NMR Spectroscopy of Polymers in Solution and in ... - ACS Publications

Chapter 16

Effect of Internal Donors in Propylene Polymerization Analyzed with NMR and the Internet

Downloaded by UNIV OF PITTSBURGH on February 25, 2016 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch016

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Kenichi Shimozawa , Masayoshi Saito , and Riichirô Chûjô

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ΤοΗο Catalyst Company, Ltd., 3-5, Chigasaki 3-Chome, Chigasaki-City, Kanagawa, 253-0041 Japan Department of Environmental and Material Engineering, Teikyo University of Science and Technology, 2525, Yatsusawa, Uenohara-machi, Kitatsuru-gun, Yamanashi 409-0193, Japan 2

The two-site model was applied to obtain stochastic parameters for MgCl /internal donor/TiCl solid catalyst component used in combination with Al(C H ) 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. 2

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Since the discovery of Ziegler-Natta catalyst, isotactic polypropylene has been widely used as a commodity material due to its low cost and excellent physical properties. As the modulus of the resin is closely related to isotacticity, an understanding in polymerization mechanism is important in the chemistry of propylene polymerization as well as in the industry.

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© 2003 American Chemical Society

In NMR Spectroscopy of Polymers in Solution and in the Solid State; Cheng, H. N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

209 Since 1962 (1), N M R has been one of the most powerful analytical tools for the determination of the microstructure of polypropylene. Many attempts have been made to describe statistical formation of isotactic polymer chains (2), and the two-site model is widely accepted as a desirable stochastic model for olefin polymerization^. This model has been proven to be valid for not only polyolefms but also vinyl-type polymers such as polystyrene. The model is especially useful to describe the polymerization mechanism by using pentad fractions of C - N M R spectra of polyolefms. Indeed, this model is effective toward a better understanding of the nature of the polymerization centers for propylene. In the area of isotactic polypropylene, there have been several publications (4-6) where the two-site model stochastic parameters are successfully utilized to explain the effect of external donors that are used in combination with solid catalyst component. However, few studies employing the two-site model have been made of the effect of internal donor on catalyst components. In this study, the authors focused on the effect of internal donors on stereoregularity of Ziegler-Natta catalyst systems with the aid of the two-site model analysis system now available through the Internet (7).


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The Two-Site Model The theory and the parameters of the two-site model have been presented elsewhere in detail (3), but is briefly reviewed here for convenience. The model is composed of three parameters; these are stochastic parameters describing the role of the first and the second sites, and the fraction of the polymers obtained from the first site. More precisely, the first parameter α is the probability of the selection of d (or Ϊ) monad in an asymmetric site; the second parameter σ is that of m diad in a symmetric site; and the last parameter ω is the fraction of the polymers obtained from the asymmetric site.

Experimental Catalyst Preparation 2

M g C l (30g, S.A. 11 m /g) and internal donor were placed in a 1 L stainless steel vibration mill pot with 50 balls (25 mm φ) under nitrogen and vibrated at room temperature for 30 h. The ground product (10 g as MgCl ) was reacted with T i C l (200 mL) in a 500 mL flask two times for two hours each at 110°C, followed by washing with η-heptane. The types of internal donor as well as the amount were varied to obtain a series of solid catalyst components. The T i and donor contents analyzed are summarized in Table 1. 2

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210 Propylene Polymerization The propylene polymerization was carried out in a 2.0-L stainless steel autoclave. In the presence of a small amount of η-heptane, A1(C H ) (1.32 mmol) and an external donor, cyclohexylmethyldimethoxysilane were placed in the autoclave, and then the catalyst (2.6 umol-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.


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C - N M R Measurements 13

C - N M R measurements were done only for p-xylene insoluble fractions. C - N M R spectra of the polymers were obtained on a JEOL GSX-270 spectrometer using 10 mm o.d. tubes. Sample concentration was about 5 wt% in l,2,4-mcWorobenzen/C D . The chemical shifts were referenced to T M S . The results were analyzed using an Internet system for two-site model analysis (7). 13

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Results and Discussion

The Effects of Internal and External Donors on α Figure 1 shows the relationship between α and the molar ratio of internal donor to T i (iD/Ti) in the solid catalyst, while Figures 2 and 3 show the relationship between ω 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 α became larger along with the increase in the iD/Ti. α 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 Bernoullian 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).


211 Table 1. The Catalysts and the Internal Donors Ti iD/Ti Internal Donor (molar ratio) Donor* Content Content (mmol/g) (mmol/g) 0.24 DEP la 0.06 0.25 DEP 0.22 0.34 0.65 lb lc DEP 0.41 0.88 0.36 2a DBP 0.10 0.31 0.03 DBP 0.37 0.27 2b 0.10 DBP 0.37 0.49 2c 0.18 *DEP, diethylphthalate; DBP, di-n-buthylphthalate.


Catalyst

Table 2. Polymerization Results XI Si/Ti Activity p-Xl* (mol/m (wt%) mmmm (g-PP/gol) cat.) (mol%) la 0 14,100 43.0 81.8 10 89.3 14,500 76.9 50 12,700 87.3 92.8 0 lb 18,800 61.8 10 92.9 23,600 92.1 93.7 50 21,600 94.7 lc 0 85.6 30,500 61.0 10 37,900 92.4 94.3 50 36,600 95.9 95.9 2a 0 19,700 80.7 37.0 10 15,600 79.6 89.9 50 14,100 91.9 87.9 0 2b 25,700 44.5 10 29,200 91.9 88.1 93.1 50 24,800 92.4 0 2c 44,700 56.5 85.9 93.7 10 41,100 92.0 95.4 50 36,500 94.4 */?-xylene insoluble. Catalyst

fraction α

ω

0.968 0.981 0.987

0.965 0.983 0.989

0.989 0.989 0.976 0.990 0.992 0.967 0.983 0.986

0.983 0.989 0.966 0.992 0.997 0.959 0.981 0.985

0.986 0.988 0.976 0.989 0.992

0.987 0.990 0.968 0.991 0.994


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1.00

0.99

Û

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0.98 Downloaded by UNIV OF PITTSBURGH on February 25, 2016 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch016

-ΤΓ"

TIC:

0.97 OSi/Ti=0 ASi/TNIO • SÎ/TN50 1

0.96

0.95 0.00

0.20

0.40 0.60 iD/Ti (molar ratio)

0.80

1.00

Figure 1. The relationship between iD/Ti and α (internal donor = DEP).

1.00

0.99

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0.97 OSi/Ti=0 ASi/Ti=10 Si/Ti=50

0.96

0.95 0.00

0.20


0.80

1.00

Figure 2. The relationship between iD/Ti and ω (internal donor = DEP).



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OSi/Ti=0 ASi/Ti=10 • Si/Ti=50

0.00

0.20


0.80

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Figure 3. The relationship between iD/Ti and ω (internal donor = DBP).


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The Effects of Internal and External Donors on ω ω shows an interesting profile for the range of this study where iD/Ti was varied using DEP as an internal donor. In the absence of external donor (Si/Ti = 0), ω becomes greater along with an increase in iD/Ti to reach its maximum at around iD/Ti = 0.5. However, it appears to drop gradually with further increase in iD/Ti. In contrast, in the case where external donor is incorporated, ω increases with an increase in iD/Ti, and drastically goes up at a point where iD/Ti exceeds ca. 0.6. These results may well imply complementary function of external donor on the formation of active isotactic polymerization centers for a specific region of iD/Ti in the catalyst component. We deduced the following hypothesis to account for such differences in the change of ω. In the region of iD/Ti

NMR Spectroscopy of Polymers in Solution and in ... - ACS Publications

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