First-order Plots Research Articles

Atom transfer radical polymerization of n-octyl acrylate under microwave irradiation

The atom transfer radical polymerization (ATRP) of n-octyl acrylate (OA) was successfully carried out using ethyl-2-bromobutyrate as an initiator, and CuBr/2,2 ′-bipyridine (bpy) as a catalyst under microwave irradiation (MI) at 76.8 °C. The polymerization of n-octyl acrylate under MI showed linear first-order rate plots, a linear increase of the number-average molecular weight M n with conversion, and low polydispersities, 1.1< M w/ M n<1.4, where M w is weight-average molecular weight. The ATRP of n-octyl acrylate is well controlled. Under the same experimental conditions, the apparent rate constant, k p app, under MI is larger apparently than that under conventional heating. In addition, the effects of concentration of catalyst and other factors on polymerization are reported.

Homogeneous Solution Reverse Atom Transfer Radical Polymerization of Methyl Methacrylate

A homogeneous reverse atom transfer radical polymerization (RATRP) of methyl methacrylate (MMA) was successfully carried out in N, N-dimethylformamide(DMF) (25%, v/v) at 69°C, using an initiating system azobisisobutyronitrile (AIBN)/CuBr2/N, N, N′, N″, N″-pentamethyldiethylenetriamine (PMDETA). The kinetics of homogeneous solution polymerizations showed linear first-order rate plots, indicating a constant number of growing species throughout the polymerization as well as a negligible contribution of termination or transfer reactions; a linear increase of the number-average molecular weight with conversion, and relatively low polydispersities, but low initiator efficiency. The dependence of the rate of polymerization on the concentrations of initiator, catalyst, ligand and temperature were presented.

Synthesis of Conjugated−Ionic Block Copolymers by Controlled Radical Polymerization

A novel series of well-defined amphiphilic block copolymers containing conjugated poly(fluorene) (PF) block and coillike poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) were successfully synthesized through atom transfer radical polymerization (ATRP) initialized by a 2-bromoisobutyrate end-capped PF macroinitiator using CuCl/HMTETA (1,1,4,7,10,10-hexamethyltriethylenetetramine) as the catalyst. The first-order kinetics plot indicated a constant number of active species present in the polymerization. The molecular weight and molecular weight distribution were well controlled, implying well-defined block structure of the prepared block copolymers. The chemical structures of the copolymers were confirmed by 1H NMR studies. The copolymers could be successfully quanternized, and the final conjugated−ionic block copolymers were found to form PF aggregates in water as confirmed by 1H NMR, UV−vis, and photoluminescence (PL) spectra. PL spectra and quantum efficiency measurement also suggested the formation of excimers within the PF aggregates.

Optimized Photoisomerization on Gold Nanoparticles Capped by Unsymmetrical Azobenzene Disulfides

Gold nanoparticles capped by an unsymmetrical azobenzene disulfide, 4-hexyl-4‘-(12-(dodecyldithio)dodecyloxy)azobenzene (C6AzSSC12), were synthesized in order to investigate the efficiency of azobenzene photoisomerization on colloidal gold surfaces. The nanoparticles were synthesized by a two-step method to avoid the direct contact of azobenzene units with a reducing agent. The average size of the particles was determined to be ∼5.2 ± 1.3 nm from transmission electron microscope (TEM) images. The CH2 antisymmetric (∼2919 cm-1) and symmetric (∼2850 cm-1) stretching bands in the FTIR spectra of the nanocomposite confirmed the all-trans conformation of alkyl chains in the C6AzSSC12 on the colloidal gold. The photoisomerization reaction of the C6AzSSC12-capped gold nanoparticles was studied by UV−vis absorption spectroscopy in toluene. The reaction kinetics was identical to that of the free C6AzSSC12 molecules dissolved in toluene, with no deviations from a first-order plot for both trans-to-cis and cis-to-trans photoisomerization, suggesting no steric hindrance throughout the whole reaction process. The free volume guaranteed by the 50% dilution of the dye functions due to the unsymmetrical disulfide structures, as well as their noncompact molecular tails owing to the assembly on the curved colloidal gold, must be responsible for such a highly efficient photoreaction. Sedimentation of the nanoparticles arose in toluene subsequent to the photoisomerization of the capping azobenzene molecules from trans to cis isomers. This phenomenon can be interpreted as resulting from differences in the degree of solvation between the azobenzene isomers.

