Polydispersity Index (polydispersity + index)

Distribution by Scientific Domains

Terms modified by Polydispersity Index

  • polydispersity index value

  • Selected Abstracts

    Quantification of Grafting Densities Achieved via Modular "Grafting-to" Approaches onto Divinylbenzene Microspheres

    Leena Nebhani
    Abstract The surface modification of divinylbenzene (DVB)-based microspheres is performed via a combination of reversible addition fragmentation chain transfer (RAFT) polymerization and rapid hetero-Diels,Alder (HDA) chemistry with the aim of quantifying the grafting densities achieved using this "grafting-to" method. Two variants of the RAFT-HDA concept are employed to achieve the functionalization of the microspheres. In the first approach, the microspheres are functionalized with a highly reactive diene, i.e., cyclopentadiene, and are subsequently reacted with polystyrene chains (number-averaged molecular weight, Mn,=,4200,g,mol,1; polydispersity index, PDI,=,1.12.) that carry a thiocarbonyl moiety functioning as a dienophile. The functionalization of the microspheres is achieved rapidly under ambient conditions, without the aid of an external catalyst. The surface grafting densities obtained are close to 1.2,,1020 chains per gram of microspheres. In the second approach, the functionalization proceeds via the double bonds inherently available on the microspheres, which are reacted with poly(isobornyl acrylate) chains carrying a highly dienophilic thiocarbonyl functionality; two molecular weights (Mn,=,6000,g,mol,1, PDI,=,1.25; Mn,=,26,000,g,mol,1, PDI,=,1.26) are used. Due to the less reactive nature of the dienes in the second approach, functionalization is carried out at elevated temperatures (T,=,60,C) yet in the absence of a catalyst. In this case the surface grafting density is close to 7,chains,nm,2 for Mn,=,6000,g,mol,1 and 4,chains,nm,2 for Mn,=,26,000,g,mol,1, or 2.82,,1019 and 1.38,,1019,chains g,1, respectively. The characterization of the microspheres at various functionalization stages is performed via elemental analysis for the quantification of the grafting densities and attenuated total reflectance (ATR) IR spectroscopy as well as confocal microscopy for the analysis of the surface chemistry. [source]

    Hybrid titanium catalyst supported on core-shell silica/poly(styrene- co -acrylic acid) carrier

    Lijun Du
    Abstract Hybrid titanium catalysts supported on silica/poly(styrene- co -acrylic acid) (SiO2/PSA) core-shell carrier were prepared and studied. The resulting catalysts were characterized by Fourier transform infrared (FTIR) spectroscopy, laser scattering particle analyzer and scanning electronic microscope (SEM). The hybrid catalyst (TiCl3/MgCl2/THF/SiO2TiCl4/MgCl2/PSA) showed core-shell structure and the thickness of the PSA layer in the two different hybrid catalysts was 2.0 ,m and 5.0 ,m, respectively. The activities of the hybrid catalysts were comparable to the conventional titanium-based Ziegler-Natta catalyst (TiCl3/MgCl2/THF/SiO2). The hybrid catalysts showed lower initial polymerization rate and longer polymerization life time compared with TiCl3/MgCl2/THF/SiO2. The activities of the hybrid catalysts were enhanced firstly and then decreased with increasing P/P. Higher molecular weight and broader molecular weight distribution (MWD) of polyethylene produced by the core-shell hybrid catalysts were obtained. Particularly, the hybrid catalyst with a PSA layer of 5.0 ,m obtained the longest polymerization life time with the highest activity (2071 kg PE mol,1 Ti h,1) and the resulting polyethylene had the broadest MWD (polydispersity index = 11.5) under our experimental conditions. The morphology of the polyethylene particles produced by the hybrid catalysts was spherical, but with irregular subparticles due to the influence of PSA layer. 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source]

    4-Acetoxystyrene nitroxide-mediated controlled radical polymerization: Comparison with styrene

    Andrew 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]

    Structural determination of ethylene-propylene-diene rubber (EPDM) containing high degree of controlled long-chain branching

