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Radical Termination (radical + 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] Ultraviolet curing of acrylic systems: Real-time Fourier transform infrared, mechanical, and fluorescence studiesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2002Carmen Peinado Abstract The photopolymerization of acrylic-based adhesives has been studied by Fourier transform infrared and fluorescence analysis in real time. Real-time infrared spectroscopy reveals the influence of the nature of the photoinitiator on the kinetics of the reaction. Furthermore, the incident light intensity dependence of the polymerization rate shows that primary radical termination is the predominant mechanism during the initial stages of the curing of the acrylic system with bis(2,4,6-trimethylbenzoyl) phenyl phosphine oxide (TMBAPO) as a photoinitiator. The fluorescence intensity of selected probes increases during the ultraviolet curing of the adhesive, sensing microenvironmental viscosity changes. Depending on the nature of the photoinitiator, different fluorescence,conversion curves are observed. For TMBAPO, the fluorescence increases more slowly during the initial stage because of the delay in the gel effect induced by primary radical termination. Mechanical tests have been carried out to determine the shear modulus over the course of the acrylic adhesive ultraviolet curing. In an attempt to extend the applications of the fluorescence probe method, we have undertaken comparisons between the fluorescence changes and shear modulus. Similar features in both curves confirm the feasibility of the fluorescence method for providing information about microstructural changes during network formation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4236,4244, 2002 [source] Kinetic Study of the Thermopolymerization of Furfuryl Methacrylate in Bulk by Mathematical Modeling.MACROMOLECULAR THEORY AND SIMULATIONS, Issue 9 2009Part A: Simulation of Experimental Data, Sensitivity Analysis of Kinetic Parameters Abstract Mathematical modeling of the thermopolymerization of FM and CMFMA was carried out using a cross-linked kinetic model proposed for the photo-initiated polymerization of acryl-furanic compounds. In this model, the photochemical initiation step was substituted by a thermal one and it was assumed that the constant of radical termination was time-dependent, which allowed the gel effect (Trommsdorff) at high monomer conversion to be simulated. Optimization of all kinetic constants was achieved and the results of simulation suitably fitted the experimental data of the monomer conversion. The contribution of each step in the mechanism and its dependence on the experimental conditions were estimated by a sensitivity analysis technique. [source] Benzoyl peroxide,p -acetylbenzylidenetriphenyl arsoniumylide initiated copolymerization of citronellol and styrenePOLYMER INTERNATIONAL, Issue 8 2001K Srivastava Abstract Alternating copolymers, containing styrene and citronellol sequences, have been synthesized by radical polymerization using benzoylperoxide (BPO),p -acetylbenzylidenetriphenyl arsoniumylide (pABTAY) as initiator, in xylene at 80,±,1,°C for 3,h under inert atmosphere. The kinetic expression is Rp , [BPO]0.88 [citronellol]0.68 [styrene]0.56 with BPO and Rp , [pABTAY]0.27 [citronellol]0.76 [styrene]0.63 with pABTAY, ie the system follows non-ideal kinetics in both cases, because of primary radical termination and degradative chain transfer reactions. The activation energy with BPO and pABTAY is 94,kJ,mol,1 and 134,kJ,mol,1, respectively. Different spectral techniques, such as IR, FTIR, 1H NMR and 13C NMR, have been used to characterize the copolymer, demonstrating the presence of alcoholic and phenyl groups of citronellol and styrene. The alternating nature of the copolymer is shown by the product of reactivity ratios r1 (Sty),=,0.81 and r2 (Citro),=,0.015 using BPO and r1 (Sty),=,0.37 and r2 (Citro),=,0.01 using (pABTAY), which are calculated by the Finemann,Ross method. A mechanism of copolymerization is proposed. © 2001 Society of Chemical Industry [source] | ||||||||||