Heterotactic poly(N-isopropylacrylamide) prepared via radical polymerization in the presence of fluorinated alcohols

Radical polymerization of N-isopropylacrylamide in toluene at −40 °C in the presence of fourfold amounts of fluorinated alcohols was investigated. The ¹³C NMR analysis of the obtained polymers suggested that the addition of fluorinated alcohols induced heterotactic specificity in radical polymerization of NIPAAm, although syndiotactic poly(NIPAAm)s were obtained by adding alkyl alcohols as we have previously reported. To the best of our knowledge, this is the first synthesis of heterotactic poly(NIPAAm).     

Metal-free isotactic-specific radical polymerization of N-isopropylacrylamide with pyridine N-oxide derivatives: The effect of methyl substituents of pyridine N-oxide on the isotactic specificity and the proposed mechanism for the isotactic-specific radical polymerization

The radical polymerizations of N-isopropylacrylamide (NIPAAm) in chloroform at low temperatures in the presence of pyridine N-oxide (PNO) derivatives were investigated. It was found that the methylation at meta-positions of PNO improved the isotactic specificity induced by PNO, whereas the methylation at ortho-positions prevented the induction of the isotactic specificity. NMR analysis revealed that NIPAAm and PNO derivatives formed predominantly 2:1 complex through a hydrogen-bonding interaction. Furthermore, the induction of the isotactic specificity was attributed to the conformationally limited propagating radicals. Based on these findings, the mechanism of the isotactic-specific radical polymerization was discussed.     

Direct synthesis of syndiotactic-rich poly(N-isopropylacrylamide) via radical polymerization of hydrogen-bond-complexed monomer

Radical polymerization of N-isopropylacrylamide (NIPAAm) in toluene was investigated in the presence of hexamethylphosphoramide (HMPA). We succeeded in directly preparing syndiotactic-rich poly(NIPAAm), the syndiotacticity of which (r=70%) is the highest among those of radically-prepared poly(NIPAAm)s so far reported, by lowering polymerization temperature to −60 °C in the presence of a two-fold amount of HMPA. The NMR analysis revealed that the induced syndiotactic-specificity was ascribed to 1:1 complex formation between NIPAAm and HMPA. Furthermore, thermodynamic analysis described that the induced syndiotactic-specificity was enthalpically achieved.     

Hydrogen-bond-assisted isotactic-specific radical polymerization of N-isopropylacrylamide with pyridine N-oxide

Radical polymerization of N-isopropylacrylamide (NIPAAm) in CHCl₃ at low temperatures in the presence of pyridine N-oxide (PNO) was investigated. An isotactic poly(NIPAAm) with meso diad content of 61% was successfully prepared at −60 °C in the presence of a two-fold amount of PNO. Thermodynamic analysis suggested that the isotactic-specificity was entropically induced, probably due to conformational fixation near the propagating chain-end through coordination by PNO.     

Effect of polymerization conditions on the syndiotactic-specificity in radical polymerization of N-isopropylacrylamide and fractionation of the obtained polymer according to the stereoregularity

Radical polymerization of N-isopropylacrylamide (NIPAAm) was examined in the presence of hexamethylphosphoramide (HMPA). The addition of an excess amount of HMPA induced syndiotactic-specificity that gradually enhanced as the feed monomer was consumed. The syndiotacticity of the obtained poly(NIPAAm)s was improved by increasing the [HMPA]₀/[NIPAAm]₀ ratio to five and prolonging the polymerization time to 96 h (racemo=72%). It was also revealed that more stereoregulated poly(NIPAAm) could be fractionated by reprecipitating the resulting polymers from hexane-THF mixture. This result suggested that more stereoregulated poly(NIPAAm) showed a lower solubility than less stereoregulated poly(NIPAAm)s. Furthermore, unusual hysteresis was observed in transmittance analysis of an aqueous solution of the fractionated syndiotactic poly(NIPAAm).     

