External donors polymerization

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). 

Chadwick, J.C. van Kessel, G.M.M. Sudmeijer, O. Regio- and stereospecificity in propene polymerization with MgCl2-supported Ziegler-Natta catalysts effects of hydrogen and the external donor. Macromol. Chem. Phys. 1995, 196,... 

Barino, L. Scordamaglia, R. Steric equivalence between internal and external donors as polymerization stereoregulators a molecular mechanics study. Macromol. Symp. 1995, 89, 101-111. 

The two-site model was applied to obtain stochastic parameters for MgCl2/intemal donor/TiCU solid catalyst component used in combination with A1(C2H5)3 and external donor for propylene polymerization. The type and the amount of internal donor were varied. With respect to the fraction for asymmetric site, the two-site model enabled us to conclude that new kinds of active centers are generated in specific cases where external donor is believed to be replacing weaker internal donor during polymerization. 

The propylene polymerization was carried out in a 2.0-L stainless steel autoclave. In the presence of a small amount of n-heptane, Al(C2Hs)3 (1.32 mmol) and an external donor, cyclohexylmethyldimethoxysilane were placed in the autoclave, and then the catalyst (2.6 pmol-Ti) was introduced at room temperature. After hydrogen (2.0 L) was charged, liquid propylene (740 g) was introduced and prepolymerization was conducted at 20 °C for 5 min. The temperature was then raised to 70 °C, and polymerization was conducted at 70 °C for 60 min. Typically, about 300 g of polypropylene powder were obtained. The results were summarized in Table 2. 

Figure 1 shows the relationship between a and the molar ratio of internal donor to Ti (iD/Ti) in the solid catalyst, while Figures 2 and 3 show the relationship between o and iD/Ti in the solid catalyst. In both cases the results are included where the polymerization was conducted with and without the external donor. Regardless of the presence of external donor, the value of a became larger along with the increase in the iD/Ti. a also increased as the Si/Ti in polymerization increases. This means that a, the probability of the selection of d (I) monad in the asymmetric Bemoullian site, is attributed to both the amount of internal donor of catalyst component and the amount of external donor during polymerization relative of active centers (Ti). 

To forecast the behavior of a catalyst system in an industrial continuous polymerization, these characteristics should be determined within a wide range of conditions eg, temperature, concentration, and ratios of the various components (activator, external donor, solid catalyst, etc). A laboratory batch-scale test can provide most of this information. A small, simple reactor suitable for these studies is shown in Figure 8. The polymerization can be carried out in a hydrocarbon or liquid propylene. In some cases the pol5unerization test can be performed in the gas phase, provided the reactor is prepared with a suitable heat transfer and catalyst dispersing bed (eg, a salt bed). 

There has been a large effort towards elucidating the roles of the external donor in the polymerization of propylene. Some of this work has centered on the... 

Tacticity measurements can be correlated with reaction mechanisms and physical properties. For example, the incorporation of an electron donor into the polymerization catalyst formulation has been found to increase isotacticity in a propylene-1-butene copolymer [123], and the distribution of propylene and 1-butene contents as a function of molecular weight varied, depending on donor type. External donors, such as dimethox-ysilane, decrease the butene content more than internal electron donors (in this case, di-n-butyl phthalate). Mechanisms of new polymerization reactions, such as the group-transfer copolymerization of methyl methacrylate and lauryl methacrylate, can be elucidated by comparing NMR-derived structural details [124]. The presence of unanticipated peaks in the spectrum of poly(ethylene-co-norbomene) suggest the occurrence of epimerization... 

A new type of catalysts contain a diether such as 2,2-disubstituted-l,3-dimethoxy-propane and have high stereospecficity even in the absence of an external donor [356]. The polypropene yield obtained under typical polymerization conditions (liquid monomer, 70 °C, 1-2 h) has increased from 30 80 kgPP/g cat for the third generation to 80-160 kg PP/g cat [342,357]. 

Correlations were sought between the structure of external alkoxy silane donors and the microstructure of the obtained polymer chain. Propylene was polymerized in liquid monomer with a heterogeneous high activity Ziegler-Natta catalyst. Fifteen different alkoxy silanes of structure RnSi(OR )4.n, where ns 1-3, R = Ph or alkyl and R = C1.3 alkyl, were used as external donors. Polymers were fractionated by boiling heptane extraction. Microstructures of the polymers were studied by means of C NMR. 

The structure of the donor had a marked effect on the catalyst activity and isotacticity of the polymer. Nevertheless, all of the alkoxy silanes tested produced qualitatively similar changes in the microstructure of PP compared to PP polymerized without external donor the size of the isotactic mmmm-pentad peak in the C NMR spectra varied with the donor, but the sizes the non-isotactic pentad peaks were more or less constant relative to one another, except for the syndiotactic rrrr-pentad peak, which relative to the other nonisotactic pentad peaks was decreased slightly in unfractionated PP and increased in the boiling heptane soluble fraction. These results suggest that all external alkoxy silane donor have the same qualitative effect on active centers, but the effectiveness and selectivity of deactivation stronc y depend on the donor structure. 

Our main purpose in this work was to study the influence of the stracture of alkoxy silanes as external donors in supported Ziegler-Natta Ti-catalyst on propylene polymerization behavior, on the microstmcture of the polypropylene, and on the active centers of the catalyst. 

---