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The study of highly strained cyclopropyl adamantane compounds with emphasis on their radical copolymerizations with oxygen Schmidt, Justin Orvel

Abstract

The synthesis of a new type of highly strained cyclopropyl group between the two remaining bridgehead positions in 1,3-dimethyl- adamantane was accomplished by an internal coupling reaction involving the removal of two bromines with sodium-potassium alloy in ether. The product, 5,7-dimethyltetracyclo [3.3.1.1⁵⁷.0 ¹³] decane, commonly named 3,5-dimethyl-l,3-dehydroadamantane (DMDHA), formed a polymer on heating the pure material to a temperature above 90°C. The polymer's properties were studied by X-ray analysis and differential scanning calorimetry. The previously reported parent compound to DHDMA, tetracyclo-5 7 13 [3.3.1.1⁵⁷.0 ¹³] decane, commonly named 1,3-dehydroadamantane (DHA), was synthesized and studied to determine the properties of the cyclopropyl bond between the bridgehead carbons. Reactions with several stable free radicals and other known reagents with affinities for radicals indicated a very high degree of radical character present at the bridgehead positions. From kinetic studies of the reaction of DHA with molecular oxygen at approximately 1 atmosphere and 22°C, first order kinetic plots were obtained which yielded the rate constants of k = 15.0 x 10⁻⁵ sec⁻¹ in octane and 32.8 x 10⁻⁵ sec⁻¹ in xylene. Oxygen was absorbed to an extent slightly greater than one mole per mole of DHA. The reaction was without ambiguity shown to be of radical nature through the use of several free radical inhibitors to retard the reaction rate. The initiation mechanism of the DHA-oxygen copolymerization could not be determined with certainty. The spontaneous reaction was either initiated by oxygen radicals attacking the highly strained, highly p-orbital-charactered carbon atom from its unprotected back side or else by the internal bond of the cyclopropyl group spontaneously ring opening to form an adamantane diradical. In the latter case the radical formed would sometimes capture an oxygen before the equilibrium shifted back to the closed form. All that is known for certainty is that the 1,3-carbon bond in DHA is very highly strained and is either in equilibrium between an open and a closed form or else is at least very radical-like in nature.

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