Polypropylene and Ethylene−Propylene Copolymer Reactor Alloys

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Polypropylene and Ethylene-Propylene Copolymer Reactor Alloys Prepared by Metallocene/Ziegler-Natta Hybrid Catalyst Lie Lu,† Hong Fan,*,† Bo-Geng Li,† and Shiping Zhu*,‡ State Key Laboratory of Chemical Engineering, Department of Chemical & Biological Engineering, Zhejiang UniVersity, Hangzhou, People’s Republic of China 310027, and Department of Chemical Engineering, McMaster UniVersity, Hamilton, Ontario, Canada L8S 4L7

Polypropylene/ethylene-propylene rubber (PP/EPR) reactor alloys were prepared with a metallocene/ Ziegler-Natta hybrid catalyst system (rac-Et(Ind)2ZrCl2/TiCl4/MgCl2) using a process composed of three stages: propylene homopolymerization, metallocene activation, and ethylene-propylene copolymerization. A series of alloy samples were produced and characterized at various copolymerization conditions by varying the methylaluminoxane (MAO)/Zr ratio and monomer composition. It was shown that the metallocene/ Ziegler-Natta hybrid system exhibited the features of both metallocene and Ziegler-Natta catalysts during copolymerization. The hybrid catalyst had better ability in incorporating R-olefin than the Ziegler-Natta catalyst owing to the action of metallocene active sites. DSC and IR analyses suggested that EPR in the alloys became random with increased MAO/Zr ratio. In addition, reducing the ethylene content in the feed decreased the activity and promoted the production of random copolymers. An operation window of reaction conditions was identified for the preparation of well-dispersed spherical PP/EPR reactor alloy particles containing up to about 40 wt % EPR. Introduction As a common plastic, polypropylene is widely used in many areas. However, its poor impact strength obviously limits its application.1,2 To toughen polypropylene (PP), ethylene-propylene rubber (EPR) or ethylene-propylene-diene monomer (EPDM) is often added as an impact modifier.3-6 In recent years, reactor granule technology (RGT) was developed based on Ziegler-Natta catalysis. The process is composed of two sequential polymerization stages: propylene homopolymerization and ethylenepropylene copolymerization. PP/rubber alloys prepared by RGT gave good impact properties, which have been successfully commercialized.7-12 Recently, some patents reported methods for the preparation of PP/rubber reactor alloys with metallocene/Ziegler-Natta (Z-N) hybrid catalysts.13,14 These hybrid catalysts were prepared by supporting metallocene and Z-N active sites on the same support. It is anticipated that a hybrid catalyst can combine the advantages of Z-N catalyst in morphological control and metallocene catalyst in polymer chain microstructure control. Our recent work demonstrated that ethylene-propylenediene elastomer prepared by metallocene catalyst (m-EPDM) is much better than the conventional EPDM prepared by Z-N catalyst for toughening PP.15 The PP/m-EPDM blend has a lower critical rubber content at the brittle-ductile transition and a much narrower brittle-ductile transition range. In addition, the elongation at break of the PP/m-EPDM blends is improved. In general, isotactic polypropylene (iPP)/rubber reactor alloys are better than mechanical blends in terms of material properties and production costs. In our previous work,16 functional polypropylene granules were synthesized through propylene copolymerization with protected dihydromyrcene alcohol by a spherical Ziegler-Natta catalyst. The functional polypropylene performed effectively as a support for anchoring metallocene * To whom correspondence should be addressed. E-mail: hfan@ zjuem.zju.edu.cn (H.F.); [email protected] (S.Z.). † Zhejiang University. ‡ McMaster University.