Living cationic polymerization of styrene with 1-phenylethyl chloride/TiCl 4 /Bu 2 O

Living polymerization of styrene was obtained in the system 1-phenylethyl chloride/TiC14/Bu2O in mixture of 1,2-dichloroethane and n-hexane (55:45 v/v) at -15°C. The number-average molecular weights (Mn) of the polymers obtained increased in direct proportion to the monomer conversion and agreed well with the calculated values. The first-order kinetic plots were linear. The molecular weight distributions were narrow (Mn/Mn=1.46-1.56).

Synthesis of Conjugated−Acidic Block Copolymers by Atom Transfer Radical Polymerization

A novel series of well-defined amphiphilic triblock copolymers containing conjugated polyfluorene (PF) and coillike poly(methacrylic acid) (PMAA) were successfully synthesized by vacuum thermolysis of the precursor block copolymers, poly(2-tetrahydropyranyl methacrylate)−PF−poly(2-tetrahydropyranyl methacrylate) (PTHPMA−PF−PTHPMA) prepared by using atom transfer radical polymerization (ATRP) with a 2-bromoisobutyrate end-capped PF as the macroinitiator. Both the linear first-order kinetic plot and the linear dependence of the Mn vs conversion indicate the controlled nature of the polymerization of THPMA. The polydispersity indices of the PTHPMA−PF−PTHPMA triblock copolymers were essentially less than 1.3, demonstrating the well-defined structures of the obtained block copolymers. The chemical structures of the PTHPMA−PF−PTHPMA block copolymers were confirmed by 1H NMR, 13C NMR, and FTIR studies. Decomposition of the precursors at about 145 °C afforded the PMAA−PF−PMAA block copolymers, and the anhydride formed during the thermolysis could be successfully transformed to the corresponding acid by further hydrolysis in warm water. PMAA−PF−PMAA and PTHPMA−PF−PTHPMA presented almost identical photophysical behavior in diluted solutions, suggesting that similar excited species were essentially involved in their luminescent processes. Aggregation of PF segments was found to occur in the aqueous solution of the PMAA−PF−PMAA as revealed by spectroscopic studies. 1H NMR results also substantiate the formation of PF aggregates stabilized by soluble PMAA.

L-selectin Dimerization Enhances Tether Formation to Properly Spaced Ligand

Selectin counterreceptors are glycoprotein scaffolds bearing multiple carbohydrate ligands with exceptional ability to tether flowing cells under disruptive shear forces. Bond clusters may facilitate formation and stabilization of selectin tethers. L-selectin ligation has been shown to enhance L-selectin rolling on endothelial surfaces. We now report that monoclonal antibodies-induced L-selectin dimerization enhances L-selectin leukocyte tethering to purified physiological L-selectin ligands and glycopeptides. Microkinetic analysis of individual tethers suggests that leukocyte rolling is enhanced through the dimerization-induced increase in tether formation, rather than by tether stabilization. Notably, L-selectin dimerization failed to augment L-selectin-mediated adhesion below a threshold ligand density, suggesting that L-selectin dimerization enhanced adhesiveness only to properly clustered ligand. In contrast, an epidermal growth factor domain substitution of L-selectin enhanced tether formation to L-selectin ligands irrespective of ligand density, suggesting that this domain controls intrinsic ligand binding properties of L-selectin without inducing L-selectin dimerization. Strikingly, at low ligand densities, where L-selectin tethering was not responsive to dimerization, elevated shear stress restored sensitivity of tethering to selectin dimerization. This is the first indication that shear stress augments effective selectin ligand density at local contact sites by promoting L-selectin encounter of immobilized ligand.

