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Transfer Constant (transfer + constant)

Distribution by Scientific Domains

Polymers and Materials Science	66%

Selected Abstracts

Telomerization of acrylic acid with mercaptans: Part 2.

POLYMER INTERNATIONAL, Issue 4 2001
Kinetics of the synthesis of star-shaped macromolecules of acrylic acid
Abstract The telomerization of acrylic acid with a polyfunctional transfer agent initiated by 2,2,-azobisisobutyronitrile, was first investigated in an organic medium (THF) at 65,°C. Transfer constants (CT) for three mercaptans, ethylene glycol bisthioglycolate, trimethylolpropane tris(2-mercaptoacetate) and pentaerythritol tetrakis(3-mercaptopropionate) of about 6.1, 6.3 and 7.3, respectively, were determined. From these results, it followed that a well-defined star poly(acrylic acid) could not be synthesized via telomerization of acrylic acid in THF. Nevertheless, the same study was performed with the tetrafunctional transfer agent in water/THF mixtures. This work emphasized that the nature of the solvent plays an important role in determining the transfer constant. Thus, the value of CT for the tetrafunctional transfer agent decreased from 7.3 in THF to 1, the ,ideal' case for telomerization (CT,,,1), for the mixture of solvents water/THF (80%, 20%, v/v). With this route, it seems that acrylic acid star-shaped macromolecules could be synthesized via telomerization. © 2001 Society of Chemical Industry [source]

Scale-up development of high-performance polymer matrix for DNA sequencing analysis

ELECTROPHORESIS, Issue 19 2006
Fen Wan
Abstract Linear polyacrylamide (LPA) has been widely used as a replaceable separation matrix in CE. An increase in the molecular weight of the separation medium favors the separation of larger DNA fragments. In order to obtain ultrahigh-molecular-weight (UHMW) LPA, a "frozen" method was developed to synthesize the LPA homopolymer. This approach has three major advantages when compared with other existing routes of LPA synthesis: (i),long LPA chains could be obtained easily, with their average molecular weight (MW) being in the high 10,MDa range; (ii),the desired MW could be adjusted over a broad range by controlling the temperature and the concentration of initiators during synthesis; (iii),the product solution contains only a tiny amount of impurity besides the solvent and LPA. Both static and dynamic laser light scattering measurements were carried out to characterize the synthesized LPA in the buffer solution. The DNA sequencing matrix prepared from LPA using this method was studied and the results were compared with the newly developed commercial product POP7 from Applied Biosystems. It should be noted that this approach can be applied to synthesize other water-soluble polymers, resulting in UHMW products because the chain transfer constant is smaller at lower temperatures. [source]

Kinetic study of the free-radical polymerization of vinyl acetate in the presence of deuterated chloroform by 1H-NMR spectroscopy

JOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2008
Mohammad Ali Semsarzadeh
Abstract The free-radical polymerization of vinyl acetate was performed in the presence of deuterated chloroform (CDCl3) as a chain-transfer agent (telogen) and 2,2,-azobisisobutyronitrile as an initiator. The effects of the initiator and solvent concentrations (or equivalent monomer concentration) and the reaction temperature on the reaction kinetics were studied by real-time 1H-NMR spectroscopy. Data obtained from analysis of the 1H-NMR spectra were used to calculate some kinetic parameters, such as the initiator decomposition rate constant (kd), kp(f/kt)1/2 ratio (where kp is the average rate constant for propagation, f is the initiator efficiency, and kt is the average rate constant for termination), and transfer constant to CDCl3 (C). The results show that kd and kp(f/kt)1/2 changed significantly with the solvent concentration and reaction temperature, whereas they remained almost constant with the initiator concentration. C changed only with the reaction temperature. Attempts were made to explain the dependence of kp(f/kt)1/2 on the solvent concentration. We concluded from the solvent-independent C values that the solvent did not have any significant effect on the kp values. As a result, changes in the kp(f/kt)1/2 values with solvent concentration were attributed to the solvent effect on the f and/or kt values. Individual values of f and kt were estimated, and we observed that both the f and kt values were dependent on the solvent (or equivalent monomer) concentration. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source]

Pharmacokinetic mapping for lesion classification in dynamic breast MRI

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2010
Matthias C. Schabel PhD
Abstract Purpose: To prospectively investigate whether a rapid dynamic MRI protocol, in conjunction with pharmacokinetic modeling, could provide diagnostically useful information for discriminating biopsy-proven benign lesions from malignancies. Materials and Methods: Patients referred to breast biopsy based on suspicious screening findings were eligible. After anatomic imaging, patients were scanned using a dynamic protocol with complete bilateral breast coverage. Maps of pharmacokinetic parameters representing transfer constant (Ktrans), efflux rate constant (kep), blood plasma volume fraction (vp), and extracellular extravascular volume fraction (ve) were averaged over lesions and used, with biopsy results, to generate receiver operating characteristic curves for linear classifiers using one, two, or three parameters. Results: Biopsy and imaging results were obtained from 93 lesions in 74 of 78 study patients. Classification based on Ktrans and kep gave the greatest accuracy, with an area under the receiver operating characteristic curve of 0.915, sensitivity of 91%, and specificity of 85%, compared with values of 88% and 68%, respectively, obtained in a recent study of clinical breast MRI in a similar patient population. Conclusion: Pharmacokinetic classification of breast lesions is practical on modern MRI hardware and provides significant accuracy for identification of malignancies. Sensitivity of a two-parameter linear classifier is comparable to that reported in a recent multicenter study of clinical breast MRI, while specificity is significantly higher. J. Magn. Reson. Imaging 2010;31:1371,1378. © 2010 Wiley-Liss, Inc. [source]

