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Jul 1, 2013 - Four R1R2Si(OMe)2 type compounds were added as an external electron donor (De) in propylene polymerization with TiCl4/Di/MgCl2 type ...
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Mechanism of Propylene Polymerization with MgCl2‑Supported Ziegler−Natta Catalysts Based on Counting of Active Centers: The Role of External Electron Donor Xian-rong Shen, Zhi-sheng Fu, Jie Hu, Qi Wang, and Zhi-qiang Fan* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Four R1R2Si(OMe)2 type compounds were added as an external electron donor (De) in propylene polymerization with TiCl4/Di/MgCl2 type supported Ziegler−Natta catalysts (Di = internal donor). Each polypropylene (PP) sample was fractionated into three parts (atactic, medium-isotactic and isotactic PP), and the number of active centers ([C*]/[Ti]) in each PP fraction was counted using 2-thiophenecarbonyl chloride as the quenching and tagging agent. The gradual decrease of [C*]/ [Ti] with De/Ti ratio is ascribed to competitive and reversible coordination of De on either central Ti of the active center or Mg adjacent to the central Ti. The former coordination leads to deactivation of C*, and the latter one leads to still living C*. The chain propagation rate constant (kp) of the active centers producing atactic, medium-isotactic and isotactic PP change with De/ Ti in different ways. Only the kp of active centers producing isotactic PP was evidently increased by De. Enhancement in isotacticity of PP product is found to be a combined result of both deactivation of active centers by De and selective activation of the active centers that produce isotactic PP. Changing the alkyl groups of R1R2Si(OMe)2 leads to an altered balance between the deactivation and activation effects of De.

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INTRODUCTION The industrial production of more than 50 million tons of isotactic polypropylene (PP) each year is based on catalyzed propylene polymerization with MgCl2-supported Ziegler− Natta catalysts.1,2 Organic electron donors like esters, ethers, and alkoxysilanes are widely used in the catalyst preparation and polymerization processes, which play key roles in enhancing isotacticity and regulating molecular weight distribution of the PP products.3−11 The electron donor added in the process of catalyst preparation is called the internal electron donor (Di), and the electron donor added in the polymerization process is called the external electron donor (De). In recent decades, the most commonly used catalyst in PP production contains phthalate as Di and alkoxysilane as De. With such catalysts, PP with high isotacticity and controllable molecular weight can be produced at very high catalysis efficiency.1 Since the discovery of TiCl4/Di/MgCl2−AlR3/De type propylene polymerization catalysts in the early 1980s, great efforts have been paid to disclosing and understanding the mechanism of electron donor effects, with an aim of further improving the chain structure of PP by applying new Di/De combinations.3−40 The main role of Di has been proposed to control the amount and spacial distribution of TiCl4 adsorbed on the MgCl2 crystallite surface.12−14 When TiCl4/Di/MgCl2 © 2013 American Chemical Society

type catalysts were treated with an AlR3/De mixture, most of Di molecules in the catalyst were found to be quickly replaced by De, implying that the De plays more important roles in the polymerization system.4,14 The effects of De on stereoselectivity of active centers have been ascribed to reversible adsorption of donor on metal atoms (Mg or Ti) neighboring the central Ti metal of the active center. Busico et al. proposed a three-site model to explain the effects of De on catalyst efficiency and polymer stereoregularity.19 In this model, successive adsorption of De on catalyst changes the stereochemical environment of the active center, turning apsecific centers into isospecific centers. A modified three-site model was proposed by Terano et al.20 The mechanism of donor effects have also been studied based on investigation of the polymerization kinetics, including the effects of donor on the number and propagation rate constant of active centers.26−29 Terano et al. investigated the effects of both Di and De on the number and propagation rate constant of different type of active centers based on stoppedflow polymerization experiments.26,27 By using a 14CO tagging method, Bukatov et al. compared the number and propagation rate constant of active centers of a series of catalysts containing Received: May 4, 2013 Revised: June 29, 2013 Published: July 1, 2013 15174