Syntheses of Functional Alkoxyamines and Application to Syntheses of Well-Defined Star Polymers

Three kinds of 2,2,6,6-tetramethylpiperidine-N-oxyl (EMPO)-based alkoxyamines, 1-(4-iodophenyl)- (3), 1-(4-ethynylphenyl)- (4), and 1-[4-(1, 3, 2-dioxaborinan-2-yl)phenyl]-(2,2,6,6-tetramethyl-1-piperidinyloxyl)ethanes (5) were prepared. The Pd-catalyzed cross-coupling reaction of 3 with 1,3,5-triethynylbenzene or 1,3,5-tribromobenzene with 4 gave the corresponding 1,3,5-tris(alkoxyaminophyenylethynyl)benzene 11, and the Pd-catalyzed cross-coupling reaction of 5 with 1,3,5-tribromobenzenegave the corresponding 1,3,5-tris(alkoxyaminophenyl)benzene 12. Bulk polymerization of styrene (St) at 120°C initiated with 11 and 12 were investigated. The first-order plots, linear relationships between and conversion, and low M w /M n s of the formed poly(St) showed that the polymerization proceeded in the living fashion, leading to formation of well-defined three-arm star polymers with M w /M n of 1.20-1.40.

Solvent Effects on the Synthesis of Poly(methyl methacrylate) by Atom-Transfer Radical Polymerization (ATRP)

Methyl methacrylate has been polymerized by ATRP at 100°C in bulk, in toluene (50%, v/v) and benzonitrile (50% and 25%, v/v), using methyl 2-halopropionate as initiator (MeXPr, X = Cl or Br), copper halide (CuX) as catalyst and 2,2′-bipyridine (bpy) as ligand. The mixed halogen system MeBrPr/CuCl has also been used. For all initiator systems used, the bulk polymerizations showed linear first-order rate plots, a linear increase of the number-average molecular weight with conversion, and relatively low polydispersities, but low initiator efficiency. The polymerizations performed in toluene do not significantly increase the polymerization efficiency, but the molecular weight distribution is broadened. When the polymerizations are performed in benzonitrile, the polymerization efficiency increases considerably, but is independent of monomer dilution. The molecular weight distributions are narrowed when the polymerizations are performed in 25% (v/v) benzonitrile solution. Experimental number-average molecular weights match the theoretical values when the polymerizations are carried out using the mixed halogen initiator system in benzonitrile solution.

Linear filtering and nonlinear interactions in direction-selective visual cortex neurons: a noise correlation analysis.

Spatial and temporal properties related to direction selectivity of both simple and complex type visual cortex neurons were assessed by cross-correlation analysis of their responses to random ternary white noise. This stimulus consisted of multiple randomly placed bars, each colored white, black, or gray with equal probability, which were rerandomized every 5-10 ms. A first-order cross-correlation analysis of a neuron's spike train with the spatiotemporal history of the stimulus provided an estimate of the neuron's linear spatiotemporal filtering properties. A nonlinear correlation analysis measured the amount of interaction for pair-wise combinations of bars as a function of their relative spatial and temporal separations. The spatiotemporal orientation of each of these functions was quantified using a "motion energy index" (MEI), which was compared to the neurons' direction selectivity measured with drifting sinewave gratings. Both first-order and nonlinear correlation plots usually showed s-t orientation whose sign was consistent with the neuron's direction preference; however, in many cases the MEI for first-order analysis was weak compared to that seen in the nonlinear interactions. The structures of the nonlinear interaction functions were also compared with predictions from a conventional model of direction selectivity based on a simple spatiotemporally oriented linear filter, followed by an intensive nonlinearity ("LN model"). These comparisons showed that some neurons' data agreed reasonably well with such a model, while others agreed poorly or not at all. Simulations of an alternative model which combines signals from idealized lagged and nonlagged front-end linear filters produce noise correlation results more like those seen in the neurophysiological data.

