4 -Bromo-6,8-dioxabicyclo octan

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

In some cases, neighboring group participation is responsible for the formation of the (3-cnfiguration. In the polymerization of a stereoisomer mixture of 4(a)- and 4(e)-bromo-6,8-dioxabicyclo[3.2.1]octane (42 58) (15a and I5e), a polymer consisting of 78% trans- and 22% cis-units was obtained even at —90 °C at which a... 

The two stereoisomers of4-bromo-6,8-dioxabicyclo[3.2.1]-octan-7-one (47a and 47b) showed different behavior in their cationic oligomerizations at — 40 or 0 °C [50] The axial isomer 47 a showed a tendency to cyclodimerize, particularly at higher temperature, whereas the equatorial isomer 47 e was much less reactive and gave only a small amount of a cyclic dimer along with other oligomers. Reductive debromination using tri- -butylstannane converted the cyclic dimer 48 of 47 a to the cyclic dimer 43 of 41, indicating that the former also consisted of a pair of the enantiomeric monomeric units. 

More recently Sumitomo and Okada studied the polymerization of 4-bromo-6,8-dioxabicyclo 3.2.1 octane with a substituent at the axial or equatorial position 40) ... 

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