    Susanta Mitra
    Abstract This work highlights an attempt to characterize the degree and nature of long-chain branching (LCB) in an unknown sample of ethylene-propylene-diene rubber (EPDM). Two EPDM rubbers selected for this study were comparable in comonomer compositions but significantly different with respect to molar mass and the presence of LCB. Both rubbers contained 5-ethylidene-2-norbornene (ENB) as diene. Solution cast films of pure EPDM samples were used for different characterization techniques. 1H-NMR, and 13C-NMR were used for assessing the comonomer ratios and LCB. Size exclusion chromatography (SEC) equipped with triple detector system was used to determine the molar mass (both absolute and relative) and polydispersity index (PDI). Presence of branching was also detected using sec-viscometry. Rheological analysis has also been used for characterizing LCB. Finally, on the basis of the experimental findings and the available theories, an attempt was made to identify the chemical nature and degree of LCB. This study reveals the possibility of detailed characterization of molecular architecture of EPDM containing LCB by comparing with an essentially linear EPDM in light of an existing theory. 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source]

    Synthesis and characterization of novel poly(arylenevinylene) derivative

    Song Se-Yong
    Abstract The new poly(arylenevinylene) derivative composed naphthalene phenylene vinylene backbone was developed. The theoretical calculation showed that the model compound of the obtained polymer was highly distorted between the stryl and naphthalene units as well as between the backbone and fluorene side units. The polymer was synthesized by the palladium catalyzed Suzuki coupling reaction with 2,6-(1,,2,-ethylborate)-1,5-dihexyloxynaphtalene and 1,2-bis(4,-bromophenyl)-1-(9,,9,-dihexyl-3-fluorenyl)ethene. The structure of the polymer was confirmed by 1H NMR, IR, and elemental analysis. The weight,average molecular weight of the polymer is 29,800 with the polydispersity index of 1.87. The new polymer showed good thermal stability with high Tg of 195C. The bright blue fluorescence (,max = 475 nm) was observed both in solution and film of new polymer with naphthalene phenylene vinylene backbone. Double layer LED devices with the configuration of ITO/PEDOT/polymer/LiF/Ca/Al showed a turn-on voltage at around 4.5 V, the maximum luminance of 150 cd/m2, and the maximum efficiency of 0.1 cd/A. 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source]

    Formation of monodisperse microbubbles in a microfluidic device

    AICHE JOURNAL, Issue 6 2006
    J. H. Xu
    Abstract The crossflowing rupture technique was first used in a microfluidic device to prepare microbubbles, and successfully prepared monodisperse microbubbles with polydispersity index (,) values of <2%. The parameters affecting the microbubble-formation process, such as two-phase flow rates, continuous-phase viscosity, surface tension, and surfactants were investigated. The microbubble-formation mechanisms of the crossflowing rupture technique with those of the techniques of both flow-focusing rupture and geometry-dominated breakup were also compared. It was also found that the bubble size decreased with increasing continuous-phase rate and its viscosity, while independent of surface tension. The different species of surfactants also influenced the microbubble-formation process. Moreover, the bubble-formation mechanism by using the crossflow rupture technique was different from the techniques of both hydrodynamic flow focusing and geometry-dominated breakup. The microbubble-formation process using the crossflowing rupture technique is controllable. 2006 American Institute of Chemical Engineers AIChE J, 2006 [source]

    Lyophilised liposome-based formulations of ,-tocopheryl succinate: Preparation and physico-chemical characterisation

    pn Koudelka
    Abstract ,-Tocopheryl succinate (,-TOS) is a semisynthetic analogue of ,-tocopherol with selective toxicity to the cancer cells and anticancer activity in vivo. Yet, no suitable formulation of ,-TOS for medical application has been reported. Various formulations, for example, solutions in organic solvents, oil emulsions and vesicules prepared by spontaneous vesiculation, polyethylene glycol conjugates and liposomes of various compositions have been tested. We developed and characterised a stable lyophilised liposome-based ,-TOS formulation. ,-TOS (15,mol%) was incorporated into large oligolamellar vesicles (OLVs) composed of soy phosphatidylcholine (SPC) by the method of lipid film hydration followed by extrusion through polycarbonate filters. Stabilised liposomal formulation was prepared by lyophilisation in the presence of sucrose (molar ratio lipid/sucrose, 1:5). The size distribution of the liposomes (130,140,nm, polydispersity index 0.14) as well as the stable lipid and ,-TOS contents were preserved during storage in the lyophilised form at 2,8C for at least 6 months. The data indicate good physical and chemical stability of the lyophilised preparation of ,-TOS liposomes that can be used in clinical medicine. 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99: 2434,2443, 2010 [source]

    Multinuclear NMR characterisation and dermal delivery of fluorinated drugs in soybean-microemulsion systems

