Home About us Contact
|
Home About us Contact | ||
|
|
||
Thermal Initiation (thermal + initiation)
Selected AbstractsThermal Initiation of MMA in High Temperature Radical PolymerizationsMACROMOLECULAR MATERIALS & ENGINEERING, Issue 4 2005Philip Nising Abstract Summary: Several researches have dealt with the thermal initiation of methyl methacrylate (MMA) in the past. Some of them already discussed the presence of peroxide containing species that are formed from dissolved oxygen and the monomer itself as main reason for this initiation. However, a more detailed investigation as well as a kinetic description of this phenomenon is still due in literature. In this paper, the formation and decomposition of methyl methacrylate peroxides are described. MMA that has been in contact with air forms macromolecular peroxides at temperatures below 100,°C from physically dissolved oxygen. These peroxides have molecular weights of approximately 3,000,5,000 g,·,mol,1, depending on the temperature during formation. Above this temperature, these peroxides decompose quickly and initiate the radical polymerization. Depending on the reaction conditions, monomer conversions from 15 to 30% are obtained. In combination with additional initiators, the MMA peroxides provoke an acceleration of the reaction rate and can also lead to bimodal molecular weight distributions. An analytical method based on UV-spectrophotometry was developed for the quantification of the peroxide content in the monomer. The kinetic rate constants for the formation were determined in batch experiments with purified, air-saturated monomer to be kf,0,=,6.28,·,107 l2,·,mol,2,·,s and EA,=,7.75,·,104 J,·,mol,1. The decomposition rate constants were determined from batch dead-end polymerizations and found to be kd,0,=,4.73,·,107 l,·,mol,1,·,s,1 and EA,=,8.56,·,104 J,·,mol,1. [source] Macroporous monolithic chiral stationary phases for capillary electrochromatography: New chiral monomer derived from cinchona alkaloid with enhanced enantioselectivityELECTROPHORESIS, Issue 17 2003Michael Lämmerhofer Abstract A new chiral monomer derived from cinchona alkaloid, namely O -9-(tert -butylcarbamoyl)-11-[2-(methacryloyloxy)ethylthio]-10,11-dihydroquinine 1, was employed for the preparation of enantioselective monolithic capillary columns by an in situ copolymerization with 2-hydroxyethyl methacrylate 2 (HEMA), ethylene dimethacrylate 3 (EDMA) in the presence of cyclohexanol and 1-dodecanol as porogens (UV or thermal initiation of azobisisobutyronitrile (AIBN) as radical initiator). The porous properties and the electrochromatographic behavior of the new chiral monoliths were comparatively evaluated with previously described analogs obtained from O -9-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine 4 as chiral monomer. Despite close structural and physicochemical similarities of the both chiral monomers, the pore distribution profiles of the resulting monoliths were shifted typically towards larger pore diameters with the new monomer 1. Once more, it was confirmed that a low cross-linking (10 wt% related to total monomers) and a pore diameter of about 1 ,m in the dry state provides the best electrochromatographic efficiency as a result of lower resistance to mass transfer (smaller C-term contribution to peak broadening) and more homogeneous flow profile (smaller A-term). Most importantly, as expected the new poly(1 - co -HEMA- co -EDMA) monoliths showed enhanced enantioselectivities and in addition faster separations as compared to poly(4 - co -HEMA- co -EDMA) analogs, which represents a significant improvement. Further, the elution order was reversed owing to the pseudoenantiomeric behavior of quinine- and quinidine-derived monomers. Fluorescence-labeled 9-fluorenylmethoxycarbonyl (FMOC), dansyl (DNS), 7-dimethylaminosulfonyl-1,3,2-benzoxadiazol-4-yl (DBD), carbazole-9-carbonyl (CC) amino acids could be separated with resolution values between 2 and 4 (with efficiencies typically between 100,000 and 200,000 plates/m) and fluorescence detection (variable wavelength fluorescence detector in-line with UV) yielding routinely a gain in detection sensitivities up to two orders of magnitude without specific optimization of the conditions with regards to fluorescence efficiency. [source] 4-Acetoxystyrene nitroxide-mediated controlled radical polymerization: Comparison with styreneJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2010Andrew S. Cho Abstract The nitroxide-mediated controlled radical polymerization (NM-CRP) of 4-acetoxystyrene with an alkoxyamine was analyzed by a combined experimental and modeling approach. At low nitroxide concentrations, thermal initiation was significant, and control of the polydispersity was poor, as was observed previously for styrene. A continuum model based on the method of moments was used to regress the parameters for the reversible nitroxide uncoupling/coupling reactions (activation energy of uncoupling), thermal initiation (activation energy of initiation), and termination (frequency factor of recombination). The model was able to capture the molecular weight averages and the polydispersity index as a function of time and the nitroxide concentration qualitatively and quantitatively. Using this mechanistic framework, we developed kinetic Monte Carlo models that allowed the molecular weight distributions to be predicted explicitly in good agreement with experimental data. A comparison of the NM-CRP of 4-acetoxystyrene and styrene is provided to illustrate the effect of the acetoxy substituent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Thermal Initiation of MMA in High Temperature Radical PolymerizationsMACROMOLECULAR MATERIALS & ENGINEERING, Issue 4 2005Philip Nising Abstract Summary: Several researches have dealt with the thermal initiation of methyl methacrylate (MMA) in the past. Some of them already discussed the presence of peroxide containing species that are formed from dissolved oxygen and the monomer itself as main reason for this initiation. However, a more detailed investigation as well as a kinetic description of this phenomenon is still due in literature. In this paper, the formation and decomposition of methyl methacrylate peroxides are described. MMA that has been in contact with air forms macromolecular peroxides at temperatures below 100,°C from physically dissolved oxygen. These peroxides have molecular weights of approximately 3,000,5,000 g,·,mol,1, depending on the temperature during formation. Above this temperature, these peroxides decompose quickly and initiate the radical polymerization. Depending on the reaction conditions, monomer conversions from 15 to 30% are obtained. In combination with additional initiators, the MMA peroxides provoke an acceleration of the reaction rate and can also lead to bimodal molecular weight distributions. An analytical method based on UV-spectrophotometry was developed for the quantification of the peroxide content in the monomer. The kinetic rate constants for the formation were determined in batch experiments with purified, air-saturated monomer to be kf,0,=,6.28,·,107 l2,·,mol,2,·,s and EA,=,7.75,·,104 J,·,mol,1. The decomposition rate constants were determined from batch dead-end polymerizations and found to be kd,0,=,4.73,·,107 l,·,mol,1,·,s,1 and EA,=,8.56,·,104 J,·,mol,1. [source] Modeling of Polymerization Kinetics and Molecular Weight Development in the Microwave-Activated Nitroxide-Mediated Radical Polymerization of StyreneMACROMOLECULAR REACTION ENGINEERING, Issue 2-3 2009Jorge J. Hernández-Meza Abstract Calculations of the polymerization rate and molecular weight development in the nitroxide-mediated radical polymerization (NMRP) of styrene (STY), using hydroxyl-TEMPO and dibenzoyl peroxide (BPO), and activated by microwave irradiation (MI), are presented. The calculations are based on a kinetic model developed in our group. Microwave activation is modeled by three approaches: microwave-activated production of free radicals from monomer molecules, microwave-enhanced thermal initiation, and microwave-enhanced dormant polymer activation. The results obtained are compared against experimental data from the literature. The first approach is the most adequate. The NMRP of STY using TEMPO, BPO, and conductive heating, and the NMRP of STY activated by MI, without initiator, are also analyzed as reference cases. [source] | ||||||||||