Macromolecular Research, Vol. 19, No. 6, pp 622-628 (2011) DOI 10.1007/s13233-011-0614-5
www.springer.com/13233
Control of Molecular Weight Distribution for Polypropylene Obtained by Commercial Ziegler-Natta Catalyst: Effect of Electron Donor He-Xin Zhang1,2, Young-Joo Lee3, Joon-Ryeo Park3, Dong-Ho Lee*,4, and Keun-Byoung Yoon*,2 1
Laboratory of Polymer Engineering, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130-022, PR China 2 Department of Polymer Science and Engineering, Kyungpook National University, Daegu 702-701, Korea 3 Catalyst&Process Development Division, R&D Center Samsung Total Petrochemicals Co. Ltd., Chungnam 356-874, Korea 4 Dongyang Mirae University, Seoul 152-714, Korea Received November 23, 2010; Revised January 30, 2011; Accepted January 31, 2011 Abstract: Polymerization of propylene was carried out using a MgCl2-supported TiCl4 catalyst in conjunction with triethylaluminium (TEA) as the cocatalyst and various types of alkoxy silane compounds as an external donor. The effect of the external donor on the performance of the catalyst with different internal donors was investigated. The polydispersity index (PDI) of polypropylene (PP) obtained with the diether and succinate based catalyst were decreased with the introduction of an external donor and the PDI increased for the phthalate based catalyst. The molecular weight and PDI increased with the introduction of an external donor. The highest PDI of PP was obtained by polymerization with di-n-propyldimethoxysilane (DnPDMS) as an external donor. In addition, a mixture of external donors was used to control the PDI of PP and the composition of the catalyst was examined after treated with TEA/external donor. Furthermore, the theoretical PDI value was calculated for a mixture of external donor systems. The PDI of PP could be controlled and predicted while retaining high activity, high isospecificity and high molecular weight by changing the structure of the external donor and/or their mixture. Keywords: polypropylene, molecular weight distribution, internal donor, external donor.
properties depending on the synthetic conditions.4-7 The nature of the added external donor can strongly modify the tacticity of the resulting polymers, which can range from atactic PP to extremely isotactic PP, the molecular weight distribution, which can be rather narrow or rather broad.8 Alkoxysilanes were cited as external donors in the large number of research for Ziegler-Natta catalyst systems.12-15 The beneficial effects of alkoxysilanes became known in the early 80’s. Aromatic ester type Lewis bases were used as external donor such as ethyl benzoate,3,6 methyl p-toluate12 or ethyl anisate16 before using silane compounds. Another group of commonly used external donors were the bulky amines, e.g. 2,2,6,6-tetramethylpiperidine.14 External donors markedly increase the stereospecificity of PP and usually decrease the activity of Ziegler-Natta catalysts.3-6,12,14 Some studies on the influence of external alkoxysilane donor on molecular weight distribution have been reported.3,12-14 Soga et al.3 reported that the gel permeation chromatography (GPC) curve tend to shift towards higher molecular weight region by addition of triethoxyphenylsilane as an external donor. Seppala et al.17 suggested that the use of external donor increases the average molecular weight of the polymer.
Introduction The world market for polypropylene (PP) was currently over 30 × 106 metric tons/year, and more than 90% world production of PP was produced by Ziegler-Natta catalyst system.1,2 Over the years, these catalysts have evolved from simple TiCl3 crystals to the multicomponent MgCl2/donor/TiCl4 catalyst systems, where the donor was a Lewis base that can be added during catalyst preparation (the so called internal donor).3 1,3-Diether,4,5 aromatic ester (benzoate and phthalate),3,6,7 and aliphatic ester (succinate)8,9 were widely used as internal donors. The internal donor plays its role during the preparation of the solid catalyst when it competes with TiCl4 for coordination to the MgCl2 support and shields the faces of the support where only aspecific centers could be formed from TiCl4 coordination.10 During polymerization, internal donor was removed from catalyst due to complex formation with the alkyl-aluminium cocatalyst, so that external donor was needed to supply for high isotacticity.4,11 The resulting catalyst system was chemical complexity and the resulting PP present very different *Corresponding Author. E-mail:
[email protected] The Polymer Society of Korea
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Control of Molecular Weight Distribution for Polypropylene Obtained by Commercial Ziegler-Natta Catalyst: Effect of Electron Donor
The effect of external donor structure on propylene polymerization with MgCl2/DiBP/TiCl4 catalyst has been studied intensively by Proto and Harkonen et al.14,17,18 However, the effectiveness of external donor types was always influenced by the structure of internal donor used. The performance of external donor was similar and reproducible with the same internal donor, while varied widely if a different internal donor was used.4 It was well known that molecular weight distribution has a significant effect on the processability and end-use properties of PP.19,20 The aim of the present work was to study the effect of external donor structure (both hydrocarbon and alkoxy substituents) on catalyst activity, isospecificity, molecular weight and molecular weight distribution of PP by using MgCl2-supported TiCl4 catalyst system. The effect of the mixture of external donors on propylene polymerization was also investigated.