    Sonja Hoeller
    Abstract The present study evaluated the effect of different commercially available soybean lecithins in microemulsion systems in terms of microstructure transformation, physicochemical properties and transport of selected entrapped fluorinated drugs through skin. Physicochemical characterisations by particle size and polydispersity index (PDI) measurements were performed and a direct correlation with NMR self-diffusion coefficients of the individual components was found. An increase of lysophosphatidylcholine (LPC), phosphatidylethanolamine (PE) and lysophosphatidylethanolamine (LPE) in the phospholipid mixtures increased the mean particle sizes and PDI. Bicontinous microemulsion structures were proven by 1H and 31P NMR in the placebo microemulsions. Reasonable permeation of the lipophilic drugs of all microemulsions systems was confirmed in standard diffusion studies using porcine skin. This could be due to the incorporation of the drugs in the surfactant structure of the lecithin based bicontinous micro textures, as proven by 19F NMR self-diffusion studies. 2008 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:2686,2695, 2009 [source]

    Determination of block size in poly(ethylene oxide)- b -polystyrene block copolymers by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

    Marion Girod
    Abstract Characterization of block size in poly(ethylene oxide)- b -poly(styrene) (PEO- b -PS) block copolymers could be achieved by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) after scission of the macromolecules into their constituent blocks. The performed hydrolytic cleavage was demonstrated to specifically occur on the targeted ester function in the junction group, yielding two homopolymers consisting of the constitutive initial blocks. This approach allows the use of well-established MALDI protocols for a complete copolymer characterization while circumventing difficulties inherent to amphiphilic macromolecule ionization. Although the labile end-group in PS homopolymer was modified by the MALDI process, PS block size could be determined from MS data since polymer chains were shown to remain intact during ionization. This methodology has been validated for a PEO- b -PS sample series, with two PEO of number average molecular weight (Mn) of 2000 and 5000 g mol,1 and Mn(PS) ranging from 4000 to 21,000 g mol,1. Weight average molecular weight (Mw), and thus polydispersity index, could also be reached for each segment and were consistent with values obtained by size exclusion chromatography. This approach is particularly valuable in the case of amphiphilic copolymers for which Mn values as determined by liquid state nuclear magnetic resonance might be affected by micelle formation. 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3380,3390, 2009 [source]

    Electrochemical and optical properties of novel donor-acceptor thiophene-perylene-thiophene polymers

    Sermet Koyuncu
    Abstract In this study, donor-acceptor type thiophene-perylene-thiophene monomers were synthesized and polymerized by both oxidative polymerization using FeCl3 as catalyst and the electrochemical process. UV,vis, FTIR, 1H NMR, and elemental analysis techniques were used for structural characterization. Thermal behaviors of these compounds were determined by using TGA system. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels and electrochemical and optical band gap values were calculated by using the results of cyclic voltammetry and UV,vis measurements, respectively. The number,average molecular weight (Mn), weight,average molecular weight (Mw), and polydispersity index (PDI) values of synthesized polymers were determined by size exclusion chromatography. Conductivity measurements of these polymers were carried out by electrometer by using a four-point probe technique. The conductivity was observed to be increased by iodine doping. 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1974,1989, 2008 [source]

    Photoinitiated dispersion polymerization of methyl methacrylate: A quick approach to prepare polymer microspheres with narrow size distribution

    Jia Chen
    Abstract Photoinitiated dispersion polymerization of methyl methacrylate was carried out in a mixture of ethanol and water as dispersion medium in the presence of poly(N -vinylpyrrolidone) (PVP) as the steric stabilizer and Darocur 1173 as photoinitiator. 93.7% of conversion was achieved within 30 min of UV irradiation at room temperature, and microspheres with 0.94 ,m number,average diameter and 1.04 polydispersity index (PDI) were obtained. X-ray photoelectron spectroscope (XPS) analysis revealed that only parts of surface of the microspheres were covered by PVP. The particle size decreased from 2.34 to 0.98 ,m as the concentration of PVP stabilizer increased from 2 to 15%. Extra stabilizer (higher than 15%) has no effect on the particle size and distribution. Increasing medium polarity or decreasing monomer and photoinitiator concentration resulted in a decrease in the particle size. Solvency of reaction medium toward stabilizer, which affects the adsorption of stabilizer on the particle surface, was shown to be crucial for controlling particle size and uniformity because of the high reaction rate in photoinitiated dispersion polymerization. 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1329,1338, 2008 [source]

    An efficient synthesis of telechelic poly (N -isopropylacrylamides) and its application to the preparation of ,,,-dicholesteryl and ,,,-dipyrenyl polymers

