Effects of Some New Alkoxysilane External Donors on Propylene

Article pubs.acs.org/IECR

Effects of Some New Alkoxysilane External Donors on Propylene Polymerization in MgCl2‑Supported Ziegler−Natta Catalysis Qian Zhou,† Tao Zheng,† Huayi Li,*,‡ Qian Li,‡ Yu Zhang,‡ Liaoyun Zhang,*,† and Youliang Hu‡ †

College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, 19A,Yuquanlu, Beijing 100049, China Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

ABSTRACT: Five new alkoxysilanes with different sizes of hydrocarbon substituents were first synthesized and employed as external donors for propylene polymerization with a Ziegler−Natta catalyst. The effects of these and industrial alkoxysilanes with different sizes of hydrocarbon substituents on the microstructure of polypropylene were studied by SSA and 13C NMR. The results showed that isotactic sequence length of polypropylene increased with the size of R2 substituent on alkoxysilanes in the five donor systems, similar to the regularity of molecular weight, isotacticity, and thermal properties of polypropylene. Although the polypropylene produced by double cycloalkane substituents on alkoxysilanes had lower lamellae thickness L1, its contents were the highest. Moreover, excess large volume of hydrocarbon substituents on alkoxysilanes might be detrimental to improving isotactic sequence length of polypropylene. Polypropylene prepared by MIPDMS had a more uniform distribution of the stereodefects, indicating that MIPDMS might be used for the industrial production of BOPP. fractionation19,20 (TREF), and nuclear magnetic resonance spectroscopy21−23 (13C NMR) are used for studying features of the microstructure. Also, lots of results have shown that there are good correspondences between them.14,24 Compared with TREF fractionation, SSA fractionation can take less time and provide important information about the lamellar thickness and its distribution of polypropylene, which has a connection with the isotactic sequence length and its distribution of polypropylene.14−16 Although alkoxysilane external donors were widely used for propylene polymerization,7 the relations between the size of hydrocarbon substituents on alkoxysilanes and the property of polypropylene needed further research. Therefore, we designed and prepared 11 alkoxysilanes with different volume for systematic study of these connections. The effects of different alkoxysilanes on the catalytic activity of the Ziegler−Natta catalyst system, molecular weight, molecular weight distribution, thermal properties, isotactic sequence length, and isotactic sequence distribution of polypropylene were studied by differential scanning calorimetry (DSC), gel permeation chromatography (GPC), self-nucleation and annealing (SSA), and nuclear magnetic resonance spectroscopy (13C NMR), respectively. The results of SSA showed that the features of polypropylene (well-distrbuted of isotactic sequence length of polypropylene) prepared by MIPDMS were similar to the requirements of biaxially oriented polypropylene.25−27 It was possible that MIPDMS as external donor was used for the industrial production of BOPP.

1. INTRODUCTION Olefin polymerization initiated by Ziegler−Natta catalyst is a most efficient chemical process for industrial production of isotactic polypropylenes during olefin polymerization.1 At present, the combinations of MgCl2-supported TiCl4 (main catalysts), phthalate (internal donor), and alkoxysilane (external donor) are a most widely used polypropylene catalyst system.2,3 Especially, alkoxysilane external donor has an important influence on controlling catalyst activity, stereospecificity, and molecular weight distribution of polypropylene.4−6 The research of new and effective alkoxysilanes was always one of the hot topics for propylene polymerization.2,7−9 Seppälä and Härkönen10,11 found that active centers of catalyst were significantly influenced by the size and number of the alkoxy substituents and the size of hydrocarbon group on alkoxysilanes. They found that at least one free nonhindered alkoxy group was required for selective deactivation in the complex (alkoxysilane and AlEt3). Large, nonlinear hydrocarbon groups could prevent the complex from deactivating sterically hindered isotactic centers. However, if hydrocarbon groups were small, both atactic and isotactic active centers deactivated about equally. Some researchers12 believed that the addition of alkoxysilane external donors could transform the atactic centers to isotactic centers by selective deactivation for the production of isotactic polypropylene. Density functional theory research9 was used by Wondimagegn to study the role of alkoxysilane external donors on stereoselectivity and molecular weight of polypropylene. He found that the alkoxysilane external donor [R1R2Si(OMe)2] with both bulky substituents on both R1 and R2 could significantly increase the stereospecificity and molecular weight of polypropylene. Alkoxysilane external donors play an important role not only on stereospecificity of polypropylene, but also on the microstructure of polypropylene. Usually, successive self-nucleating and annealing (SSA) thermal fractionation,13−18 temperature rising elution © 2014 American Chemical Society

