Synthesis and characterization of liquid crystalline copolymers with dual photochromic pendant groups

In order to study the photoreactivity and the optical properties of liquid crystalline copolymers with multiple photochromic groups, a series of novel liquid crystalline binary and ternary polyacrylates consisting of one (CC or NN) or dual (CC and NN) photochromic segments were synthesized and characterized considering their liquid crystalline, optical, and photochromic properties and their thermal stability. Achiral homopolymer P1 shows a smectic A phase (fan-shaped texture), and all chiral copolymers CP1-CP6 exhibit chiral nematic phases (cholesteric, oily streaks textures). The polymers show excellent solubility in common organic solvents such as chloroform, toluene, and THF. These polymers also exhibit good thermal stability, with decomposition temperatures (T_ds) greater than 373 °C at 5% weight loss, and beyond 440 °C at 50% weight loss under nitrogen atmosphere. UV irradiation caused E/Z photoisomerization at NN and CC segments of the synthesized photochromic copolymers leading to reversible and irreversible isomerizations, respectively. The synthesized liquid crystalline ternary copolymer CP6, containing two different photochromic NN and CC groups, is sensitive to different UV wavelengths and is notably interesting from the viewpoint of photochromic copolymers.     

Thiophene-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene)s: Synthesis, characterisation and optical properties

This work describes the synthesis and characterisation of two types of thiophene-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene)s (PAE-PAV) copolymers, whose repeating units (-Ph-CC-Th-CHCH-Ph-CHCH-)_n, 5, and (-Th-CC-Ph-CC-Th-CHCH-Ph-CHCH-)_n, 8a-c, consist, respectively, of a 1:2 and a 2:2 ratio of triple bond/double bond moieties. Comparison of their photophysical, electrochemical and photovoltaic properties has been carried out. Although similar electrochemical data (HOMO: −5.43 eV, LUMO: ∼−3.15 eV, ) as well as identical thin film absorption behaviour (λ_a=500 nm, ) were obtained for both types of materials, significant differences in their thin film photoluminescence behaviour and photovoltaic properties were observed. While polymer 5 shows a fluorescence maximum at λ_e=568 nm (with a fluorescence quantum yield of Φ_f=7%), a total fluorescence quenching was observed in 8. Far better photovoltaic performance was obtained from solar cells (set up: ITO/PEDOT:PSS/active layer/LiF/Al; active layer consisting of 5 or 8b as donor and PCBM as acceptor in a 1:3 ratio by weight) designed from 5 than from 8b. Open circuit voltage, V_OC, as high as 900 mV and power conversion efficiency, η_AM1.5, around 1.2% were obtained. This can be attributed to the 1:2 triple bond/double bond ratio as well as the grafting of shorter octyloxy and 2-ethylhexyloxy side chains in 5 and to its comparatively higher molecular-weight.     

Synthesis and optical properties of three novel functional polyurethanes bearing nonlinear optical chromophoric pendants with different π electron conjugation bridge structure

High molecular weight functional polyurethanes bearing large π electron conjugated chromophoric pendants with different conjugation bridge structure, poly(1a), poly(1b), and poly(1c), were synthesized and characterized by FTIR, ¹H NMR and UV-vis absorption spectra. Their optical properties were evaluated by optical limiting and nonlinear optical analyses. The results show that these polymers possess good optical limiting and large nonlinear optical properties, which are attributed to the long D-π-A conjugated π electron structure of the NLO-chromophoric segment. Poly(1a) with CC double bond as π electron conjugation bridge shows better optical limiting property than poly(1b) and poly(1c) with CN or NN double bond as conjugation bridge structure under the same linear transmittance, while poly(1c) with NN double bond as π electron conjugation bridge of the NLO-chromophoric segment is superior on nonlinear optical properties to poly(1a) and poly(1b) with CC and CN double bonds as π electron conjugation bridge structure, respectively.     