Dimethylaluminum Chloride Catalyzed Living Isobutylene Polymerization

The first examples of dimethylaluminum chloride catalyzed living isobutylene polymerizations are presented. The polymerizations are carried out with conventional tertiary alkyl chloride initiators and 60/40 v/v nonpolar/polar solvent mixtures, the most common solvent systems used for isobutylene triblock synthesis. Additives like proton traps and electron donors are not required. The “living” nature of these polymerizations is demonstrated at −75 to −80 °C in both 60/40 v/v hexane/methylene chloride and hexane/methyl chloride solvent systems using first-order rate plots, trends in Mn and Mw/Mn versus conversion, and delayed incremental-monomer-addition experiments. Polyisobutylenes are prepared with Mn = 150 kDa and Mw/Mn = 1.2. Since initiation from adventitious moisture is of minor concern in this system, experiments were also run in the presence of dimethyl phthalate. “Livingness” was preserved though polymerization rates are slower with dimethyl phthalate than in its absence.

Quantitative Reverse Transcription–Polymerase Chain Reaction to Study mRNA Decay: Comparison of Endpoint and Real-Time Methods

Four quantitative reverse transcription–PCR (RT-PCR) methods were compared to evaluate the time course of mRNA formation and decay. Mouse fibroblasts (NIH 3T3) transfected with the human β-globin open reading frame/c-myc 3′-untranslated region chimeric gene under control of the c-fos promoter (fos-glo-myc) were used for serum-inducible transcription. The amount of fos-glo-myc mRNA, relative to β-actin, was measured by quantitative, RT-PCR at various times following the addition of serum to serum-starved fibroblasts transfected with the chimeric gene. Both endpoint (band densitometry and probe hybridization) and real-time (SYBR green and TaqMan) PCR methods were used to assay the identical cDNA. The real-time methods produced a 4- to 5-log dynamic range of amplification, while the dynamic range of the endpoint assays was 1-log. The real-time and probe hybridization assays produced a comparable level of sensitivity that was considerably greater than band densitometry. The coefficient of variation from 22 replicate PCR reactions was 14.2 and 24.0% for the SYBR green and TaqMan detection, respectively, and 44.9 and 45.1% for the band densitometry and probe hybridization assays, respectively. The rank order for the values of r2 obtained from the linear regression of the first-order mRNA decay plots was SYBR green > TaqMan > probe hybridization > band densitometry. Real-time PCR is more precise and displays a greater dynamic range than endpoint PCR. Among the real-time methods, SYBR green and TaqMan assays produced comparable dynamic range and sensitivity while SYBR green detection was more precise and produced a more linear decay plot than TaqMan detection.

Poly(methyl methacrylates) with Ruthenium Tris(bipyridine) Cores via NiBr2(PR3)2-Catalyzed Atom Transfer Radical Polymerization (ATRP)

Ruthenium(II) tris(bipyridine) complexes derivatized with six α-bromoester functional groups were used as multifunctional initiators for the bulk and solution (toluene) polymerizations of methyl methacrylate (MMA) and methyl acrylate (MA) using NiBr2(PPh3)2 and NiBr2(PBu3)2 as atom transfer radical polymerization (ATRP) catalysts. Both tertiary (1) and secondary (2) halide metalloinitiators were prepared for investigation. The resulting orange-colored polymers were characterized by gel permeation chromatography (GPC) in chloroform. The molecular weight distributions of the poly(methyl methacrylates) (PMMAs) were narrow (toluene, Mw/Mn = 1.1−1.3; bulk, Mw/Mn < ∼1.3 at <50% conversion). The presence of [Ru(bpy)3]2+ in the eluting polymers was verified by in-line, diode-array UV/vis spectroscopic analysis. For polymerizations of MMA in toluene at 80 °C using the tertiary initiator and NiBr2(PPh3)2 as the catalyst, Mn vs percent conversion and pseudo first-order kinetics plots were linear. Thus, these conditi...