Scheme I. Structure of external donor.
Experimental Materials. The MgCl2-supported Ti-based catalyst (CatA, Cat-B, Cat-C) and external donor (phenyltriethoxysilane (PTES), dicyclopentyldimethoxysilane (DCPDMS), diisopropyldimethoxysilane (DiPDMS), propyl-i-propyldimethoxysilane (PiPDMS), di-n-propyldimethoxysilane (DnPDMS)) were kindly supplied by Samsung Total Petrochemicals Co., Ltd., Korea. The catalyst compositions were shown in Table I and the structure of external donor were given in Scheme I. Polymer grade propylene was provided by Korea Petrochemical Ind. Co., Ltd., Korea. Triethyl aluminium (TEA, 1.0 M solution in n-hexane) was purchased from Tosoh Akzo Co., Ltd., Japan. n-Hexane was distilled from sodium/ benzophenone under nitrogen prior to use. Polymerization Procedure. The polymerization was carried out in a 300 mL glass reactor equipped with a magnetic stirring bar. The reactor was back-filled three times with nitrogen and charged with the required amount of n-hexane. At the stipulated temperature of 40 oC, the reaction solution was vigorously stirred under 1 atm of propylene for the desired period of time after which the cocatalyst (TEA) was added to the reactor. After cocatalyst was added, the external donor and a catalyst solution were injected respectively and then the polymerization started with a continuous feed of propylene. Propylene pressure was kept constant throughout the polymerization through use of a bubbler. After 2 h, the polymerization was terminated by adding 10% HClTable I. Composition of MgCl2/ID/TiCl4 Catlaysts Cat-A
Type of Internal Donor
Ti Content (wt%)
Diether
3.1
Cat-B
Phthalate
2.0
Cat-C
Succinate
2.4
Macromol. Res., Vol. 19, No. 6, 2011
methanol solution, and then the mixture was poured into 500 mL of methanol to precipitate the polymer and followed by drying under vacuum at 60 oC to its constant weight. Characterization. The obtained polymers were fractionated by extraction with boiling n-heptane for 8 h to determine the isotactic index (I.I.) and the I.I. values reported for each samples were the weight percentage of n-heptane insoluble polymer. The melting temperature (Tm) of the obtained polymer was determined by differential scanning calorimetry (DSC, DuPont TA 4000, TA Instruments) operating at a heating rate of 10 oC/min. The PP cooled down from melt (200 oC) to 30 oC at a rate of 10 oC/min, and the melting point and heat of fusion (∆Hm) was determined in the second scan. The crystallinity (Xc) calculated with 100% crystalline PP (∆Hm=209 J/g).21,22 The electro-donor content in the solid catalyst was studied by gas chromatograph (GC, Agilent Technologies, 6890N). The molecular weight and PDI of n-heptane insoluble PP fractions were determined by GPC measurement with 1,2,4trichlorobenzene as solution at 135 oC using a PL-220 (Polymer Laboratories) equipped with a refractive-index detector.