    Florence Segui
    Abstract Poly(N -isopropylacrylamide)s (PNIPAMs) with cholesteryl or pyrenyl moieties at each chain end (CH-PNIPAMs or Py-PNIPAMs) were prepared via end-group modification of ,,,-dimercapto poly(N -isopropylacrylamides), ranging in molecular weight from , 7000 to 45,000 g mol,1 with a polydispersity index of 1.10 or lower. The telechelic thiol functionalized PNIPAMs were obtained by aminolysis of ,,,-di(isobutylthiocarbonylthio)-poly(N -isopropylacrylamide)s (iBu-PNIPAMs) obtained by reversible addition-fragmentation chain transfer (RAFT) polymerization of N -isopropylacrylamide in the presence of the difunctional chain transfer agent, diethylene glycol di(2-(1-isobutyl)sulfanylthiocarbonylsulfanyl-2-methyl propionate) (DEGDIM). The self-assembly of the polymers in water was assessed by fluorescence spectroscopy, using the intrinsic emission of Py-PNIPAM or the emission of pyrene added as a probe in aqueous solutions of CH-PNIPAM. 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 314,326, 2008 [source]

    Synthesis of poly(4-vinylpyridine) by reverse atom transfer radical polymerization

    Gregory T. Lewis
    Abstract Controlled radical polymerization of 4-vinylpyridine (4VP) was achieved in a 50 vol % 1-methyl-2-pyrrolidone/water solvent mixture using a 2,2,-azobis(2,4-dimethylpentanitrile) initiator and a CuCl2/2,2,-bipyridine catalyst,ligand complex, for an initial monomer concentration of [M]0 = 2.32,3.24 M and a temperature range of 70,80 C. Radical polymerization control was achieved at catalyst to initiator molar ratios in the range of 1.3:1 to 1.6:1. First-order kinetics of the rate of polymerization (with respect to the monomer), linear increase of the number,average degree of polymerization with monomer conversion, and a polydispersity index in the range of 1.29,1.35 were indicative of controlled radical polymerization. The highest number,average degree of polymerization of 247 (number,average molecular weight = 26,000 g/mol) was achieved at a temperature of 70 C, [M]0 = 3.24 M and a catalyst to initiator molar ratio of 1.6:1. Over the temperature range studied (70,80 C), the initiator efficiency increased from 50 to 64% whereas the apparent polymerization rate constant increased by about 60%. 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5748,5758, 2007 [source]

    Controlled radical polymerization of a trialkylsilyl methacrylate by reversible addition,fragmentation chain transfer polymerization

    M. N. Nguyen
    Abstract The reversible addition,fragmentation chain transfer (RAFT) polymerization of a hydrolyzable monomer (tert -butyldimethylsilyl methacrylate) with cumyl dithiobenzoate and 2-cyanoprop-2-yl dithiobenzoate as chain-transfer agents was studied in toluene solutions at 70 C. The resulting homopolymers had low polydispersity (polydispersity index < 1.3) up to 96% monomer conversion with molecular weights at high conversions close to the theoretical prediction. The profiles of the number-average molecular weight versus the conversion revealed controlled polymerization features with chain-transfer constants expected between 1.0 and 10. A series of poly(tert -butyldimethylsilyl methacrylate)s were synthesized over the molecular weight range of 1.0 104 to 3.0 104, as determined by size exclusion chromatography. As strong differences of hydrodynamic volumes in tetrahydrofuran between poly(methyl methacrylate), polystyrene standards, and poly(tert -butyldimethylsilyl methacrylate) were observed, true molecular weights were obtained from a light scattering detector equipped in a triple-detector size exclusion chromatograph. The Mark,Houwink,Sakurada parameters for poly(tert -butyldimethylsilyl methacrylate) were assessed to obtain directly true molecular weight values from size exclusion chromatography with universal calibration. In addition, a RAFT agent efficiency above 94% was confirmed at high conversions by both light scattering detection and 1H NMR spectroscopy. 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5680,5689, 2005 [source]

    First diphosphinoamine ligand bearing a polymerizable side chain: Complexation with copper(I)

