Kinetics metallocene polymerization

This interpretation explains why Ziegler, Ballard, and metallocene catalysts, which have a different initiation chemistry, can be supported on the same silicas, but display totally different kinetics (and much faster reactions) without a gradual rise in activity. It also explains how the kinetics of polymerization with a single Cr/silica catalyst can be so easily manipulated by the choice of the reaction temperature, the activation temperature, CO reduction (Figure 16), and incorporation of metal alkyls and poisons in the reaction mixture. 

Rg. 9.8. Mixed-metallocene polymerization of ethylene in a semibatch reactor branching (constrained geometry) catalyst CGC-Ti linear catalyst Et[lnd]2ZrCl2. Reactor and kinetic data initial concentration CGC-Ti 8 x 10 kmol m initial concentration Et[lnd]2ZrCl2 3.2 X 10 kmol m monomer molar feed... 

Fig. 9.9. Mixed-metallocene polymerization of ethylene in a CSTR. Kinetic data are the same as in Figure 9.8. Residence time CSTR 300 s feed concentrations identical to initial concentrations in Figure 9.8. Bivariate chain length/number of branches distribution. Hydrogen present. Based on molecular weight distribution and branching distribution from...

Like many homogeneously catalyzed reactions, the overall cycle (or cycles) in these polymerization reactions probably contains too many steps to be easily analyzed by any single approach. Both kinetics and model compound studies have thrown light on some of the steps. However, as indicated above, many of the model compounds isolated from the reactions of primary silanes with metallocene alkyls and hydrides are too unreactive to explain the polymerization results. 

Possible Back-Skip of Growing Chain. Several experimental facts relative to propene polymerization behavior of different metallocene-based catalytic systems can be rationalized by considering a disturbance of the chain migratory insertion mechanism due to a kinetic competition between the monomer coordination in the alkene-free state and a back-skip of the growing chain to the other possible coordination position (see Scheme 1.3). 

MAO is needed in large excess relative to the metallocene initiator, usually IO2 IO4 1, to achieve high activities and stable kinetic profiles. MAO is usually added first in a polymerization system, and a portion may actually serve the function of destroying deleterious impurities prior to the addition of the metallocene initiator. Otherwise, the impurities would destroy the metallocene if the metallocene were added first. 

Generally, metallocenes favor consecutive primary insertions as a consequence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the experimental conditions (especially temperature and monomer concentration). Secondary insertions cause an increased steric hindrance to the next primary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes. One of the isomerization processes observed in metallocene-catalyzed polymerization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139-142). The mechanism originally proposed to be of an elimination-isomerization-addition type is now thought to involve transition metal-mediated hydride shifts (143,144). 

Based on these kinetic and microscopic observations, olefin polymerization by supported catalysts can be described by a shell by shell fragmentation, which progresses concentrically from the outside to the centre of the support particles, each of which can thus be considered as a discrete microreactor. A comprehensive mathematical model for this complex polymerization process, which includes rate constants for all relevant activation, propagation, transfer and termination steps, serves as the basis for an adequate control of large-scale industrial polymerizations with Si02-supported metallocene catalysts [A. Alex-iadis, C. Andes, D. Ferrari, F. Korber, K. Hauschild, M. Bochmann, G. Fink, Macromol. Mater. Eng. 2004, 289, 457]. 

Because ansa metallocenes had been found to be effective polymerization catalysts, exploration of chelating bis(arylimido) complexes has been undertaken, typical examples being (48-51). In general, these have distorted octahedral structures with mo-n in the range 1.73-1.75A and ZMo=N-R in the range 155-162°. Complex (51), which is chiral, catalyzed the kinetic resolution of styrene oxide with ane.e of 30%. 

Ziegler-Natta Catalysts Kinetics of Ziegler-Natta Polymerizations Practical Features of Ziegler-Natta Polymerizations Comparisons of Cis-1,4-Polydienes Metallocene Catalysts... 

The efficacy of ansa metallocenes as polymerization catalysts has stimulated research into chelating bis(arylimido) complexes, including those with chiral ligands. The first of these, (62), and derivatives (63) and (64) (n= 1, 2) were reported by Gibson et al. in 1996.121 The complexes display distorted octahedral structures with d(Mo=N), Z(Mo=N—C) and Z(N=Mo=N) in the ranges 1.725-1.754 A, 155-162° and 100-103°, respectively. Complexes featuring strained, seven-membered, unsymmetrical ansa bis(imido ligands), e.g., (65), have also been reported.113 The first chiral bis(imido)-MoVI complex, C2-symmetric (66), catalyzes the kinetic resolution of styrene oxide and enantioselective trimethylsilylcyanation of benzaldehyde with 30% and 20% e.e., respectively.151... 

Propagation rates of first order in monomer concentration have been reported for ethylene and for propylene in the case of aspecific metallocenes266 as well as for propylene polymerization with isospecific metallocenes activated with MAO, B(C6F5)3, and [PhsCHBlCftFsL].290 297 Moreover, first-order kinetics were also observed for 1-hexene polymerization with the [ra(r-G2H4( 1 -Ind)2ZrMc [ McB(Gf,l 3)3. 156... 

A kinetic model has been proposed based on microstructural analysis, including both chain-epimerization and site-epimerization reactions in both C2- and C.-symmctric metallocenes, and rationalizing the observed pseudo-second-order kinetics of propylene polymerization promoted by C2-symmetric metallocene catalysts. This point has been extended to co-polymers.298 A thorough study of propylene polymerization with the Me2C(Cp)(9-Flu)ZrCl2 system in the presence of a large series of different counterions that rationalized the correlation between the nature of ion pair and the microstructure of the resulting PPs has been performed.104... 

For example, use of a neutral Sc-metallocene catalyst (45) shows kinetic deuterium isotope effects that were not present with Grubbs Ti system.89 It appears that deuterium isotope effects occur if insertion is rate-determining and if overall the rate of polymerization is relatively slow. 

Wang, W.-J. Yan, D. Zhu, S. Hamielec, A.E. Kinetics of long chain branching in continuous solution polymerization of ethylene using constrained geometry metallocene. Macromolecules 1998, 31 (25), 8677-8683. 

Compared to conventional heterogeneous Ziegler-Natta systems in which a variety of active centers with different structures and activities usually coexist, homogeneous metallocene-based catalysts give very uniform catalyt-ically active sites which possess controlled, well-defined ligand environments [37]. Consequently, the polymerization processes in homogeneous systems are often more simple, and kinetic and mechanistic analyses for these systems are greatly simplified [38]. 

The first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

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