Results and Discussion Effect of Internal Donor on Performance of External Donor. The contents and structure of internal donors have influenced the performance of external donor on propylene polymerization. The polymerization was carried out by using three kinds of MgCl2/ID/TiCl4 catalysts in conjunction with TEA in the absence and presence of DCPDMS as external donor. The experimental results are given in Table II. The catalyst activity of Cat-B was similar to Cat-C and the highest catalyst activity was obtained by Cat-A. It means that the catalyst activity was strongly affected by the structure 623
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Table II. Effect of Internal Donor on Performance of Catalyst in the Absence and Presence External Donora Catalyst
a
External Donor
Activityb
I.I. (wt%)
Tm (oC)
Xc (%)
Mwc
PDI
Cat-A
-
9.1
92.6
161.2
48.1
48.8
6.7
Cat-A
DCPDMS
8.8
98.2
162.6
49.3
54.1
5.7
Cat-B
-
3.7
78.2
159.6
30.3
34.8
5.4
Cat-B
DCPDMS
4.3
98.8
165.1
41.3
61.0
6.5
Cat-C
-
4.0
90.2
160.4
46.7
68.7
7.5
Cat-C
DCPDMS
3.7
98.9
161.1
48.8
75.6
-4
o
6.9 b
Polymerization Condition: [Ti]=1.2×10 mol/L, [Al]/[Ti]=400, [external donor]/[Al]=0.1, atmosphere pressure, 40 C, 2 h. Activity: Kg-polymer/(g-Ti h). c×10-4 g/mol.
of internal donor and the catalyst activity decreased in the following order: diether > phthalate ≈ succinate. In addition, the catalyst activity decreased with the introduction of external donor for Cat-A and Cat-C polymerization system, while the catalyst activity of Cat-B with phthalate as internal donor increased. It was well known that the influence of external donor on MgCl2-supported TiCl4 catalyst can be explained by the coexistence of two concurrent effects: (a) poisoning of the non-stereospecific sites and (b) activation of the isospecific one.13 When the latter effect prevails over the former one can observe not only an increase of I.I. but also an increase of the polymer yield. And then, the improved catalyst activity for Cat-B could be ascribed to the more isospecific active sites were activated by the DCPDMS. Among the catalysts used in the present research, the CatA and Cat-C with diether and succinate as internal donor showed a higher I.I. value than Cat-B with phthalate as internal donor in the absence of external donor. The higher I.I. could correspond to the higher content of internal donor remained in the solid catalyst after treated with TEA cocatalyst. Correa et al.10 reported that the donor coordination to MgCl2 fragment reflects the relative Lewis base strength of oxygen atom, while the coordination energies of the oxygen atoms were in the order of: ether > carboxyl >> ester. It means that the coordination strength of different internal donor to the solid catalyst was in the following order: diether > succinate >> phthalate. Albizzati23 found that only trace amount (ca. 2 wt%) of diether internal donor was extracted from the MgCl2/1,3-diethers/TiCl4 catalyst system after treated with TEA, while near 70 wt% of internal donor was extracted from the MgCl2/DiBP/TiCl4 catalyst system by TEA. However, the almost equaled I.I. value was obtained by polymerization with different catalysts in the presence of DCPDMS as external donor. It could be ascribed to the poison of nonstereospecific site by external donor after internal donor was extracted by TEA.4,11 The Tm and Xc of PP were slightly influenced by the kind of internal donor in the solid catalyst. The Cat-A showed a higher Tm and Xc in the absence of external donor and both of them was improved with the introduction of external donor. The Tm of PP was in the range from 159.6 to 165.1 oC, and the 624
Figure 1. Molecular weight distribution curves of PP obtained by polymerization with (a) Cat-A, (b) Cat-B, and (c) Cat-C in the absence and presence DCPDMS. Macromol. Res., Vol. 19, No. 6, 2011
Control of Molecular Weight Distribution for Polypropylene Obtained by Commercial Ziegler-Natta Catalyst: Effect of Electron Donor
Xc was in the range from 30.3% to 49.3%. Figure 1 showed the molecular weight distribution curves of the n-heptane insoluble fractions. The samples obtained with three types of MgCl2/ID/TiCl4 catalysts in the absence and presence of DCPDMS as external donor. The molecular weight distribution curves of Cat-A were unimodal and symmetrical. In addition, the molecular weight distribution curves were less changed with the introduction of external donor for Cat-A. It could be corresponded to the small amount of diether internal donor extracted by TEA and the active center of catalysts could not be changed. Albizzati23 already reported that only small amount of diether internal donors was extracted by TEA. In contrast to Cat-A, the lower molecular weight fraction (logMw ≈ 5.1) of the molecular weight distribution curve shifted to higher molecular weight region (logMw ≈ 5.7) for Cat-B. With regard to Cat-C, a small shoulder