The relative stabilization of 10 4-monoand 24 4,4'-disubstituted triphenylmethyl radicals 1 has been measured by recording the degree of dissociation of the corresponding quinonoid dimers 2 by means of ESR. The following substituents or combinations of two of them have been used: H, CF3, r-Bu, OMe, OPh, CN, COPh, COMe, Ph, SMe, and N02. Both donors and acceptors enhance the stability in the ground state of the radicals, which is evaluated in terms of ' values and a Hammett-like equation. Two donors act additively, as do two acceptors. No specific synergism of a donor with an acceptor (capto-dative stabilization) has been found. Most efficient for the relative stabilization are the electroneutral substituents Ph and SMe. Most of the ESR spectra of these trityls are new. An exact assignment became possible via the corresponding ENDOR spectra, which are listed in detail. Many of the substituted trityls including dimers and precursors have been prepared

Die Ti telradikale (4-R -CJ&),C' (I), R = F, C1, Ph, OMe, NO2, tBu, OEt, SMe, CN, CFs, wurden dargestellt, davon die vier letzten erstmalig, und mittels ESR-Spektroskopie vermessen. a!, 4 und u; werden angegeben. Die ESR-Signale bleiben im gesamten zugangljchen Temperaturbereich -30 bis + 100°C in ihrer Intensitat unverandert. Daraus folgt, dal3 alte diese Radikale im genannten Temperaturbereich kinetisch stabil sind, also nicht wie andere Trityle zu Dimerisierungen, etwa o p , up-, a,a-, neigen.

Compatibility of PVC with liquid alkyltin alkylthioglycolate stabilizers was studied by thermal methods, including isothermal calorimetry of mixing, DSC, DMA, and dielectric relaxation. The enthalpy of mixing of PVC with the series of alkyltin alkyl thioglycolates was measured over the entire concentration range at ambient conditions. It was found that all tested compounds are compatible with PVC in a broad concentration range and form homogeneous mixtures. The results are discussed in terms of the superposition of glassy state and molecular interaction contributions to the enthalpy of mixing of the glassy polymer with liquid additives. The influence of organotin stabilizers on the glassy structure of PVC films was studied by means of DSC and the dielectric relaxation spectrum. The enhancement of the glassy structure of PVC by alkyltin alkyl thioglycolates is interpreted as the result of strong multi‐site molecular complexes between organotin molecules and PVC chains, which act as additional clips in the entanglement network of polymer chains.

Additions of the substituted diphenylmethyl radicals ArlAr'CR 2 (R = CMe3, SiMe3, GeMe3, SnMe3, OSiMe3, CF3, CO'Me, CN) to various acrylonitriles CHZ=C(X)CN 3 (X = SMe, SiPr, StBu, OAc, OSiMe,, OSiEt3, OMe, OEt) lead to 1,2-5 or 1,4-adducts 6 (ketenimines), depending mainly on the steric hindrance by the substituents R and X. Bulky substituents like tBu in 2 and tBuS in the acrylonitrile favour the formation of the extended and nearly strainless ketenimine system 6 (1,Cadduct); smaller substituents like OSiMe, (radical 2) and SMe, OAc, OSiMe3, OSiEt3, OMe, OEt (acrylonitrile) allow isolation of the sterically crowded 1,2-adducts. Substituents of intermediate bulkiness like CF3 (radical 2) and SiPr (acrylonitrile) give a mixture of 1,2-adducts (6cb, hb) and dimers ? of the adduct radicals 4 (?cb, hb). The voluminous tBu group directly bound to the olefin (3j, k) prevents addition.The latter is generally reversible, and the various adducts 5, 6, or ? dissociate to the adduct radicals 4 and/or fragment to the initial radicals 2 at temperatures which reflect the steric strains of the corresponding substituents R, X. The complete inertness, even toward the electron-rich olefin 3i, of the electrophilic a-carbonyl-substituted radicals 2q -s (R = CHO, COMe, COPh) in the above additions is discussed. Additions of the radicals 2a -c, f, i, n, o to the conjugated olefin 3n are described and are in accordance with the conclusion that steric effects predominate in adduct formation, whereas electronic effects are of distinct but minor importance.Over the past decade, free-radical carbon-carbon couplings have become a powerful method for impressive chemo-, regio-, and stereoselectiveIn this context the additions of carbon-centered radicals to unsaturated systems represent one of the most useful applications of freeradical chemistry and are of great importance for polymer synthesis.Experimental resultsp-"] and theoretical calculations['2131 indicate that the addition of small alkyl radicals to alkenes is usually exothermic and irreveresible, with early, unsymmetrical, reactant-like transition states, dominated by SOMO-LUMO (nucleophilic radicals) or SOMO-HOMO (electrophilic radicals) interactions ["]. However, "the complex interplay of polar, steric and bond-strengh terms"['41 seems to govern product f~rmation[~-~'~.Additions of bulky, resonance-stabilized radicals to alkenes are rare in the literature. The trityl radical appears to add only to conjugated systems, like isoprene, whereas it remains inert toward isolated double bond^"^,'^]. We have recently reported our first results on the addition of sterically hindered diphenylmethyl radicals 2a -e to the captodative (c,d)-substituted olefin 3c. A novel, in the light of the c,dconcept an unexpected['71 1P-addition takes place providing the ketenimine adduct 6a['*] (Scheme 1). These radicals retain their nucleophilic character as may be judged from their SOMO energy (IP for Me': 9.84 eV ['9a1, PhCH2: 7.20['9b1, nh2CH: 6.80[19c1, and Ph3C': 7.26['9d1), and the bonds formed when ...

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