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synopsisThe ring-opening polymerizations of cyclooctene, cyclododecene, 1,5cyclooctadiene, 1,5,9-~yclododecatriene, 3-methylcyclooctene, and 3-phenylcyclooctene have been carried out by using a twycomponent catalyst system composed of ethylaluminum dichloride and tungsten hexachloride. NMR and infrared analyses of the respective polymers indicate structures which are consistent with a ring-cleavage mode of propagation. No evidence for double-bond shifts or transannular reactions during the polymerizations of 1,5cyclooctadiene, 1,5,9-~yclododecatriene, 3-methylcyclooctene, and 3-phenylcyclooctene was found. The polymerizability of substituted, unsaturated, mediumsized alioyclic monomers suggests a convenient method for synthesis of certain perfectly alternating terpolymers. Since polymerizations occurred rapidly with little evolution of heat, it was concluded that entropy is a substantial contributor to the free energy of the ring-opening polymerization of medium-sized, unsaturated alicyclic monomers. 2209

In this paper views regarding the nature of the transition state in olefin metathesis are discussed. Earlier concepts that assumed a pairwise scrambling mechanism have been recently challenged. Metathesis studies involving acyclic as well as cyclic olefins have been reported which strongly suggest a chain process that may involve a carbene-to-metallocycle interconversion as the pathway responsible for transalkylidenation. Catalytic systems that display a low apparent metathesis activity on terminal olefins display a unique specificity in cross reactions with internal olefins. Critical experiments indicate that terminal olefins rapidly undergo regenerative metathesis preferentially. Results suggest that in the transient metallocycle, the most stringent steric requirements occur at the j3-carbon, although the origin of this steric demand is not obvious.

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In diesem Aufsatz werden Anschauungen über die Art des Übergangszustands bei der Olefin‐Metathese diskutiert. Die früher angenommene paarweise Umordnung ist kürzlich angezweifelt worden. Metathese‐Studien sowohl an acyclischen als auch an cyclischen Olefinen legen einen Kettenmechanismus nahe, bei dem die Umwandlung eines Carbens in einen Metallocyclus für die Umalkylidenierung verantwortlich sein könnte. Einige Katalysator‐Systeme, die bei terminalen Olefinen nur eine niedrige Metathese‐Aktivität zeigen, führen in Mischungen von terminalen Olefinen und Olefinen mit innenständiger Doppelbindung dagegen selektiv zu gekreuzten Produkten. Kritische Experimente zeigen, daß terminale Olefine bevorzugt einer schnellen regenerativen Metathese unterliegen. Einige Ergebnisse deuten darauf hin, daß die strengsten sterischen Anforderungen an das β‐C‐Atom gestellt werden; einleuchtende Gründe für diese sterische Forderung gibt es jedoch nicht.