Copolymerization behavior of Cp2ZrCl2/MAO and [(CO)5WC(Me)OZrCp2Cl]/MAO: a comparative study on ethylene/1-pentene copolymers

Ethylene/1-pentene copolymers were synthesized using Cp₂ZrCl₂(1)/MAO and [(CO)₅WC(Me)OZr(Cp)₂Cl](2)/MAO catalyst systems. The copolymers were characterized by SEC, DSC, FTIR and ¹³C NMR spectroscopy. The copolymers synthesized with [(CO)₅WC(Me)OZr(Cp)₂Cl](2)/MAO had higher average molecular weights and broader polydispersities compared to those produced with Cp₂ZrCl₂(1)/MAO. The chemical heterogeneity was investigated by SEC-FTIR and fractionation techniques. All copolymers showed a higher incorporation of the 1-pentene in the low molecular weight fraction as revealed by SEC-FTIR. Crystallization analysis fractionation (CRYSTAF) showed a broad chemical composition distribution (CCD) for all the copolymers synthesized with these two catalyst systems. Selected copolymers were also analyzed using an automated preparative molecular weight fractionation.     

Enhance the performances of dye-sensitized solar cell by a new type of sensitizer to co-sensitize zinc oxide photoelectrode with ruthenium complex

We designed a new type of sensitizer for dye-sensitized solar cells based on ZnO photoelectrode. Three five-coordinate transition metal complexes [2,6-(ArNCMe)₂C₅H₃NMCl₂·nCH₃CN] (MZn, Cd, Hg) (named as Zn1, Cd1, Hg1), have been synthesized. In all complexes, the metal center is tridentately chelated by the ligand and further coordinated by two chlorine atoms, resulting in distorted trigonal bipyramidal geometry. The improvement in conversion efficiency of dye-sensitized solar cell was achieved by the complexes (M) and N719 co-sensitizing ZnO photoelectrode. In the tandem structure of M/N719/ZnO, the M forms a re-organization of energy level due to its single-crystal structure, which is advantageous to the electron injection and the hole recovery. The result demonstrates the M/N719 co-sensitized solar cell exhibited excellent photovoltaic performances with the short-circuit photocurrent density of 8.943 mA cm⁻², the open-circuit photovoltage of 591 mV and the fill factor of 0.639 under standard global AM 1.5 solar irradiation conditions.     

Determination of the interaction within polyester-based solid polymer electrolyte using FTIR spectroscopy

Characterization and interaction behavior between Li⁺ ion and CO groups of a series polyester electrolyte have been thoroughly examined using Fourier transform infrared (FTIR). The “free/Li⁺ bonded” CO absorptivity coefficient of the LiClO₄/polyester can be determined quantitatively using FTIR spectrum ranging from 1800 to 1650 cm⁻¹ at 80 °C. Results from curve fitting show that the “free/Li⁺ bonded” CO absorptivity coefficient is 0.144 ± 0.005. The CO group of polymer electrolyte shows strong interaction with Li⁺ ion and a limit value of 95% “Li⁺ bonded” CO is approached in the polymer electrolyte system when the Li⁺ ion equivalent fraction is about 0.28. The molecular structure of polyester electrolyte does not affect significantly the efficiency of interaction between Li⁺ ion and CO.     

Synthesis, light emission, and photo-cross-linking of luminescent polyacetylenes containing acrylic pendant groups

New poly(1-phenyl-1-alkyne)s bearing acrylic pendants (-{(C₆H₅)CC[(CH₂)_mOCOCHCH₂]}_n-; P1(m), m=3, 9) with high molecular weights are synthesized in high yields by WCl₆-Ph₄Sn catalyst. The structures and properties of P1(m) are characterized and evaluated by IR, NMR, TGA, UV, and PL analyses. All the polymers are solution-processable and thermally stable (T_d≥380 °C). Upon photoexcitation, the polymers emit strong blue lights with emission efficiencies (>40%) comparable to that of poly(1-phenyl-1-octyne), an emissive disubstituted polyacetylene. No excimeric absorption and emission are observed in the thin films of the polymers. The acrylic moieties of P1(m) are readily cross-linked by UV irradiation without harming their optical properties, which may enable them to find high-tech applications in photoresist, luminescent patterning, and light-emitting diodes.     