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different Di and De.28,29 According to this literature, addition of an external donor in the propylene polymerization system with MgCl2-supported Ziegler−Natta catalysts causes a decrease in the number of active centers ([C*]/[Ti]) and increase in the chain propagation rate constant (kp). These results suggest that deactivation of a part of active centers and properties alteration of the remaining active centers happen in parallel when De is added. However, because the changes of active center’s number and propagation rate constant with De/Ti molar ratio have not been experimentally determined, a detailed evaluation of the donor effects and quantitative comparisons between different external donors have not been reported before. On the other hand, many theoretical studies on the mechanism of donor effects have been reported in the past ten years, using density functional theory (DFT) calculations as the main tool.40−50 It has been proved by DFT calculation that De molecules can coordinate on lateral cuts of MgCl2 crystallites in the catalyst. Adsorption of the donor molecule on the adjacent positions of active sites increases their stereospecificity and changes their intrinsic activity. However, these conclusions are to be confirmed by more experimental evidence. In our previous works, we have developed a new method of counting active centers in propylene or ethylene polymerization with Ziegler−Natta catalysts using 2-thiophenecarbonyl chloride (TPCC) as a quenching agent.51−53 The method enables us to determine the number of active centers efficiently. Alkoxysilanes are widely used as De in industrial production of isotactic PP with TiCl4/Di/MgCl2 type Ziegler−Natta catalyst containing diester type Di. Previous studies show that the size of alkyl groups in alkoxysilane influences the catalyst activity as well as the microstructure and the molecular weight characteristics of the PP product.19,54,55 However, influence of De structure on the active center distribution is scarcely reported. In this paper, four different alkoxysilanes were used as De in TiCl4/Di/MgCl2−Al(CH2CH3)3 catalyst for propylene polymerization. The changes of active center’s number with the addition of different alkoxysilanes in systems of propylene polymerization with a TiCl4/Di/MgCl2 type Ziegler−Natta catalyst were determined. The effects of De on the catalytic activity, isospecificity and the distribution of active centers will also be investigated. The aims of this work are to disclose the details of donor effects and give more precise evaluation on the performances of different De. A mechanistic model of the donor effect will be proposed.

were supplied by Linyi Lujing Chemical Co. (Shandong, China) and distilled before use. General Procedures of Polymerization. Propylene polymerizations were performed in a 150 mL Schlenk flask containing about 80 mL of n-heptane at 60 °C under N2 atmosphere. The reagents were added in the order of solvent, TEA, external electron donor (when needed), and the catalyst with [Ti] = 0.8 mmol/L and Al/Ti = 100 (mol/mol). After the catalyst was precontacted with TEA for 30 min, propylene gas of 1 atm pressure was bubbled into the Schlenk flask for 4.5 min. Then a TPCC toluene solution (in TPCC/Al = 2) was injected into the reactor to quench the polymerization and stirred at 60 °C for 5.5 min. Subsequently an ethanol/HCl mixture (95/5) was added to decompose the catalyst, and the polymer was precipitated with excess of ethanol. Polypropylene was treated by refluxing the quenched polymer in excess of ethanol/HCl mixture for 60 min. Then the polymer was isolated, washed, dried and weighed. Each polymer sample was then thoroughly purified by one dissolution−precipitation operation, extracted with fresh ethanol in a Soxhlet extractor for 12 h, and then dried in vacuum at 60 °C. Polymer Fractionation. Each purified polypropylene sample was fractionated into three fractions in two steps. (1) About 2 g of PP was fully dissolved in 200 mL of boiling noctane, and then the solution was cooled to room temperature. After the polymer was fully crystallized, the suspension was separated into the solution part and solid part by centrifuging. Polymer recovered from the solution part was named as noctane soluble part (C8-sol). (2) The solid part was first dried in vacuum and then extracted with boiling n-heptane for 12 h in a Soxhlet extractor. The boiling n-heptane soluble part (C7-sol) was recovered from the solution, and the n-heptane insoluble part (C7-ins) was collected from the sample holder. All three fractions were dried in vacuum and weighed. Sulfur content of the fractions was also measured to count the number of active centers in each fraction. As proved by 13C NMR and DSC analysis, the C8-sol, C7-sol and C7-ins fractions are composed of atactic PP (aPP), medium-isotactic PP (miPP), and isotactic PP (iPP) chains, respectively (see the Supporting Information for the results of 13C NMR and DSC analysis on the fractions). Characterization Methods. The sulfur content of the quenched polymer was measured in a GLC-200 microcoulometry sulfur analyzer with a lower detection limit of 0.05 ppm (Jiangyan Yinhe Instrument Co., Jiangyan, China). The polymer sample for analysis was solid powder (2−4 mg, weighed to ±0.01 mg), and the average value of three parallel measurements was taken as the sulfur content. Propagation rate constant (kp) of polymerization was calculated according to the equation

