- The effect of viscosity on the coupling and hydrogen-abstraction reaction between transient and persistent radicals
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The effect of viscosity on the radical termination reaction between a transient radical and a persistent radical undergoing a coupling reaction (Coup) or hydrogen abstraction (Abst) was examined. In a non-viscous solvent, such as benzene (bulk viscosity bulk 99% Coup/Abst selectivity, but Coup/Abst decreased as the viscosity increased (89/11 in PEG400 at 25 °C [bulk = 84 mPa s]). While bulk viscosity is a good parameter to predict the Coup/Abst selectivity in each solvent, microviscosity is the more general parameter. Poly(methyl methacrylate) (PMMA)-end radicals had a more significant viscosity effect than polystyrene (PSt)-end radicals, and the Coup/Abst ratio of the former dropped to 50/50 in highly viscous media (bulk = 3980 mPa s), while the latter maintained high Coup/ Abst selectivity (84/16). These results, together with the low thermal stability of dormant PMMA-TEMPO species compared with that of PSt-TEMPO species, are attributed to the limitation of the nitroxide-mediated radical polymerization of MMA. While both organotellurium and bromine compounds were used as precursors of radicals, the former was superior to the latter for the clean generation of radical species.
- Li, Xiaopei,Kato, Tatsuhisa,Nakamura, Yasuyuki,Yamago, Shigeru
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p. 966 - 972
(2021/04/29)
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- Acrylate contact adhesive materials having tight molecular weight distribution
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The invention relates to an initiator system for radical polymerizations, which is characterized in that a combination of compounds of general formulas (I) and (II) is used, wherein: R′, R″, R′″ and R″″, are selected independently of one another, and represent; a) branched and unbranched C1 to C18 alkyl radicals, C3 to C18 alkenyl radicals, C3 to C18 alkynyl radical; b) C1 to C18 alkyl radicals, C3 to C18 alkenyl radicals, C3 to C18 alkynyl radicals, which are all substituted for at least one OH group or for a halogen atom or for a silyl ether; c) C2-C18 hetero-alkyl radicals having at least one O atom and/or an NR group in the carbon chain, whereby R is selected from one of groups a), b) or d) to g); d) C1-C18 alkyl radicals, C3-C18 alkenyl radicals, C3-C18 alkynyl radical, which are all substituted for at least one ester group, amine group, carbonate group and/or epoxy group and/or for sulfur and/or for sulfur compounds, especially thioether or dithio compounds; e) C3-C12 cycloalkyl radicals; f) C6-C10 aryl radical, and; g) hydrogen. X represents a group having at least one carbon atom and is such that free radicals X. derived from X can initiate a polymerization of ethylenically unsaturated monomers.
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- Development of a universal alkoxyamine for "living" free radical polymerizations
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Examination of novel alkoxyamines has demonstrated the pivotal role that the nitroxide plays in mediating the "living" or controlled polymerization of a wide range of vinyl monomers. Surveying a variety of different alkoxyamine structures led to α-hydrido derivatives based on a 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxy, 1, skeleton which were able to control the polymerization of styrene, acrylate, acrylamide, and acrylonitrile based monomers. For each monomer set, the molecular weight could be controlled from 1000 to 200 000 amu with polydispersities typically 1.05-1.15. Block and random copolymers based on combinations of the above monomers could also be prepared with similar control. In comparison with 2,2,6,6-tetramethylpiperidinoxy (TEMPO), these new systems represent a dramatic increase in the range of monomers that can be polymerized under controlled conditions and overcome many of the limitations associated with nitroxide-mediated "living" free radical procedures. Monomer selection and functional group compatibility now approach those of ATRP-based systems.
- Benoit, Didier,Chaplinski, Vladimir,Braslau, Rebecca,Hawker, Craig J.
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p. 3904 - 3920
(2007/10/03)
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