Analysis of the ESR line width in time-resolved (TR-ESR) experiments is shown to be a suitable tool for the measurement of addition constants of phosphinoyl and substituted benzoyl radicals. Compared with kinetic investigations, which make use of the resonance intensity or integral, observation of the line width as a function of monomer concentration has the advantage that the experimental parameter is not affected by spin-polarization processes and, therefore, a lengthy determination of these polarization parameters is avoided. The resulting addition constants are discussed with respect to the experimental hyperfine coupling constants and the geometry of the radicals. TR-ESR experiments simultaneously provide rate constants and ESR parameters and allow structure−reactivity relationships to be established.

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In memoriam Hanns FischerThe photophysical properties of three photoinitiators with a covalently linked thioxanthone sensitizer unit absorbing up to 410 nm were investigated by laser-flash photolysis and CIDNP spectroscopy. These complementary techniques revealed two competing reaction pathways of the molecular dyads 1 -3: i) triplet-energy transfer from the sensitizer to the morpholine moiety followed by a-cleavage to yield a radical pair, which initiates radical polymerization, and ii) bimolecular electron transfer from the morpholine to the thioxanthone subunit followed by proton transfer. The relative efficiency of these routes is determined by the triplet energy of the photoinitiator moiety relative to that of the sensitizer.

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(3,5-Di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)acetic acid 1a underwent on heating a new cyclodimerization reaction affording a 1 : 1 mixture of the racemic cis- and trans-isomeric lactone-acids 2. The decisive structure elucidation of 2 was carried out after its mild esterification with diazomethane and separation of the racemic isomeric methyl esters 3a and 3b. The attempted esterification of 2 with methanol and sulfuric acid gave only the methyl diarylacetate 4. In contrast to 1a, which contains the carboxylic acid functionality no cyclodimerization was observed with the corresponding methyl ester 1b or nitrile 1c.

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