Viscosity-average Molecular Weight (viscosity-average + molecular_weight)

Distribution by Scientific Domains

Selected Abstracts

Preparation of ultra-high-molecular-weight polyethylene and its morphological study with a heterogeneous Ziegler,Natta catalyst

G. H. Zohuri
Abstract Ultra-high-molecular-weight polyethylene (PE) with viscosity-average molecular weight (Mv) of 3.1 106 to 5.2 106 was prepared with a heterogeneous Ziegler,Natta MgCl2 (ethoxide type)/TiCl4/triethylaluminum catalyst system under controlled conditions. The optimum activity of the catalyst was obtained at a [Al]/[Ti] molar ratio of 61 : 1 and a polymerization temperature of 60C, whereas the activity of the catalyst increased with monomer pressure and decreased with hydrogen concentration. The titanium content of the catalyst was 2.4 wt %. The rate/time profile of the catalyst was a decay type with a short acceleration period. Mv of the PE obtained decreased with increasing hydrogen concentration and polymerization temperature. The effect of stirrer speeds from 100 to 400 rpm did not so much affect the catalyst activity; however, dramatic effects were observed on the morphology of the polymer particles obtained. A stirrer speed of 200 rpm produced PE with a uniform globulelike morphological growth on the polymer particles. The particle size distributions of the polymer samples were determined and were between 14 and 67 ,m. 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source]

Microwave-assisted ring-opening polymerization of p -dioxanone

Yu-Yan Chen
Abstract The ring-opening polymerization (ROP) of p -dioxanone (PDO) under microwave irradiation with triethylaluminum (AlEt3) or tin powder as catalyst was investigated. When the ROP of PDO was catalyzed by AlEt3, the viscosity-average molecular weight (Mv) of poly(p -dioxanone) (PPDO) reached 317,000 g mol,1 only in 30 min, and the yield of PPDO achieved 96.0% at 80 C. Tin powder was successfully used as catalyst for synthesizing PPDO by microwave heating, and PPDO with Mv of 106,000 g mol,1 was obtained at 100 C in 210 min. Microwave heating accelerated the ROP of PDO catalyzed by AlEt3 or tin powder, compared with the conventional heating method. 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3207,3213, 2008 [source]

Novel cyclohexyl-substituted salicylaldiminato,nickel(II) complex as a catalyst for ethylene homopolymerization and copolymerization

Junquan Sun
Abstract The cyclohexyl-substituted salicylaldiminato,Ni(II) complex [O(3-C6H11)(5-CH3)C6H2CHN-2,6-C6H3iPr2]Ni(PPh3)(Ph) (4) has been synthesized and characterized with 1H NMR and X-ray structure analysis. In the presence of phosphine scavengers such as bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)2], triisobutylaluminum (TIBA), and triethylaluminum (TEA), 4 is an active catalyst for ethylene polymerization and copolymerization with the polar monomers tert -butyl-10-undecenoate, methyl-10-undecenoate, and 4-penten-1-ol under mild conditions. The polymerization parameters affecting the catalytic activity and viscosity-average molecular weight of polyethylene, such as the temperature, time, ethylene pressure, and catalyst concentration, are discussed. A polymerization activity of 3.62 105 g of PE (mol of Ni h),1 and a weight-average molecular weight of polyethylene of 5.73 104 g.mol,1 have been found for 10 ,mol of 4 and a Ni(COD)2/4 ratio of 3 in a 30-mL toluene solution at 45 C and 12 105 Pa of ethylene for 20 min. The polydispersity index of the resulting polyethylene is about 2.04. After the addition of tetrahydrofuran and Et2O to the reaction system, 4 exhibits still high activity for ethylene polymerization. Methyl-10-undecenoate (0.65 mol %), 0.74 mol % tert -butyl-10-undecenoate, and 0.98 mol % 4-penten-1-ol have been incorporated into the polymer. 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6071,6080, 2004 [source]

Triisobutylaluminum as cocatalyst for zirconocenes.