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EXPERIMENTAL SECTION Chemicals. A commercial MgCl2-supported Ziegler−Natta catalyst (MgCl2/Di/TiCl4, Ti content = 2.7 wt %, produced by SINOPEC) containing a diol ester type Di was used for polymerization. Propylene (polymerization grade, supplied by Yangzi Petrochemical Co., Nanjing, China) was purified by passing through columns of molecular sieve and manganesebased deoxygen agent in a gas purification system made by Dalian Samat Chemicals Co., Ltd. 2-Thiophenecarbonyl chloride (TPCC) was purchased from Alfa Aesar Co. and distilled before use. Triethylaluminum (TEA, purchased from Albemarle Co.) was used as received and diluted in n-heptane before use. n-Heptane was first dried over 4A molecular sieves under dry N2 and then refluxed over Na before use. Diisobutyldimethoxysilane (DIBDMS), cyclohexylmethyldimethoxysilane (CHMDMS), dicyclopentyldimethoxysilane (DCPDMS), and diisopropyldimethoxysilane (DIPDMS)

R p = k p[C*][M]

where Rp is the rate of polymerization calculated by dividing the polymer yield with polymerization time, [C*] is the concentration of active center, and [M] is propylene concentration. Thermal analysis of the PP fractions was performed on a TA Q100 thermal analyzer. The polymer (about 4 mg) was sealed in aluminum pan, heated to 180 °C at 10 K/min, kept at that temperature for 5 min, and then cooled to 40 °C at 10 K/min and kept for 5 min. Then the sample was scanned from 40 to 180 °C at a heating rate of 10 K/min, and the DSC trace was recorded. 15175

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Table 1. Polymerization Results in the Presence of Different External Donorsa

a

external donor

activity (kg PP/gTi h)

C7-ins (wt %)

C7-sol (wt %)

C8-sol (wt %)

[C*]/[Ti] (mol %)

kp (L/mol s)

none DIBDMS CHMDMS DCPDMS DIPDMS

3.30 2.63 2.26 3.97 2.02

81.3 90.0 89.5 92.6 91.3

15.3 7.4 8.4 5.8 6.5

3.4 2.6 2.1 1.6 2.2

2.12 1.09 1.15 1.18 0.74

197 306 249 425 348

The conditions of the polymerization and quenching reaction are described in the experimental part. The external donor was added in Si/Ti = 5.

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RESULTS AND DISCUSSION Effect of External Electron Donors on Behavior of Propylene Polymerization. The results of propylene polymerization catalyzed by MgCl2-supported Ziegler−Natta catalysts with different external electron donors are shown in Table 1. The introduction of De caused a decrease in polymerization activity excepting DCPDMS. All four external donors enhanced the isotacticity (percentage of boiling nheptane insoluble fraction) of PP. The isotacticity of PP decreased in the order of DCPDMS (92.6%) > DIPDMS (91.3%) > DIBDMS (90.0%) and CHMDMS (89.5%). It means the bulkier the alkyl substituents of alkoxysilane are, the higher the isotacticity is. It is still difficult to explain such relationship between the bulkiness of De and its ability to improve isotacticity. A generally accepted mechanism is that alkoxysilane can complex with both the active sites and the cocatalyst (TEA in this work). Bulky substituents on alkoxysilane are required to prevent the De from leaving the catalyst surface through complexation with the cocatalyst. Bearing two bulky cyclopentyl groups, DCPDMS caused the largest extent of isotacticity improvement among the four external donors. The influence of external donor on the productivity of three PP fractions is shown in Figure 1. The activity of centers that

produce iPP was enhanced by addition of DCPDMS, whereas in other cases, the productivities of the PP fractions were all lowered by De. The deactivation effect of De may be attributed to a marked decrease in the number of active centers (see Table 1). It can be seen that quite a large portion of active centers are deactivated by the added De. By measuring sulfur content of the PP fractions, the number of active centers that produce iPP, miPP, and aPP chains were determined, and the chain propagation rate constants of the three groups of active centers were calculated (see Table 2). It is clear that number of all the three groups of active centers was more or less reduced by addition of De, among which the active center that produces miPP (Cm*) experienced the largest extent of reduction. On the other hand, the propagation rate constant (kp) of the active center that produces iPP (Ci*) was markedly enhanced by De, meanwhile kp of the other two groups of active centers (Cm* and Ca*) were unchanged or only slightly lowered. The three groups of active centers show different responses to addition of De in the polymerization system. The behavior of Ci* is especially different from that of Cm* and Ca*. Effects of De Concentration on Propylene Polymerization. To further disclose the mechanism of external donor effects, detailed studies on the influences of Si/Ti molar ratio on propylene polymerization were conducted. Figure 2a shows the change of polymerization activities with increase of De concentration. With increase in Si/Ti molar ratio, the activity decreased when CHMDMS, DIBDMS, and DIPDMA were added as De, whereas that of activity containing DCPDMS slightly increased. Similar deactivation effects of other types of De have been reported in literatures,21,56 and activation of propylene polymerization by DCPDMS has also been reported in literature.23 Figure 2b shows the change of active center concentration with the Si/Ti ratio. It is seen that each kind of De caused marked reduction of [C*], and the extent of [C*] reduction tends to level off when a large amount of De is added. Among the four types of external donors, DCPDMS caused the smallest extent of [C*] reduction. For the other three donors, more than one-half of active centers were killed when the Si/Ti

Figure 1. Influence of De on the activity of three groups of active centers (conditions of polymerization are the same as Table 1).

