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 as E-3-methyl-1,3- pentadiene. We have now polymerized isoprene with the same cobalt catalysts, and unexpectedly the polymers obtained resulted to have an 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. Aninterpretation 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.