Narrower Molecular Weight Distribution (narrower + molecular_weight_distribution)

Distribution by Scientific Domains

Selected Abstracts

Study on bulk polymerization of methyl methacrylate initiated by low intensity ultrasonic irradiation

Sude Ma
Abstract Methyl methacrylate (MMA) was polymerized in bulk solutions using low intensity ultrasonic radiation of 0.25 W/cm2. The polymerization occurred after 1 h of irradiation time was applied. The polymerization rate was greatly accelerated either by increasing the amount of poly (methyl methacrylate) (PMMA) granular added into the system or by elongating the irradiation time. However, it was found that the reaction rate increased with the decreasing of the ultrasonic frequencies when the exposure time of the polymerization under the irradiation was less than 3 h. Experimental results verified that the polymerization was initiated by free radicals, which were mainly generated from the degradation of PMMA macromolecular chains, the friction between the polymer macromolecular chains and the solvent monomer. These findings were obviously different from those obtained when high intensity ultrasonic irradiation was used. The polymers fabricated in this study by using ultrasound irradiation have a narrower molecular weight distribution compared to those obtained from the polymerizations induced by the conventional initiators. 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source]

Dimethylsilylbis(1-indenyl) zirconium dichloride/methylaluminoxane catalyst supported on nanosized silica for propylene polymerization

Kuo-Tseng Li
Abstract A dimethylsilylene-bridged metallocene complex, (CH3)2Si(Ind)2ZrCl2, was supported on a nanosized silica particle, whose surface area was mostly external. The resulting catalyst was used to catalyze the polymerization of propylene to polypropylene. Under identical reaction conditions, a nanosized catalyst exhibited much better polymerization activity than a microsized catalyst. At the optimum polymerization temperature of 55C, the former had 80% higher activity than the latter. In addition, the nanosized catalyst produced a polymer with a greater molecular weight, a narrower molecular weight distribution, and a higher melting point in comparison with the microsized catalyst. The nanosized catalyst's superiority was ascribed to the higher monomer concentration at its external active sites (which were free from internal diffusion resistance) and was also attributed to its much larger surface area. Electron microscopy results showed that the nanosized catalyst produced polymer particles of similar sizes and shapes, indicating that each nanosized catalyst particle had uniform polymerization activity. 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source]

The synthesis of poly(3-hydroxybutyrate)- g -poly(methylmethacrylate) brush type graft copolymers by atom transfer radical polymerization method

Hlya Arslan
Abstract Brush type of poly (3-hydroxy butyrate), PHB, copolymer synthesis has been reported. Natural PHB was chlorinated by passing chlorine gas through PHB solution in CHCl3/CCl4 mixture (75/25 v/v) to prepare chlorinated PHB, PHB-Cl, with the chlorine contents varying between 2.18 and 39.8 wt %. Toluene solution of PHB-Cl was used in the atom transfer radical polymerization (ATRP) of methyl methacrylate, MMA, in the presence of cuprous bromide (CuBr)/2,2,-bipyridine complex as catalyst, at 90C. This "grafting from" technique led to obtain poly (3-hydroxybutyrate)-g-poly(methylmethacrylate) (PHB- g -PMMA) brush type graft copolymers (cylindrical brush). The polymer brushes were fractionated by fractional precipitation methods and the , values calculated from the ratio of the volume of nonsolvent to volume of solvent of brushes were ranged between 2.8 and 9.5 depending on the molecular weight, grafting density, and side chain length of the brushes, while the , values of PHB, PHB-Cl, and homo-PMMA were 2.7,3.8, 0.3,2.4, and 3.0,3.9, respectively. The fractionated brushes were characterized by gel permeation chromatography, 1H-NMR spectrometry, thermogravimetric analysis (TGA), and differential scanning calorimetry techniques. PHB- g -PMMA brush type graft copolymers showed narrower molecular weight distribution (mostly in range between 1.3 and 2.2) than the PHB-Cl macroinitiator (1.6,3.5). PHB contents in the brushes were calculated from their TGA thermograms and found to be in range between 22 and 42 mol %. The morphologies of PHB- g -PMMA brushes were also studied by scanning electron microscopy. 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 [source]

