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Bimodal Molecular Weight Distribution (bimodal + molecular_weight_distribution)
Selected AbstractsCore-shell particles with glycopolymer shell and polynucleoside core via RAFT: From micelles to rodsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 6 2009Samuel Pearson Abstract Amphiphilic block copolymers were synthesized via the reversible addition fragmentation chain transfer (RAFT) copolymerisation of 2-methacrylamido glucopyranose (MAG) and 5,- O -methacryloyl uridine (MAU). Homopolymerisations of both monomers using (4-cyanopentanoic acid)-4-dithiobenzoate (CPADB) proceeded with pseudo first order kinetics in a living fashion, displaying linear evolution of molecular weight with conversion and low PDIs. A bimodal molecular weight distribution was observed for PMAU at low conversions courtesy of hybrid behavior between living and conventional free radical polymerization. This effect was more pronounced when a PMAG macroRAFT agent was chain extended with MAU, however, in both cases, good control was attained once the main RAFT equilibrium was established. A stability study on PMAU found that its hydrolysis is diffusion controlled, and is accelerated at physiological pH compared with neutral conditions. Self-assembly of four block copolymers with increasing hydrophobic (PMAU) block lengths produced micelles, which demonstrated an increased tendency to form rods as the PMAU block length increased. Interestingly, none of the block copolymers were surface-active. An initial assessment of PMAU's ability to bind the nucleoside adenosine through base pairing was highly promising, with DSC measurements indicating that adenosine is fully miscible in the PMAU matrix. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1706,1723, 2009 [source] Cyclopolymerization of perfluoroisopropenyl vinylacetateJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2006Hirotada Fujiwara Abstract To develop the polymerization exploiting the interconversion of fluorinated carbon radical to hydrocarbon radical, the radical cyclopolymerization of perfluoroisopropenyl vinylacetate [CF2C(CF3)OCOCH2CHCH2] (FIA) was investigated to afford a polymer possessing mainly five-membered ring structure with bimodal molecular weight distribution having 1 × 105 as the higher molecular weight. This may be the first example wherein the cyclopolymerization between usual allyl group and fluorinated vinyl group is performed. The degree of cyclization was between 70 and 80% determined by 19F NMR of as-polymerized products. The polymer preparation from perfluoroisopropenyl group, which shows scarce homopolymerization reactivity was accomplished. The mechanism that the addition of hydrocarbon radical to perfluoroisopropenyl group to produce fluorinated carbon radical followed by the intramolecular addition reaction onto allyl group to form five-membered ring is proposed. The hydrolysis of the FIA polymer afforded a polymer possessing hydrophobic fluoroalkyl group with hydrophilic hydroxyl and carboxylic acid groups. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3220,3232, 2006 [source] Ethylene polymerization over MgO-supported zirconocene catalystsPOLYMER ENGINEERING & SCIENCE, Issue 5 2003Soo Jin Kim Supported zirconcene catalysts on a new support, MgO, were prepared and tested in ethylene polymerization. Three types of impregnation methods were employed to find an optimum supporting method for MgO. The direct impregnation of Cp2ZrCl2 on MgO showed low metal loading and polymerization activity, while the catalyst had a higher metal loading and polymerization activity when MgO was treated with methylaluminoxane (MAO) before supporting. Treatment of MgO with MAO during the supporting step invoked two types of catalytic sites, which was evidenced by the bimodal molecular weight distribution of the polymer products. MgO is considered to have potential as a support for metallocenes. [source] Copolymerization of vinylidene difluoride with hexafluoropropene in supercritical carbon dioxideJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2006Uwe Beginn Abstract Vinylidene difluoride and hexafluoropropene are copolymerized in supercritical carbon dioxide at 280 bar and 50 °C by means of free radical copolymerization, initiated by diethyl peroxydicarbonate. The first stages of the reaction were monitored by turbidity measurements and the time/conversion curve was followed gravimetrically to measure the initial rates of polymerization. The obtained copolymers possessed bimodal molecular weight distributions, their average comonomer composition was well described by the Lewis-Mayo equation with the copolymerization parameters rVDF = 4.8 and rHFP = 0. The glass transition and melting temperatures of the copolymers are similar to that of the materials resulting from aqueous emulsion polymerization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1299,1316, 2006 [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] | ||||||||||