Received: Revised: Accepted: Published: 17929

August 29, 2014 October 21, 2014 October 24, 2014 October 24, 2014 dx.doi.org/10.1021/ie5034123 | Ind. Eng. Chem. Res. 2014, 53, 17929−17936

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL DETAILS Materials. MgCl2-supported Ziegler−Natta catalyst (SAL, Ti content of 3.38%) was provided by Sinopec Catalyst Company. The cocatalyst triethylaluminum (TEA) tetramethoxysilane, n-hexane, and cyclopentyl chloride were purchased from Aldrich. Propylene (polymerization grade) was provided by Yan Shan Petrochemical Co. Ltd. Synthesis of the Alkoxysilane External Donors. The alkoxysilane external donors with purity of above 98% (seen from Figure 1) were synthesized according to the literature as follows.28

and 0.39 mmol of alkoxysilane external donor (seen from Figure 1) were added in reactor, respectively. Then, hydrogen gas (0.3 MPa) and propylene (800 g) gas were charged with the system. The C(H2)/C(propylene) ratio was 2.59 mmol/ mol. The stirrer speed was 300 r/min, and stirring time was 60 min. The heating temperature of the autoclave reactor was 70 °C. The propylene was exhausted, and polypropylene samples were obtained. Differential Scanning Calorimetry. A Mettler 822e differential scanning calorimeter was used for DSC measurements in a nitrogen atmosphere. In order to erase previous thermal history, polypropylene (about 2−4 mg) was first heated from 50 °C up to 200 °C at a rate of 50 °C/min and was held at this temperature for 5 min. Then, samples were cooled down to 50 °C and held at 50 °C for 5 min, and finally heated again to 200 °C at a rate of 10 °C/min. The melting temperature Tm and the enthalpy of fusion ΔHm were determined from the second melting. The degree of crystallinity was calculated according to the following formula:29 Xc = ΔΗ m /ΔΗ 0m

Here ΔHm was the fusion enthalpy obtained from DSC curve, and ΔH0m was the fusion heat of a perfectly crystallization polypropylene, i.e., 209.0 J/g.29 Gel Permeation Chromatography. PL-GPC 220 hightemperature gel permeation chromatography (Polymer Laboratories Ltd.) was used for the molecular weights (Mn and Mw) and the molecular weight distribution (MWD) of polypropylene at 413.15 K. The solvent was 1,2,4-trichlorobenzene, the injection volume was 100 mL, and the flow rate was 1.0 mL/ min. Calibration was made by linear polystyrene as the standard sample. Successive Self-Nucleation and Annealing. Successive self-nucleation and annealing (SSA) technology was first reported by Müller et al. in 1997.30 It has been widely used to analyze the chain structures of semicrystallized polymers.14,24,31 The steps follow: (a) Samples were heated and held up to 200 °C for 5 min, and then they cooled down to 50 °C at 20 °C/min and were held for 5 min to give an initial standard thermal history. (b) Samples were heated to Ts at 20 °C/min and held for 10 min, and then cooled to 50 °C at 20 °C/min. (c) Repeat step b at an increasingly lower Ts for 4 times, and Ts values were from 164 to 144 °C at 5 °C intervals. (d) Samples were heated from 50 to 200 °C at 10 °C/min. Ts temperature was determined according to the procedure proposed in the literature.32 Nuclear Magnetic Resonance Spectroscopy. 13C NMR spectra of polypropylene were tested in DMX 300 M (Bruker). An 80 mg portion of polypropylene in 0.5 mL of deuterated odichlorobenzene was prepared for polypropylene solution at 383 K. The highest peak of o-dichlorobenzene solvent (132.700 ppm) was used as the standard reference. Experimental conditions were the following: number of pulses more than 5000, pulse angle 30°, spectrum width 25 000 Hz, and relaxation delay 7 s. All spectra were completely proton decoupled.