Surface-initiated catalytic ethylene polymerization within nano-channels of ordered mesoporous silicas for synthesis of hybrid silica composites containing covalently tethered polyethylene

A surface-initiated catalytic ethylene polymerization technique is successfully demonstrated herein for the covalent surface-grafting of polyethylene chains within nanochannels of mesoporous silicas to give hybrid mesoporous silica/polyethylene composite materials. In this technique, a Pd−diimine catalyst, [(ArNC(Me)-(Me)CNAr)Pd(Me)(NCMe)]⁺(Ar = 2,6-(iPr)₂C₆H₃) (1), was first covalently immobilized onto two ordered mesoporous silicas (SBA-15 and MSU-F) containing surface-bound acryloyl functionalities to render the mesoporous silica-supported chelate Pd−diimine catalysts (Pd-SBA15 and Pd-MSUF, respectively). Surface-initiated ethylene polymerizations within mesopores were subsequently carried out with Pd-SBA15 and Pd-MSUF at an ethylene pressure of 400 psi and 5 °C. A mechanistic study on the polymerization behavior and the confining effects of silica meso-structures on polymer growth has been undertaken. The covalent surface-grafting of polyethylene within silica nanochannels was confirmed by the results from Fourier-transformed infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), nitrogen adsorption-desorption, electron microscopy, and proton nuclear magnetic resonance (¹H NMR). The content of grafted polyethylene in the composites can be adjusted in a wide range by varying polymerization time. This represents the first report on the covalent surface functionalization of mesoporous silicas with polyethylene via surface-initiated ethylene polymerization.     

FTIR investigation of the influence of diisocyanate symmetry on the morphology development in model segmented polyurethanes

Novel segmented polyurethanes with hard segments based on a single diisocyanate molecule with no chain extenders were prepared by the stoichiometric reactions of poly(tetramethylene oxide)glycol (M_n=1000 g/mol) (PTMO-1000) and 1,4-phenylene diisocyanate (PPDI), trans-1,4-cyclohexyl diisocyanate (CHDI), bis(4-isocyanatocyclohexyl)methane (HMDI) and bis(4-isocyanatophenyl)methane (MDI). Time dependent microphase separation and morphology development in these polyurethanes were studied at room temperature using transmission FTIR spectroscopy. Solvent cast films on KBr discs were annealed at 100 °C for 15 s and microphase separation due to self organization of urethane hard segments was followed by FTIR spectroscopy, monitoring the change in the relative intensities of free and hydrogen-bonded carbonyl (CO) peaks. Depending on the structure of the diisocyanate used, while the intensity of free CO peaks around 1720-1730 cm⁻¹ decreased, the intensity of H-bonded CO peaks around 1670-1690 cm⁻¹, which were not present in the original samples, increased with time and reached saturation in periods ranging up to 5 days. Structure of the diisocyanate had a dramatic effect on the kinetics of the process and the amount of hard segment phase separation. While PPDI and CHDI based polyurethanes showed self-organization and formation of well ordered hard segments, interestingly no change in the carbonyl region or no phase separation was observed for MDI and HMDI based polyurethanes. Quantitative information regarding the relative amounts of non-hydrogen bonded, loosely hydrogen bonded and strongly hydrogen bonded and ordered urethane hard segments were obtained by the deconvolution of CO region and analysis of the relative absorbances in CO region.     