Table 2. Influence of De on the Number of Active Centers and Chain Propagation Rate Constants of Three Groups of Active Centersa atactic PP

a

medium-isotactic PP

isotactic PP

De

[Ca*]/[Ti] (mol %)

kpa (L/mol s)

[Cm*]/[Ti] (mol %)

kpm (L/mol s)

[Ci*]/[Ti] (mol %)

kpi (L/mol s)

none DIBDMS CHMDMS DCPDMS DIPDMS

0.21 0.12 0.10 0.10 0.12

68 76 59 82 49

0.79 0.33 0.35 0.34 0.27

80 74 69 86 62

1.12 0.64 0.70 0.75 0.35

304 468 365 624 661

Polymerization conditions of the PP samples are the same as those in Table 1. 15176

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Figure 2. Changes of polymerization activity (a) and the number of active centers (b) with Si/Ti in the presence of different external donors. Figure 3. Influence of CHMDMS and DCPDMS on fraction distribution of the PP product.

ratio was larger than 15. It seems that the disappearance of a part of the active centers is the main reason for De’s deactivation effects on propylene polymerization. In addition to the decrease of the total polymerization activity with Si/Ti ratio, the productivity of iPP was found to increase with an increase in the Si/Ti ratio (see Figure 3). Each De showed similar trends of enhancing iPP productivity with rising Si/Ti ratio, meanwhile lowering the productivities of miPP and aPP chains (see the Supporting Information for the data of fraction distribution when DIBDMS and DIPDMS were the De). Among the four donors, DCPDMS showed the strongest ability to improve isotacticity of PP. Changes of the number of three groups of active centers (Ci*, Cm*, and Ca*) with donor concentration have also been determined. Figure 4 shows the influences of Si/Ti ratio on [Ci*]/[Ti], [Cm*]/[Ti], and [Ca*]/[Ti] when CHMDMA and DCPDMS are used as De. Data of the systems containing DIBDMS and DIPDMS can be found in Supporting Information. The number of all three groups of active centers decreased with increase of external donor concentration. There are similarities between the [Cm*]/[Ti] vs Si/Ti and [Ca*]/ [Ti] vs Si/Ti curves of systems containing different De, but the [Ci*]/[Ti] vs Si/Ti curve of the systems containing CHMDMS and DCPDMS is quite different. CHMDMS caused a much larger reduction in [Ci*] than DCPDMS. It means that the active centers producing isotactic PP are more sensitive to the molecular structure of the external donor. Using the data of [Ci*], [Cm*], and [Ca*] and activity of three groups of active centers, chain propagation rate constants of each group of active centers were calculated. Figure 5 shows changes of kpi, kpm, and kpa with Si/Ti ratio for the systems containing CHMDMA and DCPDMS. Data of the systems

Figure 4. Influence of CHMDMS and DCPDMS on the number of three groups of active centers ([C*] = [Ci*], [Cm*], or [Ca*]; dots: experimental data, lines: fitting of the experimental data).

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the survived active centers is also changed by the added donor, which might be correlated with the disappearance of deactivated centers. To simplify the situation, the law of active center deactivation with De concentration will be analyzed first. To explain the effects of De in improving isospecificity of the catalyst, Busico et al. proposed a three-site model, in which reversible adsorption of De molecules on Mg atoms adjacent to Ti atoms of the active centers can turn aspecific centers into isospecific ones.19 However, this model cannot explain the deactivation effect of De on a part of active centers as reported in this work. To include De’s deactivation effect in the mechanistic model, we propose the following competitive equilibriums which occur in catalyst system containing external donor: K1

K2

De ·C1* ⇌ De + C* ⇌ De ·C2* Inactive

(1)

Active

In these equilibrium equations, C* is active center that has not been coordinated by a De molecule, De·C1* is active center that is deactivated by De, and De·C2* is active center that is coordinated by a De molecule but remain active. Considering that an active center has at least one vacant position for monomer coordination, there should be high tendency of De coordination at these vacant positions. On the other hand, coordination of De on Mg atoms adjacent to the central Ti of an active center may retain or enhance its activity, meanwhile improve its stereospecificity. If the coordinated active centers De·C2* cannot be deactivated by coordination of another De molecule, the equilibrium constants of the two reversible coordination of De on C* in eq 1 can be expressed as follows:

Figure 5. Influence of CHMDMS and DCPDMS concentration on chain propagation rate constants of three groups of active centers (kp represents kpi, kpm or kpa).

