Conjugated Diolefins and Olefins Polymerization with Copper-based Catalysts.

In recent years, the research activity in the field of stereospecific polymerization of conjugated dienes1 has particularly focused on the study of new catalytic systems based on organometallic complexes of transition metals and lanthanides with various types of organic ligands (e.g., phosphines, pyridyl-imines, bis-imines) and with a well-defined structure.2 The results obtained have clearly shown that, at least in principle, each transition metal or lanthanide could be able to provide active and selective catalytic systems for the polymerization of 1,3-dienes once the right combination of metal, ligand and diene monomer was chosen. Following this idea, in the framework of our research work dealing with the synthesis and the characterization of transition metal compounds and the study of their behavior in the polymerization of 1,3-dienes, we have taken into consideration copper. We have in particular examined the polymerization of butadiene, isoprene and myrcene with catalysts obtained by combining bipyridyl and imino-pyridyl copper complexes (see figure alongside) with methylaluminoxane (MAO), exhibiting a quite good activity.3,4 The system (I)/MAO gave a predominantly 1,2 polybutadiene, a crystalline syndiotactic 3,4-polyisoprene (Tm 114°C) and a highly crystalline cis-1,4 poly(2,3-dimethyl-1,3-butadiene) (Tm 198°C); the system (II)/MAO gave from butadiene a slightly crystalline syndiotactic 1,2 polymer; from isoprene a predominantly alternating cis-1,4/3,4 polymer in which however short cis-1,4 sequences (three units) can be detected along the polymer chain, with a Tg value of about -30°C; from myrcene a predominantly alternating cis-1,4/3,4 polymer in which however longer cis-1,4 sequences (five units) are present along the polymer chain, with a Tg value of about -60°C. The results obtained highlight once more the fundamental role played by the ligand structure and the monomer structure on the selectivity in the polymerization of 1,3-dienes. The above complexes, together with the bis-iminopyridyl ones, were also tested for the polymerization of ethylene, obtaining however not very active catalysts.