No abstract

Since the beginning of polymer chemistry, the mechanism and kinetics of the reactions that initiate the spontaneous polymerization of styrene have been investigated intensively. 1 The major steps are displayed in Scheme 1, and they are well established by product analyses, kinetic evidence, and direct observations of intermediates. 1 As first formulated by Mayo, 2 a dominant Diels-Alder reaction between two styrene molecules leads to the semi-benzene dimer 1 with the rate constant k 1 . It is spectroscopically observable, and there are actually two distinguishable isomers that differ in configuration at the chiral centers. 3 They are formed rather similarly but decay at quite different rates. 3 In pure monomer, the dimers presumably undergo the retro-Diels-Alder reaction (k Ϫ1 ), and they react with styrene into the radicals 2 and 3. 4 These radicals start the styrene propagation, couple to trimers, 5 and disproportionate to the tetraline derivative 4 and styrene. By acid-catalyzed reactions, 4 is also formed directly from 1. 6,7 In addition, the diphenyl cyclobutanes 6 result from a concurring minor reaction pathway involving the 1,4-diradical 5 that, in principle, could also lead to 1.Nitroxide radicals generally inhibit but also mediate living styrene polymerizations. 8 In the latter case, the styrene self-initiation affects the conversion rates and the control over the molecular weight and polydispersity of the resulting polymer. 9 Therefore, the reactions of Scheme 1 have also found attention in styrene polymerizations mediated by nitroxides.Moad et al. 10 reported that nitroxide radicals such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO; T⅐) disappear in styrene much faster at 100°C than expected from the thermal polymerization rate in the absence of the radical scavenger. For TEMPO, a quantitative product analysis led to the formulation of the reactions shown in Scheme 2.T⅐ abstracts the weakly bonded hydrogen atom from 1 to yield the radical 2 and the hydroxylamine 7. The coupling of 2 with a second TEMPO molecule provides 8. This mechanism agrees with other known abstractions of hydrogen atoms by TEMPO,11,12 and it is supported by the observation of practically equal amounts of TEMPO incorporated into 7 (44%) and 8 (45%). However, these equal yields can also be explained by the addition of TEMPO to the exo-methylene group of 1, which is followed by hydrogen abstraction from the resulting cyclohexadienyl-type radical. The addition of TEMPO to 1 is not unlikely because the bis-TEMPO adduct 10 to styrene was observed by Moad et al. 10 in a minor yield of 10%, and its formation was confirmed later on by other authors. 12,13 However, at this stage the two possible routes to 7 and 8 cannot be distinguished. Other products containing TEMPO groups were not detected. The biphenyl cyclobutanes 6 (Scheme 1) accounted for about 7% of the total mixture of products. 10 The 1,4-diradical 5 must have a very short lifetime because it was not trapped by TEMPO, although the nitroxide concentration was 0.05 M, and TEMPO...

EPR and kinetic absorption spectroscopy experiments reveal the simultaneous formation of transient radicals and a persistent reduced tungsten radical species during the photoreaction of W 10 O 32 4؊ with organic substrates in solution. It is shown that a special kinetic effect operates which causes the observed selective product formation. Theoretical predictions on the ratio of radical concentrations and on their time developments are confirmed. Moreover, some mechanistic aspects of the reactions are clarified. Photochemical reactions of polyoxometalate anions, such as the decatungstate W 10 O 32 4Ϫ, in liquid solution can lead to useful transformations of organic substrates, e.g., to the catalytic ethylation, vinylation, carbonylation and hydroperoxidation of alkanes. 1-4 Many details of the mechanism are still unexplored. However, for tetrakis(tetra-n-butylammonium)decatungstate in non-acidified acetonitrile the basic reactions are established and, in part, also supported by quantum chemical calculations. 5 After UV excitation in the longest wavelength band of W 10 O 32 4Ϫ (290-400 nm) a reactive excited state W 10 O 32 4ϪPaper 7/08191J

The coordination chemistry of the phosphine-tethered diamidophosphine ligands PhP(CH2CH2CH2NHPh)2 (pr[NPN]H2) and PhP(1,2-CH2-C6H4-NHSiMe3)2 (bn[NPN]H2) featuring six-membered N–C3–P chelates was explored with group 4 metals, which allowed for the consecutive development of a new trimethylene-methane-tethered [PN2] scaffold. In the case of the propylene-linked system pr[NPN]H2, access to the sparingly soluble dibenzyl derivative pr[NPN]ZrBn2 (3-Zr) was gained, while thermally sensitive zirconium and hafnium diiodo complexes bn[NPN]MI2 (5-M, M = Zr, Hf) were isolated in the case of the benzylene-linked derivative bn[NPN]H2. Despite the related phosphine-tethered backbone architectures of both of these ligands, their group 4 complexes were found to exhibit either C1-symmetric (bn[NPN]MX2) or averaged CS-symmetric (pr[NPN]MX2) structures in solution. To restrain the overall flexibility of these systems and thereby control the properties of the resulting complexes without disrupting the six-membered chelates, the new trimethylene-methane-tethered N,N′-di-(tert-butyl)-substituted [PN2]H2 protioligand was designed. This tripodal ligand system was prepared on a gram scale and its CS-symmetric dichloro complexes [PN2]MCl2 (6-M, M = Ti, Zr, Hf) were isolated subsequently. The benzene-soluble dibenzyl derivative [PN2]ZrBn2 (7-Zr) was synthesised as well and characterised by X-ray diffraction. These results are discussed not only in conjunction with the known [NPN]-coordinated group 4 complexes incorporating five-membered chelates, but also in the context of “molecular claws” that are related to the new [PN2] tripod.