K1 =

containing DIBDMS and DIPDMS as De can be found in the Supporting Information. It is seen that kp of the active centers producing atactic and medium-isotactic PP only slightly changed with an increase in the Si/Ti ratio, but kpi markedly increased with the Si/Ti ratio. It also means that the active centers producing isotactic PP are more sensitive to the interaction of external donor than the active centers producing atactic and medium-isotactic PP. By comparing Figure 5 to Figures 4 and 3, one can find that the increase in kpi is the main reason for the increase of iPP content with De concentration. Mechanism of External Donor’s Interaction with Active Centers. Summing up the results presented above, it can be found that the external donor interacts with the active centers in a rather complicated way. All three groups of active centers are partly deactivated by the donor, but the extent of deactivation rises with De concentration rather slowly. Considering that the concentration of De is 50 times higher than that of the active centers, it means that the donor cannot deactivate the active centers efficiently. The intrinsic activity of

[De ·C1*] [De][C*]

K2 =

[De ·C2*] [De][C*]

(2)

As De·C1* is unable to form a propagation chain, it is reasonable to assume that [De·C1*] is not included in the experimentally determined active center concentration ([C*]e). Both the uncoordinated active centers (C*) and De·C2* produce polymer chains and can be counted into [C*]e measured by the quenching experiment, namely [C*]e = [C*] + [De ·C2*]

(3)

According to this model, [De·C1*] should equal to the reduced active center concentration in the presence of De [De ·C1*] = [C*]o − [C*]e

(4)

([C*]o is the measured active center concentration in the absence of De). When eqs 2−4 are combined, the following equations can be obtained: [C*] =

[C*]e 1 + K 2[De]

(5)

Table 3. Equilibrium Constants of De Coordination on Three Groups of Active Centers Ca*

a

Cm *

Ci *

De

K1aa (L/mol)

K2ab (L/mol)

K1ma (L/mol)

K2mb (L/mol)

K1ia (L/mol)

K2ib (L/mol)

DIBDMS CHMDMS DCPDMS DIPDMS

310 310 310 310

390 245 260 290

1730 695 685 1680

900 500 350 530

350 350 670 860

170 180 1050 180

Equilibrium constant K1 of De coordination on Ca*, Cm*, or Ci*; bEquilibrium constant K2 of De coordination on Ca*, Cm*, or Ci*. 15178

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The Journal of Physical Chemistry C K1 = (1 + K 2[De])

[C*]o − [C*]e [De][C*]e

[C*]e 1 = + K 2/K1 [C*]o − [C*]e K1[De]

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in isospecificity of Ci* by coordination of De on Mg adjacent to the central Ti. As the Si/Ti ratio increases, fraction of the active centers with higher isospecificity increases, leading to higher chain propagation constant. Chain propagation constant of αolefin polymerization with supported Z-N catalyst has been found to be roughly proportional to the isotacticity of polymer.57 The much higher propagation rate constant of Ci* than Ca* and Cm* (see Figure 5) is also an evidence for this correlation. Such positive correlation between k p and isotacticity should be related with slower chain propagation after a stereochemical mistaken insertion. As an evidence to this explanation, melting temperature of the iPP fraction was found to be higher in the presence of De (see Table 4), namely, the isospecificity of Ci*was improved.

(6)

(7)