Syndiotactic‐specific radical polymerization of N‐isopropylacrylamide in toluene at low temperatures in the presence of silyl alcohols

The radical polymerization of N‐isopropylacrylamide was carried out in toluene at low temperatures in the presence of silyl alcohols, such as triethylsilanol. Poly(N‐isopropylacrylamide) with a racemo dyad content of 75% was obtained at ? 80 °C with a 4:1 triethylsilanol to monomer ratio loading. NMR analysis suggests that the mechanism for syndiotactic induction, in the presence of silyl alcohols, may be similar to that observed with alkyl alcohols. In this case, a 1:2 complex formation, via hydrogen bonding interactions, leads to the induction of syndiotactic specificity. Copyright © 2012 Society of Chemical Industry     

Metal-free isotactic-specific radical polymerization of N-alkylacrylamides with 3,5-dimethylpyridine N-oxide: The effect of the N-substituent and solvent on the isotactic specificity

Radical polymerization of N-methylacrylamide (NMAAm), N-n-propylacrylamide, N-isopropylacrylamide (NIPAAm) and N-benzylacrylamide was investigated in CHCl₃, CH₂Cl₂ and CH₃CN, in the presence of 3,5-dimethylpyridine N-oxide (35DMPNO) to examine the effects of the N-substituent and the solvent on the isotactic specificity induced by 35DMPNO. With addition of 35DMPNO to radical polymerization of N-alkylacrylamides in CHCl₃, isotactic specificity was significantly induced in NIPAAm polymerization but only slightly induced in NMAAm polymerization. Furthermore, mixed solvents of CH₃CN and halomethanes such as CHCl₃ and CH₂Cl₂ enhanced the ability of 35DMPNO to induce isotactic specificity, and poly(NIPAAm) with 74% meso dyad was obtained.     

Effect of composition and stereoregularity on phase-transition behavior of aqueous N-ethylacrylamide/N-n-propylacrylamide copolymer solutions

Radical copolymerizations of N-ethylacrylamide and N-n-propylacrylamide (NNPAAm) at various ratios were carried out at −40 °C, in toluene in the presence of 3-methyl-3-pentanol, or in N-ethylacetamide. Syndiotactic-rich copolymers with racemo diad contents of 67.1–70.2%, and isotactic-rich copolymers with meso diad contents of 60.9–64.5% were prepared. Syndiotactic-rich copolymers with NNPAAm compositions of ≥92.9 mol% exhibited large hystereses in the phase-transition temperatures of their aqueous solutions. Isotactic-rich copolymers with NNPAAm compositions of 39.2–67.6 mol% exhibited large hystereses in the phase-transition temperatures of their aqueous solutions. Those of composition >67.6 mol% were insoluble in water. Stereosequence analysis suggested that isotactic sequences favored intramolecular hydrogen bonding between contiguous NNPAAm units, more than syndiotactic sequences. Enhanced intramolecular hydrogen bonding in isotactic sequences was responsible for the large hystereses and insolubility of isotactic-rich copolymers with high NNPAAm compositions.     

Hydrogen-bond-assisted stereocontrol in the radical polymerization of N-isopropylacrylamide with bidentate Lewis base

Radical polymerization of N-isopropylacrylamide (NIPAAm) in toluene was investigated in the presence of bidentate Lewis base such as diphosphonates. Isotacticity of the obtained poly(NIPAAm)s slightly increased at −80 °C, whereas syndiotactic-rich poly(NIPAAm)s were obtained at −40-0 °C. This result corresponded to the results observed in the presence of primary alkyl phosphates. NMR analysis revealed that NIPAAm monomer and tetraisopropyl methylenebisphosphonate formed mono-binding hydrogen-bond-assisted complex at 0 °C, but a chelate complex at −80 °C. Thus, it was concluded that the stereospecificity in NIPAAm polymerization strongly depended on the complexation mode of the added bidentate Lewis base.     

Acid-catalysed dehydration of alcohols in supercritical water

At pressures exceeding its critical pressure water retains its ionic properties to temperatures of 400 °C or more. In water under these conditions trace amounts of Arrhenius acids dissociate and selectively catalyse the dehydration of alcohols, diols, and polyols. High yields of the desired dehydration product (ethene from ethanol, propene from propanol, acetaldehyde from ethylene glycol, and acrolein from glycerol) can be obtained with a residence time of less than one minute. However, for ethanol the equilibrium conversion appears to be less than predicted by ideal solution thermochemical calculations. This may be due to catalyst deactivation, or it may be an effect of hydrogen bonding between the water and the reactant alcohol. The dehydration of n-propanol proceeds by a first order reversible reaction whose equilibrium is close to that predicted by thermodynamics. Because these dehydration reactions proceed rapidly with a high degree of specificity, they appear to be good candidates for industrial exploitation.     