Selective activation of metallic center in heterobinuclear cobalt and nickel complex in ethylene polymerization

Heterobinuclear cobalt and nickel complex {2-[2,6-R₂-C₆H₃NC(CH₃)-(CH₃)CN-(3,5-R₂′)C₆H₂-CH₂-(3′,5′-R₂)C₆H₂NC(CH₃)]-6-[2,6-R₂-C₆H₃NC(CH₃)] pyridine}CoCl₂NiBr₂ (R = isopropyl) (N₅CoNi) was prepared by reaction of pentadentate nitrogen ligand containing 2,6-bis(imino)-pyridine and α-diimine moieties with CoCl₂ and NiBr₂(DME) in turn. The complex was applied as catalyst for ethylene polymerization activated by AlEt₃, MMAO and AlEt₃/[PhMe₂NH] [B(C₆F₅)₄] respectively. The performance of the heterobinuclear complex in ethylene polymerization was compared with corresponding mononuclear complexes (α-diimine nickel bromide and 2,6-bis(imino)-pyridine cobalt chloride) and their equivalent mixture (binary complexes). When the complex N₅CoNi was activated by AlEt₃ or MMAO, its ethylene polymerization activity was lower than its control, the binary complexes. Both heterobinuclear complex and binary complex produced PE with bimodal molecular weight distribution. The amount of high-molecular-weight polyethylene produced by nickel center of N₅CoNi was less than the binary complexes, which reveals that productivity of nickel center of N₅CoNi is selectively suppressed. When the heterobinuclear complex N₅CoNi is activated by AlEt₃/[PhMe₂NH][B(C₆F₅)₄], the relative productivity of nickel center increased, although the total activity of catalyst decreased compared with AlEt₃ as cocatalyst. With respect to AlEt₃, [PhMe₂NH][B(C₆F₅)₄] can preferably activate nickel center of heterobinuclear complex. The results suggest that metal site in the heterobinuclear complex is selectively activated by cocatalyst.     

Solution and bulk rheological behavior of poly(ethylenes) based on VERSIPOL™ catalysts

Palladium catalysts [(ArNC(Me)-C(Me)NAr)Pd(CH₂)₃(COOMe)]⁺ (VERSIPOL™) or [(ArNC(Me)-C(Me)NAr)Pd(CH₂)₃(COOMe)]⁺ (Ar=2,6-i-Pr₂-C₆H₃, 2,6-Me₂-C₆H₃ or C₆H₅ and Ar′=3,5-(CF₃)₂-C₆H₃) were synthesized and tested, in dichloromethane, for the polymerization of ethylene. The influence of the substituent present on the diimine ligand on the molar mass of the resulting polymers was examined first. Poly(ethylene)s obtained in the presence of catalysts containing the bulky 2,6-i-Pr₂ group, prepared at different ethylene pressures, exhibited almost identical weight average molar mass values, but were characterized by great differences in hydrodynamic volume, radius of gyration and intrinsic viscosity values. These differences were attributed to the evolution of the topology going from hyperbranched to almost linear. Similar observations were made earlier. The major part of the work dealt with new results on the behavior of these PE samples examined in terms of particle scattering function q^5/3I(q) (Kratky-Porod) plot based on small angle neutron scattering experiments and on the semi-dilute solution behavior. Some results on the bulk rheological properties of these polymers were presented in the last section and corroborated the results obtained in dilute or semi-dilute solution. The data were compared also to PE obtained with other catalysts.     

Stimuli-responsive conjugated polymers. Synthesis and chiroptical properties of polyacetylene carrying l-glutamic acid and azobenzene in the side chain

A glutamic acid- and azobenzene-containing novel N-propargylamide, (S)-CHCCH₂NHCOCH(CH₂CH₂CO₂CH₂C₆H₅)NHCO₂CH₂CH₂-p-C₆H₄NNC₆H₅ (1) was synthesized and polymerized with (nbd)Rh⁺[η⁶-C₆H₅B⁻(C₆H₅)₃] as a catalyst to obtain the corresponding polymer [poly(1)] with the moderate number-average molecular weight of 12,200 in 93% yield. The chiroptical studies revealed that poly(1) took a helical structure in THF, CHCl₃, CH₂Cl₂ and toluene. Poly(1) underwent solvent and heat-driven helix-helix transition. The trans-azobenzene of the side chain isomerized into cis upon UV-irradiation, accompanying decrease in helicity. The cis-azobenzene moiety reisomerized into trans upon visible-light irradiation, while the polymer did not recover the original helicity.     