Figure 1. Structures of 11 alkoxysilane external donors with different substituent groups.

In a typical example, the synthesis of methylcyclopentyldimethoxysilane (MIPDMS) was carried out as Scheme 1. A 500 Scheme 1. Synthesis of 11 Alkoxysilane External Donors with Different Substituent Groups

mL flask was placed with 40 mL of THF, 0.5 mol of magnesium powder, 8 mL of cyclopentyl chloride (R1Cl), and a small amount of iodine as initiator. The mixture was heated to 70 °C, and then 0.5 mol of cyclopentyl chloride (R1Cl), 50 mL of THF, and 0.5 mol of tetramethoxysilane were dropped. After the completion of the dropwise addition, the mixture was stirred at 90 °C for 4 h. After confirmation by gas chromatography, the mixture was filtered by a G4 glass filter at room temperature to remove the precipitate. The syntheses of MIBDMS, IPCPDMS, MCPDMS, IPIBDMS, and DCHDMS were similar to the synthesis of MIPDMS. Propylene Bulk Polymerization. A reactor equipped with a mechanical stirrer was degassed 3 times. A 20 mg portion of Ziegler−Natta catalyst, 12 mmol of TEA in n-hexane solution,

3. RESULTS AND DISCUSSION Effect of Alkoxysilane External Donors on the Catalytic Activity of the Ziegler−Natta Catalyst System and the Isotacticity of Polypropylene. There were 11 17930

dx.doi.org/10.1021/ie5034123 | Ind. Eng. Chem. Res. 2014, 53, 17929−17936


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Table 1. Comparison of Catalyst Performance and Isotacticity of Polypropylene for Alkoxysilane External Donors

a

R1

R2

acronym

activity ratio kg/g cat.

IIa (%)

cyclopentyl (CP) iso-butyl (IB) iso-propyl (IP) cyclopentyl cyclohexyl (CH) iso-butyl iso-propyl iso-propyl iso-propyl cyclopentyl cyclohexyl

methoxy (MO) methyl (M) methyl methyl methyl iso-butyl iso-butyl iso-propyl cyclopentyl cyclopentyl cyclohexyl

MOCPTMS MIBDMS MIPDMS MCPDMS MCHDMS DIBDMS IPIBDMS DIPDMS IPCPDMS DCPDMS DCHDMS

32.0 29.8 31.5 33.0 35.0 38.0 38.5 39.0 40.5 41.0 36.7

97.8 98.4 98.9 99.4 99.4 99.2 99.3 99.4 99.7 99.7 99.6

The isotactic index (II) values were tested by extraction with boiling n-heptane for 6 h.

the Ziegler−Natta catalyst system and the isotacticity of polypropylene. Effect of Alkoxysilane External Donors on Molecular Weight and Molecular Weight Distribution of Polypropylene. Table 2 displayed the molecular weight and the

