Search results for "Monomer"
showing 10 items of 857 documents
Aggregation Behavior of Halogenated Squaraine Dyes in Buffer, Electrolytes, Organized Media, and DNA
2002
Aggregation properties of bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (1) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (2) have been examined in buffer and in the presence of electrolytes, β-cyclodextrin, micelles and DNA. These dyes were found to form aggregates in buffer and methanol−water solutions that have absorption bands blue-shifted to those of the monomeric forms. The iodo derivative 2 forms aggregates at much lower concentrations (1.7 × 10-6 M) compared to the bromo derivative 1 (2.35 × 10-6 M) in 20% (vol/vol) methanol−buffer solution. Increase in methanol concentration in methanol−water solutions resulted in the disruption of the aggregates. The intermediate dimer in…
Competition between inter- and intra- molecular energy exchanges in a simple quantum model of a dimer
2006
Abstract We propose a fully quantum model to describe the dynamics of a possible radiationless energy transfer process between identical and nearly localized molecules or monomers coupled through a dipole–dipole term. The system is studied as an environmentally isolated dimeric pair and we find that its dynamics exhibits a competition between the process ruling out the transfer of energy among different degrees of freedom of a given monomer and the one steering the intermolecular passage of excitations from a monomer to the other one. Such a competition is quantitatively characterized investigating on the temporal behaviour of quantum covariances of some couples of appropriate observables h…
Control over the oxidative reactivity of metalloporphyrins. Efficient electrosynthesis of meso,meso-linked zinc porphyrin dimer.
2011
The electrochemical oxidation of zinc(II) 5,15-p-ditolyl-10-phenylporphyrin at its first oxidation potential leads to the formation of the corresponding meso-meso porphyrin dimer as the main product. The number of electrons abstracted, the addition of the hindered base 2,6-lutidine as well as operating in DMF, instead of a CH(2)Cl(2)/CH(3)CN mixture are the key parameters to obtain high yields of the desired coupling product. Indeed, when the electrolyses are carried out in the CH(2)Cl(2)/CH(3)CN mixture, the unexpected zinc(II) 5-chloro-10,20-p-ditolyl-15-phenyl porphyrin is produced as a by-product, the chlorine atom originating from the CH(2)Cl(2) solvent. The monomer and the dimer are c…
Die kinetik der allendimerisation, ein beitrag zum mechanismus
1971
Zusammenfassung The reaction kinetics of the dimerization of some polyhalogenated allenes with the general formula Cl2CCCCIX (X = Cl, Br, C6H5, COOEt) and of tetrabromoallene was studied by an IR spectrophotometric method. The reaction was found to be of strictly second order kinetics. The activation parameters were determined and the experimental data and the influence of the substituents were discussed in terms of the accepted general theory of allene cycloaddition. All data are in agreement with a non-concerted two-step electrocyclic addition mechanism. Within the experimental limits no paramagnetic species like a diradical could be detected by the ESR-method during the dimerization re…
Copolymerization of VDF and HFP in Supercritical Carbon Dioxide: A Robust Approach for Modeling Precipitation and Dispersion Kinetics
2012
A kinetic model is developed for the heterogeneous free-radical copolymerization of vinylidene fluoride and hexafluoropropylene in supercritical CO 2. The model accounts for polymerization in both the dispersed (polymer-rich) phase and in the continuous (polymer-free) supercritical phase, for radical interphase transport, diffusion limitations, and chain-length-dependent termination in the polymer-rich phase. A parameter evaluation strategy is developed and detailed to estimate most of the kinetic parameters a priori while minimizing their evaluation by direct fitting. The resulting model predictions compare favorably with the experimental results of conversion and MWD at varying monomer fe…