    Ritu Ahuja
    Abstract A diphosphinoamine ligand with a polymerizable side chain, (PPh2)2NCH2C6H4CHCH2 (vbzpnp or 1), was synthesized. The ligand could be polymerized by anionic polymerization with n -butyllithium as the initiator. Polyvbzpnp was soluble in tetrahydrofuran and chloroform but was insoluble in methanol and was characterized with NMR, IR, and gel permeation chromatography. The number-average and weight-average molecular weights were 40,050 and 55,690, respectively, and the polydispersity index was 1.39. [Cu(CH3CN)4]ClO4 formed a bischelated complex with the monomer and produced [Cu(1)2]ClO4 (2), and CuCl formed a tetramer, Cu4(1)2Cl4 (3). All the compounds (1, 2, and 3) were characterized with single-crystal-structure determination, NMR, and IR spectroscopy. The addition of [Cu(CH3CN)4]ClO4 to polyvbzpnp resulted in an insoluble crosslinked polymer, which was characterized with solid-state 31P {1H} magic-angle-spinning NMR. The copolymerization of styrene and 1 produced a styrene,vbzpnp copolymer that was found to be soluble in common organic solvents. 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3411,3420, 2005 [source]

    Dendrimers as scaffolds for multifunctional reversible addition,fragmentation chain transfer agents: Syntheses and polymerization

    Xiaojuan Hao
    Abstract The synthesis and characterization of novel first- and second-generation true dendritic reversible addition,fragmentation chain transfer (RAFT) agents carrying 6 or 12 pendant 3-benzylsulfanylthiocarbonylsulfanylpropionic acid RAFT end groups with Z-group architecture based on 1,1,1-hydroxyphenyl ethane and trimethylolpropane cores are described in detail. The multifunctional dendritic RAFT agents have been used to prepare star polymers of poly(butyl acrylate) (PBA) and polystyrene (PS) of narrow polydispersities (1.4 < polydispersity index < 1.1 for PBA and 1.5 < polydispersity index < 1.3 for PS) via bulk free-radical polymerization at 60 C. The novel dendrimer-based multifunctional RAFT agents effect an efficient living polymerization process, as evidenced by the linear evolution of the number-average molecular weight (Mn) with the monomer,polymer conversion, yielding star polymers with molecular weights of up to Mn = 160,000 g mol,1 for PBA (based on a linear PBA calibration) and up to Mn = 70,000 g mol,1 for PS (based on a linear PS calibration). A structural change in the chemical nature of the dendritic core (i.e., 1,1,1-hydroxyphenyl ethane vs trimethylolpropane) has no influence on the observed molecular weight distributions. The star-shaped structure of the generated polymers has been confirmed through the cleavage of the pendant arms off the core of the star-shaped polymeric materials. 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5877,5890, 2004 [source]

    Controlled polymerizations of 2-(dialkylamino)ethyl methacrylates and their block copolymers in protic solvents at ambient temperature via ATRP

    Baowei Mao
    Abstract Very well-controlled polymerizations of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and 2-(diethylamino)ethyl methacrylate (DEAEMA) in aqueous and methanolic solutions via atom transfer radical polymerization (ATRP) at ambient temperature were demonstrated. Poly(DMAEMA) and poly(DEAEMA) of low polydispersity index (PDI) of ,1.07 were obtained using the p -toluenesulfonyl chloride/CuCl/1,1,4,7,10,10-hexamethyl-triethylenetetramine (p -TsCl/CuCl/HMTETA) system. Excellent control of polymerization was achieved even in pure methanol. This is in contrast with the very poor control of DMAEMA ATRP in methanol reported previously using a different intiator/catalyst/ligand system. The initiator p -TsCl underwent hydrolysis reaction in aqueous methanolic solutions with a second-order rate constant of 6.1 10,4 dm3 mol,1 s,1 at 25 C. Both poly(DMAEMA) and poly(DEAEMA) retained almost full chlorine-functionization at the chain ends. Well-defined block copolymers of DEAEMA and DMAEMA were successfully obtained by starting with either macroinitiators of DEAEMA or DMAEMA. Other well-defined diblock copolymers could be prepared using these macroinitiators. 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5161,5169, 2004 [source]

    Controlled, radical reversible addition,fragmentation chain-transfer polymerization in high-surfactant-concentration ionic miniemulsions

    J. B. McLeary
    Abstract Living free-radical polymerization of methacrylate and styrenic monomers with ionic surfactants was carried out with reversible addition,fragmentation chain transfer in miniemulsion with different surfactant types and concentrations. The previously reported problem of phase separation was found to be insignificant at higher surfactant concentrations, and control of the molar mass and polydispersity index was superior to that of published miniemulsion systems. Cationic and anionic surfactants were used to examine the validity of the argument that ionic surfactants interfere with transfer agents. Ionic surfactants were suitable for miniemulsion polymerization under certain conditions. The colloidal stability of the miniemulsions was consistent with the predictions of a specific model. The living character of the polymer that comprised the latex material was shown by its transformation into block copolymers. 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 960,974, 2004 [source]

