Kinetic propylene polymerization

Propylene Polymerization Kinetics in Gas Phase Reactors Usii Titanium Trichloride Catalyst... 

The semibatch model GASPP is consistent with most of the data published by Wisseroth on gas phase propylene polymerization. The data are too scattered to make quantitative statements about the model discrepancies. There are essentially three catalysts used in his tests. These BASF catalysts are characterized by the parameters listed in Table I. The high solubles for BASF are expected at 80 C and without modifiers in the recipe. The fact that the BASF catalyst parameters are so similar to those evaluated earlier in slurry systems lends credence to the kinetic model. 

The kinetics of the reaction has been studied by IR as well as laser reflection interferometry (LRI) [21,145]. The amount of polymer grown on the surface was measured from the LRI signal as a function of time. It was shown that propylene polymerization was about 30 times slower than ethylene polymerization [145]. hi addition, Kim et al. estimated the polymerization ac-... 

Fio. 14. Apparatus used for kinetic measurements of propylene polymerization (reaction vessel rocking 45 times per min. through a 45° angle). PI — pressure gage, PC = pressure control, FI = flow indicator, TC = temperature control. 

The data here related on the kinetics of the propylene polymerization and of the transfer processes and the studies of the catalysts carried out with C-labelled alkylaluminums, derive from a series of researches mostly carried out some time ago, when the knowledge of the mechanism of the considered catalytic processes was still rather limited. Nevertheless, it helped remarkably to know these new processes of anionic coordinated polymerization their true catalytic nature (which regard to a-TiCU) differentiates them from the more usual polymerization processes (radicalic) which, actually, are not catalytic. They substantially contributed to demonstrate that the anionic coordinated polymerization is a step-wise addition process in which each monomeric unit inserts itself into a metal carbon bond of the catalytic complex. 

The alkylaluminum component combined with V(acac)3 has an influence on the kinetic behavior of the propylene polymerization at —78 °C47 82 83). In the polymerization with V(acac)3 and dialkylaluminum monohalide like Al(i-C4H9)2C1, Al(n-C3H7)2C1, A1(C2HS)2C1 or Al(C2H5)2Br, M of polypropylene increased proportionally to the polymerization time, and the polydispersity (M Mj was as narrow as 1.15 0.05 (see Fig. 9). This is the case of living polymerization. As can be seen from Fig. 9, the rate of increase of Mn, i.e. the rate of propagation of living chains as expressed by lvln/(42 t), is influenced by the kind of aluminum component and decreases in the series... 

Other catalysts, highly active in ethylene polymerization, have been obtained by co-milling MgCl2 with Ti compounds other than chlorides37). Even though these catalysts are active for the propylene polymerization, they are stereospecifically poor and have mainly been used to determine kinetic parameters at short polymerization times 38). 

For example, supported TiCl4/MgCl2 catalysts show a short period of acceleration, followed by a prolonged steady period 92,93). However, in the presence of electron donors, they may show the typical decay rate kinetics observed during propylene polymerization 93). Bulk catalysts prepared by interaction of TiCU with Mg(OR)2 show either a stationary rate, or a non-stationary rate, according to the titanium content 88,94). Bulk catalysts prepared by reduction of TiCl4 with organomagnesium compounds show a decay type rate 92-95>. 

Kinetic studies concerning TiCk/MgClj catalysts for propylene polymerization are relatively scarce. 

Galli100) found that the kinetic curve, regarding propylene polymerization in a slurry under industrial conditions, can be adequately described by an expression of this type ... 

Actually, studies on the propylene polymerization at atmospheric pressure carried out in our laboratories 101 > have demonstrated that R0 and the deactivation rate depend, in a complex manner, on both the organoaluminum and external donor concentrations (see Sect. 6.1.2 and 6.1.3). The kinetic curves obtained cannot be reduced to a single model for the deactivation of active centers according to a simple 1 st and 2nd order law, but rather they seem to follow a more complicated behavior. This is not surprising if one considers that the decay of polymerization rate is probably the effect of an evolution, in time, of a plurality of different catalytic species having different stability, reactivity and stereospecificity (see Sect. 6.3). 

Fig. 33. Kinetics of propylene polymerization with the vatalyst system TiCU/MgClj—TEA at different TEA concentrations. (Polymerization conditions T = 50°C,P = 1.05 bar,Ti = 0.05 mmol/1, solvent = hexane)...

Fig. 39. Kinetics of propylene polymerization with the catalyst system TiC /MgClj at different MPT/TEA molar ratios. (Polymerization conditions as in Fig. 38).

The existence of a further type of active centers was demonstrated by Pino and Rotzinger93> by polymerizing ethylene with a MgQ2-supported catalyst in the presence of an electron donor. A comparison of the ethylene and propylene kinetic curves shows that, while propylene polymerization is characterized by the well known rapid decrease in rate, the ethylene polymerization rate increases reaching a constant value after about 30 min. This has been attributed to the existence of active... 

From the above it is clear that the Cp and k values reported in the preceding section are only average values which do not reflect the real situation, although they are quite useful in understanding certain phenomena. The active species not only consist of isospecific and non-specific centers in the case of the propylene polymerization but, rather, by a plurality of species having different reactivities, which cannot be completely identified by kinetic studies or by catalyst poisoning. 

All this has been experimentally proved (in agreement with the proposed kinetic models though they cannot all be referred to the same mechanism) both in ethylene and propylene oligomerization (homophase system) and in some cases of ethylene and propylene polymerization, provided homogeneous catalytic systems are used. 

The kinetics of olefin polymerization are the subject of several studles>104,153-156,162,182,221,226,240,241,246,252,255,266,28 12 and of an excellent book by Keii.17 The most relevant studies will be discussed below. However, we first note that the precise description of the kinetics of catalytic olefin polymerization under industrially relevant polymerization conditions has proved to be very difficult. For a given catalytic system, one has to identify all possible insertion, chain-release, and chain-isomerization reactions, and their dependence on the polymerization parameters (most importantly, temperature and monomer concentration). Once the kinetic laws for each elementary step have been determined, they have to be combined in one model in order to be able to predict the catalyst performance. This has been attempted for both ethylene and propylene polymerizations. The case of propylene polymerization with a chiral, isospecific zirconocene is shown in Figure 14.162... 

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... 

Figure 9.7 Typical kinetic curves obtained during propylene polymerization by TiClj. (A decay type B build-up or acceleration type I build-up period II decay period III stationary period.)...

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. 

Figure 9.6 Typical kinetic curves observed during propylene polymerization with ground and unground TiCls- Curve A decay type curve B build-up or acceleration type zone I build-up period zone II decay period zone HI stationary period. (After Keii, 1973.)...

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