Controlled degradation in tailor‐made macromolecules elaboration. Controlled chain‐cleavages of polydienes by oxidation and by metathesis

A chain cleavage reaction allowing the transformation of high molecular weight polymers in well‐defined oligomers can be considered as a step in a synthetic scheme. The control of oligomers with respect to their average molecular weights, their molecular weights distributions and their chain‐end microstructures implies the control of the cleavage reaction with respect to its yield, its regiospecificity and its chemiospecificity. Among several examples of efficient uses of this principle for the preparation of liquid oligomers from unsaturated polymers, attention is focused (I) on controlled oxidative degradation of rubber and (II) on metathetical controlled degradation of 1,4‐polydienes and polyakenamers. (I.a) The mechanism of the phenylhydrazine accelerated oxidation of rubber in the latex phase is described according to accurate structural studies on molecular model molecules. (I.b) The epoxidation of rubber and periodic acid cleavages of epoxides in the latex phase are interpretated referring to the influence of the biphasic medium. The results are in agreement with interfacial blocky epoxidation by the water‐soluble reagent and interfacial arrangement of the epoxidized block to be cleaved. Finally the metathetical degradations resulting from back‐bitting cyclisation and cross metathesis with acyclic alkenes are presented as preparative methods to get (II.a) well‐defined cyclic oligomers and (II.b) telechclic oligomers (i.e. terminally functionalized oligomers).