Abstract An investigation of the catalytic behavior of the dimethylated zirconocenes Me2SiCp*NtBuZrMe2 [Cp* = C5(CH3)4; 1Me], Me2SiCp2ZrMe2 (2Me), Cp2ZrMe2 (3Me), Ind2ZrMe2 (4Me), Me2SiInd2ZrMe2 (5Me), Et(2-MeInd)2ZrMe2 (6Me), and Me2Si(2-MeInd)2ZrMe2 (7Me) with the combined activator triisobutylaluminum (TIBA)/CPh3B(C6F5)4 (Al/Zr = 250; B/Zr = 1) in ethylene polymerizations at increased monomer pressures (5,11 bar, 30 C) was carried out. Sterically opened zirconocenes in ternary catalysts gave rise to active species effective in the formation of low molecular weight polyethylenes (PEs). These active species tended to increase the PE molecular weight [1Me (2100) < 2Me (20,000) < 5Me (89,000) < 3Me (94,500)] under similar conditions. PE obtained with 4Me showed a bimodal gel permeation chromatography curve with a 64% peak area [weight-average molecular weight (Mw) = 43,000] and a 36% peak area (Mw = 255,000). The increase in sterical demands from the zirconocenes was also demonstrated by the reduction of the chain transfer to monomer, the reinsertion of vinyl-ended PE chains, and their ability for isomerization. These reactions were most pronounced for the zirconocenes 1Me and 2Me. The active species responsible for the formation of low molecular weight PEs deactivated quickly. The zirconocenes 6Me, 7Me, and (2-PhInd)2ZrMe2 (8Me) bearing substituent at the 2-position of the indenyl ring was activated with TIBA alone, yielding active species effective in ethylene and propylene polymerizations. PEs formed with 6Me,8Me complexes activated with TIBA had high molecular weights. An increase in the Al/Zr ratio in the catalytic system 8Me/TIBA from 50 to 300 led to an enhancement of the molecular weight of polypropylene (PP) samples from oligomeric products to an viscosity-average molecular weight of 220,000. The increase in the molecular weights of PPs with an increase in the propylene concentration was also observed. An analysis of the catalytic performance of the 8Me/TIBA system showed first-order dependency of the initial polymerization rates on the TIBA concentration and close to second-order dependency on propylene. The second-order dependency on the monomer concentration is explained in terms of the monomer participation in the initiation step of the polymerization reaction. 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1915,1930, 2001 [source]

Poly(vinyl acetate)/Silver Nanocomposite Microspheres Prepared by Suspension Polymerization at Low Temperature

Jeong Hyun Yeum
Abstract Summary: High molecular weight (HMW) poly(vinyl acetate)/silver nanocomposite microspheres (PVAc/Ag), which are promising precursors of embolic materials with radiopacity, were prepared via a suspension polymerization approach in the presence of silver nanoparticles. It was found that a high yield and high molecular weight PVAc/Ag could be concurrently obtained even using a low-temperature initiator 2,2,-azobis(2,4-dimethylvaleronitrile) (,30,C). In the case of presence of silver nanoparticles, the rate of polymerization was slightly slower than that without Ag. The suspension polymerization approach introduced could produce PVAc/Ag composite with conversion and viscosity-average molecular weight () up to 95% and 1,300,000, respectively, in spite of the low polymerization temperature (,30,C), in sharp contrast with an only ,30% conversion of VAc under bulk polymerization. Morphology studies revealed that except normal suspension microspheres with a smooth surface, a golf ball-like appearance of the microspheres was observed, due to the migration and aggregating of the hydrophilic Ag nanoparticles at the sublayer beneath the microsphere's surface. [source]

Functionalization of high density polyethylene with maleic anhydride in the melt state through ultrasonic initiation

Yuncan Zhang
Grafting of maleic anhydride (MAH) onto high density polyethylene (HDPE) performed in the melt state through ultrasonic initiation by a laboratory-scale ultrasonic extrusion reactor was studied in this paper. The effect of sonic intesity on the amount of grafted MAH, viscosity-average molecular weight and melt flow index of the grafted product was investigated. The results show that the ultrasonic waves can obviously decrease the molecular weight of the grafted product and cause the increase of the amount of grated MAH, implying that the grafting reaction consists of the chain scission and the grafting reaction of the produced macroradicals with MAH. The percentage of grafting of the product amounts to 0.6%; its melt-flow index is between 0.5 and 2.0 g/10 min, depending upon ultrasonic intensity, MAH content and grafting temperature. Compared with the method of peroxide initiation, in this method the crosslinking reaction can be prevented easily through the allocation of ultrasonic intensity. The mechanical properties of the improved HDPE/GF composite produced by ultrasonic initiatives are higher than in those produced by peroxide initiatives. [source]

Synthesis of Ultra-high Molecular Weight Polystyrene with a Catalyst System Based on Calixarene-yttrium Complex

Chen Yao-Feng
Abstract Polymerization of styrene (St) with a new catalyst system composed of calixarene-yttrium complex, magnesium-aluminium alkyls and hexamethyl phosphoramide was studied. Hie catalyst system shows extremely high activity (> 7 106 g PSt/mol Y h) and gives polystyrene with very high viscosity-average molecular weight (>5 105). [source]