external donors with different hydrocarbyl substituents on alkoxysilanes prepared, and the properties of the obtained polypropylene were studied. Table 1 lists the effects of those alkoxysilanes [R1R2Si(OCH3)2] on the Ziegler−Natta catalyst activity and isotacticity of polypropylene. In order to discuss easily, alkoxysilanes were classified into five groups (fixed R1) in this study according to their structures: the methyl dimethoxysilane (MDMS) system including IBMDMS, IPMDMS, CPMDMS, and CHMDMS; the iso-butyl dimethoxysilane (IBDMS) system including DIBDMS, IPIBDMS, and MIBDMS; the iso-propyl dimethoxysilane (IPDMS) system including MIPDMS, IBIPDMS, DIPDMS, and CPIPDMS; the cyclopentyldimethoxysilane (CPDMS) system including MCPDMS, MOCPTMS, DCPDMS, and IPCPDMS; and the cyclohexyldimethoxysilane (CHDMS) system including MCHDMS and DCHDMS. It could be seen from Table 1 that the catalytic activity of the Ziegler−Natta catalyst system increased from 29.8 kg/g cat. to 35.0 kg/g cat. for the methyl dimethoxysilane (MDMS) system, and their catalytic activities increased in the following order: MIBDMS < MIPDMS < MCPDMS < MCHDMS. Interestingly, the regularity for the other four donor groups on the catalytic activity of the Ziegler− Natta catalyst system also increased with the size of hydrocarbyl substituents on alkoxysilanes the same as the order of the MDMS system, such as MIBDMS < MIPDMS < MCPDMS < MCHDMS, MIBDMS < DIBDMS < IPIBDMS, MIPDMS < IBIPDMS < DIPDMS < CPIPDMS, MCHDMS < DCHDMS, and MOCPTMS < MCPDMS < IPCPDMS < DCPDMS. Table 1 also showed that the isotacticity of polypropylene with different alkoxysilanes followed the same regularity as the catalytic activity of the Ziegler−Natta catalyst system, indicating that suitable bulky hydrocarbyl substituents on alkoxysilanes benefited the isotacticity of polypropylene. That might be because suitable bulky hydrocarbyl substituents on alkoxysilanes tended toward the formation of isotactic active centers, which could increase the content of isotactic active centers and decrease the content of atactic active centers.33 Also, the isotacticity of polypropylene increased with the increase of the content of isotactic active centers, causing alkoxysilanes with more bulky hydrocarbyl substituents to give higher isotacticity of polypropylene in five donor systems. The catalytic activity of the Ziegler−Natta catalyst system and the isotacticity of polypropylene prepared by DCHDMS were lower than those prepared by DCPDMS, suggesting that excess volume of incorporating hydrocarbyl substituents on both R1 and R2 in [R1R2Si(OCH3)2] was detrimental to the catalytic activity of

Table 2. Molecular Weight and Molecular Weight Distribution of Polypropylene with Alkoxysilane External Donors donor

Mn × 10−3 (g/mol)

Mw × 10−3 (g/mol)

Mw/Mn


70.8 78.0 94.4 72.2 89.8 70.0 84.0 109.4 128.5 123.8 123.5

269.4 256.3 317.7 291.8 301.0 304.2 367.3 392.5 440.3 488.4 666.9

3.8 5.1 3.4 4.1 3.4 4.3 4.4 3.6 3.4 3.9 5.4

molecular weight distribution (MWD) of polypropylene samples prepared by the 11 alkoxysilane external donors. From all of the alkoxysilane external donors considered in this investigation, the polypropylene prepared by IPCPDMS, DCPDMS, and DCHDMS gave the higher molecular weight, indicating that the simultaneous existence of two bulky substituents, such as cyclopentyl, cyclohexyl, or isopropyl, on alkoxysilanes might benefit the increasing molecular weight of polypropylene. Especially, the polypropylene prepared by IPCPDMS possessed the highest molecular weight (128.5 × 103 g/mol), suggesting that the simultaneous presence of cyclopentyl and isopropyl substituents on alkoxysilane was most beneficial for the increasing molecular weight of polypropylene. The polypropylene prepared by IPCPDMS, DCPDMS, and DCHDMS also gave the higher stereoselectivity of polypropylene and catalytic activity of the Ziegler−Natta catalyst system as discussed above. The molecular weight distributions (MWDs) of polypropylene samples prepared by different alkoxysilanes were between 3 and 5. Also, the molecular weight distribution of polypropylene prepared by IPCPDMS, DIPDMS, and MIPDMS was slightly more narrow than those prepared by other alkoxysilanes, suggesting that using alkoxysilanes with isopropyl substituent favors obtaining narrow molecular weight 17931



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Table 3. DSC Results of Polypropylene with Alkoxysilane External Donors donor

Tma (°C)

ΔHmb (J/g)

Tcc (°C)

ΔHcd (J/g)

Xce (%)