Checking eq 7 using the experimental data of [C*]o, [C*]e, and [De], a good linear relationship between [C*]e/([C*]o −[C*]e) and 1/[De] has been observed. This means that the gradual decreases of [C*]e with Si/Ti ratio in Figure 4 is very likely to be the result of the above-mentioned competitive twoway coordination of De on the active centers. By fitting the experimental data according to eq 7, the two equilibrium constants K1 and K2 of each group of active centers were determined and listed in Table 3. The data in Table 3 show that in most cases the equilibrium constant K1 representing the ability of De to deactivate active centers is larger than K2 representing the ability of De to modify stereospecificity of the active centers. Considering that the central Ti atoms bearing coordination vacancies are more electron deficient than their adjacent Mg atoms, such a phenomenon is quite expectable. It means that deactivation of a part of active centers by De is inevitable even if only a small amount of De is added in the system. Judging by the equilibrium constants, DCPDMS, the external donor bearing the bulkiest alkyl substituents, still shows unusual properties as compared to the other three donors. The K2i value of catalyst system containing DCPDMS is evidently higher than the K1i value, meaning that its active centers Ci* can be more efficiently protected through coordination of De on their adjacent Mg atoms, resulting in lower extent of reduction in [Ci*]. This may be related with the ability of bulky cyclopentyl groups to shield the coordinated DCPDMS from leaving the active center through coordination with TEA. Complexation of De with the cocatalyst has been well recognized and considered in explaining the external donor effects.6 It is interesting to find that bulky R groups in R2Si(OMe)2 type De also enhanced the K1i value. Both DCPDMS and DIPDMS show much higher K1i than those of DIBDMS and CHMDMS. Though the K1i value of DIPDMS is even higher than that of DCPDMS, its K2i value is still low. It seems that larger R groups are needed to effectively protect De’s coordination on the adjacent Mg than on the central Ti atom. It can be seen that the K1m value is larger than K1a and K1i values for all four De systems in Table 3. It implies that among the three groups of active centers the one that produces medium-isotactic PP chains can be deactivated by De most easily. This can be explained by the medium steric hindrance in the chemical environment of Cm* among the three groups of active centers. When the steric hindrance is low (the situation of Ca*), though De can access the central Ti easily, there will be still plenty of space for TEA to access the inactive De·C1* and remove the adsorbed De. In the case of Ci*, its large steric hindrance will prevent the De molecule from approaching the central Ti, leading to lower a K1i value. The differences between K1 values of Ca*, Cm*, and Ci* cause a quicker decrease in the amount of miPP fraction with Si/Ti than that of aPP fraction (see Table 1 and Figure 3). The change of propagation rate constant with De concentration can also be rationalized based on the competitive two-way coordination model. The marked increase of kpi with De concentration (Figure 5) can be attributed to improvement

Table 4. Thermal Properties of PP Fractions Obtained in the Presence of Different External Donora atactic PP

medium-isotactic PP

isotactic PP

De

ΔH (J/g)

Tm (°C)

ΔH (J/g)

Tm (°C)

ΔH (J/g)

Tm (°C)

none DIBDMS CHMDMS DCPDMS DIPDMS

26.7 12.7 13.8 7.7 8.4

114.0 103.2 100.9 98.5 101.0

85.7 79.7 78.7 80.8 82.5

144.3 141.2 142.2 142.0 142.4

97.8 98.7 98.5 102.6 95.7

159.7 164.0 163.7 163.9 165.0

a

The PP samples in this table are synthesized in the same conditions as those of Table 1.

With regard to the aPP and miPP fractions, their propagation rate constants are slightly lowered or nearly unchanged by the added De (see Figure 5). Meanwhile their isotacticity becomes lower as compared to that of the sample produced without De (see Table 4). This could mean that coordination of De on Ca* or Cm* cannot lead to improvement in their isospecificity. However, there is another possible reason. As mentioned previously, active center with low steric hindrance has relatively smaller K1 value. It is highly possible that either Ca* or Cm* is composed of more than one types of active center. When they meet the De molecules, those centers with relatively bulkier environment will be deactivated more than those having smaller steric hindrance. As a result, the survived centers are those having lower isospecificity. As mentioned in the Introduction, the widely accepted threesite model suggests that the stereochemical environment of the active centers are changed by successive adsorption of De on catalyst, leading to transformation of the apsecific centers into isospecific centers.19 If this mechanism applies to the systems studied in this work, Ca* will be turned into Cm* and Cm* be turned into Ci* as the De/Ti ratio increases. In other words, [Ci*] should decrease with De/Ti ratio much more slowly than [Ca*] and [Cm*], because most of Cm* will join the Ci* group in the presence of De. However, as seen in Figure 4, the decreases of [Ca*], [Cm*], and [Ci*] with Si/Ti ratio are roughly parallel with each other. In fact, when CHMDMS is used as De, the decrease of [Ci*] at De/Ti = 15 is even larger than that of [Cm*] (see Figure 4). Therefore, it is more reasonable to say that transformations of Ca* into Cm* and Cm* into Ci* did not happen in large scale. However, the possibility of active center transformation by De in small scale cannot be ruled out, as partial transformation of Ca* and Cm* will not significantly change the [C*] ∼ De/Ti profiles. Based 15179