Swelling Characteristics of pH- and Thermo-Sensitive Crosslinked Poly(Vinyl Alcohol) Grafts

Grafting of N-isopropylacrylamide, NIPAAm, onto partially and fully hydrolyzed poly(vinyl alcohol–g-maleic anhydride), PVA–MA, was carried out in presence of ammonium persulfate as initiator. The crosslinked PVA–MA–NIPAAm copolymers were prepared in presence of different weight percentages of methylene bisacrylamide, MBA, as crosslinker and N,N,N,N′-tetramethylethylenediamine, TMEA, as accelerator. Crosslinked PVA–MA–NIPAAm copolymers were prepared at two different temperatures 5 and 55 °C. The structural features of these grafts were confirmed by ¹H NMR analysis. Solution behaviors of both PVA–MA and PVA–MA–NIPAAm were evaluated from viscosity measurements. The swelling ratios of the crosslinked polymers were measured at different temperatures and pH values. The phase transition of the crosslinked gels was measured from DSC analysis. Crosslinked PVA–MA–NIPAAm grafts show different pH and temperature sensitivity. The swelling behaviors of PVA–MA–NIPAAm were referred to formation of hydrogen bonding between amide and carboxylic groups and also to hydrophobic aggregation of NIPAAm grafts.     

Synthesis and characterization of biresponsive graft copolymer gels

Graft copolymer gels with different compositions were prepared by the radical polymerization of N-isopropylacrylamide (NIPAAm) and poly(2-vinylpyridine) (P2VP) macromonomers in dioxane with 1 mol% N,N′-methylenebisacrylamide (BIS) as the crosslinking agent. The graft copolymer gels were analyzed at different temperatures and pH values. They demonstrated the typical swelling behavior for poly(N-isopropylacrylamide) (PNIPAAm) gels with changing temperature. In addition to the temperature dependent measurements, the graft copolymer with a high P2VP content showed a pronounced swelling transition with changing pH value. By separating the temperature and the pH sensitive component, it was possible to obtain a gel which could be swelled independently in response to temperature and pH.     

Solvents effect on the physical properties of semi-dilute poly(N-isopropyl acryl amide) solutions

The physical properties of 5 wt% poly(NIPAM) (M_v=3.22×10⁵) semi-dilute solutions in H₂O, D₂O, and THF (tetrahydrofuran) solvents were studied using dynamic light scattering (DLS) and dynamic shear viscosity (DSV) measurements. The DLS data showed that there were poly(NIPAM) slow mode inter-polymer chains associations in H₂O and D₂O solvents. However, no DLS slow mode was observed in poly(NIPAM)/THF solutions. The DSV data showed that there are shear thickening behavior in these three poly(NIPAM) solutions, resulting in a maximum shear viscosity η_peak in the viscosity η′(ω) versus shear frequency ω curve. The slow mode hydrodynamic radius 〈R_hs〉 of DLS measurements and the zero shear rate viscosity η₀ and maximum viscosity η_peak data of DSV measurements from poly(NIPAM)/H₂O and poly(NIPAM)/D₂O solutions show two critical transition temperatures with T_cr1=30-32 °C and T_cr2=32-34 °C. Poly(NIPASM)/D₂O has higher T_cr1 and T_cr2 than poly(NIPASM)/H₂O. However, no transition temperatures of poly(NIPAM)/THF solution were observed. The different temperature dependencies of these three solutions were attributed to the ‘solubility’ and ‘hydrogen bonding’ effects between poly(NIPAM) with H₂O, D₂O, and THF solvents. Without considering the polymer-solvent hydrogen bonding, the solubility of poly(NIPAM) in solvents decreases in the following sequence: THF>H₂O>D₂O and the degree of polymer-solvent hydrogen bonding increases in the following sequence: THF<H₂O<D₂O. The effects of the degree of ‘hydrogen bonding’ and the ‘solubility’ of polymer in solvents on the physical properties of poly(NIPAM) solutions are discussed.     

