Ziegler-Natta polymerizations mechanistic studies

Various mechanisms have been proposed to explain the stereoselectivity of Ziegler-Natta initiators [Boor, 1979 Carrick, 1973 Corradini et al., 1989 Cossee, 1967 Ketley, 1967a,b Tait and Watkins, 1989 Zambelli and Tosi, 1974]. Most mechanisms contain considerable details that distinguish them from each other but usually cannot be verified. In this section the mechanistic features of Ziegler-Natta polymerizations are considered with emphasis on those features that hold for most initiator systems. The major interest will be on the titanium-aluminum systems for isoselective polymerization, more specifically, TiCl3 with A1(C2H5)2C1 and TiCLt with A1(C2H5)3—probably the most widely studied systems, and certainly the most important systems for industrial polymerizations. 

Mechanistic studies on Ziegler-Natta-like polymerization soon focused on the role of cationic alkylzirconocene species in the catalysis. In the 1980s, Jordan and co-workers were the first to isolate and characterize an alkylzirconocene cation in the form of the salt [Cp2Zr(CH3)(THF)]+ [BPhJ- (Scheme 7) [84,85]. 

Much effort has been devoted during the last 30 years toward understanding the mechanisms operative in the coordination catalysis of ethylene and a-olefin polymerization using Ziegler-Natta systems (metal halide and aluminum alkyl, sometimes with Lewis base modifiers). Aspects of the complex heterogeneous reactions have been elucidated (jL- ) but the intimate mechanistic detail - for example the role of inhibitors and promoters, kinetics and thermodynamics of chain growth, modes of chain transfer and termination - comes primarily from studies of homogeneous catalysts ... 

Metallocene catalysts for the polymerizations of olefins have been known since early 1957 when Natta and co-workers first reacted triethyl aluminum (AlEts) and bis(cyclopentadienyl) titanium dichloride ( -C5H5)2TiCl2 to form a complex that polymerized ethylene. The structure of this complex was described and the polymerization results reported. With ethylene they reported to have made 7 g of crystalline polyethylene in about 8 h at 95°C with 40 atm ethylene pressure in n-heptane. Later in the same year, Breslow and co-workers repeated Natta s experiments. They found that the blue complex described by Natta was a somewhat poor catalyst (in agreement with Natta s findings), but discovered that small amoimts of oxygen in the ethylene boosted polymerization activity. When compared to the heterogeneous Ziegler-Natta catalyst system, these metallocene catalysts were poor with respect to polymerization activity. They were used essentially for mechanistic studies because of their simplicity and ease of structure elucidation. 

The advent of homogeneous Ziegler-Natta catalysts has provided new opportunities for the direct polymerization of functionalized monomers. The basis for the development of more tolerant homogeneous catalysts grew out of mechanistic studies which addressed the nature of the active site for homogeneous catalysts derived from metallocenes and... 

---