Novel Isoprene Polymers with an Alternating Cis-1,4/3,4 Structure: Synthesis, Characterization and Properties

The stereospecific polymerization of isoprene can give polymers having different structure and properties]. A cis-1,4 polymer with a very high cis content (>=98%), very similar to natural rubber, can be obtained with catalysts and titanium (e.g., TiCl4/AlEt3); a highly trans-1,4 polymer was prepared with vanadium catalysts (e.g., V(acac)3-MAO); 3,4 iso- and syndiotactic polymers were instead synthesized with catalysts based on silylene- linked cyclopentadienyl-phosphido rare earth metal alkyl complexes and on Fe(bipyridyl)2Cl2 complexes, respectively. Trans-1,4 and 3,4 iso- and syndiotactic polymers are crystalline, while the cis polymer is amorphous, exhibiting the typical elastomeric properties making it extremely useful for the production of truck tires. Very recently a 1,2 enriched polyisoprene was obtained with catalysts based on cobalt complexes carrying aminophosphoryl fused (PN3) ligands Our recent years research activity, which was particularly focused on the polymerization of conjugated dienes with catalysts based on transition metal and lanthanide organometallic complexes having a well-defined structure, has now allowed us to synthesize a novel isoprene polymer showing a particular structure not reported yet. We synthesized and characterized several types of cobalt phosphine complexes and imino-pyridyl iron complexes, and successively we used them, in combination with methylaluminoxane, for the polymerization of isoprene. By varying the nature of the ligands on the metal atom, we were able to synthesize perfectly alternating cis-1,4/3,4 polyisoprenes or predominantly alternating cis-1,4/3,4 polymers in which however some cis-14 sequences of different lengths are present along the polymer chain, as indicated by the NMR analysis. This result was quite interesting since, depending on the catalyst structure, it was possible to obtain crystalline polymers or amorphous polymers with tunable transition glass temperature in a large range of temperatures. 13C NMR of a predominantly alternating cis-1,4/3,4 polyisoprene obtained with iron-based catalysts. These features make these novel polymers of potential interest in the automotive field for the preparation of elastomeric tire compounds.