Synthesis of hydrophilic polymers in supercritical carbon dioxide in the presence of a siloxane-based macromonomer surfactant: Heterogeneous polymeri…
2003
Batch free radical polymerization of 1-vinyl-2-pyrrolidone (VP) in super- critical carbon dioxide (scCO2) was studied in the presence of a reactive polysiloxane surfactant (PDMS-mMA). Phase behavior investigation showed that when the initial concentration of the surface active macromonomer was higher than 2.5% w/w with respect to the monomer, the reaction mixture, in the absence of efficient stirring, was initially opaque to the visible light, and it slowly turned to an orange tint. Polymeriza- tion experiments carried out with surfactant concentration higher than the aforemen- tioned value proceeded with a fast kinetics, and led to the formation of spherical nanoparticles with almost quant…
Copolymerization of vinylidene fluoride and acrylic acid in supercritical carbon dioxide
2009
The free-radical copolymerization of vinylidene fluoride (VDF) and acrylic acid (AA) was carried out in supercritical carbon dioxide using both precipitation and dispersion techniques in the presence of an ammonium carboxylate perfluoropolyether compound. Formation of a copolymer was confirmed by variable contact time CP MAS NMR spectroscopy. Macromolecular matrices were significantly enriched in AA with respect to the initial feed composition and we found that the nonfluorinated monomer has a much higher reactivity ratio with respect to VDF. The cumulative concentration, crystallinity, and water affinity of the synthesized copolymers could be modified changing the initial feed composition,…
Dispersion polymerization of vinyl monomers in supercritical carbon dioxide in the presence of drug molecules: A one-pot route for the preparation of…
2008
The polymerization of 1-vinyl-2-pyrrolidone in supercritical carbon di- oxide in the presence of ibuprofen as a model drug was investigated as a new one-pot process for the preparation of polymer-based drug delivery systems (DDSs). The com- posites were prepared at 65 � C and P ¼ 31-42 MPa by changing the initial con- centration of the drug and the concentration of a crosslinking agent and that of a hydrophobic comonomer. The effects of these parameters on the performances of the polymerization and on the in vitro release kinetics of ibuprofen were studied. In all the experiments, part of the drug was entrapped inside the polymer particles and dissolved more slowly with respect to the pure …
Zur kinetik der anionischen polymerisation von methyl-methyacrylat in tetrahydrofuran bei −75°C
1973
Die anionische Polymerisation von Methylmethacrylat (MMA) in Tetrahydrofuran (THF) verlauft bei −75°C mit Cumylcasium bzw. oligomerem α-Methylstyrylcasium als Initiatoren ohne erkennbare Nebenreaktionen analog der anionischen Polymerisation von unpolaren Monomeren in atherischen Losungsmitteln. Als Beweis wird hauptsachlich die enge Molekulargewichtsverteilung der unter diesen Bedingungen hergestellten Polymethylmethacrylate angefuhrt. Bei −75°C betragen die Geschwindigkeitskonstanten fur die Monomeranlagerung an das Ionenpaar kw(±) = 80 dm3 mol−1 s−1 und an das freie Anion etwa kw(−) ≈ 6 · 104 dm3 mol−1 s−1. Leitfahigkeitsmessungen ergeben, das die Dissoziationskonstante des Polymethylmeth…
Syntheses and catalytic oxotransfer activities of oxo molybdenum(vi) complexes of a new aminoalcohol phenolate ligand.
2017
The new aminoalcohol phenol 2,4-di-tert-butyl-6-(((2-hydroxy-2-phenylethyl)amino)methyl)phenol (H2L) was prepared by a facile solvent-free synthesis and used as a tridentate ligand for new cis-dioxomolybdenum(vi)(L) complexes. In the presence of a coordinating solvent (DMSO, MeOH, pyridine), the complexes crystallise as monomeric solvent adducts while in the absence of such molecules, a trimer with asymmetric Mo[double bond, length as m-dash]O→Mo bridges crystallises. The complexes can catalyse epoxidation of cis-cyclooctene and sulfoxidation of methyl-p-tolylsulfide, using tert-butyl hydroperoxide as oxidant.