158.2 157.3 160.5 159.1 160.2 160.9 163.2 164.0 164.0 164.0 163.6

99.1 99.1 104.2 105.1 106.2 99.1 104.2 105.0 109.3 109.8 108.2

109.9 109.9 112.5 113.7 114.7 113.8 113.5 115.3 115.2 116.3 114.7

−90.6 −90.6 −93.3 −98.9 −101.6 −99.2 −99.9 −115.3 −115.2 −120.9 −107.0

47.4 47.4 49.9 50.2 50.8 47.4 49.9 50.2 52.3 52.5 51.7

a Melting temperature as determined from the peak maximum value in the endothermic curves of DSC. bThe value of the endothermic enthalpy as determined from DSC. cCrystallization temperature as determined from the peak maximum value in the exothermic curves of DSC. dThe value of the exothermic enthalpy as determined from DSC. eThe degree of crystallization calculated from the value of the endothermic enthalpy.

polypropylene and small volumes of hydrocarbon substituents (for example, methyl) on alkoxysilanes result in a double melting peak on melting curve of polypropylene. DIPDMS, IPCPDMS, DCPDMS, and DCHDMS also gave the higher stereoselectivity and average molecular weight of polypropylene as discussed above, and the changing regularities of melting temperature on melting curves were consistent with those of the average molecular weight of polypropylene. Table 3 showed that the melting enthalpy, crystallization temperature, crystallization enthalpy, and the degree of crystallinity of polypropylene by the four alkoxysilane external donors in MDMS system increased in the following order: MIBDMS < MIPDMS < MCPDMS < MCHDMS. The other four systems followed the same orders too, and the change trend of stereoregularity of polypropylene was the same as this order. It might be because there were some relationships between the thermal properties of polypropylene and stereoregularity of polypropylene. In general, when there was higher isotacticity of polypropylene, there were better arrangements of segments of polypropylene and higher thermal properties of polypropylene. Table 3 also showed that the melting enthalpies, the degree of crystallinity, crystallization temperature, and crystallization enthalpy of polypropylene prepared by DCHDMS were significantly lower than those prepared by DCPDMS, indicating that excess volumes of double hydrocarbon substituents were detrimental to the improvement of thermal properties of polypropylene. Effect of Alkoxysilane External Donors on Isotactic Sequence Length and Isotactic Sequence Length Distribution of Polypropylene. Successive self-nucleation and annealing34,35 is an effective method in analyzing features of the microstructure and the isotactic sequence distribution of polypropylene. The melting curves of polypropylene samples prepared by alkoxysilanes after SSA thermal fractionation are shown in Figure 3. Each peak of the melting temperature and the enthalpy of fusion ΔHm of each sample are listed in Table 4. We could find that the enthalpy of fusion ΔH m of polypropylene increases from 115.5 to 149.2 J/g, which increased with the increasing of volume of R2 substituent on alkoxysilanes in the five donor systems. The enthalpy of fusion ΔHm of polypropylene prepared by DCPDMS was 149.2 J/g, which was higher than that prepared by DCHDMS and also the highest one among those prepared by all of alkoxysilanes, indicating that a suitable large volume of substituents on alkoxysilanes favors increasing enthalpy of fusion ΔHm of polypropylene, but excess large volume of substituents was

distribution of polypropylene. However, the molecular weight distribution of the polypropylene prepared by DCHDMS with two large cyclopentyl-substituted groups was about 5.4, which was broader than those prepared by other alkoxysilanes, indicating that double cyclohexyl substituents on alkoxysilane might be bad for synthesizing narrow molecular weight distribution of polypropylene. Effects of Alkoxysilane External Donors on the Thermal Properties of Polypropylene. The influences of different size of hydrocarbon substituents on alkoxysilanes on the thermal properties of polypropylene were studied by DSC. The results are shown in Table 3, and the melting curves are shown in Figure 2. The melting curves of polypropylene

Figure 2. DSC curves of polypropylene with alkoxysilane external donors.

prepared by MOCPTMS and MIBDMS both had double melting peaks, and those prepared by MIPDMS, MCPDMS, MCHDMS, and DIBDMS had an evident melting peak and a fuzzy one, while those prepared by IBIPDMS, DIPDMS, IPCPDMS, and DCPDMS had only one evident melting peak with higher melting temperature. It suggested that large volumes of double hydrocarbon substituents on alkoxysilanes were a benefit for obtaining narrow melting peak of 17932



