Metallocenes volatility

Applications involving ring transfer or loss. The kinetic lability, volatility, and Lewis acidity of heavy alkaline earth metallocenes have been the properties most important to their applications. The gas-phase decomposition of volatile metallocenes is useful in the preparation of thin films of alkaline earth-containing materials and in doping semiconductors. Reviews are available on the use of group 2 organometallic compounds as precursors for chemical-vapour deposition (CVD).2 3... 

Cyclopentadienyl compounds (i.e. metallocenes) (Fig. 5), which have at least one direct metal-carbon bond to the C5H5 ligand, were first synthesised in the 1950s [79,80]. Since then, reactions of cyclopentadienyl reagents have been applied for almost every element [123]. The main application of metallocenes is their use as catalysts in the polymerisation of olefins by Ziegler-Natta polymerisation processes. As many metallocene compounds are volatile and thermally stable, they are also suitable for use as precursors in MOCVD [124-127]. Although cyclopentadienyl compounds have attracted considerable interest as precursors in CVD depositions they are sometimes too reactive [128]. However, high reactivity and thermal stability make cy-... 

Since the discovery of ferrocene and the development of metallocene chemistry, the compound cyclopentadienyllithium, C5H5Li, has been of considerable interest to both experimentalists and theoreticians. In its physical properties cyclopentadienyllithium qualitatively behaves like a typical salt. It is nearly insoluble in noncoordinating solvents and possesses a high melting (decomposition) point and a very low volatility. To our knowledge the crystal structure of C5H5Li is unknown. 

Similarity in chemical structure between the sorbate and the polymeric packaging material enhances sorption. As the molecular weight distribution (MWD) of the polymer broadens, sorption also tends to increase. LLDPEs produced with metallocene catalysts, which are characterized by a narrow MWD, have a reduced tendency to sorb volatile compounds, compared to LDPE. Sorption studies on benzaldehyde, citral, and ethyl butyrate revealed that sorption by ionomers is similar to sorption by LDPE. As in the case of migration, the extent to which the sorption process occurs depends on the initial concentration of the sorbate and the partition coefficient between the polymer and the product, as well as on the rate of transfer. 

The volatile metal or semimetal compounds used as precursors in ALE can be selected from a wide range of compounds including metal chlorides, P-diketonates, alkyls, alkoxides, and metallocenes etc. [16,17]. The precursor can be liquid, solid, or gas at room temperature and must be stable enough to withstand heating. It should also be free of impurities that volatilize below or at the temperatures used in the processing. Often moisture is the most serious impurity, and the compound should be dried before use. Any solvent coordinated to metal compounds such as P-diketonates needs to be removed. 

Polymethylene and linear PE exhibit a maximum of the volatilization rate (at about 20 26% conversion), which is absent in branched PE. Thermal and thermo-oxidative stability of metallocene-prepared syndiotac-tic PP is higher than Ziegler-prepared IPP. That means that the stereoregularity is one of the dominant facts determining the stability of PP [177, 178]. 

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