High-yield hydrogen production by supercritical water gasification of various feedstocks: Alcohols,glucose, glycerol and long-chain alkanes

Continuous supercritical water gasification (SCWG) of various feedstocks of C1–C16 was conducted to produce hydrogen-rich gas. These feedstocks represent model compounds of biomass such as methanol/ethanol (alcohol-type), glucose and glycerol (byproducts of biodiesel synthesis), and model compounds of petroleum fuels such as iso-octane/n-octane (gasoline), n-decane/n-dodecane (jet fuels) and n-hexadecane (diesel). Almost complete gasification of all the feedstocks was achieved at 25 MPa, 740 °C and 10 wt% with low total organic carbon values of their liquid effluents. The hydrogen gas yields of each feedstock were very similar to the theoretical equilibrium yields estimated by Gibbs free energy minimization. SCWG at different gasification temperatures (650 and 740 °C) and concentrations (10 and 20 wt%) revealed that methanol and ethanol (alcohols), the simple oxygenated hydrocarbons, were easier to be gasified, producing negligible amounts of liquid products, when compared with long-chain hydrocarbons (iso-octane and n-decane) under the identical conditions. When the feedstock concentration was increased from 10 to 20 wt%, the equilibrium hydrogen ratio from iso-octane gasification decreased from 1.02 to 0.79 while that of n-decane increased from 1.12 to 1.50, implying that a branched hydrocarbon may be more resistant to gasification in supercritical water.     

Photopolymerization synthesis of poly(N-isopropylacrylamide) hydrogels

Poly(N-isopropylacrylamide) (NIPAAm) gels were formed by photopolymerization of NIPAAm in the absence of a crosslinker using a water solvent at 25°C. Factors affecting formation were the wavelength region of irradiated light, the type of photoinitiators, and the concentrations of the photoinitiator and monomer. A high-pressure mercury lamp (400 W) was used as a light source. An NIPAAm concentration of 10 wt % and irradiation time of 15 h was used for the photopolymerization. The gel (68% yield) was formed when the quartz glass system was used, but no gelation was observed for the Pyrex glass system that transmits light with π > 290 nm. The gel (100% yield) was easily formed, even in the latter system, when 30 mmol/L of hydrogen peroxide and potassium persulfate were used as the photoinitiator. Water soluble photoinitiators such as ferric chloride and sodium anthraquinone-2,7-disulfonate were not effective for the gel formation. Yield of the gel increased with increasing the potassium persulfate concentration (1–30 mmol/L), but it decreased when a high concentration of hydrogen peroxide (60 mmol/L) was used. The gel yield increased with the NIPAAm concentration (5–20 wt %). The degree of swelling of the resultant poly(NIPAAm) gels, which was measured by immersing the gels in water at various temperatures (0–50°C) for 24 h, steeply decreased at about 30°C with increasing temperature, exhibiting a temperature-responsive character. The gels swelled and shrank in water below and above the temperature, respectively. The extent of the character depended on the concentrations of hydrogen peroxide and monomer. The formation mechanism of the gel in the photopolymerization of NIPAAm using hydrogen peroxide photoinitiator was discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1313–1318, 1997     

What is the state of aggregation of ethanol molecules in ethanol–supercritical carbon dioxide mixtures? An FTIR investigation in the full molar fraction range

The evolution of the degree of hydrogen bonding of ethanol molecules in scCO₂–ethanol mixtures for different molar fraction (from 0.5 to 100% in ethanol) in the temperature range 40–200 °C and at two different constant pressures P = 15 and 20 MPa is reported in this paper. For a given pressure, we observe a strong dependence of the degree of hydrogen bonding as a function of temperature and ethanol molar fraction. We emphasize that a detailed knowledge of the degree of hydrogen bonding of ethanol molecules in these binary systems is important for the understanding and the further development of the supercritical fluid technology.     

