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138 Time dependence of molecular weight in propylene bulk polymerization with MgCl2-supported ziegler-natta catalyst
533
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
138 Time Dependence of Molecular Weight in Propylene Bulk Polymerization with MgCI2-
Supported Ziegler-Natta Catalyst Fumihiko Shimizu I and Satoru Yamamoto 2
Mitsubishi Chemical Corporation, Science and Technology Research Center i 1000, Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan 2 1, Toho-cho, Yokkaichi, 510-8530, Japan INTRODUCTION In the chemistry of Ziegler-Natta catalyst, little has been known about the influence of time on molecular weight. We explored this influence and found that Mw value for the resultant polypropylene (PP) decreased with time even when 1-12 concentrations were kept constant. For the explanation of this phenomenon, a three-site model was proposed. In this paper, the formation of stereo-block polypropylene is reported. This experimental fact would be a good proof of the existence of a fluctuating site, which is one of the key points of the proposed three-site model. RESULTS AND DISCUSSION Polymerizations were carried out in liquid propylene at 70~
at constant H2 concentration
with the catalyst of type MgCl2/TiCl4/Diethyl Phthalate - AIEt3/RR'Si(OMe)2. Table 1 shows that both weight and number average molecular weight (Mw and Mn, correlated as PP in odichlorobenzene, respectively) decrease with time [1]. Furthermore, we found that the decrease in molecular weight occurs mainly in isotactic part of the produced PP. Table 1. Decrease in Mw and Mn values with Time ii
.
Time
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Mn x 10-3
.
.
.
.
.
.
Mw x 10-3
h
Run 1
Run 2
Run 3
Run 1
Run 2
Run 3
0.5
72.2
49.4
41.3
402
269
221
1.0
64.6
47.3
41.8
385
246
211
1.5
68.2
47.7
39.0
376
244
199
2.0
66.7
46.3
234
199
39.9 .
.
.
.
.
.
360 .
.
.
.
.
.
.
.
.
.
.
Condition: Catalyst = 12 mg, A1Eta= 2.0 retool, t-BuEtSi(OMe)2= 0.05 retool. Propylene bulk polymerization, 70~ H2 concentration in the gas phase of the reactor in the Run 1-3 was 4.7, 8.0 and I 1.0 vol%, respectively.
534 F. Shimizu et al. Figure 1 shows a probable three-site model that can explain why Mw decreases with time [1]. Dimethoxydialkoxysilane as an external donor reacts with A1Et3 during polymerization and monomethoxysilane is formed. Thus part of type C site (highly isotactic site) is transformed into another type C site (type C' site). The PP produced with the newly formed type C' site having the interaction with monoalkoxysilane is less isotacfic and has lower Mw than that of the original type C site probably because of flexible structure of type C' site (an equilibrium between type B and C' site) and the less electron-donating effect of monoalkoxysilane. ata~c PP
potential type A site
atactic PP low Mw
type A site
type A site
less isotactic PP
less isotactic PP Ti medium Mw
,u~
potential type B site - - - ' I
/
catalyst surface
potential type C site + RR,Si(OR)2 Before starting
polymerization
type B site
f l
!
R~
type B site ! i - RR'EtSi(OR)
AIEt~
~ IB
highly isotactic PP
type C site
' isota~cPP reduced Mw
type C' site
Initial stage in polymerization
RT ~ j
R' highlyisota~cPP R high Mw
type C site Later stage in polymerization
Figure 1. Three-Site Model of the Time Dependency of Mw. We ffactionated the obtained PP by n-heptane at 95~
and removed the insoluble part as a
highly isotactic part. Then we analyzed the soluble part by CFC, 13C-NMR, WAXD, and SAXS. The elution peak by CFC was at 110~ and the isotactic pentad mmmm was 0.39. Besides mmmm, all other pentads were also observed. WAXD showed that the soluble part contained distinct crystallites (or-form) but the crystailinity was fairly low (ca 20%). SAXS showed that there was no long-period structure. These features are typical for a stereo-block polypropylene having isotactic sequences [2]. The formation of a stereo-block polypropylene would be a good proof that there certainly is a fluctuating site as proposed in Figure 1. [3] REFERENCES [ 1] F. Shimizu et al., J. Appl. Polym. Sci., 81, 1035 (2001). [2] R. L. Kravchenko, Macromol., 33, 11 (2000). [3] V. Busico et al., Macromol., 32, 4173 (1999).