peak can be observed at lower molecular weight region (logMw ≈ 5.1) of the molecular weight distribution curve for PP obtained without external donor. However, the shoulder was disappeared by the introduction of external donor, while the peak of higher molecular weight fraction (logMw ≈ 5.7) was improved. The shift of molecular weight distribution curve could be ascribed to the internal donor were replaced by the alkoxysilane compound lead to increase the molecular weight of PP. With respect to Cat-B, a large amount of phthalate internal donor was replaced by alkylsilane external donor.23 It is considered that the replacement of phthalate internal donor by external donor leads to form different catalyst active site. In addition, the coordination strength of different internal donor to the solid catalyst was in the following order: diether > succinate >> phthalate.10 Thus, the molecular weight distribution curve of PP obtained with Cat-B was more shifted, however, curves of Cat-A and Cat-C almost no shift and slightly shifts, respectively. The increased molecular weight of n-heptane insoluble fraction of PP obtained with MgCl2/DiBP/TiCl4 catalyst system in the presence of alkoxysilane was also reported by Sacchi et al..13 On the other hand, we suggested that the magnitude of the molecular weight distribution curve change was related to the effectiveness of the external donor in the different catalyst. Among the whole experiments, the effectiveness of external donor in the different catalyst was in the following order: Cat-B >> Cat-C > Cat-A. As can be seen in Table II, the highest weight average molecular weight (Mw) and PDI were obtained by Cat-C (Internal donor: succinate). In general performances of different internal donor catalysts, phthalate and diether based catalysts were characterized by provide medium and narrow molecular weight distribution of PP, while succinate based catalysts were found to provide broad molecular weight distribution of PP.8 The different PDI could be ascribed to the different active sites and/or numbers of active sites in every catalyst.24,25 The PDI value of PP increased with the introduction of external donor for Cat-B, while the PDI decreased Macromol. Res., Vol. 19, No. 6, 2011
for Cat-A and Cat-C. It may presumably be due to the different effectiveness of external donor in the different catalyst. With regards to Cat-A and Cat-C, the molecular weight distribution curve was less influenced by the external donor and only partial low molecular weight fraction shifted to higher molecular weight region. With respect to Cat-B, most of the phthalate internal donor was replaced by alkoxysilane and the solid catalyst including alkoxysilane always lead to yield higher molecular weight, thus almost 40% of low molecular weight fraction shifted to the higher molecular weight region. Effect of External Donor Structure. The external donor shows strong effect on the catalyst activity, I.I. and molecular weight, as well as molecular weight distribution of the PP. In contrast to Cat-B with phthalate as internal donor, the composition of Cat-A and Cat-C with diether and succinate as internal donors were less changed after react with TEA. Thus, the effect of external alkoxysilane donor structure on propylene polymerization was carried out by using Cat-B (internal donor: DiBP) in conjunction with TEA. The polymerization behaviors were examined with various kinds of external donor, PTES, DCPDMS, DiPDMS, PiPDMS, and DnPDMS. The results are summarized in Table III. The catalyst activity was influenced by the structure of external donors. The results could correspond to the size and number of alkoxy group, size of hydrocarbon group of alkoxysilane. With regards to the polymerization results, the lowest catalyst activity was obtained by PTES and the highest catalyst activity was obtained by DnPDMS. As reported by Seppala et al.17 the most important factor of the catalyst activity and the properties of obtained polymer were the number of alkoxy groups attached to the silicon atom, the more alkoxy groups lead to effectively deactivate the active centers of catalyst. In addition, the size of hydrocarbon groups bonded to the silicon atom was an important factor too. Small and even long linear alkyl groups do not deactivate polymerization centers, while the activity was slightly low, when small alkyl groups were replaced by the longer ones. However, the important factor was not the absolute Table III. Effect of External Donor Structures on Propylene Polymerizationa External Donor
a
Activityb
I.I. (wt%)
Mwc
PDI
None
3.7
78.2
34.8
5.4
PTES
2.6
96.9
50.6
6.2
DCPDMS
4.3
98.8
61.0
6.5
DiPDMS
3.1
97.6
67.5
8.1
PiPDMS
4.4
97.6
66.4
8.2
DnPDMS
4.5
97.4
59.0
9.5
-4
Polymerization condition: [Ti]=1.2×10 mol/L, [Al]/[Ti]=400, [external donor]/[Al]=0.1, atmosphere pressure, 40 oC, 2 h. bActivity: Kgpolymer/(g-Ti h). c×10-4 g/mol.
