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Polymer Radicals (polymer + radical)
Selected AbstractsLiving Radical Polymerization of Acrylates Mediated by 1,3-Bis(2-pyridylimino)isoindolatocobalt(II) Complexes: Monitoring the Chain Growth at the MetalCHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2008Björn Abstract A new type of mediator for cobalt(II)-mediated radical polymerization is reported which is based on 1,3-bis(2-pyridylimino)isoindolate (bpi) as ancillary ligand. The modular synthesis of the bis(pyridylimino)isoindoles (bpiH) employed in this work is based on the condensation of 2-aminopyridines with phthalodinitriles. Reaction of the bpiH protio-ligands with a twofold excess of cobalt(II) acetate or cobalt(II) acetylacetonate in methanol gave [Co(bpi)(OAc)], which crystallize as coordination polymers, and a series of [Co(acac)(bpi)(MeOH)], which are mononuclear octahedral complexes. Upon heating the [Co(acac)(bpi)(MeOH)] compounds to 100,°C under high vacuum, the coordinated methanol was removed to give the five-coordinate complexes [Co(acac)(bpi)]. The polymerization of methyl acrylate at 60,°C was investigated by using one molar equivalent of the relatively short-lived radical source 2,2,-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70) as initiator (monomer/catalyst/V-70: 600:1:1). The low solubility of the acetato complexes inhibits their significant activity as mediators in this reaction, whereas the acetylacetonate complexes control the radical polymerization of methyl acrylate more effectively. The radical polymerizations of the hexacoordinate complexes did not show a linear increase in number-average molecular weight (Mn) with conversion; however, the polydispersities were relatively low (PDI=1.12,1.40). By using the pentacoordinate complexes [Co(acac)(bpi)] as mediators, a linear increase in Mn values with conversion, which were very close to the theoretical values for living systems, and very low polydispersities (PDI<1.13) were obtained. This was also achieved in the block copolymerization of methyl acrylate and n -butyl acrylate. The intermediates with the growing acrylate polymer radical (.PA) were identified by liquid injection field desorption/ionization mass spectrometry as following the general formula [Co(acac)(4-methoxy-bpi)-(MA)n -R] (MA: methyl acrylate; R: C(CH3)(CH2C(CH3)2OCH3)CN), a notion also confirmed by NMR end-group analysis. [source] Grafting of polymers onto and/or from silica surface during the polymerization of vinyl monomers in the presence of ,-ray-irradiated silicaJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 9 2006Akiko Tsuchida Abstract The effects of radicals on silica surface, which were formed by ,-ray irradiation, on the polymerization of vinyl monomers were investigated. It was found that the polymerization of styrene was remarkably retarded in the presence of ,-ray-irradiated silica above 60 °C, at which thermal polymerization of styrene is readily initiated. During the polymerization, a part of polystyrene formed was grafted onto the silica surface but percentage of grafting was very small. On the other hand, no retardation of the polymerization of styrene was observed in the presence of ,-ray-irradiated silica below 50 °C; the polymerization tends to accelerate and polystyrene was grafted onto the silica surface. Poly(vinyl acetate) and poly(methyl methacrylate) (MMA) were also grafted onto the surface during the polymerization in the presence of ,-ray-irradiated silica. The grafting of polymers onto the silica surface was confirmed by thermal decomposition GC-MS. It was considered that at lower temperature, the grafting based on the propagation of polystyrene from surface radical ("grafting from" mechanism) preferentially proceeded. On the contrary, at higher temperature, the coupling reaction of propagating polymer radicals with surface radicals ("grafting onto" mechanism) proceeded to give relatively higher molecular weight polymer-grafted silica. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2972,2979, 2006 [source] Construction of a polymer skeleton that is cut in half by ionizing radiationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 5 2005Takashi Shimizu Abstract Polystyrene with a benzyl ester of carboxylic acid at the center of a polymer skeleton was synthesized by living radical polymerization. The initiator used had two functional groups for 2,2,6,6-tetramethylpiperidinoxyl (TEMPO)-mediated living radical polymerization on the benzyl and the carboxylic sides of the benzyl ester. Introduction of the benzyl ester changed the polystyrene from a crosslink type to a scission type polymer on ,-irradiation. Irradiation of the polymer resulted in a binary change of the molecular weight because of the dissociative capture of secondary electrons by the benzyl ester, as: The binary change of the molecular weight suggests that the polymer can be used as a new type of radiation resist with high sensitivity and spatial resolution to ionizing and high resistivity to plasma etching. The number of scissions per 100 eV radiation energy absorbed was 0.29, which was about one fourth of the yield of secondary electrons. The low efficiency was because of the recombination of polymer radicals generated by the dissociative electron attachment. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1068,1075, 2005 [source] UV stabilising synergies between carbon black and hindered light stabilisers in linear low density polyethylene filmsMACROMOLECULAR SYMPOSIA, Issue 1 2003A. Richard Horrocks Abstract The combined effects of selected carbon black pigments and hindered light stabilisers (HALS) on the UV stabilities of linear low density polyethylene film have been studied under UVA and UVB fluorescent radiation sources. While the presence of HALS do not change the chemistry of film photodegradation, whether they are low or high molecular variants, their presence significantly extends film lifetime relative to the sum of the effects of carbon black and HALS individually. These lifetime extensions may be defined in terms of a synergy factor defined with respect to film time to lose a specific percentage of a tensile property, namely t20, the time to lose 20% of initial elongation-at-break, or the carbonyl index associated with this condition. It is proposed that possible causes of this synergy are a result of the UV screening effect of the carbon black particles which provide lower concentrations of polymer radicals for the HALS component to interact with and/or an accompanying thermal stabilising effect by the latter as a consequence of the higher polymer local temperature during irradiation of pigmented films. [source] Homolysis of N -alkoxyamines: A Computational StudyCHEMPHYSCHEM, Issue 2 2006Anouk Gaudel-Siri Dr. Abstract During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO., the reversible formation of N -alkoxyamines [PON(R1)R2] reduces significantly the concentration of polymer radicals (P.) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the CON(R1)R2bond. In this study, theoretical BDEs of a large series of model N -alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (Tc,), and the dissociation rate constant (kd,), of the N -alkoxyamines are established. The homolytic cleavage of the NOC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N -alkoxyamines with a OCON(R1)R2fragment, and the strengthening of their CON(R1)R2bond is interpreted in terms of stabilizing anomeric interactions. [source] | |||||||||||||