Synthesis of Thermochromic Iron(II) Tris(bipyridine)-Centered Star-Shaped Polyoxazolines and Their Bipyridine-Centered Macroligand Counterparts

Iron tris(bipyridine) complexes [Fe{4,4‘-bis(chloromethyl)-2,2‘-bipyridine}3], 1a, and the corresponding iodide analogue generated in situ using NaI, 1b, were explored as initiators for the polymerization of a series of 2-R-2-oxazoline monomers (R = ethyl, EtOX; methyl, MeOX; phenyl, PhOX; and undecyl, UnOX). The resulting labile core, red-violet Fe-centered star polymers fragment during molecular weight analysis by gel permeation chromatography (GPC). Thus, samples were subjected to chemical cleavage in aqueous K2CO3 to generate metal-free bipyridine-centered polyoxazolines, bpyPROX2. When combined with ferrous ammonium sulfate, these bpyPROX2 macroligands chelate to Fe(II), regenerating the [Fe(bipyridine)3]2+ chromophores. Both preparative and analytical kinetics experiments generally produce polymers with reasonably narrow molecular weight distributions (∼1.1−1.5). Molecular weight vs % monomer conversion plots with either the iodide or chloride initiating system were nearly linear for all monomers ; however, only PEOX and PUOX products show good correlation with Mn(calcd) based on monomer/initiator loading. For most monomers, reactions with iodide initiators are faster than the chlorides, and linear first-order kinetics plots were observed. Polymerization of oxazolines with 4,4‘-bis(halomethyl)-2,2‘-bipyridines produced polymers with very narrow molecular weight distributions but with molecular weights higher than targeted based on monomer loading. 1H NMR data illustrates that termination with dipropylamine is efficient for EtOX polymerizations. Thermal analysis by DSC and TGA reveal few differences between Fe-centered stars and their bpy-centered PROX macroligand counterparts. Variable-temperature UV/vis data is provided for an Fe-centered PEOX thin film sample.

A Reverse ATRP Process with a Hexasubstituted Ethane Thermal Iniferter Diethyl 2,3-Dicyano-2,3-di(p-tolyl)succinate as the Initiator

“Living”/controlled radical polymerization of methyl methacrylate (MMA) was successfully carried out using the diethyl 2,3-dicyano-2,3-di(p-tolyl)succinate (DCDTS)/CuCl2/bipy initiation system, in which DCDTS is a hexasubstituted ethane thermal iniferter. The polymerization mechanism belongs to a new reverse atom transfer radical polymerization (ATRP) process with the initiator DCDTS reversibly decomposed to primary radicals in the initiation step. The molecular weights of the resulting PMMA increased with conversion, the molecular weight distributions of PMMA were quite narrow (Mw/Mn < 1.30), and the first-order plots of ln([M]0/[M]) versus time were linear. 1H NMR analysis of PMMA revealed the presence of an α-(carbethoxy−cyano−tolyl)methyl end group from DCDTS fragments and an ω-chlorine group from catalyst. Since polymer is end-functionalized with a chlorine atom, it can then proceed the chain extension reaction with fresh MMA or the block copolymerization with the second monomer styrene (St) in the p...

Gas production from thermal decomposition of explosives: Assessing the thermal stabilities of energetic materials from gas production data

The gas formation associated with the thermal decompositions of nineteen energetic materials was determined at three temperatures (120°C, 220°C and 320°C). Although there was considerable variability within classes, among the largest producers of gas were the nitrate esters. PETN (pentaerythritol nitrate) generated about 6.3mole gas per mole, while nitrocellulose, produced almost no gas. Second in gas production were the nitramines, followed by nitroarenes and lastly, energetic salts. NTO (5-nitro-2, 4-dihydro-3H-1, 2, 4-triazol-3-one), which does not fit into the four main classes of energetic materials, exhibited gas production (2.13 mole gas per mole NTO) comparable with some nitroarenes and the energetic salt, ammonium dinitramide (ADN). For selected compounds gas evolution data was used to construct first-order plots, from which Arrhenius parameters were determined and compared with previously reported values.

