Homogeneous polymerization pathways

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

C to form the dinuclear (butadienyl)zirconium system 120. Treatment with B(C6F5)3 leads to the formation of a mono-addition product (121), even in the presence of excess borane. Complex 121 (Scheme 40) shows only a marginal ethene polymerization activity. This led to the notion that such formation of dimeric zirconium complexes might represent desactivation pathways in homogeneous Ziegler-Natta catalyst chemistry.125... 

In homogeneous coordination polymerization, the L are ligands such as cycloocta-diene. The pathway of termination by hydrogenolysis is shown dashed. 

Keaton, R. J. Jayaratne, K. C. Fettinger, J. C. Sita, L. R. Structural characterization of zirconium cations derived from a living Ziegler-Natta polymerization system New insights regarding propagation and termination pathways for homogeneous catalysts. J. Am. Chem. Soc. 2000, 122, 12909-12910. 

In Sect. 2.3 it was shown by the example of complexes 1 (2) and 6 (7), that the phenomenon of hapticity change or bond order variation between the transition metal and the aromatic ligands can change the kinetic pathways of the polymerization process and its outcome dramatically. The haptotropic behavior of metallocenes is a known phenomenon in transition organometaUic chemistry and homogeneous catalysis [149-156]. 

The international symposium on Recent Developments in Olefin Polymerization Catalysts was held in Tokyo in October 1989. This volume includes 38 i>apers fi"om the 31 lectures and 18 posters presented at the symposium, which covered the following topics Overview of super-active homogeneous and heterogeneous catalysts, kinetic profile of olefin polymerization including copolymerization, characterization of catalysts and polymers, methods for the determination of active center concentration, role of Lewis bases on the catalyst isospecificity, polymerization mechanisms, and synthetic pathways for functionalized polyolefins. We believe the contents are well balanced between fundamental research and application as well as between homogeneous and heterogeneous catalyst systems. 

In recent years, much attention has been focused on electrochemical studies of metalloporphyrins, not only as mimetic compounds of the iron porphyrin unit in heme proteins but also as potential electrocatalysts . Metalloporphyrins have been found to be applicable in both homogeneous and heterogeneous catalysis - and, because oxygen can be reduced directly through a 4-electron pathway on some transition metal porphyrins, catalysis in the heterogeneous electrochemical oxygen reduction reaction has received particular attention The application of metalloporphyrins to heterogeneous electrocatalysis requires their attachment to solid electrodes which can be realized based on chemisorption, chemical reactions with previously functionalized electrodes, chemical reactions with a functionalized polymer, incorporation of the porphyrin with the polymer film and electrochemical polymerization. 

---