Effect of the Combined External Electron Donors on the Structure and

Article pubs.acs.org/IECR

Effect of the Combined External Electron Donors on the Structure and Properties of Polypropylene/Poly(ethylene-co-propylene) InReactor Alloys Prepared by High-Efficiency Industrial Ziegler−Natta Catalyst Zhisheng Fu,† Songtao Tu,†,‡ and Zhiqiang Fan*,† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Nanjing Research Institute of SINOPEC Yangzi Petrochemical Co., Ltd., SINOPEC Beijing Institute of Chemical Industry Yangzi Branch, Nanjing 210048, China S Supporting Information *

ABSTRACT: In this work, a series of isotactic polypropylene/ethylene−propylene rubber (iPP/EPR) in-reactor alloys were prepared by high-efficiency industrial Ziegler−Natta catalyst with diphenyldimethoxysilane/dicyclopentyldimethoxysilane (DDS/ D-donor) mixtures as external electron donors. The effects of the external electron donor on the structure and mechanical properties of the iPP/EPR in-reactor alloys were studied. According to the characterization results, the iPP/EPR in-reactor alloys were mainly composed of random poly(ethylene-co-propylene), multiblock poly(ethylene-co-propylene), and highly isotactic PP. For DDS/D-donor mixtures as external electron donors and triethylaluminum (TEA) as the cocatalyst, as the amount of Ddonor in DDS/D-donor mixtures increased, the molecular weight of polypropylene homopolymer and the structural uniformity of multiblock poly(ethylene-co-propylene) increased, whereas when D-donor alone was used as the external electron donor, they decreased. However, the isotacticity of polypropylene homopolymer increased as the amount of D-donor in the DDS/D-donor mixtures increased. Therefore, as the amount of D-donor in the DDS/D-donor mixtures increased, the impact strength of the iPP/EPR in-reactor alloy increased, but when D-donor alone was used as the external electron donor, the impact strength of the alloy decreased. An optimum feed ratio between DDS and D-donor was found, namely, DDS/D-donor = 1:3 (Si/Ti = 5). The iPP/EPR in-reactor alloy prepared under these conditions was the toughest. The influence of the external electron donor on the flexural modulus and flexural strength could be ignored.

1. INTRODUCTION

molecular weight and isotacticity exhibit a much better toughness/stiffness balance.17 The desired polypropylene characteristics can be achieved by modifying the supported titanium catalyst systems used for the polymerization with structurally different internal and external donors in addition to the polymerization processes.18−22 Alkoxysilanes are used as external electron donors for supported titanium catalyst systems based on dialkyl phthalate as the internal electron donor. Studies of the relationship among structure, performance, and product properties indicate that the nature and number of alkoxy groups and the steric/ electronic nature of the aryl/alkyl group bonded to silicon in the alkoxysilane influences the catalyst activity, as well as the microstructure and molecular-weight characteristics of the resulting polypropylene.23−27 The most effective alkoxysilane external donors for high catalyst stereospecificity are methoxysilanes containing relatively bulky groups in the α position to the silicon atom.28−30 Typical examples include diphenyldimethoxysilane (DDS)31 and dicyclopentyldimethoxysilane (Ddonor).32,33 The latter is less sensitive to H2 and gives

Modifications of isotactic polypropylene (iPP) to make it tougher have been widely studied in both academia and industry.1−6 Among the methods of toughening iPP, the inreactor blending of PP with other polyolefins (e.g., ethylene− propylene random copolymer) by sequential multistage polymerization has proven to be superior with respect to both polymer properties and production cost.7−9 A typical inreactor alloy is prepared by sequential polymerization of propylene in the first reactor, followed by ethylene−propylene copolymerization in a second reactor. In the first reactor, spherical iPP particles with high porosity are formed; in the second reactor, ethylene and propylene copolymerize and consequently fill the tiny holes in iPP particles with poly(ethylene-co-propylene). The resulting in-reactor alloy is composed of three portions: random copolymer (i.e., ethylene−propylene rubber, EPR), multiblock copolymer, and iPP.10−13 The properties of in-reactor alloys mainly depend on the content and chain structure of these components.14−16 Universally, the stiffness of iPP/EPR in-reactor alloys is lower than that of iPP. As the matrix of the iPP/EPR in-reactor alloy, the nature (molecular weight, isotacticity, and etc.) of the PP formed in the first reactor governs the stiffness of the alloy. iPP/EPR in-reactor alloys containing iPP matrix with a higher © 2013 American Chemical Society

Received: Revised: Accepted: Published: 5887

November 21, 2012 March 11, 2013 March 29, 2013 March 29, 2013 dx.doi.org/10.1021/ie303216c | Ind. Eng. Chem. Res. 2013, 52, 5887−5894