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Li is the lamellar thickness, the equilibrium melting temperature T0m = 460 K (estimated values between 459 and 467 K),37 ΔH0 = 184 × 106 J/m3, and the surface energy σ = 0.0496 J/m2.38 It had been proven that the thicker lamellae value of polypropylene samples was corresponding to the higher isotactic sequence length in its macromolecular chains. The lamellae thicknesses of all samples after SSA thermal fractionation were calculated and listed in Table 5. Table 5 Table 5. Lamellar Thicknesses of Polypropylene Samples after SSA Thermal Fractionation

Figure 3. SSA melting curves of polypropylene samples prepared by 11 alkoxysilane external donors.

donor

L1

L2

L3

L4

L5


22.65

14.21 16.26 18.17 14.30 15.36 15.45 16.70 18.04 18.58 16.70 15.16

11.05 12.07 14.38 11.15 11.84 12.05 12.75 13.09 13.37 13.34 12.05

9.03 9.71 13.02 9.10 9.41 10.10 10.06 10.12 10.12 11.01 10.06

7.35 7.84 10.10 7.41 8.92 8.98 8.12 8.17 8.12 8.81 8.16

22.65 24.43 25.70 27.40 29.35 31.60 23.07 20.93

L6 6.48 8.17

showed that the lamellae thickness L1 of the longest isotactic sequence length of polypropylene prepared by DCPDMS and DCHDMS was 23.07 and 20.93 nm, lower lamellar thickness L1 than other alkoxysilanes. However, as could be seen from Figure 3, the relative contents of peak 1 corresponding to the lamellae thickness L1 prepared by DCPDMS and DCHDMS were obviously larger than those prepared by other alkoxysilanes, suggesting that double cycloalkane substituents on alkoxysilane had a disadvantage to increasing highest lamellae thickness L1 and a significant advantage to increasing its content. Comparing polypropylene prepared by the other seven alkoxysilanes (except for MIBDMS, MIPDMS, DCPDMS, and DCHDMS) in five donor systems, every lamellar thicknesses (L1 to L5) of polypropylene separately increased by increasing volume of substituent on alkoxysilanes, indicating that every isotactic sequence length corresponding different lamellae thickness (from L1 to L5) of polypropylene increased by increasing volume of substituent on alkoxysilanes. Moreover, for further analysis, the statistical parameters describing the lamellar thickness, the arithmetic average Ln, weighted average Lw, the broadness index I, the arithmetic average meso sequence length (MSLn, propylene unit), and

detrimental. It might be because the right volume of hydrocarbon substituents on alkoxysilanes was conducive to the coordination between alkoxysilane donors and the active center, but excessively bulky hydrocarbon substituents on alkoxysilanes could prevent the coordination. Then, alkoxysilanes with the right volume of hydrocarbon substituents had more effects on the enthalpy of fusion ΔHm of polypropylene, so the former gave larger enthalpy of fusion ΔHm of polypropylene than the latter. There was no peak 1 in the melting curves of polypropylenes prepared by MIBDMS and MIPDMS, indicating that the highest isotactic sequence lengths of polypropylenes prepared by MIBDMS and MIPDMS were lower than those prepared by other donors. It might be because the smaller volumes of substituents on MIBDMS and MIPDMS were causing the absence of the highest isotactic active centers in the catalytic system with the two donors. The lamellar thickness distribution of polypropylene could be obtained with the Thomson−Gibbs equation:36

⎛ 2σ ⎞ Tm = Tm0⎜1‐ ⎟ ⎝ ΔΗ 0Li ⎠

Table 4. SSA Results of Polypropylene Prepared by 11 Alkoxysilane External Donors donor

ΔHm (J/g)

Tm1 (°C)

Tm2 (°C)

Tm3 (°C)

Tm4 (°C)

Tm5 (°C)