Prostate cancer detection with multi-parametric MRI: Logistic regression analysis of quantitative T2, diffusion-weighted imaging, and dynamic contrast-enhanced MRI

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2009
Deanna L. Langer MSc
Abstract Purpose To develop a multi-parametric model suitable for prospectively identifying prostate cancer in peripheral zone (PZ) using magnetic resonance imaging (MRI). Materials and Methods Twenty-five radical prostatectomy patients (median age, 63 years; range, 44,72 years) had T2-weighted, diffusion-weighted imaging (DWI), T2-mapping, and dynamic contrast-enhanced (DCE) MRI at 1.5 Tesla (T) with endorectal coil to yield parameters apparent diffusion coefficient (ADC), T2, volume transfer constant (Ktrans) and extravascular extracellular volume fraction (ve). Whole-mount histology was generated from surgical specimens and PZ tumors delineated. Thirty-eight tumor outlines, one per tumor, and pathologically normal PZ regions were transferred to MR images. Receiver operating characteristic (ROC) curves were generated using all identified normal and tumor voxels. Step-wise logistic-regression modeling was performed, testing changes in deviance for significance. Areas under the ROC curves (Az) were used to evaluate and compare performance. Results The best-performing single-parameter was ADC (mean Az [95% confidence interval]: Az,ADC: 0.689 [0.675, 0.702]; Az,T2: 0.673 [0.659, 0.687]; Az,Ktrans: 0.592 [0.578, 0.606]; Az,ve: 0.543 [0.528, 0.557]). The optimal multi-parametric model, LR-3p, consisted of combining ADC, T2 and Ktrans. Mean Az,LR-3p was 0.706 [0.692, 0.719], which was significantly higher than Az,T2, Az,Ktrans, and Az,ve (P < 0.002). Az,LR-3p tended to be greater than Az,ADC, however, this result was not statistically significant (P = 0.090). Conclusion Using logistic regression, an objective model capable of mapping PZ tumor with reasonable performance can be constructed. J. Magn. Reson. Imaging 2009;30:327,334. © 2009 Wiley-Liss, Inc. [source]

Synthesis of dithiocarbamate bearing azobenzene group and use for RAFT polymerization of vinyl monomers

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 14 2007
Xiaoming Wan
Abstract A novel dithiocarbamate bearing azobenzene group, 2-(phenylazo-phenoxy-carbonyl) prop-2-yl 9H-carbazole-9-carbodithioate (APCDT), was synthesized and used as a RAFT agent in the polymerization of methyl methacrylate (MMA). The results showed that the controllability to the polymerization of MMA was promoted with APCDT as RAFT agent compared to 2-(ethoxycarbonyl) prop-2-yl 9H-carbazole-9-carbodithioate (EPCDT) under the same experimental conditions. The reason was attributed to the higher chain transfer constant of APCDT than that of EPCDT in the presence of more bulkier and more electrophilic azobenzene moiety. In addition, the RAFT polymerizations of St and methylacrylate (MA) using APCDT as the RAFT agent were also carried out. The ultraviolet spectrum and fluorescence spectrum of the obtained polymers were investigated. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2886,2896, 2007 [source]

Radical Telomerisation of Vinyl Phosphonic Acid with a Series of Chain Transfer Agents

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 6 2007
Ghislain David
Abstract The use of CTAs for the radical polymerisation of VPA has been successfully applied for the first time. Several CTAs were investigated, including thiol, disulfide and halogenated compounds. The use of mercaptoacetic acid led to the synthesis of the mercaptan VPA monoadduct, which was attributed to a high transfer constant (CT,=,ktr/kp) of approximately 7. Transfer reactions with VPA appeared to be unsuccessful when using benzyldisulfide, 1,1,1-trichloroethane and trichloroacetic acid. Application of bromotrichloromethane as a transfer agent led to the oligomerisation of VPA, with final conversion being approximately 80%. The transfer events were characterised by 13C NMR, MALDI-TOF and elemental analysis. Molecular weights of the successful VPA polymerisations showed values ranging from 600 to 5,000 g,·,mol,1, which were in close agreement with the theoretically calculated values. [source]

Telomerization of acrylic acid with mercaptans: Part 2.

Free Radical Polymerization of Acrylonitrile in Green Ionic Liquids

MACROMOLECULAR SYMPOSIA, Issue 1 2004
Liang Cheng
Abstract Free radical polymerization of acrylonitrile (AN) in ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 2,2;m1-azobisisobutyronitrile (AIBN) as initiator was investigated. Early investigations on polymerizations using ionic liquids indicate that they serve as especially good solvents to achieve high molecular weight polymers. Free radical polymerizations result in higher molecular weight polymers, for ionic liquids have low chain transfer constants and act to stabilize the active radical during the process of polymerization. The thermal stability of polymers synthesized in ionic liquids have be improved obviously than that in traditional solvents. [source]