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 molecula r weight polymers in well-defined oligomers can be considered as a ste p in a synthetic scheme. The control of oligomers with respect to thei r average molecular weights, their molecular weights distributions and their chain-end microstructures implies the control of the cleavage r eaction with respect to its yield, its regiospecificity and its chemio specificity. Among several examples of efficient uses of this principl e for the preparation of liquid oligomers from unsaturated polymers, a ttention 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 accur ate structural studies on molecular model molecules. (I.b) The epoxida tion of rubber and periodic acid cleavages of epoxides in the latex ph ase are interpretated referring to the influence of the biphasic mediu m. The results are in agreement with interfacial blocky epoxidation by the water-soluble reagent and interfacial arrangement of the epoxidiz ed block to be cleaved. Finally the metathetical degradations resultin g from back-hitting cyclisation and cross metathesis with acyclic alke nes are presented as preparative methods to get (II.a) well-defined cy clic oligomers and (II.b) telechelic oligomers (i.e. terminally functi onalized oligomers).