Molecular and crystal structure of poly(tetramethylene adipate) α form based on synchrotron X-ray fiber diffraction

The molecular and crystal structure of the α form of poly(tetramethylene adipate) (PTMA) was analyzed using synchrotron X-ray fiber diffraction data. The crystals belong to the monoclinic system of space group P2₁/n. The unit cell constants are a=0.6776(6), b=0.7904(6), c (fiber axis)=1.442(1) nm and β=135.6(1)°. The final crystal structure was obtained by the linked-atom least-squares refinement, which gave an R-factor of 0.130 for 103 observed spots and 64 unobserved reflections. The molecular structure deviates slightly from the fully extended conformation in the ester part. The torsional angle CH₂-CH₂-O-C(O) was found to be 155°. The CO groups of the corner and center chains in a unit cell are closely located along the c-axis and are related by the crystallographic 2₁-axes along the b-axis at z=1/4 and z=3/4. The total dipole moment arising from the CO groups is oriented in one direction at z=1/4, and in the opposite direction at z=3/4. Owing to the close arrangement of the CO groups between neighboring chains along the fiber axis, the c-projected cell dimensions and the setting angle of the polymer chain are different from those of the orthorhombic form of polyethylene and the β form of PTMA.     

Photo-polymerisation effects on the carbonyl CO bands of composite resins measured by micro-Raman spectroscopy

The spectra of photo-polymerised composite resins, measured by the micro-Raman spectroscopy, are analysed in the frequency region of the CO carbonyl stretching bands. The band intensities at about 1700 and 1715 cm⁻¹ (the peak frequencies of the hydrogen bonded and free CO bands, respectively) decrease with the irradiation time and with the methacrylate monomer conversion. Consequently, a degree of apparent conversion (DAC) can be defined. It is shown that the intensity variations conserve the band-shape and allow the DAC measurements via the single frequency intensity ratio. Both the bands give rise to DAC values which exhibit a one-to-one dependence on the degree of CC conversion, in partial disagreement with previous results. The microscopic mechanisms, associated to the intensity decrease, are critically revised and discussed on the basis of the experimental findings.     

Synthesis and characterization of low- and medium-molecular-weight hyperbranched polyethylenes by chain walking ethylene polymerization with Pd-diimine catalysts

We report in this article the synthesis and characterization of a range of hyperbranched polyethylenes having various low and medium molecular weights by chain walking ethylene polymerization with Pd-diimine catalysts of reduced ligand steric crowdedness, which are intended for potential applications as novel synthetic base stocks. Four Pd-diimine catalysts featured with different ligand crowdedness, ([(RC₆H₃NC(R′)-C(R′)NC₆H₃R)Pd(CH₃) (NCMe)]SbF₆) (1, R = 2,6-(iPr)₂, R′ = CH₃; 2, R = CH₃, R′ = H; 3, R = 2,6-(iPr)₂, R′ = H; 4, R = CH₃, R′ = CH₃), were employed herein for ethylene polymerizations at different conditions. Generally, reducing ligand steric crowdedness (in the order 1 > 4 > 3 > 2) leads to decreased catalyst activity and dramatically reduced polymer molecular weight. As opposed to high-molecular-weight polymers (weight-average molecular weight (M_w): about 150 kg/mol) obtained with catalyst 1, low-molecular-weight polymers (M_w: below 1.0 kg/mol) were obtained with 2 and 3, and medium-molecular-weight polymers (M_w: about 25 kg/mol) were produced with 4. Despite their various reduced molecular weights, the polymers are all featured with highly branched chain structures with a total branching density of above 100 branches per 1000 carbons. The low- and medium-molecular-weight hyperbranched polymers synthesized with 2-4 display good potential for applications as synthetic base stocks. In comparison with three commercial poly(α-olefin) based synthetic base stocks, they exhibit similar thermal and viscosity properties. Meanwhile, it is also discovered that a subsequent one-pot hydrogenation step can be incorporated in the process after the Pd-diimine catalyzed polymerization step to render nearly fully saturated hyperbranched polymers without the use of additional hydrogenation catalysts.     

