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The present article reports on dynamic mechanical and infrared spectroscopic tests of natural rubber crosslinked by sulfur, peroxide or high energy radiation. The effect of these vulcanization methods upon the rate of crosslinking, the position of the second order transition temperature and the double bond structure is discussed. Whereas the temperature displacement of the transition range from the second order transition phase to the rubber elastic state during crosslinking by high energy radiation or by peroxide arises from the increasing hindrance of chain movement by the crosslinks, the intramolecular addition of sulfur predominantly is responsible for the much greater temperature displacement—related to the same crosslink density—during sulfur vulcanization. The change of the double bonds which proceeds in the sulfur vulcanization at a much faster rate than in the crosslinking by irradiation also can be considered as indication of intramolecular rings being formed along the chain during sulfur vulcanization. Vulcanization by sulfur and crosslinking by irradiation appear, principally, to be based on the same mechanism. The influence of oxidation on the mechanical properties of the vulcanizates is studied in natural rubber irradiated in an oxygen atmosphere. The two damping maxima observed at the dynamic mechanical test can be ascribed, with the aid of infrared spectroscopy, to the influence of crosslinks and oxidation products on the chain mobility.

(RS)- 6-acetoxl.-5,.6-dimeth?.l-, (RS)-6-ucetoxy-2,4,6-1rimeth~.l-, 3,4,6-tet r m n i 4 i j~l -. and (RS)- 6-oci~tox~-2,3,4,5,6-pentumeth~l-2,4-c~clo/ie.~udii~n-I-oni~.~ serve as representative educts.Thcrc arc two separate main photochemical routes conveniently designated as l(n*, n) or '(n*, n ) tracks. The latter may also he attained by sensitization and leads to phenols. The former, by a-cleavage furnishes dien-ketens as indispensable phototransients. Photolysis of dien-ketens follows one or more of three reaction channels, each of which yields a particular type of photoproduct: heat-induced monocyclization affords 2,4-cyclohexadien-1-ones, heat-induced hicyclization stereoselectively furnishes bicyclo[3.1.0]hex-3-en-2-ones, and multi-step addition of protic nucleophiles stereosclcctively gives l,4-, I,6-and/or 1,2-adducts. By X-ray analysis or NOE studies, the structure of isolated photoproducts is establishcd. Conventional spectroscopy at low or flash spectroscopy at normal temperature yield information on the Cormation and decay of kinetically unstable intermediates. Photoproduct composition depends on the pattern o C substitution of the educts, o n thc solvents, and on the nucleophiles that might he present. Substituents primarily exert an influence upon the population of the various conformers of the dien-ketcn. Solvents affect the rate of the divers reaction paths competing for the phototransient. Nucleophiles play more than a trivial role when adducts are formed. With the detailed view of a dien-keten's role on hand, the photoproduct from a given o-quinol-acetate -or more general from a linear conjugatcd cyclohexadienone -is now predictable.

Zwei Varianten einer dreistufigen Synthese von Crocetin-dimethylester (1 c) werden beschrieben. 2,6-Dimethylphenol (2a) oder 4-Brom-2,6-dimethylphenol (2 b) und (E)-1,4-Dibrom-2-buten (3a) dienen als bequem zugangliche Synthesebausteine. Die photochemische dffnung der linearkonjugierten Cyclohexadienone 6 und 7 zu suco-isomeren Dienylketenen, die mit Frankfurter Sonnenlicht muhelos erreicht werden kann, spielt die Schliisselrolle. Die Konfiguration der stereoisomeren Schliisselverbindungen 6 und 7 wird durch enantioselektiven Abbau von 7 mit zirkular polarisiertem Licht sowie durch enantioselektiven Aufbau von 7 festgelegt. Light-induced Reactions, XII') A Convenient Synthesis of DimethylcrocetinTwo variant schemes of a three-step synthesis of crocetin dimethyl ester (Ic) are described. 2,6-Dimethylphenol (2a) or 4-bromo-2,6-dimethylphenol (2 b), respectively, and (E)-l,4-dibromo-but-2-ene (3 a) serve as conveniently available starting material. The photochemical cleavage of the linearly conjugated cyclohexadienones 6 and 7 affording seco-isomeric dienyl ketenes plays the key role, and is smoothly achieved by sun light of Frankfurt. The configuration of the stereoisomeric key compounds 6 and 7 is determined by enantioselective degradation, using circularly polarized light, as well as by enantioselective synthesis of 7.