Industrial & Engineering Chemistry Research

Article

product with ethanol three times. Subsequently, the copolymer was dried in a vacuum at 60 °C for 12 h. Homopolymerization of Propylene. Polypropylene homopolymer was prepared by a two-stage polymerization process. In the prepolymerization and homopolymerization stage, the slurry polymerization of propylene was the same as in the preparation of the iPP/EPR in-reactor alloy. At the end of homopolymerization, propylene and solvent were removed by vacuum, and the soild product was poured into an excess of ethanol containing 5% HCl, filtered, and washed with ethanol three times. Subsequently, the PP was dried in a vacuum at 60 °C for 12 h. Determination of the Isotacticity Index of PP. About 1 g of PP was dissolved in 250 mL of boiling n-heptane and heated under reflux for 12 h. The solution was then cooled to 25 °C. The crystallized PP was collected and weighed. The isotacticity index of PP was calculated as the weight percentage of crystallized PP relative to the initial PP. Fractionation of PP or iPP/EPR In-Reactor Alloy. About 2 g of PP or iPP/EPR in-reactor alloy was heated under reflux in 200 mL of n-octane for 2 h, then cooled to room temperature (25 °C), and held at 25 °C for 24 h. The suspension was separated into a solution and a solid by centrifugation. The solid was dried in a vacuum and is denoted as the n-octane-insoluble part (C8-insol). The solution was distilled to remove n-octane and then dried in a vacuum. The corresponding solid is denoted as the n-octane-soluble part (C8-sol). C8-insol was extracted with boiling n-heptane for 12 h on a modified Kumagawa extractor.20 Then, the n-heptanesoluble part (C7-sol) was recovered by rotary evaporation. Both the n-heptane-soluble part and the n-heptane-insoluble part (C7-insol) were dried in a vacuum. 13 C NMR Analysis of the Fractions. 13C NMR spectra of the fractions were measured on a Varian Mercury Plus 300 NMR spectrometer at 75 MHz. o-Dichlorobenzene-d4 was used as the solvent, and the concentration of the polymer solution was 10 wt %. The spectra were recorded at 120 °C with hexamethyldisiloxane as an internal reference. Chromium triacetylacetone (4−5 mg) was added to each sample to decrease the relaxation time and ensure quantitative results. Broadband decoupling with a pulse delay of 3 s was employed. Typically, 5000 transients were collected. Measurement of the Molecular Weight. The molecular weights and molecular weight distributions of the fractions were measured by gel permeation chromatography (GPC) in a PL 220 GPC instrument (Polymer Laboratories, Ltd., Church Stretton, United Kingdom) at 150 °C in 1,2,4-trichlorobenzene with 0.0125% butylated hydroxy toluene (BHT). Three PL mixed B columns (500−107) were used. Universal calibration against narrow polystyrene standards was employed. Measurement of the Mechanical and Physical Properties. The notched Charpy impact strength of each polymer sample was measured on a model ZBC1251-2 pendulum impact testing machine (Shenzhen SANS Testing Machine Co., Ltd., Shenzhen, China) according to standard method ASTM D256. The flexural modulus was measured following standard method ASTM D790 on a model CMT4104 electromechanical universal testing machine (Shenzhen SANS Testing Machine Co., Ltd., Shenzhen, China). The polymer granules were heatmolded at 180 °C into sheets, which were then cut into pieces, put into a 280 × 280 × 4 mm mold, and pressed under 20 MPa at 180 °C for 5 min. The sample plates were then slowly cooled to room temperature in the mold. Sample strips for the tests

particularly higher molecular weights and broader molecular weight distributions (MWDs) but slightly lower stereospecificity.33 Most studies in this area have been limited to the use of a single alkoxysilane as the external electron donor for controlling the polymer characteristics. However, a few patents34−40 and articles32,33 have reported the use of combinations of alkoxysilanes for regulating the molecular weight distribution, melt flow index, and mechanical properties of polypropylene. Moreover, the molecular weight and microstructure of poly(ethylene-co-propylene) are conditioned by alkylaluminium (used as a cocatalyst).41,42 The structure of alkylaluminium is changed upon being complexed with alkoxysilane. Therefore, the nature of the external electron donor influences not only the chain structure of polypropylene formed in the first reactor but also the chain structure of poly(ethylene-co-propylene) formed in the second reactor. In this work, a new-generation spherical high-efficiency polypropylene catalyst developed by SINOPEC that is widely used in the production of homopolymers, random copolymers, and impact copolymers with high ethylene contents was used as the catalyst. DDS/D-donor mixtures of different molar ratios were used as external electron donors to prepare iPP/EPR in-reactor alloys. Our scientific interest was focused on the influence of the combined alkoxysilanes on the structure and properties of the iPP/EPR in-reactor alloy.