Thermodynamic modeling of liquid-fluid phase equilibrium in supercritical ethylene + copolymer + co-solvent systems using the PC-SAFT equation of state

Copolymers are important in the manufacture of new polymeric materials with specific characteristics. For linear polymers, thermodynamic models based on the thermodynamic perturbation theory are interesting, since this theory regards the association between monomers. In this work, cloud points of mixtures of copolymers (PEH, PEP, PEAA and PEVA) (PEP: poly(ethylene-co-propylene); PEAA: poly(ethylene-co-acrylic acid); PEH: poly(ethylene-co-1-hexene); PEVA: poly(ethylene-co-vinyl acetate)), a supercritical fluid (C₂) and co-solvents (C₁, C₂, C₃, nC₄, 1C₄, 1C₆, AA, VA, He, N₂, CO₂) (C₁: methane; C₂: ethane; C₂: ethylene; C₃: propane; nC₄: n-butane; 1C₄: 1-butene; 1C₆: 1-hexene; AA: acrylic acid; VA: vinyl acetate; He: helium; N₂: nitrogen; CO₂: carbon dioxide) were modeled using the PC-SAFT equation of state (Perturbed Chain-Statistical Associating Fluid Theory) with a one-type van der Waals mixing rule by fitting one single interaction parameter. Pure component parameters for the supercritical fluid and co-solvents were obtained by regression of vapor pressure and density data of saturated liquid, while pure component parameters for polymers that compose the copolymers were obtained by regression of pure liquid PVT data. Binary interaction and pure component parameter estimation was performed by using the modified maximum likelihood method. Relative deviations between the calculated and experimental cloud points show that the PC-SAFT model had an excellent performance.     

Electrochemical and spectral studies on the catalytic oxidation of nitric oxide and nitrite by high-valent manganese porphyrins at an ITO electrode

Catalytic oxidation of nitric oxide and nitrite by water-soluble manganese(III) meso-tetrakis(N-methylpyridinium-4-yl) porphyrin (Mn(III)(4-TMPyP) was first studied at an indium-tin oxide (ITO) electrode in pH 7.4 phosphate buffer solutions. A stepwise oxidation of Mn(III)(4-TMPyP) through high-valent manganese porphyrin species has been observed by electrochemical and spectroelectrochemical (OTTLE) techniques. The formal potential of 0.63 V for the formation of OMn(IV)(4-TMPyP) has been estimated from OTTLE data. The product, oxoMn(IV) porphyrin, was relatively stable decaying slowly to Mn(III)(4-TMPyP) with a first-order rate constant of 3.7 × 10⁻³ s⁻¹. OMn(IV)(4-TMPyP) has been found to oxidize NO catalytically at potentials about 70 mV more negative than that previously reported for OFe(IV)(4-TMPyP) with good selectivity against nitrite. Nitrite was catalytically oxidized at potentials higher than 1.1 V presumably by OMn(V)(4-TMPyP). OMn(IV)(4-TMPyP) was observed as an intermediate species. Nitrate has been confirmed to be a final product of the electrolysis at 1.2 V, while at 0.8 V nitrite left unchanged, demonstrating that OMn(IV)(4-TMPyP) could not oxidize nitrite. A possible schemes of the catalytic oxidation of NO by OMn^IV(4-TMPyP) and NO₂⁻ by OMn(V)(4-TMPyP) have been proposed.