118.5 115.5 115.8 116.2 117.3 121.3 129.9 133.2 133.8 149.2 135.0

175.9

169.4 171.6 173.2 169.5 170.7 170.9 172.0 173.1 173.5 172.1 170.3

164.4 166.3 169.6 164.6 165.9 166.2 167.4 167.9 168.3 168.2 166.2

159.4 161.3 165.7 159.6 160.5 160.9 162.2 162.8 162.8 164.0 162.0

153.1 155.2 160.9 153.4 154.1 154.7 156.3 156.5 156.3 158.7 156.7

175.9 176.7 176.5 177.8 178.4 179.0 176.3 174.5

17933

Tm6 (°C) 148.6 155.6



Article

polypropylene prepared by MIPDMS was the most narrow thickness distribution (I = 1.041), suggesting that isotactic sequence length distribution of polypropylene prepared by MIPDMS was the most well-distributed. Therefore, compared with other alkoxysilanes in MDMS system, the results of SSA suggested that the polypropylene prepared by MIPDMS had a smaller amount of high isotacticity, a greater amount of relative medium isotacticity, and a more uniform distribution of the stereodefects. Because the polypropylene prepared by MIPDMS had structural features for the requirements of BOPP materials, therefore, it was possible that MIPDMS as external donor would be used for the industrial production of BOPP. Comparing polypropylene prepared by the other nine alkoxysilanes (except for MIBDMS and MIPDMS) in five donor systems, the arithmetic average Ln of polypropylene increased with increasing volume of substituent on alkoxysilanes, indicating that isotactic sequence length of polypropylene increased with the increasing volume of substituent on alkoxysilanes. Nuclear magnetic resonance spectroscopy was always used to study the macrostructure of polypropylene. The methyl regions results of the 300 MHz 13C NMR spectrum of the polypropylene samples are listed in Table 7. The content of mmmm and mm increased with the size of hydrocarbyl substituents on alkoxysilanes for four donor groups (except for MDMS system), which were as same as the regularity of Ln and Lw calculated from SSA data. For further comparison with the results of NMR and SSA, the average isotactic sequence length (m) was calculated from the NMR results with the following equation,40 and the results are listed in Table 6.

weighted average meso sequence length (MSLw, propylene unit) are calculated using the following equations:39 Ln =

Lw = I=

n1L1 + n2L 2 + n3L3 + n4L4 + ... njLj n1 + n2 + n3 + n4 + ... + nj

=

∑ fi Li

n1L12 + n2L 2 2 + n3L32 + n4L4 2 + ... njLj 2 n1L1 + n2L 2 + n3L3 + n4L4 + ... + njLj

=

∑ fi Li2 ∑ fi Li

Lw Ln

MSL =

3L L helix

Here the Li is the lamellar thickness for each fraction, and ni is the normalized area of each fraction on the final SSA curve. Lhelix is the length of one crystal cell along the chain direction. For the crystal cell of monoclinic form, the c-axis is 0.65 nm, which corresponds to chain direction. In this direction, one cell contains 3 monomers, Lhelix = 0.65 nm. Table 6 showed that the arithmetic average Ln and the weighted average Lw of polypropylene prepared by MIBDMS Table 6. Ln, Lw, I, MSLn, and MSLw Calculated from SSA and m Calculated from 13C NMR of Polypropylene Prepared by 11 Alkoxysilane External Donors donor

Ln/(nm)

Lw/(nm)

MSLn

MSLw

m

I


12.75 13.24 14.88 12.83 14.00 15.23 15.43 15.87 16.62 18.42 14.72

13.60 14.20 15.56 13.67 14.98 16.43 17.18 16.98 18.23 20.03 17.05

58.85 61.75 71.45 59.21 64.62 66.46 70.29 73.25 76.71 82.94 64.11

62.77 66.00 74.40 63.09 69.14 79.29 75.83 78.37 84.14 89.86 74.26

20.70 19.34 54.23 44.78 53.20 48.33 48.54 56.04 62.44 62.75 43.28

1.067 1.073 1.046 1.065 1.070 1.079 1.113 1.070 1.097 1.087 1.158

m= mmmm + 3 ×

1 2

1 mmmr 2

1

1

× mrrr + 2rmmr + 2 rmrm + 2 rmrr 1

1

+ rmmr + 2 rmrm + 2 rmrr

It could be seen that the average isotactic sequence length (m) also increased with the size of hydrocarbyl substituents on alkoxysilanes for four donor groups (except for MDMS system) in Table 6. Also, the regularity of m values calculated from NMR were in the same order as MSLn and MSLw calculated from SSA. In other words, the average meso sequence length calculated from SSA was in good agreement with that calculated from high-resolution 13C NMR. SSA fractionation was an effective method for studying the stereodefect distribution of polypropylene synthesized with different alkoxysilanes.