2. EXPERIMENTAL DETAILS Synthesis of iPP/EPR In-Reactor Alloy. iPP/EPR inreactor alloy was synthesized in a multistage subsequential polymerization process. In the prepolymerization stage, the slurry polymerization of propylene was conducted in a 0.8 L stainless steel jacketed autoclave for 15 min. In each case, 50− 80 mg of a high-efficiency Ziegler−Natta catalyst, TiCl4/ MgCl2/diester (SINOPEC, Beijing, China), was used in the polymerization. The catalyst had a Ti content of 2.9 wt %. Diphenyldimethoxysilane (DDS, Huabang Chemistry Ltd., Hubei, China), dicyclopentyldimethoxysilane (D-donor, Huabang Chemistry Ltd., Hubei, China), or a DDS/D-donor mixture with a specified molar ratio was used as the external electron donor (Si/Ti = 5). Triethylaluminum (TEA, Albemarle, Baton Rouge, Louisiana, USA) was used as the cocatalyst (Al/Ti = 100). n-Heptane was used as the solvent. The pressure of propylene (containing 6.25 mol % H2) in the prepolymerization stage was 1 atm, and the temperature was 25 °C. The mechanical stirring speed was about 300 rpm. After the prepolymerization, propylene (containing 1 mol % H2) was introduced into an autoclave to a pressure of 0.6 MPa. Propylene homopolymerization was carried out for 2 h at 75 °C. During this period, the mechanical stirring speed was adjusted to 100 rpm after 10 min. At the end of this stage, propylene and solvent were removed by evacuation to 5 mmHg for 5 min, and the copolymerization of ethylene and propylene was started. In advance, gaseous ethylene and propylene, containing 1 mol % H2, had been completely mixed in a container in the molar ratio of 1:2 (ethylene/propylene). The ethylene/propylene mixture was continuously supplied at a constant pressure (0.3 MPa) to the autoclave through an inlet pipe in the bottom of the autoclave. At the same time, the ethylene/propylene mixture was discharged by a vent pipe on the cover of the autoclave, so that the mixture composition remained constant. After 20 min at 75 °C, the copolymerization was terminated by pouring the resulting polymer into an excess of ethanol containing 5% HCl, filtering, and washing the solid 5888

dx.doi.org/10.1021/ie303216c | Ind. Eng. Chem. Res. 2013, 52, 5887−5894


Article

active centers in the supported titanium catalyst, the MWD profiles of C7-insol in PP-1−PP-5 were deconvoluted into five Flory components (labeled A−E). The basic theory of the deconvolution of the MWD profiles is as follows: The MWD of a polymer produced by one type of active center in Ziegler− Natta polymerization is a Schulz−Flory distribution, and the distribution index is 2. Thus, the MWD of a polymer produced by a catalyst with multiple active centers is the superposition of the distributions for each type of active center.47 Therefore, the distribution index of each component deconvoluted by the Flory function is 2, where the number of peaks indicates the number of types of active centers and the relative peak heights indicate the relative weights of polymer produced by the various types of active centers. This type of deconvolution has been shown to be a useful and reliable method for differentiating the multiple active sites of heterogeneous Ziegler−Natta catalysts.48−50 Each Flory component with particular molecular weight and Schulz−Flory MWD is produced by a type of active center. The weight fractions and average molecular weights of the five Flory components in samples PP-1−PP-5 are listed in Table S2 (Supporting Information). The molecular weights of Flory components A and B in PP-2−PP-4 were much higher than those in PP-1 and PP-5. However, the molecular weights of Flory components C−E in PP-2−PP-4 were between those in PP-1 and PP-5. There was not much difference in the molecular weights of Flory components C−E in the five samples. However, the combined content of Flory components A and B increased significantly as the amount of D-donor in the DDS/D-donor mixtures increased and then decreased to some extent when Ddonor alone was used as the external electron donor. Therefore, polypropylene homopolymers prepared with combined alkoxysilanes had higher molecular weights and broader MWDs. PP4 had the highest molecular weight and narrowest MWD among samples PP-2−PP-4. Synthesis of iPP/EPR In-Reactor Alloys. After the influence of the external electron donor on the chain structure of polypropylene homopolymer had been determined, iPP/ EPR in-reactor alloys were prepared under the same range of conditions. The polymerization and fractionation results are summarized in Table 1. The catalytic activities of the catalytic

were cut from the plates following standard method ASTM D256. For each test point, five parallel measurements were made, and the average values were recorded. Morphology Analysis. The morphology and dispersion state of the EPR phase of the iPP/EPR in-reactor alloys were investigated by scanning electron microscopy (SEM; S-4800, Hitachi, Tokyo, Japan). The SEM samples were prepared as follows: Strips of the polymer were prepared as described in the preceding section and fractured in liquid nitrogen. The fractured surface was dipped into toluene at room temperature and etched by toluene under ultrasound for 5 min. Then, the fractured surface was coated with platinum and observed by SEM.