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size of the hydrocarbon group, but the steric hindrance which it creates, inhibiting the electron donating effect of the alkoxy groups.17 Due to the co-effect of the hydrocarbon and alkoxy substituents in the alkoxysilane compound, the catalyst activity decreased in the following order: DnPDMS > DCPDMS > None > DiPDMS > PTES. PiPDMS = The I.I. value increased significantly when external donor was employed in the polymerization. According to the Proto,14 the stereospecificity of the catalyst system was affected by the structure of alkoxysilane, while the bulkiness of the alkyl and alkoxy substituents on the silicon seems to play different roles. It was found that an increase of bulkiness of the alkyl group had increased the I.I., while the I.I. decreased with increasing bulkiness of alkoxy groups. The maximum I.I. value (98.8 wt%) was obtained by the DCPDMS as external donor. It was well known that the presence of two bulky alkyl substituents was important for the selective poisoning of the non-stereospecific sites.13 The molecular weight distribution curves of the n-heptane insoluble fractions are shown in Figure 2. As given in Table III, the molecular weight of PP was improved with the introduction of Lewis bases. In addition, Figure 2 showed the molecular weight distribution curve slightly shifted to higher molecular weight fraction when the external donor added. This phenomenon could correspond to the introduction of external donor lead to increase in isotactic PP yield, it is well known that the isotactic polymer has a higher molecular weight than atactic polymer in the heterogeneous catalyst system.13,26,27 The highest molecular weight of PP was obtained with DiPDMS as external donor, which consisted two bulky iso-propyl substituents. Additionally, the Mw increased with increasing the size of hydrocarbon groups in external donor for DiPDMS, PiPDMS, and DnPDMS. However, the Mw of PP obtained with DCPDMS was lower than PiPDMS, while the PP obtained with PTES donor showed a lower Mw. Seppala et al.17 sug-
Figure 2. Molecular weight distribution curves of isotactic PP obtained by polymerization in the absence and presence of external donors. 626
gested that no clear correlation was found between the average molecular weight and the structure of alkoxysilane donor and the molecular weight of that composed with methoxy group was higher than that with ethoxy group. The Mw of PP was in the range from 50.6×104 to 67.5×104 g/mol by changing the structure of external donor, while the Mw of PP obtained without external donor was only 34.8×104 g/ mol. The PDI of PP obtained with external donor was higher than the polymerization without external donor. As shown in Figure 2, the molecular weight distribution curve of medium molecular weight fraction (log Mw ≈ 4.5~5.5) shifted to higher molecular weight fraction (log Mw > 5.5), while the lower molecular weight fraction (log Mw < 4.5) was less changed. It was suggested that the increment of PDI could correspond to the activation of the active sites by external donor lead to increase the medium to higher molecular weight fraction of PP, and then the broad molecular weight was obtained. The PDI value of obtained PP could be controlled from 6.2 to 9.5 by changing the external donor, while the PDI of PP obtained in the absence of alkoxysilane donor was 5.4. Effect of the Mixture of External Donors. The effect of external donor on propylene polymerization was reported by many researchers.3-6,12-14,17,18 According to the previous section, the molecular weight was affected by the structure of external donor. In order to produce PP with broad molecular weight distribution, the polymerization was carried out with Cat-B and the mixture of external donors. The influence of external donors on composition of catalyst was studied by GC analysis of the base content in the solid catalyst under the polymerization condition in the absence of propylene monomer. The composition of catalyst is shown in Table IV. As reported by Sacchi et al.,13 the Ziegler-Natta catalyst was reacted with a mixture of TEA and external donor, two kinds of chemical transformations took place. The internal donor was extracted from the solid catalyst surface by the reaction with TEA, and external donor took its place. The content of internal donor (DiBP) significantly decreased after treated with TEA/external donor, while the total electro donor (internal + external donor) content increased. The total electron donor content was in the ranged from 4.46 to 5.75 wt% after treated with TEA/external donor. The effect of PTES, PiPDMS and their mixtures on propylene polymerization was given in Table V. The catalyst activity was diminished by the introduction of PTES as external donor, while the activity was improved by PiPDMS. As expected, the catalyst activity increased with the increase of PiPDMS feed mole ratio. The I.I. value was improved by the introduction of PiPDMS as a second external donor. However, future increase the amount of PiPDMS lead to less changed in I.I. value. The I.I. value of obtained PP was in the range from 96.9 to Macromol. Res., Vol. 19, No. 6, 2011
Control of Molecular Weight Distribution for Polypropylene Obtained by Commercial Ziegler-Natta Catalyst: Effect of Electron Donor
Table IV. Base Content of the Catalyst Treated with Cocatalyst and External Donora Feed Mole Ratio (PTES:PiPDMS)
Donorb
1:0
4:1
2:1
1:1
1:2
1:4
0:1
ED Free
Internal Donor
0.63
0.34
0.51
0.21
0.36
0.34
0.23
2.41
PTES
5.12
3.16
2.87
2.66
2.41
1.29
-
-
PiPDMS
-
1.01
1.08
1.80
2.39
3.46
5.33
-
Total
5.75
4.46
4.67
5.16
5.09
5.56
a
4.51 -4
o
2.41 b
Donor analysis by GC: [Ti]=1.20×10 mol/L, 100 mL, 40 C, atmosphere pressure, [Al]/[Ti]=400, [external donor]/[Al]=0.1, 1 h. ×103 mol/gcatalyst.