catalysts, which was applied in slurry copolymerization of ethylene and propylene to produce the iPP/m-EPR reactor alloy. This approach allowed the metallocene catalyst to effectively copolymerize ethylene and R-olefin. However, the complicated steps involved in protecting the polar comonomer could limit its application in industrial production. In RGT, Z-N catalyst does very well in controlling PP morphologies in the homopolymerization stage; however, EP copolymer produced by Z-N catalyst in the subsequent copolymerization stage often has poor elastomeric properties due to the presence of long ethylene sequences, restricting the PP reactor alloy impact properties. Compared to Z-N catalyst, metallocene is known to have better ability in incorporating R-olefins and to have comonomer units randomly distributed along the copolymer chain, thus limiting the formation of long ethylene sequences. The metallocene copolymer gives excellent elastomeric properties.17-20 Ethylene-R-olefin copolymers prepared by metallocene catalyst are proven to be very effective impact modifiers for PP.21-23 It appears to be logical thinking and a worthy effort to combine the advantages of Z-N catalyst for PP morphological control in the homopolymerization stage and metallocene catalyst for copolymer chain microstructure control in the copolymerization stage, for the preparation of PP/ rubber reactor alloys through RGT. The resulting PP/rubber reactor alloys are expected to have better material properties than the conventional counterparts prepared by a single Z-N catalyst. In this work, we combine a metallocene with a Z-N catalyst in the catalyst preparation stage by immobilizing rac-ethylenebis(indeny1)zirconium dichloride (rac-Et(Ind)2ZrCl2) onto TiCl4/MgCl2 particles. The resulting metallocene/ZieglerNatta hybrid catalyst system is applied in reactor granule technology to prepare PP/EPR reactor alloys. The structure and properties of the prepared polymers are evaluated. Experimental Section Materials. A high-yield spherical TiCl4/MgCl2-ID (where ID is an internal donor) catalyst was donated by the Beijing

10.1021/ie900579h CCC: $40.75  2009 American Chemical Society Published on Web 08/14/2009

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Scheme 1. Preparation of Metallocene/Ziegler-Natta Hybrid Catalyst

Research Institute of Chemical Industry, Beijing, China. The catalyst contained 2.4 wt % Ti content. The Z-N catalyst was used in the polymerization with triethylaluminum (TEA) as a cocatalyst and cyclohexylmethyldimethoxysilane (CHMDMS) as an external donor. The metallocene catalyst rac-Et(Ind)2ZrCl2 was purchased from Sterm Chemicals Co. Methylaluminoxane (MAO) from J&K Chemical Co. was used as received. Polymerization-grade ethylene and propylene from Yangzi Petrochemical Chemical Engineering Company, China, were purified by passing through CuO and molecular sieves. Toluene (anhydrous grade, from Hangzhou Chemical Reagents Factory, China) was refluxed over potassium for 24 h prior to use. Preparation of Metallocene/Ziegler-Natta Hybrid Catalyst. The Z-N catalyst (1 g) and toluene (80 mL) were added to a glass flask, followed by addition of TEA (3.6 mmol) and rac-Et(Ind)2ZrCl2 (50 mg) in toluene solution under magnetic stirring. The temperature was subsequently raised to 60 °C, and the reaction proceeded at this temperature for 6 h with stirring. The resulting slurry was washed eight times with 100 mL of toluene, and finally dried under vacuum. Polymerization. PP was synthesized with the metallocene/ Z-N hybrid catalyst system or the net Z-N catalyst by the homopolymerization of liquid propylene. In this process, the catalyst together with cocatalyst and cyclohexylmethyldimethoxysilane (CHMDMS) as an external donor were introduced to an autoclave equipped with a magnetic stirrer under a flow of nitrogen. Liquid propylene was then introduced to start prepolymerization for 15 min at room temperature. The reactor temperature was then raised to 70 °C and maintained at this temperature for a defined period of time. The PP/EPR reactor alloys were synthesized by the hybrid catalyst using a three-stage reaction process. The first stage was the homopolymerization in liquid propylene, the second stage was the activation of metallocene catalyst, and the third stage was the copolymerization of ethylene and propylene in toluene solution. TEA was used as the cocatalyst in the propylene homopolymerization stage. The liquid propylene was vented out at the end of polymerization. A defined amount of MAO in toluene solution was immediately introduced and mixed with polypropylene particles at 70 °C for 1 h to activate metallocene sites inside the particles. Finally, the copolymerization was initiated by introducing a mixture of ethylene and propylene with a targeted composition, and was conducted at 50 °C at a constant pressure. The copolymerization was finally stopped by adding an excess amount of hydrochloric acid solution diluted with ethanol. Characterization. The intrinsic viscosity of polypropylene was measured with an Ubbelohde viscometer in a decalin solution at 135 °C. The viscosity-average molecular weights (Mv) of the PP samples were estimated from their intrinsic viscosity data using the Mark-Houwink equation. Isotacticity