3. RESULTS AND DISCUSSION Homopolymerization of Propylene. In iPP/EPR inreactor alloys, PP is the matrix. PP can be isotactic, syndiotactic, or atactic, depending on the orientation of the pendant methyl groups attached to alternate carbon atoms. Because of its structure, isotactic PP has the highest crystallinity, resulting in good mechanical properties such as stiffness and tensile strength. Syndiotactic PP is less stiff than isotactic PP but has better impact strength and clarity. Because of its irregular structure, the atactic form has low crystallinity, resulting in a sticky, amorphous material used mainly for adhesives and roofing tars.43,44 Increasing the amount of atactic PP in a predominantly isotactic formulation increases the roomtemperature impact resistance and stretchability but decreases the stiffness, haze, and color quality.45 In industrial propylene polymerization catalyzed by MgCl2/TiCl4/diester-type highyield catalysts, triethylaluminum (TEA) is the first choice for cocatalyst, because it gives PP of higher isotacticity than does triisobutylaluminum (TIBA) cocatalyst.46 Electron donors are major components in propylene polymerization catalyst systems, where a high stereospecificity is required to produce isotactic PP. In this work, two silanes, namely, diphenyldimethoxysilane (DDS) and dicyclopentyldimethoxysilane (Ddonor), were used as external electron donors. First, homopolymerizations of propylene were performed using DDS, D-donor, or DDS/D-donor mixtures with different molar ratios. The results obtained from these experiments regarding the influence of the external electron donor on propylene polymerization are reported in Table S1 (Supporting Information). According to Table S1 (Supporting Information), the catalytic activities of the catalytic systems consisting of DDS, D-donor, and DDS/D-donor mixtures as external electron donors were similar to each other. The isotacticity indexes of samples PP-2−PP-4 were intermediate between those of PP-1 and PP-5. That is, the combined alkoxysilanes produced PP samples with intermediate isotacticities. The molecular weights and the distributions of C7-insol (part of PP insoluble in boiling n-heptane) in PP-2−PP-4 were higher than those of PP-1 and PP-5. However, the molecular weights and the distributions of C7-sol (part of PP soluble in boiling nheptane) in PP-2−PP-4 were not higher than those of PP-1 and PP-5. This indicates that the polypropylene homopolymers prepared using the combined alkoxysilanes had higher molecular weights and broader MWDs because the contents of C7-sol were just about 1 wt %. Supported titanium catalysts have different types of active centers that provide polypropylene homopolymer with a wide range of chain structure distributions. To determine the influence of the combined alkoxysilanes on the distribution of

Table 1. Preparation and Fractionation of PP/EPR InReactor Alloys

sample

external donor (DDS/D-donor ratio)

activity [kg of polymer (g of catalyst)−1]

Ea (wt %)

C8sol (wt %)

C7sol (wt %)

C7insol (wt %)

alloy-1 alloy-2 alloy-3 alloy-4 alloy-5

1:0 3:1 1:1 1:3 0:1

2.11 2.16 1.95 2.50 2.16

4.2 5.0 5.9 4.5 4.6

9.5 11.9 14.9 11.7 10.0

4.7 3.8 3.0 2.9 3.4

85.8 84.3 82.1 85.4 86.6

a Content of ethylene in PP/EPR in-reactor alloys calculated from 13C NMR results.

systems using DDS, D-donor, and DDS/D-donor mixtures as external electron donors were similar to each other. C7-insol (part insoluble in boiling n-heptane) was the main component of iPP/EPR in-reactor alloys. The content of C7-sol (part soluble in boiling n-heptane) was low. The contents of C8-sol (part soluble in n-octane at room temperature) in alloy-2− alloy-4 (especially alloy-3) were higher than those in alloy-1 5889



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inflection point (as shown in Figure 1). According to the areas of these two parts, the weight percentages of low-isotacticity PP and multiblock poly(ethylene-co-propylene) could be estimated, as summarized in Table 3. As the amount of D-donor

and alloy-5. The contents of C7-sol in alloy-3 and alloy-4 were slightly lower than those in alloy-1 and alloy-5. To some extent, the combined alkoxysilanes were able to increase the incorporation of ethylene. Chain Structure of iPP/EPR In-Reactor Alloys. Every fraction of alloy-1−alloy-5 was measured by GPC. The molecular weights and distributions of these fractions are summarized in Table 2. The molecular weights of C8-sol, C7-

Table 3. Estimated Contents of Multiblock Poly(ethylene-copropylene) and Low-Isotacticity PP in PP/EPR In-Reactor Alloysa

Table 2. Molecular Weight and Its Distribution for Every Fraction in Alloy-1−Alloy-5 C8-sol

C7-sol

C7-insol

sample

M̅ w (×104)

MWD

M̅ w (×104)

MWD

M̅ w (×104)

MWD


8.2 12.3 13.1 16.0 12.7

3.8 3.8 5.6 5.2 3.9

4.1 4.9 7.5 9.3 8.5

6.7 7.8 8.6 8.8 8.8

20.0 20.9 27.1 33.7 24.4

5.6 4.8 6.8 6.0 4.2

sample

multiblock EP (wt %)

LiPP (wt %)

SPP

multiblock EP /C8-sol


1.9 1.9 2.0 2.2 2.3

2.8 1.9 1.0 0.7 1.1

0.60 0.50 0.33 0.24 0.32

0.20 0.16 0.13 0.19 0.23

a

Multiblock EP, multiblock poly(ethylene-co-propylene); LiPP, lowisotacticity polypropylene; SPP, fraction of low-isotacticity PP in C7sol; C8-sol, soluble part in n-octane at room temperature.

