Nitroxide units

A QD-based sensor for carbon-centered radicals (alkyl or phenyl radicals) was developed by the use of 4-amino-2,2,6,6-tetramethylpiperidine oxide (4-amino-TEM-PO)-functionalized CdSe QDs. The presence of a nitroxide moiety in the functional capping layer leads to quenching of the luminescence of the QDs. The nitroxide units of the capping layer read with the carbon-centered radical to yield the diamagnetic alkoxyamine-modified protecting layer. As the resultant units are unable to quench the luminescence of the QDs, this leads to an intensified luminescence upon interadion with readive carbon-centered radicals. 

Alternatively (Chart 8.16) one can prepare the TEMPO-substituted monomer 44 from 42 and perform a Sonogashira coupling using 1,4-bis(ethylhexyl)-2,5-diethynylbenzene 14b to obtain the nitroxide-functionalized polymer 45. " Surprisingly, the presence of the nitroxide units does not at all inhibit the polymerization PPE 45 forms in almost quantitative yields with a of 29. Addition of an excess of styrene to 45 shows that around 12 styrene units can be incorporated per repeat, before the nitroxide group in 46 becomes inactive, even though a large excess of styrene was added to 45. 

Novel metal-free paramagnetic ILs based on imidazolium cations containing a chiral cyclic nitroxide unit, cf. structure 1 in Fig. 11, with various different anions have been prepared. ... 

Monomers not amenable to direct homopolymerization using a particular reagent can sometimes be copolymcrizcd. For example, NMP often fails with methacrylates (e.g. MMA, BMA), yet copolymerizalions of these monomers with S are possible even when the monomer mix is predominantly composed of the methacrylate monomer,15j This is attributed to the facility of cross propagation and the relatively low steady state concentration of propagating radicals with a terminal MMA (Section 7.4.3.1). MMA can also be copolymerized with S or acrylates at low temperature (60 C).111 Under these conditions, only deactivation of propagating radicals with a terminal MMA unit is reversible, deactivation of chains with a terminal S or acrylate unit is irreversible. Molecular weights should then be controlled by the reactivity ratios and the comonomer concentration rather than by the nitroxide/alkoxyamine concentration. 

Concerning molecule-based magnets, the first spin-ladder was synthesized (/ -EPYNN)[Ni (dmit)2] (/ -EPYNN = / -7V-ethylpyridinium o-nitronyl nitroxide). Within the crystal lattice, the radical cation /7-EPYNN units are arranged in ID chains with ferromagnetic interactions. The chains of [Ni(dmit)2] moieties in the ladder formation exhibit coexistent antiferromagnetic interactions.1031,1032... 

Dynamic formation of graft polymers was synthesized by means of the radical crossover reaction of alkoxyamines by using the complementarity between nitroxide radical and styryl radical (Fig. 8.13) [40]. Copolymer 48 having alkoxyamine units on its side chain was synthesized via atom transfer radical polymerization (ATRP) of TEMPO-based alkoxyamine monomer 47 and MMA at 50°C (Scheme 8.9). The TEMPO-based alkoxyamine-terminated polystyrene 49 was prepared through the conventional nitroxide-mediated free radical polymerization (NMP) procedure [5,41], The mixture of copolymers 48 and 49 was heated in anisole... 

Scheme 12 Simplified through-space exchange mechanisms for nitroxide (aminoxyl) units.

By contrast a nitroxide attached to monomeric units rigidly held to a solid surface should show a spectral line shape similar to the bulk polymer in the glassy state (Figure IE). Thus a polymer molecule adsorbed at a solid-liquid interface should exhibit a composite spectrum, from which one can deduce the fraction of the monomeric units in loops and tails, and their mean motion. The sensitivity of the method to small amounts of units with motional freedom can be seen from Figure 2. 

To realize the above-mentioned systems, we carefully chose suitable switching units and radical moieties. As an initial attempt, we employed l,2-bis(2-methyl-l-benzothiophen-3-yl)perfluorocyclopentene (9a) as a photochromic spin coupler (Scheme 9.2). Compound 9a is one of the most fatigue-resistant diarylethenes [21]. Nitronyl nitroxide was chosen for the spin source, because this radical is jr-conjugative. Thus, we designed molecule 10a, which is an embodiment of the simplified model 8a [37, 62]. 

Polymerization can be started using an alkoxyamine as initiator such that, ideally, no reactions other than the reversible activation of dormant species and the addition of monomer to carbon-centered radicals take place. The alkoxyamine consists of a small radical species, capable of reacting with monomer, trapped by a nitroxide. Upon decomposition of the alkoxyamine in the presence of monomer, polymeric dormant species will form and grow in chain length over time. Otherwise, polymerization can be started using a conventional free-radical initiator and a nitroxide. The alkoxyamine will then be formed in situ when an initiator molecule decomposes, and, after adding a monomer unit or two, is trapped by a nitroxide. 

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