Recent Progresses in Cayalyzed Butadiene and Isoprene Polymerization

The first generation of catalysts for the polymerization of butadiene and isoprene was obtained by a combination of transition metal compounds (e.g., chlorides, alcoholates, carboxylates) with aluminum-alkyls [1]. These systems permitted to prepare all the stereoisomers of polybutadiene, but their activity was rather low, and the selectivity too was not very high, since the stereoregular polymer often represented only a small fraction of the crude polymer. With the advent of MAO as alkylating agent, in the 1980s, new catalytic sytems were introduced, in some cases much more active and stereospecific than those based on common aluminum-alkyls. Metallocene catalysts could also be used for the polymerization of 1,3-dienes, allowing to obtain novel polymers from substituted butadienes which were not possible to prepare with the conventional Ziegler-Natta catalysts. The results obtained, however, were particularly interesting from the scientific point of view - useful for a better comprehension of the diene polymerization mechanism - but did not lead to any real improvement with regard to the catalysts used for the polymerization of butadiene and isoprene, the only monomers of industrial interest. Starting from the 2000s, a new generation of catalysts, based on well defined complexes of transition metals and lanthanides with various ligands containing donor atoms such as P and/or N and/or O (e.g., phosphines, imines, imino-pyridines, keto-imines), has been introduced [2,3]. These novel catalysts permitted better control of the polymerization regio- and stereo-selectivity, of the molecular weight and molecular weight distribution of the resulting polymers. It was possible to prepare polybutadienes and polyisoprenes with a higher cis content, 99.9% and ?99% respectively, with catalysts based on lanthanide (Nd, Pr, La) complexes with bis-imino, bis-iminopyridine, keto-imino and keto-imino-pyridine ligands, upon activation with various types of aluminum-alkyls (i.e., AliBu2H, MAO, TIBAO). This result is extremely important from the industrial point of view since even a slight increase in the polymer cis content may lead to a great improvement in the elastic properties of the polymers. A high cis-1,4 polybutadiene (?85%) was also prepared with catalysts based on various types of titanium and vanadium complexes (e.g., phosphine, keto-imines, bis-imines, pyridine-imines complexes) with MAO; in particular the obtaining of a cis polymer from vanadium was quite unexpected since vanadium catalysts are mainly known in the field of stereospecific diene polymerization for their ability to give highly trans-1,4 polymers, and are by far the most important systems for preparing trans-1,4 polybutadiene. With these novel catalytic systems it was possible to obtain new, highly stereoregular polymeric structures that were not possible to prepare before. This is the case with syndiotactic and isotactic 3,4 polyisoprene obtained by iron and lanthanide catalysts, respectively, and cis-1,4-alt-3,4 polyisoprene obtained by cobalt catalysts. The catalytic systems based on well-defined cobalt complexes were however, without a doubt, those giving, up to now, the most interesting results. New catalytic systems have been developed by combination of MAO with phosphine-CoCl2 complexes. The peculiarity of such novel complexes lies in their ability i) to allow the formation of poly(butadiene)s with controlled microstructure (cis-1,4 or 1,2) by simply varying the type of ligand coordinated to the Co atom and ii) to give living polymerizations. Taking advantage of these features, we were able to prepare new butadiene di-block stereoregular polybutadienes (BDSPs), in which the two blocks, linked through a single junction point, have different characteristics (amorphous cis-1,4 and crystalline syndiotactic 1,2); these novel polymers will allow to prepare compounds for applications in several fields such as tires, soles and technical articles, with improved properties compared to those nowadays available. Based on what reported above, it is clearly evident that investigations into the synthesis of novel transition metal and lanthanide complexes, with particular regard to their environmental sustainability, and their use as catalyst components for the polymerization of 1,3-dienes, seem to pave the way for future studies. Preparation of homo- and multi-block copolymers exploiting the "living" character of these catalytic systems, and the possibility of controlling and/or modifying the catalytic regio- and stereoselectivity during polymerization, through an appropriate choice of type of ligand, type of aluminum-alkyl and Al/Mt ratio, nature of the monomer, will represent another way to go in the future.