in the DDS/D-donor mixtures increased, the content of multiblock poly(ethylene-co-propylene) increased slightly, whereas the content of low-isotacticity polypropylene homopolymer tended to decrease. However, the ratio between multiblock poly(ethylene-co-propylene) and C8-sol was lower for alloy-2−alloy-4 than for alloy-1 and alloy-5. The C8-sol fractions of alloy-1−alloy-5 were all characterized by 13C NMR spectroscopy. The analysis results are summarized in Table 4.52 In these fractions, the molar ratio between

sol, and C7-insol in alloy-4 were the highest among the corresponding fractions in the five alloy samples. The MWDs of C8-sol and C7-insol in alloy-4 were moderate among the corresponding fractions in the five samples. The MWD of C7sol in alloy-4 was the highest. Because this fraction was the smallest component of the iPP/EPR in-reactor alloys, its influence on the molecular weight and MWD of the overall alloys could be ignored. In our previous work,17 it was found that C8-sol is a mixture of random poly(ethylene-co-propylene) (≥90 wt %) and atactic polypropylene homopolymer. C7-sol was found to be a mixture of multiblock poly(ethylene-co-propylene) (40−70 wt %) and low-isotacticity polypropylene homopolymer. As shown in Figure 1, the GPC curves of C7-sol in these five alloys were bimodal. The left and right peaks can be attributed to lowisotacticity PP and multiblock poly(ethylene-co-propylene), respectively.51 These bimodal GPC curves were cut into two parts by a straight line perpendicular to the x axis at the

Table 4. 13C NMR Results for C8-Sol in iPP/EPR In-Reactor Alloys E P EE EP PP EEE EEP + PEE PEP EPE PPE + EPP PPP rErP nE nP

alloy-1

alloy-2

alloy-3

alloy-4

alloy-5

42.8 57.2 21.2 41.8 37.0 13.1 16.1 13.6 7.8 24.6 24.8 1.8 2.0 2.7

42.7 57.3 22.4 43.4 34.2 14.3 16.2 12.3 11.2 23.6 22.4 1.6 2.0 2.6

42.0 58.0 22.2 44.5 33.3 13.0 18.4 10.7 10.4 28.6 18.9 1.5 1.9 2.6

39.4 60.6 19.0 41.2 39.8 9.7 18.6 11.0 8.9 23.9 27.9 1.8 1.9 2.9

43.7 56.3 20.3 46.3 33.4 10.7 19.4 13.7 10.0 25.8 20.4 1.3 1.9 2.4

ethylene and propylene units was close to 1:1, and the product of rE (reactivity ratio of ethylene) and rP (reactivity ratio of propylene) was in the range of 1−2. This is the characteristic sequence distribution of random poly(ethylene-co-propylene). The contents of propylene units in alloy-2−alloy-4 were higher than those in alloy-1 and alloy-5. The contents of PP dyads and PPP triads in C8-sol in alloy-4 were the highest. In addition, the number-average sequence length of propylene units in C8-sol in alloy-4 was the highest. All in all, the external electron donor was found to have a minimal influence on the chain structure of C8-sol. The C7-sol fractions of alloy-1−alloy-5 were also characterized by 13C NMR spectroscopy. The analysis results are summarized in Table S3 (Supporting Information).52 Because this fraction still contained 30−60 wt % of low-isotacticity PP,

Figure 1. GPC curves of C7-sol in iPP/EPR in-reactor alloys. 5890



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ethylene units and propylene units was close to 1:1, and the contents of PP′ dyads and PPP′ triads were the highest. Moreover, as the amount of D-donor in the DDS/D-donor mixtures increased, the values of PP′/EE′, PPP′/EEE′, and nP′ / n′E increased, but when D-donor alone was used as the external electron donor, these values decreased. The values of PP′/EE′, PPP′/EEE′, and nP′ /nE′ in alloy-4 were 0.79, 0.69, and 0.90, respectively. These results show that the multiblock poly(ethylene-co-propylene) in alloy-4 was the most uniform in chain structure and contained the longest polypropylene segments among the five alloy samples. Thus, it can be deduced that the multiblock poly(ethylene-co-propylene) in alloy-4 might be the best compatibilizer between the iPP matrix and random poly(ethylene-co-propylene). As mentioned previously, the external electron donor to some extent influences the chain structure, especially the sequence distribution, of the multiblock poly(ethylene-co-propylene) in iPP/EPR in-reactor alloys. Mechanical Properties of iPP/EPR In-Reactor Alloys. The mechanical properties of the iPP/EPR in-reactor alloys were measured and are summarized in Table 6. As the amount

the signal of PPP triads in poly(ethylene-co-propylene) overlapped the signal of PPP triads in low-isotacticity PP. Consequently, the data listed in Table S3 (Supporting Information) might not represent the real chain structure of multiblock poly(ethylene-co-propylene) in this fraction. If the contribution of polypropylene homopolymer to the sequence distribution (as listed in Table S3, Supporting Information) were excluded, a relatively precise sequence distribution of multiblock poly(ethylene-co-propylene) in C7-sol would be available. The detailed calculation method was as follows: First, the mole percentage of each triad (EEE, EEP, PEP, EPE, PPE, and PPP) as listed in Table S3 (Supporting Information) was transformed into the corresponding weight percentage. Then, the weight percentage of each triad in poly(ethylene-copropylene) (expressed as mEEE ′ , mEEP ′ , mPEP ′ , mEPE ′ , mPPE ′ , and ′ ) in the C7-sol fraction was calculated by the equations mPPP ′ = mEEE /(1 − SPP) mEEE