    Reversible addition,fragmentation chain-transfer graft polymerization of styrene: Solid phases for organic and peptide synthesis

    Leonie Barner
    Abstract The ,-initiated reversible addition,fragmentation chain-transfer (RAFT)-agent-mediated free-radical graft polymerization of styrene onto a polypropylene solid phase has been performed with cumyl phenyldithioacetate (CPDA). The initial CPDA concentrations range between 1 10,2 and 2 10,3 mol L,1 with dose rates of 0.18, 0.08, 0.07, 0.05, and 0.03 kGy h,1. The RAFT graft polymerization is compared with the conventional free-radical graft polymerization of styrene onto polypropylene. Both processes show two distinct regimes of grafting: (1) the grafting layer regime, in which the surface is not yet totally covered with polymer chains, and (2) a regime in which a second polymer layer is formed. Here, we hypothesize that the surface is totally covered with polymer chains and that new polymer chains are started by polystyrene radicals from already grafted chains. The grafting ratio of the RAFT-agent-mediated process is controlled via the initial CPDA concentration. The molecular weight of the polystyrene from the solution (PSfree) shows a linear behavior with conversion and has a low polydispersity index. Furthermore, the loading of the grafted solid phase shows a linear relationship with the molecular weight of PSfree for both regimes. Regime 2 has a higher loading capacity per molecular weight than regime 1. 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4180,4192, 2002 [source]

    Atom transfer radical polymerization of n -butyl acrylate catalyzed by CuBr/N -(n -hexyl)-2-pyridylmethanimine

    Huiqi Zhang
    Abstract The homogeneous atom transfer radical polymerization (ATRP) of n -butyl acrylate with CuBr/N -(n -hexyl)-2-pyridylmethanimine as a catalyst and ethyl 2-bromoisobutyrate as an initiator was investigated. The kinetic plots of ln([M]0/[M]) versus the reaction time for the ATRP systems in different solvents such as toluene, anisole, N,N -dimethylformamide, and 1-butanol were linear throughout the reactions, and the experimental molecular weights increased linearly with increasing monomer conversion and were very close to the theoretical values. These, together with the relatively narrow molecular weight distributions (polydispersity index , 1.40 in most cases with monomer conversion > 50%), indicated that the polymerization was living and controlled. Toluene appeared to be the best solvent for the studied ATRP system in terms of the polymerization rate and molecular weight distribution among the solvents used. The polymerization showed zero order with respect to both the initiator and the catalyst, probably because of the presence of a self-regulation process at the beginning of the reaction. The reaction temperature had a positive effect on the polymerization rate, and the optimum reaction temperature was found to be 100 C. An apparent enthalpy of activation of 81.2 kJ/mol was determined for the ATRP of n -butyl acrylate, corresponding to an enthalpy of equilibrium of 63.6 kJ/mol. An apparent enthalpy of activation of 52.8 kJ/mol was also obtained for the ATRP of methyl methacrylate under similar reaction conditions. Moreover, the CuBr/N -(n -hexyl)-2-pyridylmethanimine-based system was proven to be applicable to living block copolymerization and living random copolymerization of n -butyl acrylate with methyl methacrylate. 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3549,3561, 2002 [source]

    Reverse atom transfer radical solution polymerization of methyl methacrylate under pulsed microwave irradiation

    Zhenping Cheng
    Abstract The reverse atom transfer radical polymerization (RATRP) of methyl methacrylate (MMA) was successfully carried out under pulsed microwave irradiation (PMI) at 69 C with N,N -dimethylformamide as a solvent and with azobisisobutyronitrile (AIBN)/CuBr2/tetramethylethylenediamine as an initiation system. PMI resulted in a significant increase in the polymerization rate of RATRP. A 10.5% conversion for a polymer with a number-average molecular weight of 34,500 and a polydispersity index of 1.23 was obtained under PMI with a mean power of 4.5 W in only 52 min, but 103 min was needed under a conventional heating process (CH) to reach a 8.3% conversion under identical conditions. At different [MMA]0/[AIBN]0 molar ratios, the apparent rate constant of polymerization under PMI was 1.5,2.3 times larger than that under CH. 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3823,3834, 2002 [source]

