Gap aperture

In the case of catalyst 3, the thiophene substitution in the 6,7 position controls the gap aperture between fluorenyl and indenyl ligands by repulsing steric interactions at the complex backside [10, 11]. This leads to increased stereoselectivities [9] and is responsible for a C2-symmetric-like polymerization mechanism, characterized by increasing isotacticities when the polymerization temperature is reduced [5, 11] (Fig. 7). 

Figure 3.43 Coordination gap aperture angle in rac. -ethylcnchis l -(4,5,6,7-tetrahydroin-denyl)]zirconium dichloride [rac.-(ThindCH2)2ZrCl2] (left) and in rac.-ethylenebis[l -(4,7-dimethyl-4,5,6,7-tetrahydroindenyl)]zirconiumdichloride [rac.-(Me2 ThindCH2),ZrCl2] (right). For the sake of clarity, only the C-C bonds are sketched for the re ligands. 0 - Zr O - CL O C, CH, CH2 or CH3. Side view. Reproduced by permission from Ref. 30. Copyright 1995 Wiley-VCH Weinheim...

Hortmann, K. Brintzinger, H. H. Steric effects in a/wa-metaUocene-based Ziegler-Natta catalysts Coordination gap aperture and obliquity angles as parameters for structure-reactivity correlations. NewJ. Chem. 1992,16,51-55. 

Siloxy substitution at the 3-position of the indenyl ligand (17) was found to remarkably improve the 1-olefin copolymerization ability, whereas substitution at the 2-position (15) slightly reduced the copolymerization ability as compared to the unsubstituted 5. The reason for this was suggested to be mainly the increased coordination gap aperture of the 3-siloxy-substituted complexes. Table 1 summarizes the ethylene reactivity ratio data obtained for the siloxy-substituted complexes 15, 16, and 17 The large difference in the ethylene and comonomer reactivity ratio values, the product of which is much below unity, emphasizes the prevailing tendency of the catalysts to produce copolymers with isolated comonomer units. The reason for the 15 0% lower incorporation of 1-hexadecene than 1-hexene was explained by the higher steric bulk and lower rate of diffusion of the longer a-olefin. 

Although titanocene complexes are less active than half-titanocenes, Miyashita et al. showed that a methylene-bridged titanocene 19 is effective for syndiospecific styrene polymerization [97]. A large gap aperture is probably necessary for coordination of styrene. 

Hhcker et al. independently reported single-component catalysts that consist of methylene-bridged zirconocene complexes 40 and 41 [138, 139]. It is noteworthy that these catalysts polymerize MMA with the BPh4 anion instead of fluorinated boranes. The advantage of a methylene bridge seems to be its large gap aperture [140]. 

Dry Type Flame Arrester A flame arrester that rises an element consisting of small gaps or apertures to quench and extinguish the flame, as distinguished from hydranlic flame arresters. 

Loch, n. hole, opening cavity eye pore prison, -blende, /. perforated screen or diaphragm. -eisen, n. puncher, pimch. locben, v.t. perforate, punch, pierce, locherig, a. full of holes, perforated, porous. Locherstelle,/. open place, gap, hole. Loch-frass, m. pitting (as of metals), -karte,/. punch (ed) card, -kultur,/. (Bact.) stab culture. -scheibe, /. perforated disk, apertured disk. 

PI. 2.2 (A) Oral aperture of Prosimian AOS Naso-Palatine Papilla (Median Sulcus), without incisor-gap, in Aye-Aye (Dauhentania), (B) Rhinarium and Sulcus, with incisor-gap, in Angwantibo (Arctocebus). Arrow —> = Papillary Sulcus P = Naso-Palatine Papilla and RS = Rhinarial Sulcus (courtesy of Alain Schilling3). 

Figure 2.2.13 shows an overview of the MRI system developed for salmon selection. A 0.2-T C-shaped yoked permanent magnet with a 25-cm gap [50-ppm homogeneity for 15-cm DSV (diameter spherical volume), weight 1.4 tons] is used for the magnet. For the rf coils, two solenoid coils with a 14-cm circular aperture and 14 cm x 18 cm oval aperture were developed. 

The incoming light reflects from the end surface of a fibre, which is caused by the difference in refractive indexes of a fibre, a light source and a gap between them. Such losses are usually small as compared with other losses occurring in the system. The losses caused by different numerical apertures cannot be avoided. As higher NA of the fibre, as lower losses of... 

The gas reaction chamber and the objective aperture assembly occupied the gap between the upper and lower objective pole pieces, leading to a gas reservoir around the sample. Such ECELL systems were a major step forward in scientific capability, being used by Gai et al. (3,73-78), Doole et al. (79), Crozier et al. (80), and Goringe et al. (81) to characterize catalysis. Other developments for catalytic studies include an ex situ reaction chamber attached externally to the column of a TEM, for example, by Parkinson and White (82) and Colloso-Davila et al. (83). Reactions were carried out in the ex situ chamber (and not in situ), and the sample was cooled to room temperature and inserted into the column of the TEM (without exposure to the atmosphere) under vacuum. Baker et al. (84) used ETEM at gas pressures of a few mbar with limited resolution, and, in these experiments, representative higher gas pressures were not employed. 

Figure 8.19 Cell homogenizers. The cell membranes are disrupted by the shearing effects of being forced through narrow apertures. In the Potter homogenizer (a) the gap between the plunger and the walls of the tubes is critical in determining the extent of the disruptive effect and in the cell press systems, (b) disruption is caused by rapidly forcing a frozen cell suspension through a small hole.

The gas reaction chamber and the objective aperture assembly occupied the gap between the upper and lower objective pole pieces, leading to a gas reservoir aroimd the sample. Such ECELL systems were a major step forward in scientific capability, being used by Gai et al. (3,73-78), Doole et al. (79), Crozier et al. 

The aperture of an electron microscope is a piece of metal with a small hole in the center, similar to that in a light microscope. The primary functions of the aperture are to limit light scattering and/or to select the non-diffracted or diffracted beams. The objective aperture in a TEM is located in the gap between the pole pieces of the electromagnetic lens, as illustrated in Figure 3.5. The hole diameter of the objective aperture in the TEM is only in the order of tens of micrometers. 

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