(1)

′ = mEEP /(1 − SPP) mEEP

(2)

′ = mPEP /(1 − SPP) mPEP

(3)

′ = mEPE /(1 − SPP) mEPE

(4)

′ = mPPE /(1 − SPP) mPPE

(5)

′ = (mPPP − SPP × 100)/(1 − SPP) mPPP

(6)

Table 6. Mechanical Properties of iPP/EPR In-Reactor Alloys Measured at 10 °C

where SPP is the fraction of low-isotacticity PP in C7-sol. The contribution of polypropylene homopolymer to the content of PPP triads was excluded by eq 6. Finally, the values of m′EEE, m′EEP, m′PEP, m′EPE, m′PPE, and m′PPP were transformed into the mole percentages of the corresponding triads (expressed as EEE′, EEP′ + PEE′, PEP′, EPE′, PPE′ + EPP′ and PPP′). Subsequently, rErP, nE, and nP were calculated. The calculation results are summarized in Table 5. The contents of EE′ dyads, PP′ dyads, EEE′ triads, and PPP′ triads

alloy-1

alloy-2

alloy-3

alloy-4

alloy-5

66.4 33.6 57.6 23.4 19.0 53.3 8.5 4.6 4.8 19.4 9.4 8.1 5.7 2.9

64.3 35.7 57.2 18.7 24.1 53.2 8.0 3.1 4.0 15.2 16.5 15.8 6.9 3.8

63.5 36.5 54.7 22.5 22.8 50.7 7.9 4.9 4.1 19.1 13.3 9.9 5.6 3.2

52.6 47.4 44.7 20.2 35.1 40.9 7.5 4.2 5.3 13.9 28.2 15.4 5.2 4.7

63.3 36.7 53.7 20.0 26.3 48.9 9.5 4.9 6.5 7.7 22.5 14.1 6.3 3.7

impact strength (kJ/m2)

flexural modulus (MPa)

flexural strength (MPa)


4.6 7.7 9.5 12.4 9.2

1072 1117 1099 988 1027

43.5 46.0 43.0 45.4 44.3

of D-donor in DDS/D-donor mixtures increased, the impact strength increased; the impact strength then decreased when Ddonor alone was used as the external electron donor. The flexural moduli and flexural strengths of the five samples were similar to each other. In particular, alloy-4 showed the highest impact strength, but its flexural modulus was slightly lower than those of the other alloys. Alloy-4 was thus the best iPP/EPR inreactor alloy synthesized in this work, with an excellent toughness/stiffness balance. Morphology of iPP/EPR In-Reactor Alloys. Figure 2 displays SEM images of the cryogenically fractured surfaces of strips of alloy-2−alloy-4 strips etched by toluene. In these images, a biphasic structure can be clearly seen. The EPR phase, which was soluble in toluene at 50 °C, was removed by toluene etching. The cavities on the sample surface were left by EPR after toluene etching. The cavities on the surface of alloy-4 were much smaller than those on the surfaces of alloy-2 and alloy-3. At the same time, the number of cavities on alloy-4 was much larger than the numbers on alloy-2 and alloy-3. That is, the dispersion of the rubber phase in alloy-4 was much more uniform than the dispersion in alloy-2 and alloy-3. The contents of EPR and multiblock poly(ethylene-co-propylene) in alloy-4 were not the highest among the five alloys (as shown in Tables 1 and 2). However, the chain structure of the multiblock poly(ethylene-co-propylene) in alloy-4 was special. The highest number-average sequence length of propylene units (as shown in Table 5) might enable the multiblock poly(ethylene-copropylene) in alloy-4 to be the best compatibilizer between the iPP matrix and EPR. Therefore, the toughness of alloy-4 was the highest among the five alloys. Moreover, the molecular

Table 5. Estimated Results for the Composition of Multiblock Poly(ethylene-co-propylene) in C7-Sol E′ P′ EE′ EP′ PP′ EEE′ EEP′ + PEE′ PEP′ EPE′ PPE′ + EPP′ PPP′ rE′rP′ nE′ nP′

sample

were very high. The value of rE′rP′ was greater than 8. This is the characteristic sequence distribution of multiblock poly(ethylene-co-propylene). As the amount of D-donor in the DDS/D-donor mixtures increased, the contents of propylene units, PP′ dyads, and PPP′ triads increased, and these contents then decreased when D-donor alone was used as the external electron donor. Notably, in alloy-4, the molar ratio between 5891