    Preparation of aromatic polyimides highly soluble in conventional solvents

    Wei Huang
    Abstract Several highly soluble polyimides were synthesized from various aromatic tetracarboxylic dianhydrides and an aromatic diamine containing tert -butyl pendent groups [4,4,-methylenebis(2- tert -butylaniline)]. All the polyimides showed excellent solubility in common solvents such as chloroform, tetrahydrofuran, and dioxane at room temperature. The number-average molecular weight ranged from 3.6 104 to 1.3 105 according to gel permeation chromatography relative to a polystyrene standard, and the polydispersity index was between 1.9 and 2.5. The glass-transition temperatures of the resulting polyimides ranged from 213 to 325 C, as measured by differential scanning calorimetry, and little weight loss was observed up to 450 C in N2 by thermogravimetric analysis. These experimental data indicated that the tert -butyl pendent groups reduced the interactions among polymer chains to improve their solubility in organic solvents without the loss of thermal stability. Transparent and flexible films of these polyimides were obtained via casting from solution. 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 229,234, 2002 [source]

    Sedimentation field-flow fractionation and granulometric analysis of PLGA microspheres

    Nathalie Faisant
    Abstract Sedimentation field flow fractionation operated in the steric hyperlayer mode was used to obtain fractions of defined characteristics from crude samples of poly(D,L-lactic-co-glycolic acid) microspheres which were polydisperse in size. In less than ten minutes, Sedimentation Field Flow Fractionation (SdFFF) separation yielded three analytical fractions of very different size and particle size distribution (PSD) characteristics, as determined by granulometric analyses (Coulter Counter and image analysis of SEM). A crude sample (average size = 45 ,m, 105% size polydispersity index) was separated into fractions of 73 ,m, 56 ,m, 8 ,m average diameters which showed a PSD of 39%, 33%, 30%, respectively. Our results demonstrated that SdFFF used in conjunction with particle size analysis offers a new approach to laboratory scale production of drug vectors of a specified average size and reduced size dispersity. In the future, this could be used to select the most convenient particles for drug loading and release. [source]

    Reduction-Sensitive Reversibly Crosslinked Biodegradable Micelles for Triggered Release of Doxorubicin

    Yanmin Xu
    Abstract Reduction-responsive reversibly crosslinked biodegradable micelles were developed and applied for triggered release of doxorubicin (DOX). An amphiphilic block copolymer of poly(ethylene glycol) (PEG) and poly(, -caprolactone) (PCL) that contains two lipoyl functional groups at their interface (PEG-L2 -PCL) has been synthesized. 1H NMR spectroscopy and gel permeation chromatography (GPC) measurements show that the PEG-L2 -PCL block copolymer had a controlled composition (PEG 5 kDa and PCL 5.4 kDa) and a polydispersity index (PDI) of 1.36. PEG-L2 -PCL formed micelles with sizes that ranged from 20 to 150,nm in aqueous solutions, wherein a critical micelle concentration (CMC) of 16,mgL,1 was determined. The micelles were readily crosslinked by adding 7.6,mol % of dithiothreitol (DTT) relative to the lipoyl groups. Notably, micelles after crosslinking demonstrated a markedly enhanced stability against dilution, physiological salt concentration, and organic solvent. In the presence of 10,,10,3,M DTT, however, micelles were subject to rapid de-crosslinking. In vitro release studies showed minimal release of DOX from crosslinked micelles at a concentration of 10,mg,L,1 (C,<,CMC, analogous to intravenous injection), wherein less than 15% of the DOX was released in 10,h. In contrast, rapid release of DOX was observed for DOX-loaded non-crosslinked micelles under otherwise the same conditions (,80% release in 0.5,h). In the presence of 10,,10,3,M DTT mimicking an intracellular reductive environment, sustained release of DOX from crosslinked micelles was achieved, in which 75% of the DOX was released in 9,h. These novel reduction-sensitive reversibly crosslinked biodegradable micelles are highly promising for targeted intracellular delivery of anticancer drugs. [source]

    Synthesis and Characterization of a Novel Degradable Aliphatic Polyester that Contains Monomeric Lactate Sequences

