Stereospecific polymerizations heterogeneous/homogeneous

He was a Professor of Industrial Chemistry, School of Engineering, Polytechnic Institute of Milan, Milan, Italy since 1937. He became involved with applied research, which led to the production of synthetic rubber in Italy, at the Institute in 1938. He was also interested in the synthesis of petrochemicals such as butadiene and, later, oxo alcohols. At the same time he made important contributions to the understanding of the kinetics of some catalytic processes in both the heterogeneous (methanol synthesis) and homogeneous (oxosynthesis) phase. In 1950, as a result of his interest in petrochemistry, he initiated the research on the use of simple olefins for the synthesis of high polymers. This work led to the discovery, in 1954, of stereospecific polymerization. In this type of polymerization nonsymmetric monomers (e.g., propylene, 1-butene, etc.) produce linear high polymers with a stereoregular structure. 

Other types of complex catalysts that have received attention for stereospecific polymerization are the reduced metal oxides and the alfin catalysts (prepared from compounds of sodium). All three types are mainly used in heterogeneous polymerization, although some homogeneous processes are also commercially important. 

Unlike heterogeneous Ziegler-Natta catalysts, homogeneous matallocene catalysts often produce only low molecular weight polymers, especially in stereospecific polymerizations. The molecular weight is given by... 

Corradini, P Busico, V. Cavallo, L. Guerra, G Vacatello, M. Venditto, V. Structural analogies between homogeneous and heterogeneous catalysts for the stereospecific polymerization of 1 -aUcenes. J. Mol. Catal. 1992, 74, 433-442 and references therein. 

It is well known that atactic polystyrene (APS) is obtained with the catalyst such as radical initiater and IPS is obtained with Ziegler- Natta catalyst. SPS can be produced by a transition metal compound and MAO catalyst. As a transition metal, titanium(Ti) is excellent for the catalytic activity. We studied the stereospecific polymerization of styrene with homogeneous Ti and heterogeneous Ti compound and MAO catalyst. 

The Sharpless epoxidation of allylic alcohols with tert-butylhydroperoxide and titanium(IV) isoproxide has been carried out with a polymeric tartrate made from tartaric acid and 1,8-octanediol.114 The yield of epoxide (92%) was comparable with that when dimethyl tartrate was used (91%), but the 79% ee was lower than the 98% ee found with the dimethyl tartrate. It may be possible to raise the ee by further variations in the structure of the polymer. The heterogenization of a catalyst for a homogeneous reaction often requires optimization to obtain comparable or higher yields and stereospecificity. 

Conclusions on the stereospecificity of the catalyst cannot be drawn in a simple way from the aggregate state, but heterogeneous catalyst systems seem to be necessary for the polymerization of isotactic poly(a-oleflns). Conversely, syndiotactic poly(propylene) has so far only been produced with a homogeneous catalyst system. Other syndiotactic poly (a-olefins) are unknown. 

Corradini, R Guerra, G. Models for stereospecificity in homogeneous and heterogeneous Ziegler-Natta polymerizations. Progr. Polym. Sci. 1991,16, 239-251. 

At the same time, the fact that the homogeneous catalyst precursors are structurally well-defined has provided an extraordinary opportunity to investigate the origin of stereospecificity in olefin polymerization at a level of detail that was difficult if not impossible with the conventional heterogeneous catalysts. For example, NMR analysis of the isotactic polymer produced with HI revealed the stereochemical errors mmmr, mmrr, and mrrm in the ratios of 2 2 1 (Fig.5). This observation is consistent with an enantiomorphic site control mechanism, where the geometry of the catalyst framework controls the stereochemistry of olefin insertion.6 30,31 These results established unambiguously a clear experimental correlation between the chirality of the active site, which could be established by x-ray crystallography of the metallocene catalyst precursor, and the isotacticity of the polymer produced. 

---