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and then decreased when D-donor alone was used as the external electron donor. However, the isotacticity of polypropylene homopolymer increased as the amount of D-donor in the DDS/D-donor mixtures increased. The supported titanium catalytic system is very complicated, including TiCl4, MgCl2, triethylaluminum, and internal and external electron donors. Although the role of the external electron donor is complex, the following essential reactions seem to proceed in MgCl2/internal electron donor/TiCl4−AlEt3/external electron donor systems: complex formation with trialkylaluminium, leading to a decrease in reducing capacity with respect to the titanium entities, and deactivation of some specific centers by selective complex formation or even conversion of nonspecific sites into isospecific sites.53 When the catalyst is brought into contact with trialkylaluminium, a large proportion of the internal donor is lost as a result of alkylation and/or complexation reactions. To a large extent, the external donor replaces the internal donor in the solid catalyst, thereby maintaining high catalyst stereospecificity.54 According to BattCoutrot et al.’s molecular model, a bulky and rigid group on the external electron donor is required to control monomer access by blocking the vacant space near the empty orbital of titanium.32 For alkoxysilanes [R2(CH3O)2Si, R = cyclopentyl (Cpy), phenyl (Ph), etc.], cyclopentyl (Cpy) is bulkier than phenyl (Ph). That is, the steric hindrance of D-donor is higher than that of DDS. Therefore, D-donor is more effective in controlling propylene access by blocking the vacant space near the empty orbital of titanium and, consequently, in making higher-isotacticity polypropylene. Therefore, as the amount of D-donor in the DDS/D-donor mixtures increased, the isotacticity of the polypropylene homopolymer increased. Because DDS and D-donor can deactivate some specific centers or convert nonspecific sites into isospecific sites, the distribution of active center s changes after the catalyst is mixed with DDS or D-donor. The types of active centers transformed by DDS and D-donor might be different from each other, because of their structural differences. The active centers transformed by DDS and D-donor are denoted here as A and B centers, respectively. A and B centers provide polypropylene not only with low isotacticity, but also with low molecular weight. As shown in Table S2 (Supporting Information), the molecular weight of PP-5 was much higher than that of PP-1. That is, the catalyst without B centers can produce PP with higher molecular weight. When DDS and D-donor are added into the catalytic system together, both A and B centers can be transformed. Thus, the fraction of PP with low molecular weight can be further diminished. As the amount of D-donor in the DDS/D-donor mixtures increases, the amount of B centers transformed by D-donor increases. At an optimum ratio of DDS to D-donor (DDS/D-donor = 1:3 in this work), both A and B centers can be transformed the best, so the molecular weight of PP prepared under these conditions is the highest. At the same time, because both A and B centers are deactivated and/or converted into isospecific sites, the distribution of active centers is the narrowest at DDS/D-donor = 1:3. As a result, multiblock poly(ethylene-co-propylene) prepared under these conditions is the most uniform in chain structure.

Figure 2. SEM images of fractured surfaces of the iPP/EPR in-reactor alloys etched by toluene at 50 °C: (a) alloy-2, (b) alloy-3, (c) alloy-4.

weight of the iPP matrix in alloy-4 was the highest. Therefore, the flexural modulus of alloy-4 decreased only slightly. Proposed Mechanism for the Influence of the External Electron Donor on the Structure of iPP/EPR In-Reactor Alloy. The experimental results show that, the molecular weight of polypropylene homopolymer and the structural uniformity of poly(ethylene-co-propylene) increased as the amount of D-donor in DDS/D-donor mixtures increased

4. CONCLUSIONS In conclusion, strong external electron-donor effects were found in the preparation of iPP/EPR in-reactor alloys with high-efficiency industrial Ziegler−Natta catalyst by multistage sequential polymerization. The structure and mechanical 5892