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Scheme 1. Coordination of De Molecule on a Mono-Ti Active Site Located on the (110) Lateral Cut of MgCl2 Crystallite

fractionated into three parts (aPP, miPP, and iPP) in the order of increasing stereoregularity by simple dissolution and extraction. The number of active centers ([C*]/[Ti]) in all three fractions decreased with increasing De/Ti molar ratio from 0 to 15, and the plateau value of [C*]/[Ti] that was higher than 0 exists in all cases. Such a change in [C*] with De/Ti can be reasonably depicted by a mechanistic model that includes competitive and reversible coordination of a De molecule on either the central Ti of the active center or the Mg adjacent to the central Ti. In this model, the former coordination leads to deactivation of C*, the latter one leads to still active centers with altered kp values and isospecificity, and coordination of another De on active centers that already have an adsorbed De can be neglected. The equilibrium constants of De coordination on either Ti (K1) or Mg (K2) for three groups of active centers in polymerizations using four types of De were determined. The K1 values of all three groups of active centers are larger than the K2 values, excepting the equilibrium constants of De coordination on C*i when DCPDMS was used as De. It means that De has a stronger tendency to coordinate on the central Ti than the adjacent Mg, as the former has a larger Lewis acidity than the latter. The much larger K2 value on Ci * of the DCPDMS system can be ascribed to a strong protection effect of the bulky cyclo-pentyl groups on the adsorbed De. The change in the kp value by De also disclosed important information on the external donor effects. The kp values of active centers producing atactic (Ca*) and medium-isotactic PP (Cm*) only slightly changed with De/Ti ratio, but kpi increased with De/Ti evidently. Meanwhile, the isotacticity of the iPP fraction was increased by De, but those of the aPP and miPP fractions were slightly lowered. It means that the catalysis properties of Ci * are influenced by De in much larger extent than those of Ca* and Cm*. Enhancement in the isotacticity of the PP product is a combined result of both reversible deactivation of three groups of active centers by De and selective activation of the active centers that produce isotactic PP. Changing the alkyl groups of R1R2Si(OMe)2 leads to an altered balance between the deactivation and activation effects and an altered PP chain structure and polymerization kinetic parameters as consequences.

on the results of this work, the improvement of the polymer isotacticity (percentage of iPP fraction) by an external donor can be largely attributed to the increase in kpi at a relatively high De/Ti ratio. Considering the coordination of De molecule on a mono-Ti active site located on (110) lateral cut of MgCl2 crystallite, the model of Scheme 1 may be plausible. In this model, the external donor R2Si(OMe)2 can either coordinate to the central Ti and its adjacent Mg in chelate form, leading to deactivated center (b), or coordinate to two adjacent Mg in chelate form, leading to active center (c). These two kinds of De coordinated active centers are relatively stable because both of the two methoxy groups of De have been fixed. When a second De molecule coordinates with Ti in active center (c), only the monodentate complex (d) can be formed. This monodentate De will be easily removed by the cocatalyst. Therefore, formation of (d) can be neglected when the De concentration is not very high. There is another question concerning the above-mentioned mechanistic model: why the stereoselectivity of Ci* can be further improved by De, as Ci* already has high stereoselectivity in the absence of De? One possible reason is that diethylaluminum chloride (DEAC) formed by the reactions of titanium species and TEA may coordinate on Mg adjacent to the central Ti of active centers, and turning them into isospecific centers (Ci*).20 When De is introduced in the system, it may replace the coordinated DEAC. Because the coordination of De on Ci* is more stable than that of DEAC, the stereoselectivity of Ci* and its chain propagation rate constant will be further enhanced. In summary, experimental data on changes of [C*] and kp with [De] disclose the coexistence of deactivation and activation effects of De on the active centers. Enhancement in the isotacticity of PP is a combined result of both temporary elimination of three groups of active centers by De and selective activation of the active centers that produce isotactic PP.

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CONCLUSIONS Adding an R1R2Si(OMe)2 type external electron donor (De) in TiCl4/Di/MgCl2 type supported Ziegler−Natta catalysts caused either decreases or increases the propylene polymerization activity, which is a combined result of the reduction in the number of active centers and enhancement in the chain propagation rate constant (kp). The PP product can be 15180

