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Molecular Weight Control (molecular + weight_control)
Selected AbstractsEfficient Molecular Weight Control with Trialkylaluminum in Ethylene/Norbornene Copolymerization by [Ph2C(Flu)(3-MeCp)]ZrCl2/Methylaluminoxane CatalystMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 19 2010Tomoyuki Tada Abstract Ethylene/norbornene copolymerizations were conducted with [Ph2C(Flu)(3-RCp)]ZrCl2 [R: Me (1) or Me3Si (2)], which give alternating EN copolymers. The activity of 1 in the absence of R3Al was approximately twice that of 2 and increased further upon addition of R3Al. Et3Al increased the activity most effectively, and the of the produced polymer decreased from 100 000 to 25 000,g,·,mol,1. On the other hand, the value increased upon addition of iBu3Al from 100 000 to 209 000,g,·,mol,1 accompanied by an ,2.5-fold increase of activity. Consequently, 1 was found to be one of the most promising complexes for the synthesis of alternating EN copolymers, of which value was efficiently controlled by the kind and the amount of R3Al added. [source] Kinetic Simulations of Reversible Chain Transfer Catalyzed Polymerization (RTCP): Guidelines to Optimum Molecular Weight ControlMACROMOLECULAR THEORY AND SIMULATIONS, Issue 1 2010Philipp Vana Abstract Kinetic simulations of reversible chain transfer catalyzed polymerization (RTCP) were performed using the program package Predici. Mimicking the RTCP of styrene in bulk at 80,°C, the full molecular weight distributions, the polydispersities of resulting polymer and the time evolutions of monomer conversion and participating species were simulated. The influence of the kinetic coefficients governing the RTCP equilibrium , specifically, the rate coefficients of activation, ka, and deactivation, kda , on the controlled polymerization behavior was probed in detail by varying their respective simulation input values over five orders of magnitude. It was found that optimum results for molecular weight control are obtained for K,=,ka/kda in the range 1 to 10 and with ka and kda being of the order of 106 L,·,mol,1,·,s,1 or above. The influence of degenerative chain transfer on the process was found to be significant only in poorly controlled systems, but is small in well-controlled RTCP. Based on the finding that the catalyst is depleting during the polymerization due to cross-termination, guidelines for obtaining high molecular weight material via repeated addition of catalyst were developed. [source] The kinetics of enhanced spin capturing polymerization: Influence of the nitrone structureJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2009Edgar H. H. Wong Abstract Several nitrones and one nitroso compound have been evaluated for their ability to control the molecular weight of polystyrene via the recently introduced radical polymerization method of enhanced spin capturing polymerization (ESCP). In this technique, molecular weight control is achieved (at ambient or slightly elevated temperatures) via the reaction of a growing radical chain with a nitrone forming a macronitroxide. These nitroxides subsequently react rapidly and irreversibly with propagating macroradicals forming polymer of a certain chain length, which depends on the nitrone concentration in the system. Via evaluation of the resulting number-average molecular weight, Mn, at low conversions, the addition rate coefficient of the growing radicals onto the different nitrones is determined and activation energies are obtained. For the nitrones N - tert -butyl-,-phenylnitrone (PBN), N -methyl-,-phenylnitrone (PMN), and N -methyl-,-(4-bromo-phenyl) nitrone (pB-PMN), addition rate coefficients, kad,macro, in a similar magnitude to the styrene propagation rate coefficient, kp, are found with spin capturing constants CSC (with CSC = kad,macro/kp) ranging from 1 to 13 depending on the nitrone and on temperature. Activation energies between 23.6 and 27.7 kJ mol,1 were deduced for kad,macro, congruent with a decreasing CSC with increasing temperature. Almost constant Mn over up to high monomer to polymer conversions is found when CSC is close to unity, while increasing molecular weights can be observed when the CSC is large. From temperatures of 100 °C onward, reversible cleavage of the alkoxyamine group can occur, superimposing a reversible activation/deactivation mechanism onto the ESCP system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1098,1107, 2009 [source] Iron halide mediated atom transfer radical polymerization of methyl methacrylate with N -alkyl-2-pyridylmethanimine as the ligandJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 19 2004Huiqi Zhang Abstract The controlled atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) catalyzed by iron halide/N -(n -hexyl)-2-pyridylmethanimine (NHPMI) is described. The ethyl 2-bromoisobutyrate (EBIB)-initiated ATRP with [MMA]0/[EBIB]0/[iron halide]0/[NHPMI]0 = 150/1/1/2 was better controlled in 2-butanone than in p -xylene at 90 °C. Initially added iron(III) halide improved the controllability of the reactions in terms of molecular weight control. The p -toluenesulfonyl chloride (TsC1)-initiated ATRP were uncontrolled with [MMA]0/[TsC1]0/[iron halide]0/[NHPMI]0 = 