Kinetics of Fe(III) and Mn(IV) reduction by the Black Sea strain of Shewanella putrefaciens using in situ solid state voltammetric Au/Hg electrodes

Abstract Solid state gold-amalgam (Au/Hg) microelectrodes were used in situ to analyze dissolved Fe2+ and Mn2+ release from Fe(III) (oxy)hydroxides and MnO2 during reduction by strains of Shewanella putrefaciens, in particular the Black Sea strain, MR-4. Different strains (common to freshwater or seawater environments) showed a lag time in the release of Mn2+ but a similar first-order rate constant for Mn2+ production. These data suggest that all strains use a common enzyme system. An increase in the initial amount of cells added to the solid phase showed an increase that was not linear with initial cell concentration, indicating surface limitation effects. Fe(III) citrate was reduced faster than Fe(OH)3 than α-FeOOH. Addition of nitrate to the cultures inhibited both Mn(IV) and Fe(III) reduction. These data indicate that metal oxide reduction should be an important process when nitrate and oxygen concentrations are low. Calculations were performed using both first-order kinetic plots for data after the lag phase and a two-parameter exponential growth equation ([Mn]=AeBt). The slope B gives similar information to kobs found from first-order log [Mn] vs. time plots whereas A, the pre-exponential factor, gives information on the lag phase commonly observed in culture experiments. A is shown to be a function of several variables depending on the experiment performed.

Photoprobe Study of Siloxane Polymers. 3. Local Free Volume of Polymethylsilsesquioxane Probed by Photoisomerization of Azobenzene

We have investigated the local free volume and its thermal fluctuation in an amorphous polymethylsilsesquioxane (PMSQ) consisting only of T3 (CH3SiO3/2) and T2 (CH3Si(OH)O2/2) units using photoisomerization of azobenzene molecularly dispersed in the PMSQ films as a photoprobe over a wide temperature range (4−300 K). Photoisomerization profiles are discussed on the basis of three parameters, i.e., first-order plots, final cis fractions, and three-component analysis, for uncured and silanol-condensation-cured PMSQ's. The final cis fraction in the uncured PMSQ decreases markedly below 250 K, which has never been observed in linear polysiloxanes, MQ resins, or carbon-based polymers. The effect of low-molecular-weight components in the PMSQ was evaluated using fractionated PMSQ's by molecular weight. The effect of hydrogen bonding by a silanol group was also studied by temperature dependence of infrared spectra. These results suggest that the low-molecular-weight components with silanol groups fill local vacan...

Glass transition temperatures and fermentative activity of heat-treated commercial active dry yeasts.

Differential scanning calorimetry thermograms of various samples of commercial instant active dry yeasts revealed a clear glass transition typical of amorphous carbohydrates and sugars. The resulting glass transition temperatures were found to decrease with increasing moisture content. The observed glass curve was similar to that of pure trehalose, which is known to accumulate in large amounts in baker's yeast. The effect of heat treatment at various temperatures on the fermentative activity (as measured by the metabolic production of CO(2)) of dry yeast was studied. First-order plots were obtained representing the loss of fermentative activity as a function of heating time at the various temperatures assayed. Significant losses of fermentative activity were observed in vitrified yeast samples. The dependence of rate constants with temperature was found to follow Arrhenius behavior. The relationship between the loss of fermentative activity and glass transition was not verified, and the glass transition was not reflected on the temperature dependence of fermentative activity loss.

Anionic polymerization of methyl methacrylate using tetrakis[tris(dimethylamino)phosphoranylidenamino] phosphonium (P5+) as counterion in tetrahydrofuran

A novel metal-free initiator, i.e. the salt of the tetrakis[tris(dimethylamino)phosphoranylidenamino]phosphonium (P5+) cation with the 1,1-diphenylhexyl (DPH–) anion was prepared by cation metathesis. It initiates a very fast and controlled anionic polymerization of methyl methacrylate in THF. Kinetic investigations between –20 and +20°C using a flow tube reactor provide nearly linear first-order time-conversion plots with half-lives below 0.1 s, a linear dependence of the number-average degree of polymerization, and rather narrow molecular weight distributions (Mw/Mn ≈ 1.2). 13C NMR measurements on a model of the active chain end (the P5+ salt of ethyl isobutyrate) in THF-d8 show 15 and 25 ppm upfield shifts of the α-carbon compared to the dimers and tetramers of the lithium ester enolate, respectively, indicating a non-aggregated structure and an increased charge density on the α-carbon.