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Bimolecular Termination (bimolecular + termination)
Selected AbstractsBimolecular radical termination: New perspectives and insightsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2008Geoffrey Johnston-Hall Abstract The reversible addition-fragmentation chain transfer-chain length dependent termination (RAFT-CLD-T) method has allowed us to answer a number of fundamental questions regarding the mechanism of diffusion-controlled bimolecular termination in free-radical polymerization (FRP). We carried out RAFT-mediated polymerizations of methyl acrylate (MA) in the presence of a star matrix to develop an understanding of the effect of polymer matrix architecture on the termination of linear polyMA radicals and compared this to polystyrene, polymethyl methacrylate, and polyvinyl acetate systems. It was found that the matrix architecture had little or no influence on termination in the dilute regime. However, due to the smaller hydrodynamic volumes of the stars in solution compared to linear polymer of the same molecular weight, the gel onset point occurred at greater conversions, and supported the postulate that chain overlap (or c*) is the main cause for the observed autoacceleration observed in FRP. Other theories based on "short,long" termination or free-volume should be disregarded. Additionally, since our systems are well below the entanglement molecular weight, entanglements should also be disregarded as the cause of the gel onset. The semidilute regime occurs over a small conversion range and is difficult to quantify. However, we obtain accurate dependencies for termination in the concentrated regime, and observed that the star polymers (through the tethering of the arms) provided constriction points in the matrix that significantly slow the diffusion of linear polymeric radicals. Although, this could at first sight be postulated to be due to reptation, the dependencies showed that reptation could be considered only at very high conversions (close to the glass transition regime). In general, we find from our data that the polymer matrix is much more mobile than what is expected if reptation were to dominate. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3155,3173, 2008 [source] Outer-sphere electron transfer metal-catalyzed polymerization of styrene using a macrobicyclic ligandJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 1 2008Craig A. Bell Abstract The CuBr-catalyzed polymerizations of styrene in the presence of a macrobicyclic mixed donor (N and S) encapsulating ligand, NH2capten, were carried out in toluene at 60 and 100 °C. The macrobicyclic nature of the ligand ensures that a transition metal ion is effectively encapsulated (caged) within the three-dimensional cavity, resulting in activation of radicals through an outer-sphere electron transfer mechanism. The kinetic data showed that the polymerizations were uncontrolled with little "living" behavior. The external orders of reaction in [CuBr], [NH2capten], and [CuBr2] were 0.5, 0.5, and close to zero, respectively, in agreement with the postulated mechanism of little or no deactivation of polymeric radicals and a significant amount of bimolecular termination. Although "living" behavior was not found using the cage ligand, it was decided that it would provide an ideal method for radical coupling experiments to make high-molecular weight multiblock copolymers from a difunctional PSTY (Br-PSTY-Br, PDI = 1.11). The coupling reaction of Br-PSTY-Br using CuBr/NH2capten and excess Cu(0) in toluene at 100 °C gave no loss of the starting Br-PSTY-Br. Changing the solvent to the aprotic DMSO resulted in a significant increase in the rate of consumption of starting Br-PSTY-Br, with over 87% being used in under 10 min at 60 °C. In addition, higher molecular weight species were also formed, suggesting that OSET gives little or no side reactions on this time scale. It was initially thought that to get such high rates of reaction that the SET-LRP disproportionation mechanism (2Cu(I) , Cu(0) + Cu(II)) was at play. However, UV,Vis experiments of the CuBr/NH2capten showed little or no disproportionation products. This important result suggests that DMSO catalyzes the OSET reaction through the stabilization of the radical-anion intermediate, which then rapidly fragments to a polymeric radical. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 146,154, 2008 [source] Styrylpyridinium borate salts as dye photoinitiators of free-radical polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2002Beata J, drzejewska Abstract Styrylpyridinium borate salts photoinitiate free-radical polymerization. The rate of photopolymerization depends on the ,Go of electron transfer between a borate anion and a styrypyridinium cation. This latter value was estimated for a series of styrylpyridinium borate salts, and the relationship between the rate of polymerization and the free energy of activation gives the dependence predicted by the classical theory of electron transfer. This relation was independently observed for the two series of styrylpyridinium borate salts tested,one for the photoredox pair with an iodine atom and the second without. Styrylpyridinium borate salts were stable at ambient temperature in the formulations prepared for the photopolymerization experiments. Photopolymerization initiated by the photoredox pairs tested proceeded by the conventional mechanism in which bimolecular termination occurs by a reaction between two macroradicals. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1433,1440, 2002 [source] Compartmentalization in Atom Transfer Radical Polymerization (ATRP) in Dispersed Systems,MACROMOLECULAR THEORY AND SIMULATIONS, Issue 8 2006Yasuyuki Kagawa Abstract Summary: Compartmentalization in atom transfer radical polymerization (ATRP) in dispersed systems at low conversion (<10%) has been investigated by means of a modified Smith,Ewart equation focusing on the system n -butyl acrylate/CuBr/4,4,-dinonyl-2,2,-dipyridyl at 110,°C. Compartmentalization of both propagating radicals and deactivator was accounted for in the simulations. As the particle diameter (d) decreases below 70 nm, the polymerization rate (Rp) at 10% conversion increases relative to the corresponding bulk system, goes through a maximum at 60 nm, and thereafter decreases dramatically as d decreases further. This behavior is caused by the separate effects of compartmentalization (segregation and confined space effects) on bimolecular termination and deactivation. The very low Rp for small particles (d,<,30 nm) is due to the pseudo first-order deactivation rate coefficient being proportional to d,3. Simulated propagating radical concentration ([P,]) as a function of particle diameter (d) at 10% conversion for ATRP of n -butyl acrylate ([nBA]0,=,7.1 M, [PBr]0,=,[CuBr/dNbpy]0,=,35.5 mM) in a dispersed system at 110,°C. The dotted line indicates the simulated [P,] in bulk at 10% conversion. [source] | ||||||||||