Effects of external donors and hydrogen concentration on oligomer formation and chain end distribution in propylene polymerization with Ziegler-Natta catalysts

Torvald Vestberg
Abstract The effect of type and concentration of external donor and hydrogen concentration on oligomer formation and chain end distribution were studied. Bulk polymerization of propylene was carried out with two different Ziegler-Natta catalysts at 70 C, one a novel self-supported catalyst (A) and the other a conventional MgCl2 -supported catalyst (B) with triethyl aluminum as cocatalyst. The external donors used were dicyclopentyl dimethoxy silane (DCP) and cyclohexylmethyl dimethoxy silane (CHM). The oligomer amount was shown to be strongly dependent on the molecular weight of the polymer. Catalyst A gave approximately 50 % lower oligomer content than catalyst B due to narrower molecular weight distribution in case of catalyst A. More n -Bu-terminated chain ends were found for catalyst A indicating more frequent 2,1 insertions. Catalyst A also gave more vinylidene-terminated oligomers, suggesting that chain transfer to monomer, responsible for the vinylidene chain ends, was a more important chain termination mechanism for this catalyst, especially at low hydrogen concentration. Low site selectivity, due to low external donor concentration or use of a weak external donor (CHM), was also found to increase formation of vinylidene-terminated oligomers. 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 351,358, 2010 [source]

Synthesis of PMMA- b -PBA block copolymer in homogeneous and miniemulsion systems by DPE controlled radical polymerization

Ying-Da Luo
Abstract In this research, poly(methyl methacrylate)- b -poly(butyl acrylate) (PMMA- b -PBA) block copolymers were prepared by 1,1-diphenylethene (DPE) controlled radical polymerization in homogeneous and miniemulsion systems. First, monomer methyl methacrylate (MMA), initiator 2,2,-azobisisobutyronitrile (AIBN) and a control agent DPE were bulk polymerized to form the DPE-containing PMMA macroinitiator. Then the DPE-containing PMMA was heated in the presence of a second monomer BA, the block copolymer was synthesized successfully. The effects of solvent and polymerization methods (homogeneous polymerization or miniemulsion polymerization) on the reaction rate, controlled living character, molecular weight (Mn) and molecular weight distribution (PDI) of polymers throughout the polymerization were studied and discussed. The results showed that, increasing the amounts of solvent reduced the reaction rate and viscosity of the polymerization system. It allowed more activation,deactivation cycles to occur at a given conversion thus better controlled living character and narrower molecular weight distribution of polymers were demonstrated throughout the polymerization. Furthermore, the polymerization carried out in miniemulsion system exhibited higher reaction rate and better controlled living character than those in homogeneous system. It was attributed to the compartmentalization of growing radicals and the enhanced deactivation reaction of DPE controlled radical polymerization in miniemulsified droplets. 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4435,4445, 2009 [source]

Ethylene polymerization behavior of Cr(III)-containing montmorillonite: Influence of chromium compounds

Kazuhiro Yamamoto
Abstract Montmorillonite was treated with Cr(NO3)3, Cr(acetate)3, and Cr(acac)3 to give three catalyst precursors, Cr-MMT-1, Cr-MMT-2, and Cr-MMT-3, respectively. Application of these catalysts to the ethylene polymerization reaction revealed Cr-MMT-1 to be much more reactive than the other two while the molecular weight distributions of the polymers were practically the same. Elemental analysis, XRD, and TEM measurements suggested that chromium occupied the interlayer section in Cr-MMT-1 and mostly the outer surface region for the other two catalysts. Aluminosilicate-supported Cr catalysts exhibited reactivity similar to that of Cr-MMT-2 and Cr-MMT-3. However, more of the low-molecular-weight polymer was formed. These data suggested that there is a relationship between the sites of the Cr ions and catalytic reactivity, and between supporting solid identity and molecular weight distribution of the polymer. The use of n -Bu2Mg and Et2Zn in the place of Et3Al led to lower activity but gave polymers of narrower molecular weight distribution, with more of the high-molecular-weight material. 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2272,2280, 2009 [source]