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(11) Lou, J. Q.; Tu, S. T.; Fan, Z. Q. Polypropylene Chain Structure Regulation by Alkoxysilane and Ether Type External Donors in TiCl4/ DIBP/MgCl2-AlEt3 Ziegler-Natta Catalyst. Iran. Polym. J. 2010, 19, 927−936. (12) Busico, V.; Corradini, P.; Martino, L. D.; Proto, A.; Savino, V.; Albizzati, E. Polymerization of Propene in the Presence of MgCl2Supported Ziegler-Natta Catalysts, 1. The Role of Ethyl Benzoate as “Internal” and “External” Base. Makromol. Chem. 1985, 186, 1279− 1288. (13) Albizzati, E.; Giannini, U.; Morini, G.; Galimberti, M.; Barino, L.; Scordamaglia, R. Recent Advances in Propylene Polymerization with MgCl2 Supported Catalysts. Macromol. Symp. 1995, 89, 73−89. (14) Sacchi, M. C.; Tritto, I.; Shan, C. J.; Mendichi, R. Role of the Pair of Internal and External Donors in Magnesium ChlorideSupported Ziegler-Natta Catalysts. Macromolecules 1991, 24, 6823− 6826. (15) Sacchi, M. C.; Fan, Z. Q.; Forlini, F.; Tritto, I.; Locatelli, P. Use of Different Alkoxysilanes as External Donors in MgCl2-Supported Ziegler-Natta Catalysts to Obtain Propene/1-Butene Copolymers with Different Microstructure. Macromol. Chem. Phys. 1994, 195, 2805− 2816. (16) Fan, Z. Q.; Forlini, F.; Tritto, I.; Locatelli, P.; Sacchi, M. C. Effect of Ethoxy- and Methoxysilane Donors in Propene/1-Hexene Copolymerization with High-Yield Supported Ziegler-Natta Catalysts. Macromol. Chem. Phys. 1994, 195, 3889−3899. (17) Barino, L.; Scordamaglia, R. Modeling of Isospecific Ti Sites in MgCl2 Supported Heterogeneous Ziegler-Natta Catalysts. Macromol. Theory Simul. 1998, 7, 407−419. (18) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, H. J. W.; Thune, P. C. The Role of Electron Donors on Lateral Surfaces of MgCl2Supported Ziegler−Natta Catalysts: Observation by AFM and SEM. J. Catal. 2008, 257, 81−86. (19) Busico, V.; Cipullo, R.; Monaco, G.; Talarico, G.; Vacatello, M.; Chadwick, J. C.; Segre, A. L.; Sudmeijer, O. High-Resolution 13C NMR Configurational Analysis of Polypropylene Made with MgCl2Supported Ziegler-Natta Catalysts. 1. The “Model” System MgCl2/ TiCl4-2,6-Dimethylpyridine/Al(C2H5)3. Macromolecules 1999, 32, 4173−4182. (20) Liu, B. P.; Nitta, T.; Nakatani, H.; Terano, M. Precise Arguments on the Distribution of Stereospecific Active Sites on MgCl2-Supported Ziegler-Natta Catalysts. Macromol. Symp. 2004, 213, 7−18. (21) Ferreira, M. L.; Damiani, D. E. Effect of Different Donors on Kinetics of ZN Catalysts and Molecular Weight of the Obtained Polypropylene. J. Mol. Catal. A: Chem. 1999, 150, 53−69. (22) Chadwick, J. C.; Morini, G.; Balbontin, G.; Camurati, I.; Heere, J. J. R.; Mingozzi, I.; Testoni, F. Effects of Internal and External Donors on the Regio- and Stereoselectivity of Active Species in MgCl2-Supported Catalysts for Propene Polymerization. Macromol. Chem. Phys. 2001, 202, 1995−2002. (23) Garoff, T.; Virkkunen, V.; Jaaskelainen, P.; Vestberg, T. A Qualitative Model for Polymerization of Propylene with a MgCl2Supported TiCl4 Ziegler−Natta Catalyst. Eur. Polym. J. 2003, 39, 1679−1685. (24) Matsuoka, H.; Liu, B. P.; Nakatani, H.; Terano, M. Variation in the Isospecific Active Sites of Internal Donor-Free MgCl2-Supported Ziegler Catalysts: Effect of External Electron Donors. Macromol. Rapid Commun. 2001, 22, 326−328. (25) Kang, K. K.; Shiono, T.; Jeong, Y. T.; Lee, D. H. Polymerization of Propylene by Using Mg(OEt)2−DNBP−TiCl4 Catalyst with Alkoxy Disilanes as External Donor. J. Appl. Polym. Sci. 1999, 71, 293−301. (26) Nishiyama, I.; Liu, B. P.; Natsuoka, H.; Nakatani, H.; Terano, M. Kinetic Evaluation of Various Isospecific Active Sites on MgCl2Supported Ziegler Catalysts. Macromol. Symp. 2003, 193, 71−80. (27) Wang, Q.; Murayama, N.; Liu, B. P.; Terano, M. Effects of Electron Donors on Active Sites Distribution of MgCl2-Supported Ziegler-Natta Catalysts Investigated by Multiple Active Sites Model. Macromol. Chem. Phys. 2005, 206, 961−966.

ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures of polymer purification and sulfur content determination, conditions of 13C NMR analysis of polypropylene fractions, experiment data of fraction distribution of the PP product and sulfur content of the fractions, experimental data of the number and chain propagation rate constants of three groups of active centers, and results of DSC and 13C NMR analysis on the three kinds of PP fractions. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by the National Natural Science Foundation of China (Grant No. 21074108) and the Major State Basic Research Programs (Grant No. 2011CB606001) is gratefully acknowledged.

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