cis-1,4-alt-3,4 Polyisoprene: Synthesis and Characterization

We recently reported on the synthesis and characterization of COCl2(PRPh2)(2) (R = methyl, ethyl, n-propyl, isopropyl, and cyclohexyl), The catalyst systems obtained by combining the above complexes with methylaluminoxanes (MAO) were highly active in the polymerization of 1,3-dienes, giving 1,2 syndiotactic polymers from butadiene and terminally substituted butadienes (e.g., E-1,3-pentadiene and E-1,3-hexadiene) and 1,2 isotactic polymers from internally substituted 1,3-dienes such its E-3-methyl-1,3-pentadiene. We have now polymerized isoprene with the same cobalt catalysts, and unexpectedly the polymers obtained resulted to have in equibinary, alternating cis-1,4/3,4 structure, as indicated by the two-dimensional NMR data. A structure of this type is rather unusual in the field of diolefins stereospecific polymerization, and only few cases have been reported in the literature. An interpretation for the formation of such a polymer is proposed. The structure of cis-1,4-alt-3,4 polyisoprene, with alternating sequences of in-chain and side-chain double bonds. makes it of potential interest in view of a possible selective functionalization of the double bonds.