Syndiotactic poly(alpha-olefin)s by hydrogenation of highly stereoregular 1,2 (3,4) syndiotactic polydienes

In recent years we have studied the polymerization of 1,3-dienes (i.e., butadiene and several types of substituted butadienes) with catalytic systems obtained by combining methylaluminoxane (MAO) with various types of transition metal (e.g., Ti, V, Cr, Co, Fe) complexes (e.g., metallocene complexes, phosphine complexes, bypiridyl complexes)1. We were able to prepare highly stereoregular polymers having different structure (cis-1,4; trans-1,4; 1,2; 3,4; iso- and syndiotactic), mainly depending on the type of monomer and on the catalyst structure. Specifically, syndiotactic 1,2 polymers were obtained from terminal substituted 1,3-butadienes (e.g., 1,3-pentadiene, 1,3-hexadiene, 5-methyl-1,3-hexadiene, 1,3-heptadiene, 1,3-octadiene) with the system CoCl2(PiPrPh2)2/MAO; syndiotactic 3,4 polyisoprene and syndiotactic 1,2 poly(3-methyl-1,3-pentadiene) were instead obtained with the system FeCl2(bipy)2/MAO (bipy = bipyridine). The great availability of all these stereoregular polydienes may represent a considerable source for highly regular olefin polymers; in fact through the hydrogenation of the syndiotactic 1,2 (or 3,4) poly(1,3-diene)s with diimide, formed by thermal decomposition of p- toluene-sulfonyl-hydrazide, we were able to obtain highly stereoregular syndiotactic polyolefins (see Scheme below), which in some cases are difficult to synthesize by simple stereospecific polymerization of the corresponding monomers. All the syndiotactic polyolefins obtained were fully characterized by FT-IR, NMR (1H, 13C and 2D), GPC, DSC and RX, and their mechanical properties evaluated.