporarily mask pentaerythritol derived branch cells which are used to construct the interior of this new dendrimer family. These BCRs with multiplicities =3 (Nb=3), are organized and amplified around an initiator core with multiplicity = 4 (Nc=4). The initiator core, pentaerythritol tetrabromide (Nc=4), is allowed to react with four equivalents of 4-acetothiomethyl-2,6,7-trioxabicyclo[2.2.2]octane (Nb=3) in the presence of base, to form the first generation possessing four bicyclic orthoester groups. After deprotection and transformation to bromide surface groups via tosylate intermediates, the second generation possessing twelve bicyclic orthoester groups is formed. Surprisingly, attempted displacement of all 36 surface groups at the third generation level showed that only one third of the end groups could be substituted. These experimental data suggest that this (Nc=4, Nb=3) type dendrimer family exhibits de Gennes dense packing properties at the third generation level. Elemental analysis, FTIR, H/C13 nmr spectroscopy and mass spectroscopy were used to confirm the structures. Molecular simulation data suggest that this dendrimer family should not undergo de Gennes dense-packing until the fourth generation. Steric requirements of the bulky mercaptomethyl anion used in the SN2 displacement of terminal bromides on this congested surface are proposed as the reason for incomplete formation of the third generation. This observation illustrates another example of “sterically induced stoichiometry” (SIS).

Thermodynamic control of the polymerizability of five-, six-, and seven-membered lactones.

The thermodynamics of polymerization of epsilon-caprolactone and 1,4-dioxan-2-one has been investigated theoretically and compared with that recently reported for delta-valerolactone and gamma-butyrolactone. Specifically, the ability of these monomers to polymerize has been related to the strain of the rings, the Gibbs free energy of simple models for ring-opening reactions of the cyclic lactones, and the conformational preferences of linear model compounds of the corresponding homopolyesters. The results are fully consistent with the lack of polymerizability of gamma-butyrolactone, while the ring openings of epsilon-caprolactone and delta-valerolactone have been found to be exergonic processes. Polymerizability of 1,4-dioxan-2-one has been found to be favored, even though less than that of epsilon-caprolactone and delta-valerolactone. Two factors explain these features: (i) the strain of the ester group in the lactones increases with the exergonic character of the ring-opening process, and (ii) the stability of coiled conformations in model compounds follows this order: poly-4-hydroxybutyrate > poly(1,4-dioxan-2-one) > poly-6-hydroxycaproate approximately poly-5-hydroxyvalerate. Finally, the influence of the environment on the polymerizability of the three cyclic lactones is discussed in detail.

Photorefractive effect in a poled polymer containing the tricyanovinylcarbazole group

The photorefractive properties of a poled polymer containing the tricyanovinylcarbazole group with an alkylene spacer are investigated by four‐wave mixing experiments and Mach–Zehnder type electro‐optic measurements. The photorefractive effect is demonstrated in a polymer that shows both the photoconductivity and the electro‐optic effect intrinsically.

6-alkoxy-3,4-dihydro-2h-pyrans from substituted α,β-unsaturated esters

Abstract Cycloaddition reactions of substituted α,β-unsaturated esters with various electronich olefins lead to 6-alkoxy-3,4-dihydro-2H-pyrans.

Polymerized planar suspended lipid bilayers for single ion channel recordings: comparison of several dienoyl lipids.

The stabilization of suspended planar lipid membranes, or black lipid membranes (BLMs), through polymerization of mono- and bis-functionalized dienoyl lipids was investigated. Electrical properties, including capacitance, conductance, and dielectric breakdown voltage, were determined for BLMs composed of mono-DenPC, bis-DenPC, mono-SorbPC, and bis-SorbPC both prior to and following photopolymerization, with diphytanoyl phosphocholine (DPhPC) serving as a control. Poly(lipid) BLMs exhibited significantly longer lifetimes and increased the stability of air-water transfers. BLM stability followed the order bis-DenPC > mono-DenPC ≈ mono-SorbPC > bis-SorbPC. The conductance of bis-SorbPC BLMs was significantly higher than that of the other lipids, which is attributed to a high density of hydrophilic pores, resulting in relatively unstable membranes. The use of poly(lipid) BLMs as matrices for supporting the activity of an ion channel protein (IC) was explored using α-hemolysin (α-HL), a model IC. Characteristic i-V plots of α-HL were maintained following photopolymerization of bis-DenPC, mono-DenPC, and mono-SorbPC, demonstrating the utility of these materials for preparing more durable BLMs for single-channel recordings of reconstituted ICs.

Reconstitution of Rhodopsin into Polymerizable Planar Supported Lipid Bilayers: Influence of Dienoyl Monomer Structure on Photoactivation

G-protein-coupled receptors (GPCRs) play key roles in cellular signal transduction and many are pharmacologically important targets for drug discovery. GPCRs can be reconstituted in planar supported lipid bilayers (PSLBs) with retention of activity, which has led to development of GPCR-based biosensors and biochips. However, PSLBs composed of natural lipids lack the high stability desired for many technological applications. One strategy is to use synthetic lipid monomers that can be polymerized to form robust bilayers. A key question is how lipid polymerization affects GPCR structure and activity. Here we have investigated the photochemical activity of bovine rhodopsin (Rho), a model GPCR, reconstituted into PSLBs composed of lipids having one or two polymerizable dienoyl moieties located in different regions of the acyl chains. Plasmon waveguide resonance spectroscopy was used to compare the degree of Rho photoactivation in fluid and poly(lipid) PSLBs. The position of the dienoyl moiety was found to have a significant effect: polymerization near the glycerol backbone significantly attenuates Rho activity whereas polymerization near the acyl chain termini does not. Differences in cross-link density near the acyl chain termini also do not affect Rho activity. In unpolymerized PSLBs, an equimolar mixture of phosphatidylethanolamine and phosphatidylcholine (PC) lipids enhances activity relative to pure PC; however after polymerization, the enhancement is eliminated which is attributed to stabilization of the membrane lamellar phase. These results should provide guidance for the design of robust lipid bilayers functionalized with transmembrane proteins for use in membrane-based biochips and biosensors.