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
138 Time Dependence of Molecular Weight in Propylene Bulk Polymerization with MgCI2-
Supported Ziegler-Natta Catalyst Fumihiko Shimizu I and Satoru Yamamoto 2
Mitsubishi Chemical Corporation, Science and Technology Research Center i 1000, Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan 2 1, Toho-cho, Yokkaichi, 510-8530, Japan INTRODUCTION In the chemistry of Ziegler-Natta catalyst, little has been known about the influence of time on molecular weight. We explored this influence and found that Mw value for the resultant polypropylene (PP) decreased with time even when 1-12 concentrations were kept constant. For the explanation of this phenomenon, a three-site model was proposed. In this paper, the formation of stereo-block polypropylene is reported. This experimental fact would be a good proof of the existence of a fluctuating site, which is one of the key points of the proposed three-site model. RESULTS AND DISCUSSION Polymerizations were carried out in liquid propylene at 70~
at constant H2 concentration
with the catalyst of type MgCl2/TiCl4/Diethyl Phthalate - AIEt3/RR'Si(OMe)2. Table 1 shows that both weight and number average molecular weight (Mw and Mn, correlated as PP in odichlorobenzene, respectively) decrease with time [1]. Furthermore, we found that the decrease in molecular weight occurs mainly in isotactic part of the produced PP. Table 1. Decrease in Mw and Mn values with Time ii
.
Time
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Mn x 10-3
.
.
.
.
.
.
Mw x 10-3
h
Run 1
Run 2
Run 3
Run 1
Run 2
Run 3
0.5
72.2
49.4
41.3
402
269
221
1.0
64.6
47.3
41.8
385
246
211
1.5
68.2
47.7
39.0
376
244
199
2.0
66.7
46.3
234
199
39.9 .
.
.
.
.
.
360 .
.
.
.
.
.
.
.
.
.
.
Condition: Catalyst = 12 mg, A1Eta= 2.0 retool, t-BuEtSi(OMe)2= 0.05 retool. Propylene bulk polymerization, 70~ H2 concentration in the gas phase of the reactor in the Run 1-3 was 4.7, 8.0 and I 1.0 vol%, respectively.
534 F. Shimizu et al. Figure 1 shows a probable three-site model that can explain why Mw decreases with time [1]. Dimethoxydialkoxysilane as an external donor reacts with A1Et3 during polymerization and monomethoxysilane is formed. Thus part of type C site (highly isotactic site) is transformed into another type C site (type C' site). The PP produced with the newly formed type C' site having the interaction with monoalkoxysilane is less isotacfic and has lower Mw than that of the original type C site probably because of flexible structure of type C' site (an equilibrium between type B and C' site) and the less electron-donating effect of monoalkoxysilane. ata~c PP
potential type A site
atactic PP low Mw
type A site
type A site
less isotactic PP
less isotactic PP Ti medium Mw
,u~
potential type B site - - - ' I
/
catalyst surface
potential type C site + RR,Si(OR)2 Before starting
polymerization
type B site
f l
!
R~
type B site ! i - RR'EtSi(OR)
AIEt~
~ IB
highly isotactic PP
type C site
' isota~cPP reduced Mw
type C' site
Initial stage in polymerization
RT ~ j
R' highlyisota~cPP R high Mw
type C site Later stage in polymerization
Figure 1. Three-Site Model of the Time Dependency of Mw. We ffactionated the obtained PP by n-heptane at 95~
and removed the insoluble part as a
highly isotactic part. Then we analyzed the soluble part by CFC, 13C-NMR, WAXD, and SAXS. The elution peak by CFC was at 110~ and the isotactic pentad mmmm was 0.39. Besides mmmm, all other pentads were also observed. WAXD showed that the soluble part contained distinct crystallites (or-form) but the crystailinity was fairly low (ca 20%). SAXS showed that there was no long-period structure. These features are typical for a stereo-block polypropylene having isotactic sequences [2]. The formation of a stereo-block polypropylene would be a good proof that there certainly is a fluctuating site as proposed in Figure 1. [3] REFERENCES [ 1] F. Shimizu et al., J. Appl. Polym. Sci., 81, 1035 (2001). [2] R. L. Kravchenko, Macromol., 33, 11 (2000). [3] V. Busico et al., Macromol., 32, 4173 (1999).