Intra- and Intermolecular Hydrogen Bonding in Miscible Blends of CO2/Epoxy Cyclohexene Copolymer with Poly(Vinyl Phenol)

In this study, we synthesized a poly(cyclohexene carbonate) (PCHC) through alternative ring-opening copolymerization of CO₂ with cyclohexene oxide (CHO) mediated by a binary LZn₂OAc₂ catalyst at a mild temperature. A two-dimensional Fourier transform infrared (2D FTIR) spectroscopy indicated that strong intramolecular [C–H···O=C] hydrogen bonding (H-bonding) occurred in the PCHC copolymer, thereby weakening its intermolecular interactions and making it difficult to form miscible blends with other polymers. Nevertheless, blends of PCHC with poly(vinyl phenol) (PVPh), a strong hydrogen bond donor, were miscible because intermolecular H-bonding formed between the PCHC C=O units and the PVPh OH units, as evidenced through solid state NMR and one-dimensional and 2D FTIR spectroscopic analyses. Because the intermolecular H-bonding in the PCHC/PVPh binary blends were relatively weak, a negative deviation from linearity occurred in the glass transition temperatures (T_g). We measured a single proton spin-lattice relaxation time from solid state NMR spectra recorded in the rotating frame [T_1ρ(H)], indicating full miscibility on the order of 2–3 nm; nevertheless, the relaxation time exhibited a positive deviation from linearity, indicating that the hydrogen bonding interactions were weak, and that the flexibility of the main chain was possibly responsible for the negative deviation in the values of T_g.     

Recognition property and preparation of Staphylococcus aureus protein A-imprinted polyacrylamide polymers by inverse-phase suspension and bulk polymerization

Staphylococcus aureus protein A-imprinted polyacrylamide gel beads (SpA-IPGB) and imprinted polyacrylamide particles (SpA-IPGP) were synthesized by inverse-phase suspension polymerization and bulk polymerization, using SpA as template, acrylamide (AM) as functional monomers. Recognition capacity studies of the prepared SpA-IPGB and SpA-IPGP on SpA and S. aureus were conducted to determine recognition specificity. Computer imitation docking studies were conducted between template proteins and acrylamide monomer to reveal the recognition mechanism of SpA molecularly imprinted polymer. The results showed that the imprinted gel beads had high adsorption capacity and specificity to the SpA and S. aureus, and the adsorption quantity could reach 6.85 × 10⁻³ μmol/g and 10³-10⁴ CFU/g. The adsorption capacities of SpA-IPGB on SpA and S. aureus were higher than that of SpA-IPGP and Non-imprinted polyacrylamide gel beads (Non-IPGB). The recognition specificity of SpA-IPGB and SpA-IPGP on S. aureus was much higher than on Escherichia coli and Streptococcus thermophilus. The formation of complementary shape, and hydrogen bonding, electrostatic, and hydrophobic interactions between the imprinting cavities and the template proteins is the driving force for effective and specific recognition of protein imprinted polymer.     

Synthesis of isotactic polystyrene in hydrocarbons by initiation with t-BuLi in the presence of sodium dodecylbenzenesulfonate

Isotactic rich polystyrene was synthesized at 30 °C by the t-BuLi initiated polymerization in the presence of sodium dodecylbenzenesulfonate (SDBS) in hexane or cyclohexane. The polymerization rates at 30 °C were very fast and typically gave quantitative conversions. At SDBS/t-BuLi molar ratios between 0.5 and 1.0, the unfractionated polystyrene had almost the same stereoregularity, with triad and pentad contents of about 45% and 19%, respectively. The polymers could be fractionated into a more isotactic PS (mm = 0.77) and other fractions with lower isotactic content. The influence on the stereochemistry of lithium and potassium dodecylbenzenesulfonates or other sulfonate derivatives, reaction temperature and solvent have also been investigated. Density functional theory (DFT) was used to simulate the reactive sites using 1-lithio-1,3-diphenylbutane and 4-methylbenzenesulfonate (SBS) were used as simplified models of polystyryllithium and SDBS respectively. DFT calculation results indicated that unlike the simpler LDPB model and LDPB-styrene complexes, the pro-m-SBS-LDPB-styrene complexes were the preferred stereochemical configuration consistent with the formation of long isotactic pentad sequences.