Chien catalysts

As written. Equation 19 Implies a simultaneous loss of two sites of the same type. On a heterogeneous catalyst this is only realistic for adjacent sites, as has recently been suggested by Chien (15). Equation 19 assumes adjacent sites are the same species, which appears consistent with active site structural models appearing in the literature (17-18). Performing the same... 

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

Chien JCW, Babu GN, Newmark RA, Cheng HH, Llinas GH (1992) Microstructure of elastomeric polypropylenes obtained with nonsymmetric ansa-titanocene catalysts. Macromolecules 25 7400-7402... 

Chien JCW, Llinas GH, Rausch MD, Lin YG, Winter HH, Atwood JL, Bott SG (1992) Metallocene catalysts for olefin polymerizations. XXIV. Stereoblock propylene polymerization catalyzed by rac-anri -ethylidene(l-T 5-tetramethylcyclopentadienyl)(l-r 5-indenyl) dimethyltitanium A two-state propagation. J Polym Sci A 30 2601-2617... 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

The authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

Chien, MW Pearson, IM Nobe, K. Reduction and absorption kinetics of nitric oxide on colbalt perovskite catalysts. Ind. Eng. Chem., Prod. Res. Dev., 1975, Volume 14, 131-134. 

A Iky nes have been polymerized using ionic and radical initiators, but the polymer molecular weights are low. High molecular weights are obtained by using Ziegler-Natta coordination catalysts (Sec. 8-4d-2) [Chien et al., 1980]. The polymers are of considerable interest in terms of their potential as (semi)conducting materials. 

If it is borne in mind that usually less than % of the titanium atoms are part of a catalyst center, the weak basis of all of the mechanistic and kinetic approaches can be understood. It is still quite unknown how the propagation centers are formed. Whatever is measured, the result is obtained with a substance that contains only a small concentration of the moiety of interest. Nevertheless, the kinetic results summarized by Keii (S), Cooper (50), and Chien and Hsieh (20) are of great value and have been extended with new and more precise results (76-78, 88, 89). 

Breslow (139) discovered a homogeneous system well suited for kinetic analysis. He realized that bis(cyclopentadienyl)titanium(IV) compounds, which are very soluble in aromatic hydrocarbons, could be used instead of titanium tetrachloride as the transition-metal compound together with aluminum alkyls to give Ziegler catalysts. Subsequent research on this and other systems with various alkyl groups has been conducted by Natta et al. (140, 141), Belov el at. (142-144), Patat (145), Patat and Sinn (146) Sinn et al. (119, 147), Shilov and co-workers (148-150), Chien and Hsieh (20), Adema (151), Clauss and Bestian (152), Henrici-Olive and Olive (153), and Reichert and Schoetter (154) and Fink (155). 

Chien, J. C. W., Most Advanced Magnesium Chloride Supported Ziegler-Natta Catalyst , in Catalysis in Polymer Synthesis, ACS Symp. Ser. 496, Washington, DC, 1992, pp. 25-55. 

Chien, J. C. W., Advances in Ziegler Catalysts. Syndiotactic Styrene Polymerisation , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 209-210. 

Subsequent research on this and other systems with various alkyl groups was conducted by Natta (39), Belov et al. (40,41), Patat and Sinn (42), Shilov et al. (43, 44), Chien (45), Adema (46), Clauss and Bestian (47), Henrici-Olive and Olive (48), Reichert and Schoetter (49), and Fink et al. (50, 51). Investigations of kinetics and various other methods have helped to define the nature of the active centers of some homogeneous catalysts, to explain aging effects of solid Ziegler catalysts, to establish the mechanism of the interaction of the catalyst with olefins, and to provide quantitative evidence of some elementary steps (10). 

A similar picture can be drawn for supported type catalysts. Some relevant data concerning propylene polymerization are plotted in Fig. 6 using the same approach as in Fig. 5. Data of Chien and Kuo 64) are compared with those of Bukatov et al. 97). Two scanty sets of experimental points obtained at 70 °C are complemented by the dashed curve calculated from Fig. 1 of paper 64 ) obtained at 50 °C. Even though it must be admitted that kp values may not necessarily be the same for the two systems investigated, the tendency of obtaining lower (and perhaps more realistic) kp figure for longer contact times is quite clear. 

Such a different conclusion can be understood by considering the difficulties connected to the experimental determination and the definition of Thiele modulus parameters, such as So and D. According to Chien, S means the catalyst primary particle size with a value of about 10 cm for a-TiCIj instead, in the Multigrain model, Sp seems to correspond to the size of the whole catalyst granule. 

There is little information on rates of initiation in coordination polymerization. With the soluble catalyst (7r-CsHs)2TiCl2—AlMejCl for ethylene, Chien [111] reported = 4.99 x 10 1 mole s at 15°C Ei = 15.5 kcal mole ). This was determined from the rate of incorporation of C14 labelled AlMe2 Cl into the polymer. In most systems examined the initiating centres have been preformed and reaction starts immediately. This is the case with Natta type catalysts from transition metal subhalides with the alkyls of aluminium, beryllium and zinc. With some catalysts the rate rises rapidly to a steady value but with others there is a rapid rise to a maximum followed by a decline to a constant rate, the latter being... 

There is some disagreement as to how firmly metal alkyls are adsorbed on titanium trichloride. Keii et al. [112] state that AlEtj is removed by washing with hydrocarbon solvent with loss of polymerization activity which is restored by the addition of further AlEts- Natta [81] found that washed catalyst contained substantial amounts of adsorbed aluminium alkyl and retained its polymerization activity. Chien [86] likewise concluded that treatment of TiCl3 with AlEtjCl resulted in surface alkylation. With the precipitates from TiCl4 and aluminium alkyls there are similar discrepancies. Most data (Table 1) show the presence of substantial amounts of adsorbed alkyl, but Saltman et al. [41] and Tepenitsyna et al. [225] both reported that washing with hydrocarbon solvent removed adsorbed aluminium alkyl and polymerization activity. ... 

J. C. W. Chien, Recent advances in supported high mileage catalysts for olefin polymerization. Transition Metal Catalyzed Polymerizations (R. P. Quirk, ed.), Cambridge University Press. Cambridge, 1988, pp. 55-83. 

Chien s kinetic model [48,49], unlike Ewen s model described above, is for the systems in which more than one active species is present. The model assumes the presence of multiple active center types, chain transfer to MAO, chain transfer by /3-H elimination (see p. 801), and first-order deactivation reactions of active centers. Chien applied the model in the study of ethylene polymerization with Cp2ZrCl2/MAO catalyst and propylene polymerization with Et(Ind)2ZrCl2/MAO and Et(H4lnd)2ZrCl2/MAO catalysts. 

Based on the analyses of experimental results, some researchers have suggested that there are at least two types of active species, which have diflferent activity and stereospecificity, formed in the homogeneous catalyst systems. Chien [49] indicated that in the case of the Et(H4lnd)2ZrCl2/MAO system, two types of active species (see Table 9.8) coexist in about equal amounts one has higher selectivity, 10-20 times greater rate constant of propagation, and a factor of 5-15 times faster chain transfer to MAO than the second type of active species. Metallocene complexes with different... 

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