The stereospecific polymerization of 1,3-dienes began in 1954: since then several catalytic systems were prepared using transition metal and lanthanide compounds in association with opportune alkylating agents. With the introduction of methylaluminoxane (MAO), at the beginning of 1980s, new catalysts were introduced, in some cases much more active and stereospecific than those based on common aluminum-alkyls. 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, N, O (e.g., phosphines, imines, imino-pyridines, keto-imines) has been introduced1. These systems were found to be particularly active and able to provide polymers with controlled microstructure (i.e., cis-1,4; 1,2 (3,4); mixed cis-1,4/1,2(3,4) with a variable 1,2(3,4) content), molecular weight and molecular weight distribution. Our contribution in the field can be summarized as follows: (i) we have synthesized and characterized several new lanthanide (Nd, Pr, La) complexes with bis-imino, bis-iminopyridine, keto-imino and keto-imino-pyridine ligands which, upon activation with various types of aluminum-alkyls (i.e., AliBu2H, MAO, TIBAO), provided polymers with a very high cis-1,4 content from butadiene (?99.9%) and isoprene (?98%), and this result represents a quite important achievement 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, and the limited supply of natural rubber is causing an increase in demand for synthetic polyisoprene; (ii) we were also able to obtain high cis-1,4 polybutadiene with catalysts based on titanium bis-imine complexes; the cis content in this case was not extremely high (85-90%), nevertheless some dependence of the regioselectivity on the bis-imine ligand structure was observed, which bodes well for the future; (iii) we have synthesized and characterized several new cobalt complexes with different mono-and bi-dentate ligands containing P and/or N and/or O as donor atoms. Depending on the ligand nature we were able to obtain polybutadienes with different structures (cis-1,4; syndiotactic 1,2; mixed cis-1,4/1,2 structure) and properties, but above all some of these new systems exhibited "living" features, and, being at the same time able to drastically change their selectivity during the polymerization process, allowed to prepare stereoblock (co)polymers having the properties of thermoplastic elastomers; (iv) finally we also synthesized and characterized several new iron complexes with various ligands containing N and or O as donor atoms (i.e., bipyridine, phenanthroline, ?- and ?- ketoimines, pyridinimines) which in combination with MAO were able to give most of the polymers provided by cobalt catalysts. This result too is quite important from the industrial point of view, since it would be nice to substitute a toxic metal such as cobalt with iron, undoubtedly a more sustainable metal and with a lower environmental impact.
An overview of the latest developments in the stereospecific polymerization of 1,3-dienes
Giuseppe Leone;
2019
Abstract
The stereospecific polymerization of 1,3-dienes began in 1954: since then several catalytic systems were prepared using transition metal and lanthanide compounds in association with opportune alkylating agents. With the introduction of methylaluminoxane (MAO), at the beginning of 1980s, new catalysts were introduced, in some cases much more active and stereospecific than those based on common aluminum-alkyls. 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, N, O (e.g., phosphines, imines, imino-pyridines, keto-imines) has been introduced1. These systems were found to be particularly active and able to provide polymers with controlled microstructure (i.e., cis-1,4; 1,2 (3,4); mixed cis-1,4/1,2(3,4) with a variable 1,2(3,4) content), molecular weight and molecular weight distribution. Our contribution in the field can be summarized as follows: (i) we have synthesized and characterized several new lanthanide (Nd, Pr, La) complexes with bis-imino, bis-iminopyridine, keto-imino and keto-imino-pyridine ligands which, upon activation with various types of aluminum-alkyls (i.e., AliBu2H, MAO, TIBAO), provided polymers with a very high cis-1,4 content from butadiene (?99.9%) and isoprene (?98%), and this result represents a quite important achievement 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, and the limited supply of natural rubber is causing an increase in demand for synthetic polyisoprene; (ii) we were also able to obtain high cis-1,4 polybutadiene with catalysts based on titanium bis-imine complexes; the cis content in this case was not extremely high (85-90%), nevertheless some dependence of the regioselectivity on the bis-imine ligand structure was observed, which bodes well for the future; (iii) we have synthesized and characterized several new cobalt complexes with different mono-and bi-dentate ligands containing P and/or N and/or O as donor atoms. Depending on the ligand nature we were able to obtain polybutadienes with different structures (cis-1,4; syndiotactic 1,2; mixed cis-1,4/1,2 structure) and properties, but above all some of these new systems exhibited "living" features, and, being at the same time able to drastically change their selectivity during the polymerization process, allowed to prepare stereoblock (co)polymers having the properties of thermoplastic elastomers; (iv) finally we also synthesized and characterized several new iron complexes with various ligands containing N and or O as donor atoms (i.e., bipyridine, phenanthroline, ?- and ?- ketoimines, pyridinimines) which in combination with MAO were able to give most of the polymers provided by cobalt catalysts. This result too is quite important from the industrial point of view, since it would be nice to substitute a toxic metal such as cobalt with iron, undoubtedly a more sustainable metal and with a lower environmental impact.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