Hyperbranched polycarbosiloxane with dendritic boron cores: Synthesis, characterization, and structure regulation

Jie Kong
Abstract The synthesis, characterization, and structure regulation of hyperbranched polycarbosiloxane with dendritic boron cores were realized in this paper. First, dendritic boron core was synthesized via hydroboration with borane dimethylsulfide and bis(allyloxy)dimethylsilane. Then, the hyperbranched polycarbosiloxanes with dendritic boron cores were synthesized via hydrosilylation with AB2 type monomer of bis(allyloxy)methylsilane and dendritic boron cores. The molecular structures of the dendritic boron core and resulting hyperbranched polymers were characterized by using Fourier transform infrared spectroscopy, 1H nuclear magnetic resonance, and 13C nuclear magnetic resonance spectroscopies. Size exclusion chromatography/multiangle laser light scattering analysis reveals that the structures of hyperbranched polycarbosiloxane can be regulated effectively by incorporation of functional dendritic boron cores. Compared with hyperbranched polycarbosiloxane of the same molecular weight level, the hyperbranched polycarbosiloxane with dendritic boron cores presents narrower molecular weight distribution as well as much smaller hydrodynamic radius and intrinsic viscosity. Thermalgravimetric analyzer analysis indicates that both the decomposition temperature and ceramic yields are increased as the results of the incorporation of dendritic boron cores into hyperbranched polycarbosiloxane. 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3930,3941, 2006 [source]

Effect of Solvents on Living Polymerization of Propylene with [t -BuNSiMe2Flu]TiMe2 -MMAO Catalyst System

Kei Nishii
Abstract Summary: Batchwise polymerization of propylene was conducted at 0,C in heptane using [tert -BuNSiMe2Flu]TiMe2 (1) combined with dried methylaluminoxane (MAO) and dried modified methylaluminoxane (MMAO), which were prepared from the toluene solutions of MAO and MMAO by removing the solvent and the free trialkylaluminium. The dried MMAO system quantitatively gave the polymer with higher molecular weight and narrower molecular weight distribution (MWD), ca. 1.3, compared with the dried MAO system. In addition, the polymerization with dried MMAO in toluene, chlorobenzene (CB) and ortho -dichlorobenzene (o -DCB), as solvent, also produced polymers quantitatively with MWD of ca. 1.1,1.3. The number-average molecular weight () of the polymers, obtained with these systems, increased linearly against the polymer yield, while the narrow MWD was retained and the number of polymer chains was almost constant. The results of post-polymerization in heptane and o -DCB proved that the living polymerization proceeded irrespective of the solvent used. The semi-batch polymerizations under an atmospheric pressure of propylene indicated that the propagation rate was more than 10 times higher in o -DCB than in heptane. The 13C NMR analysis of the polymers indicated that the syndiotacticity also depended on the polarity of the solvent and increased in the following order: CB,,,o -DCB,<,toluene,<,heptane. A plausible scheme for the interaction between active species and solvents. [source]

Characterization of Ethylene Copolymers with Liquid Chromatography and Melt Rheology Methods

Yefim Brun
Abstract Summary: Melt rheology and polymer chromatography methods were applied to characterize molecular heterogeneities in products of free radical copolymerization of ethylene with methyl acrylate and vinyl acetate comonomers performed in continuously stirred tank and tubular reactors. We found that the ethylene,vinyl acetate copolymers made in both reactors had similar linear viscoelastic properties typical to branched products of the high pressure process. But the ethylene,methyl acrylate copolymers obtained in the tubular reactor had unusually high melt viscosity at low shear rate and much lower onset of shear thinning despite the narrower molecular weight distribution and the lower overall amount of long-chain branches compare to their autoclave counterparts with similar average molecular weight and chemical composition. Using interaction polymer chromatography method called gradient elution at critical point of adsorption we found that ethylene-acrylate copolymers from the tubular reactor had very broad chemical composition distribution, which was consistent with a significant difference in reactivity ratios between ethylene and acrylate comonomers. Such chemical composition heterogeneity can be a reason for the observed unusual rheological properties of these copolymers. [source]