and MIPDMS were 13.24 and 14.88 nm, which were larger than these prepared by MCPDMS and MCHDMS. It might be because the lamellae thicknesses L2 of polypropylene prepared by MIBDMS and MIPDMS (see from Table 5) were larger and their contents (see from Figure 3) were larger. Interestingly, the thickness distribution of lamellar (broadness index I) of

Table 7. 13C NMR Results of the Stereosequence Distribution of Polypropylene Prepared by 11 Alkoxysilane External Donors mm

mr

rr

sample

mm

mmmm

mmmr

rmmr

mr

mmrr

mrmm + rmrr

mrmr

rr

rrrr

rrrm

mrrm


86.03 91.24 97.42 96.87 96.96 96.44 96.38 98.17 98.38 98.96 97.95

80.59 84.23 94.76 93.45 94.39 93.07 93.45 95.40 95.63 96.19 96.32

4.57 6.14 2.18 3.03 1.83 3.11 2.62 2.77 2.65 2.61 1.60

0.87 0.87 0.48 0.39 0.74 0.26 0.31 0.21 0.10 0.16 0.03

7.92 5.05 1.51 1.92 1.67 2.13 2.41 0.61 0.85 0.65 1.80

4.66 3.31 0.80 1.22 1.13 1.35 1.41 0.40 0.49 0.38 0.39

2.53 1.01 0.38 0.54 0.27 0.67 0.69 0.15 0.25 0.23 1.13

0.73 0.73 0.33 0.16 0.27 0.11 0.31 0.06 0.11 0.04 0.28

6.07 3.25 1.06 1.21 1.06 1.42 1.22 1.01 0.77 0.39 0.25

1.99 0.92 0.25 0.33 0.42 0.39 0.32 0.35 0.21 0.08 0.04

1.75 1.04 0.21 0.33 0.16 0.29 0.10 0.24 0.25 0.14 0.13

2.33 1.29 0.60 0.55 0.48 0.74 0.80 0.52 0.21 0.16 0.08

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From the SSA and 13C NMR results, we could find that the different distributions of stereodefects along the molecular chains of polypropylene were attributed to the alkoxysilanes used. It was known that multiple active centers existed in heterogeneous Ziegler−Natta catalyst systems, which were different from each other in stereospecificity and stability. In our study, MIPDMS as external donor was the most beneficial to the formation of a uniform insertion of the stereodefects along the molecular chains. From the results of the four measurements, we could find that the catalytic activities of the Ziegler−Natta catalyst system, molecular weight, thermal properties, and isotactic sequence length of polypropylene prepared by IBIPDMS and DIPDMS were higher than those prepared by DIBDMS. The results probably suggested that the branching degree of the hydrocarbon substituents attached to central silicon atom played a major role in controlling the structure and property of polypropylene. We also found that the catalytic activity of the Ziegler−Natta catalyst system, thermal properties, isotacticity, and isotactic sequence length of polypropylene prepared by DCPDMS were almost the highest, beyond those prepared by DCHDMS. It indicated that suitable large volume of hydrocarbon substituents on alkoxysilanes might favor improving the property of polypropylene and excess large volume of hydrocarbon substituents on alkoxysilanes might be detrimental to the property of polypropylene.

structure and property of polypropylene. Polypropylene prepared by MIPDMS had a smaller amount of high isotacticity, greater amount of relative medium isotacticity, and a more uniform distribution of stereodefects, which were similar to the requirements of BOPP material. There was a possibility that MIPDMS as donor would be used on the industrial production of BOPP.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86-10-62562697. *E-mail: [email protected]. Tel.: +86-10-88256321. Fax: +86-10-88256321. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from National Science Foundation of China (Nos. 51343006 and 51403216) are gratefully acknowledged.

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REFERENCES

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Effects of Some New Alkoxysilane External Donors on Propylene

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