    Congming Xiao
    Abstract Summary: A novel degradable aliphatic polyester that contains monomeric lactate sequences is synthesized via melt-polycondensation of ethylene glycol lactate diol (EGLD) with succinic anhydride without the use of catalyst. The structure of the EGLD precursor and the polyester are verified with FT-IR and 1H NMR spectra. Gel permeation chromatography reveals that the weight-average molecular weight of the polyester is 5.5,,104 with a polydispersity index (PDI) of 1.7. Differential scanning calorimetry profiles reveal that the polyester is a semicrystalline polymer with a glass transition temperature of ,12,C and melting temperature of 101,C. The weight loss percentage of the polyester after immersing for 208 d in active sludge is 2.7%, which suggests degradation has occurred. The synthesis route of the polyester synthesized here (see inset for structure). [source]

    Production of Long-Chain Branched Polyolefins with Two Single-Site Catalysts: Comparing CSTR and Semi-Batch Performance

    Saeid Mehdiabadi
    Abstract We developed a mathematical model to describe the solution polymerization of olefins with two single-site catalysts in a series of two CSTRs. The model was used to simulate processes where semi-crystalline macromonomers produced in the first reactor are incorporated as long chain branches onto amorphous (or lower crystallinity) chains in the second reactor (cross-products). The simulation results show that CSTRs are more efficient to make chains with high LCB density and high weight percent of cross-products. The model can also predict the polydispersity index, average chain lengths, and fractions of the different polymer populations, and help the polymer reactor engineer formulate new products with complex microstructures. [source]

    Mathematical Modeling of Atom-Transfer Radical Copolymerization

    Mamdouh Al-Harthi
    Abstract A comprehensive mathematical model for atom transfer radical copolymerization in a batch reactor is presented using the concept of pseudo-kinetic rate constants and the method of moments. The model describes molecular weight, monomer conversion, polydispersity index, and copolymer composition as a function of polymerization time. Model predictions were compared with experimental data for styrene and butyl acrylate copolymerization and excellent agreement was obtained. We have also tested the model with styrene-acrylonitrile copolymerization data obtained in our laboratory. Finally, we used the model to study the effect of comonomer reactivity ratio, feed composition, activation and deactivation rate constants on the copolymer composition. [source]

    Synthesis and Characterization of Ethylene/Propylene Copolymers in the Whole Composition Range

    Ma. Joaquina Caballero
    Abstract Summary: The incorporation of comonomer molecules in the backbone of a homopolymer can influence the final properties of the material, decreasing its crystallinity and the melting and glass transition temperatures, and increasing its impact resistance and transparency. In the present work, ten ethylene/propylene copolymers have been synthesized using a supported metallocene catalytic system covering the whole composition range. Any desired composition was obtained by controlling the feed composition during the reaction. These synthesized copolymers have been characterized by different techniques in order to study the effect of the comonomer incorporation onto their final properties. When the comonomer content is low, the behaviour of the copolymer is similar to that of the corresponding homopolymer. Nevertheless, if the comonomer content increases, the copolymer becomes more amorphous (low crystallization temperature and soft XRD signals) and easily deformable, reaching a behaviour close to that corresponding to an elastomeric material. In order to corroborate these results the samples have been characterized by TREF and GPC-MALS. TREF analysis showed that copolymers containing less than 10% and more than 80% of ethylene are semicrystalline, with elution temperatures typical of this kind of polymers. Molecular weights are higher for homopolymers and they decrease as the comonomer concentration increases, whereas the polydispersity index keeps almost constant at the expected value for this kind of samples. [source]

    Dynamic Monte Carlo Simulation of Graft Copolymers Made with ATRP and Metallocene Catalysts

    Mamdouh Al-Harthi
    Abstract The synthesis of polyolefin graft copolymers made with coordination polymerization was studied by dynamic Monte Carlo simulation. Narrow molecular weight distribution macromonomers, containing terminal vinyl groups made with atom-transfer radical polymerization (ATRP), were incorporated randomly into the polyolefin backbone. In addition to average molecular weights and polydispersity index, the model predicts the complete molecular weight distribution (MWD) and branching density of the graft copolymer. The effect of the concentration of macromonomers on the grafting efficiency was also studied. [source]

    Advances in the Synthesis and Characterization of Polypeptide-Based Hybrid Block Copolymers

    Ivaylo Dimitrov
    Abstract Linear polystyrene- block -poly(Z-L-lysine) copolymers with a very narrow molecular weight distribution (polydispersity index < 1.03) could be obtained via the ring-opening polymerization of Z-L-lysine- N -carboxyanhydride using ,-(primary amino hydrochloride)-polystyrenes as macroinitiators in N,N -dimethylformamide as the solvent at 40-80 C. The block copolymer samples were analyzed by means of NMR, size exclusion chromatography, and analytical ultracentrifugation. [source]