Catalyst Initiator Site Control

Syndioselective polymerization by a Cs metallocene such as Me2C(Cp)(Flu)ZrCl2 proceeds by catalyst site control. A statistical model for syndioselective catalyst site control has been described in terms of the parameter p [Resconi et al., 2000]. Parameter p is the probability of a monomer with a given enantioface inserting at one site of the initiator p is also the probability of the monomer with the opposite enantioface inserting at the other site of the initiator. The pentad fractions are given by 

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The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

The polymer stereosequence distributions obtained by NMR analysis are often analyzed by statistical propagation models to gain insight into the propagation mechanism [Bovey, 1972, 1982 Doi, 1979a,b, 1982 Ewen, 1984 Farina, 1987 Inoue et al., 1984 Le Borgne et al., 1988 Randall, 1977 Resconi et al., 2000 Shelden et al., 1965, 1969]. Propagation models exist for both catalyst (initiator) site control (also referred to as enantiomorphic site control) and polymer chain end control. The Bemoullian and Markov models describe polymerizations where stereochemistry is determined by polymer chain end control. The catalyst site control model describes polymerizations where stereochemistry is determined by the initiator. 

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

Some chiral initiators have structures such that alternate monomer placements occur with opposite faces of the monomer to yield the syndiotactic polymer. This is syndioselective polymerization proceeding with catalyst site control and is usually observed only with some homogeneous initiators, both traditional Ziegler-Natta and metallocene. 

Not all syndioselective polymerizations proceed with polymer chain end control. Some metallocene initiators yield syndioselective polymerization through catalyst site control (Sec. 8-5). 

The open nature of the metal site limits catalyst site control by CpA initiators. Polymerization of propene proceeds with weak chain end control at low temperatures. The highest stereoselectivity reported is (mmmm) — 0.77 using Me2Si(Flu)(N-t-Bu)ZrCl2. 

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond of the monomer together with an electrophilic attack by the cationic counterion (G) on the alkene tt-electrons. The catalyst fragment acts essentially as a template or mold for the orientation and isotactic placement of incoming successive monomer units. Isotactic placement occurs because the Initiator fragment forces each monomer unit to approach the propagating center with the same face. This mechanism is referred to as catalyst site control or enantiomorphic site control. 

Sulfated titania has been investigated much less extensively than sulfated zirconia. Desmartin-Chomel et al. [97] have studied the acidic properties of sulfated titania using ammonia adsorption calorimetry and FTIR spectroscopy. The number of acid sites on the sulfated catalyst was noticeably increased, and dependent on the surface area of the original titania. The dispersion of the initial oxide controls the amount of sulfur retained by the solid and the thermal stability of the resulting sulfate. Ammonia adsorption is commonly used to determine the acidity of sulfated oxides however, it is also well-known that NH3 is a powerful reductmt, and that the acidity of sulfated zirconia is decreased by reduction. At low ammonia coverage, sulfated titanias exhibit a much lower heat of adsorption, and the IR study of NH3 adsorption showed that the first doses of NH3 dissociate at the surface with the formation of OH species. The lower heat of adsorption was then attributed to the contribution of NH3 dissociation to the differential heat of adsorption. This phenomenon has been observed for sulfated aluminas [109]. 

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