Table V. Effect of External Donor on Propylene Polymerizationa Feed Mole Ratio (PTES:PiPDMS)
Activityb
I.I. (wt%)
Mwc
PDI
Theoretical PDI
1:0
2.3
96.9
50.6
6.2
6.2
4:1
2.2
97.6
50.1
7.1
6.9
2:1
2.4
97.9
60.3
8.0
7.3
1:1
3.1
97.8
65.4
8.6
7.7
1:2
3.5
97.6
67.3
8.3
8.1
1:4
4.1
97.5
63.4
9.0
8.3
0:1
4.0
97.8
66.4
8.6
8.6
a
Polymerization condition: [Ti]=1.2×10-4 mol/L, [Al]/[Ti]=400, [external donor]/[Al]=0.1, atmosphere pressure, 40 oC, 2 h. bActivity: Kg-polymer/(g-Ti h). c×10-4 g/mol.
Figure 3. Molecular weight distribution curves of PP obtained by polymerization in the presence of PTES:PiPDMS=(a) 1:0, 0:1, (b) 4:1, (c) 2:1, (d) 1:1, (e) 1:2, (f)1:4 mixture.
97.9 wt%. The effect of PTES:PiPDMS feed mole ratio on Mw of PP was given in Table V and the molecular weight distribution Macromol. Res., Vol. 19, No. 6, 2011
curves were shown in Figure 3. The Mw and PDI of the obtained PP were improved by the addition of PiPDMS as second external donor, while the Mw 627
H.-X. Zhang et al.
and PDI increased with increasing PiPDMS feed mole ratio. On the other hand, the Mw and PDI were less changed with increasing PiPDMS feed mole ratio after the PiPDMS feed mole ratio more than half. It is considered that the PiPDMS donor play a dominant role in the modification of solid catalyst after the feed mole ratio of PiPDMS more than half. It may presumably be due to the higher bulkiness of PTES than that of PiPDMS (both the alkyl and alkoxy groups) and the content of replaced PiPDMS was higher than PTES. As shown in Figrue 3, the molecular weight distribution curves were unimodal and the curve shape was less changed with the different feed mole ratio of PTES:PiPDMS. The theoretical PDI values of PP obtained with the mixture of external donors were also calculated from the combined GPC curve of PP obtained by the PTES and PiPDMS. The ratio of combined GPC curve was determined by the ratio of external donor in the solid catalyst system. The highest PDI of PP was obtained at PTES:PiPDMS=1:4. It was showed that the PDI of PP could be controlled from 6.2 to 9.0 by changing PTES:PiPDMS feed mole ratio.
Conclusions The performance of external donor was influenced by the type of internal donor due to the different coordination strength. The catalyst activity with different internal donor decreases in the following order: diether > phthalate ≈ succinate. Although the internal donor was different, the molecular weight was improved with the introduction of external donor. Among the three catalysts, the succinate based catalyst has the highest molecular weight and PDI. The catalyst activity was dependent on the structure of external donor, while the more alkoxy groups or the bulkiness of alkoxy groups lead to decrease the catalyst activity, I.I. value and molecular weight of obtained PP. The linear alkyl groups do not deactivate the active centers of catalyst, while the molecular weight increased with the increasing the alkyl group size in external donor. With regards to the mixture of external donors, the highest PDI was obtained in presence of PTES:PiPDMS=1:4. It was showed that the molecular weight and molecular weight distribution of PP could be controlled by changing the structure of external donor and/ or the mixture of external donors.
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Macromol. Res., Vol. 19, No. 6, 2011