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(9) Fan, Z. Q.; Zhang, Y. Q.; Xu, J. T.; Wang, H. T.; Feng, L. X. Structure and properties of polypropylene/poly(ethylene-co-propylene) in-situ blends synthesized by spherical Ziegler−Natta catalyst. Polymer 2001, 42, 5559. (10) Fan, Z. Q.; Deng, J.; Zuo, Y. M.; Fu, Z. S. Influence of copolymerization conditions on the structure and properties of polyethylene/polypropylene/poly(ethylene-co-propylene) in-reactor alloys synthesized in gas-phase with spherical Ziegler−Natta catalyst. J. Appl. Polym. Sci. 2006, 102, 2481. (11) Fu, Z. S.; Xu, J. T.; Zhang, Y. Z.; Fan, Z. Q. Chain structure and mechanical properties of polyethylene/polypropylene/poly(ethyleneco-propylene) in-reactor alloys synthesized with a spherical Ziegler− Natta catalyst by gas-phase polymerization. J. Appl. Polym. Sci. 2005, 97, 640. (12) Dong, Q.; Wang, X. F.; Fu, Z. S.; Xu, J. T.; Fan, Z. Q. Regulation of morphology and mechanical properties of polypropylene/poly(ethylene-co-propylene) in-reactor alloys by multi-stage sequential polymerization. Polymer 2007, 48, 5905. (13) Fu, Z. S.; Fan, Z. Q.; Zhang, Y. Q.; Feng, L. X. Structure and morphology of polypropylene/poly(ethylene-co-propylene) in situ blends synthesized by spherical Ziegler−Natta catalyst. Eur. Polym. J. 2003, 39, 795. (14) Feng, Y.; Hay, J. N. The characterisation of random propylene− ethylene copolymer. Polymer 1998, 39, 6589. (15) Feng, Y.; Jin, X.; Hay, J. N. Crystalline structure of propylene− ethylene copolymer fractions. J. Appl. Polym. Sci. 1998, 68, 381. (16) Cai, H. J.; Luo, X. L.; Chen, X. X.; Ma, D. Z.; Wang, J. M.; Tan, H. S. Structure and properties of impact copolymer polypropylene. II. Phase structure and crystalline morphology. J. Appl. Polym. Sci. 1999, 71, 103. (17) Tu, S. T.; Fu, Z. S.; Fan, Z. Q. Influence of cocatalyst on the structure and properties of polypropylene/poly(ethylene-co-propylene) in-reactor alloys prepared by MgCl2/TiCl4/diester type Ziegler− Natta catalyst. J. Appl. Polym. Sci. 2012, 124, 5154. (18) 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. (19) Ribour, D.; Monteil, V.; Spitz, R. Detitanation of MgCl2supported Ziegler−Natta catalysts for the study of active sites organization. J. Polym. Sci. A: Polym. Chem. 2008, 46, 5461. (20) Olalla, B.; Broyer, J.; McKenna, T. F. L. Heat Transfer and Nascent Polymerisation of Olefins on Supported Catalysts. Macromol. Symp. 2008, 271, 1. (21) Taniike, T.; Terano, M. Reductive formation of isospecific Ti dinuclear species on a MgCl2 (110) surface in heterogeneous Ziegler− Natta catalysts. Macromol. Rapid Commun. 2008, 29, 1472. (22) Gao, M. Z.; Liu, H. T.; Wang, J.; Li, C. X.; Ma, J.; Wei, G. S. Novel MgCl2-supported catalyst containing diol dibenzoate donor for propylene polymerization. Polymer 2004, 45, 2175. (23) Härkö nen, M.; Seppälä, J. V.; Väänänen, T. External Alkoxysilane Donors in Ziegler−Natta Catalysis. Effects on Poly(propylene) Microstructure. Macromol. Chem. 1991, 192, 721. (24) Seppala, J. V.; Harkonen, M. Effect of the Structure of External Alkoxysilane Donors on the Polymerization of Propene with HighActivity Ziegler−Natta Catalysts. Makromol. Chem. 1989, 190, 2535. (25) Abedi, S.; Daftari-Besheli, M.; Shafiei, S. Highly active Ziegler− Natta catalyst for propylene polymerization. J. Appl. Polym. Sci. 2005, 97, 1744. (26) Virkkunen, V.; Laari, P.; Pitkanen, P.; Sundholm, F. Tacticity distribution of isotactic polypropylene prepared with heterogeneous Ziegler−Natta catalyst. 2. Application and analysis of SSA data for polypropylene. Polymer 2004, 45, 4623. (27) Ferraro, A.; Camurati, I.; Dall’occo, T.; Piemontesi, F.; Cecchin, G. Advances in Ziegler−Natta catalysts for polypropylene. Kinet. Catal. 2006, 47, 176. (28) Härkönen, M.; Seppälä, J. V.; Väan̈ änen, T. Effects of the Structure of External Alkoxy Silane Donor in High Activity Ziegler− Natta Catalyst on the Microstructure of Polypropylene. In Catalytic

properties of the iPP/EPR in-reactor alloys were significantly influenced by the external electron donor. Using DDS/D-donor mixtures as external electron donors and TEA as the cocatalyst, the molecular weight of polypropylene homopolymer and the structural uniformity of multiblock poly(ethylene-co-propylene) increased as the amount of D-donor in the DDS/D-donor mixtures increased but then decreased when D-donor alone was used as the external electron donor. However, the isotacticity of polypropylene homopolymer increased as the amount of Ddonor in the DDS/D-donor mixtures increased. Therefore, as the amount of D-donor in the DDS/D-donor mixtures increased, the impact strength of iPP/EPR in-reactor alloy increased, but the impact strength then decreased when Ddonor alone was used as the external electron donor. An optimum feed ratio between DDS and D-donor was found, namely, DDS/D-donor = 1:3 (Si/Ti = 5). The iPP/EPR inreactor alloy prepared under these conditions was the toughest, because its multiblock poly(ethylene-co-propylene) was the most uniform in chain structure, containing a maximum of propylene units and PPP triads and acting as the best compatibilizer between iPP and EPR. The influence of the external electron donor on the flexural modulus and flexural strength, however, can be ignored.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. 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 from the National High-Tech R&D Program of China (Grant 2012AA040305) and the Major State Basic Research Programs (Grant 2011CB606001) is gratefully acknowledged.

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REFERENCES

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