150/1/1/2 in 2-butanone at 90 °C. In contrast to the EBIB-initiated system, the initially added iron(III) halide greatly decreased the controllability of the TsC1-initiated ATRP. The ration of iron halide to NHPMI significantly influenced the controllability of both EBIB and TsC1-initiated ATRP systems. The ATRP with [MMA]0/[initiator]0/[iron halide]0/[NHPMI]0 = 150/1//1/2 provided polymers with PDIs , 1.57, whereas those with [iron halide]0/[NHPMI]0 = 1 resulted in polymers with PDIs as low as 1.35. Moreover, polymers with PDIs of approximately 1.25 were obtained after their precipitation from acidified methanol. The high functionality of the halide end group in the obtained polymer was confirmed by both 1H NMR and a chain-extenstion reaction. Cyclic voltammetry was utilized to explain the differing catalytic behaviors of the in situ -formed complexes by iron halide and NHPMI with different molar ratios. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4882,4894, 2004 [source] Rapid Synthesis and MALDI-ToF Characterization of Poly(ethylene oxide) Multiarm Star PolymersMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 1 2010Maria Doycheva Abstract Multiarm PEO star polymers with a purely aliphatic polyether structure have been synthesized using hyperbranched polyglycerol (PG) with different molecular weights as a multifunctional initiator. Different degrees of deprotonation of the initiator were studied with respect to molecular weight control. The results show that the degree of deprotonation is a crucial parameter for the synthesis of well-defined polymers with controlled molecular weights. Partial deprotonation of the PG hydroxyl groups (5,8%) was proven to represent an optimum for the synthesis of star polymers with molecular masses close to the theoretical values. Molecular weights of the stars ranged between 9,000 and 30,000,g,·,mol,1. MALDI-ToF spectra confirmed that the PEO arms in the star polymers possess homogeneous lengths. [source] Atom Transfer Radical Polymerization of Glycidyl Methacrylate: A Functional MonomerMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 16 2004Pedro Francisco Cañamero Abstract Summary: A detailed investigation of the polymerization of glycidyl methacrylate (GMA), an epoxy-functional monomer, by atom transfer radical polymerization (ATRP) was performed. Homopolymers were prepared at relatively low temperatures using ethyl 2-bromoisobutyrate (EBrIB) as the initiator and copper halide (CuX) with N,N,N,,N,,N,-pentamethyldiethylenetriamine (PMDETA) as the catalyst system. The high polymerization rate in the bulk did not permit polymerization control. However, homopolymerization in solution enabled us to explore the effects of different experimental parameters, such as temperature, solvent (toluene vs. diphenyl ether) and initiator concentration, on the controllability of the ATRP process. SEC analysis of the homopolymers synthesized confirmed the importance of solvent character on molecular weight control, the lowest polydispersity indices () and the highest efficiencies being found when the polymerizations were performed in diphenyl ether in combination with a mixed halide technique. A novel poly(glycidyl methacrylate)- block -poly(butyl acrylate) (PGMA- b -PBA) diblock copolymer was prepared through ATRP using PGMA-Cl as a macro-initiator. This chain growth experiment demonstrated a good living character under the conditions employed, while simultaneously indicating a facile synthetic route for this type of functional block copolymer. In addition, the isotacticity parameter for the PGMAs obtained was estimated using 1H NMR analysis which gave a value of ,GMA,=,0.26 in agreement with that estimated in conventional radical polymerization. SEC chromatograms of PGMA-Cl macroinitiator and PGMA- b -PBA diblock copolymer. [source] Kinetic Simulations of Reversible Chain Transfer Catalyzed Polymerization (RTCP): Guidelines to Optimum Molecular Weight ControlMACROMOLECULAR THEORY AND SIMULATIONS, Issue 1 2010Philipp Vana Abstract Kinetic simulations of reversible chain transfer catalyzed polymerization (RTCP) were performed using the program package Predici. Mimicking the RTCP of styrene in bulk at 80,°C, the full molecular weight distributions, the polydispersities of resulting polymer and the time evolutions of monomer conversion and participating species were simulated. The influence of the kinetic coefficients governing the RTCP equilibrium , specifically, the rate coefficients of activation, ka, and deactivation, kda , on the controlled polymerization behavior was probed in detail by varying their respective simulation input values over five orders of magnitude. It was found that optimum results for molecular weight control are obtained for K,=,ka/kda in the range 1 to 10 and with ka and kda being of the order of 106 L,·,mol,1,·,s,1 or above. The influence of degenerative chain transfer on the process was found to be significant only in poorly controlled systems, but is small in well-controlled RTCP. Based on the finding that the catalyst is depleting during the polymerization due to cross-termination, guidelines for obtaining high molecular weight material via repeated addition of catalyst were developed. [source] | ||||||||||