(II) was determined as the weight fraction of polymer materials that was insoluble in boiling heptane. The differential scanning calorimetry (DSC) analysis of the polymers was made by a Perkin-Elmer DSC-7 thermal analyzer under N2 atmosphere. The sample was heated from 30 to 180 °C at a heating rate of 10 °C/min, kept at 180 °C for 5 min, and then cooled to 30 °C at 10 °C/min for recrystallization. This was followed by reheating from 30 to 180 °C at 10 °C/ min. The Ti and Zr concentrations in the hybrid catalyst system were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using an IRIS Intrepid II XSP type ICP. The xylene soluble (XS) measurement was conducted by dissolving the alloy samples in xylene at 130 °C. The flask containing the dissolved polymer was then immersed in a water bath at 25 °C and maintained for 24 h, during which the insoluble portion was precipitated. The precipitate was collected by filtration. The soluble part in the filtrate was collected by evaporation of the solvent followed by vacuum drying. The residue was then weighed. The part of XS reactor alloy was random copolymer. Fourier transform infrared (FTIR) spectra of the alloy samples were recorded on a Bruker Vector 22 FTIR spectrometer. A thin film of each sample was prepared through hot pressing. The 300 MHz 13C NMR analysis was conducted on a Varian Mercury 300 pulsed NMR spectrometer at 120 °C. The polymer sample was dissolved in o-dichlorobenzene in a 10 mm NMR tube with a concentration of about 10 wt %. At least 3000 scans were applied for each acquisition to obtain a good signal-tonoise ratio. Polymer molecular weight (Mw) and molecular-weight distribution (MWD) were measured by gel permeation chromatography (GPC) with a PL-GPC 220 coupled with an in-line capillary viscometer. The analyses were performed at 150 °C using 1,2,4-trichlorobenzene as solvent with a flow rate of 1 mL/min. A calibration curve was established with monodisperse polystyrene standards. Results and Discussion Propylene Homopolymerization Catalyzed by Hybrid Catalyst. The Ziegler-Natta catalyst (TiCl4/MgC12) is wellknown to yield polypropylene with high isotacticity. In this work, this catalyst was modified with rac-Et(Ind)2ZrCl2 catalyst to obtain a metallocene/Z-N hybrid catalyst system. The supporting mechanism was illustrated by some investigators as described in Scheme 1.24 The ICP result showed that the weight contents of Ti and Zr in the hybrid system were 1.83% and 0.45%, respectively. Liquid phase polymerization of propylene was performed with this hybrid catalyst combined with various cocatalysts. The polymerization results and some analytical data of the resulting polymers are summarized in Table 1.

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Table 1. Reaction Conditions and Polymer Properties in Bulk Polymerization of Propylene Catalyzed by Net Z-N and Metallocene/Z-N Hybrid Catalysta catalyst

run

cocatalyst

Al/Mb

productivity (kg of PP/(g of catal · h))

activity (106 g of PP/(mol of M · h))

Mv (×104 g/mol)

II (%)

aggc

Z-N

1 2 3 4 5 6

TEA MAO TEA MAO MAO MAO

600 600 600 600 1000 2000

7.37 6.71 1.17 0.95 1.95 3.24

14.73 13.42 2.71 2.21 4.53 7.51

73.1 61.6 65.2 38.0 26.4 13.6

97.0 89.6 94.0 87.2 80.0 82.9

no yes no yes yes yes

hybrid

a Homopolymerization conditions: T ) 70 °C, t ) 1 h, and liquid propylene ) 1.2 L. b M represents metal; M ) Ti for runs 1 and 2 and M ) Ti + Zr for runs 3-6. c Agglomeration.

Figure 1. DSC heating curves of polypropylene samples prepared by hybrid and Z-N catalyst with different cocatalysts.

As shown in runs 1-4 in Table 1, the activities of the hybrid catalyst system were much lower than those of the conventional Z-N catalyst regardless of the cocatalyst type (TEA or MAO). This suggests some unfavorable interactions between racEt(Ind)2ZrCl2 catalyst and TiCl4 active center in the hybrid system, which yielded a negative effect on the activity of TiCl4 species. In addition, the hybrid system yielded polypropylene materials having lower molecular weight and lower isotacticity than those prepared by the Z-N catalyst. The cocatalyst type influenced the polypropylene properties. MAO gave lower Mw and lower isotacticity than TEA. The effect of MAO on isotacticity was presumably due to reversible complexation of MAO molecule with the external donor. The latter became unable to deactivate the less stereospecific active sites of the Ziegler-Natta catalyst. Figure 1 shows the DSC heating curves of the PP samples. Polypropylenes prepared by the net Z-N catalyst with both TEA and MAO (runs 1 and 2) gave single melting peaks around 160 °C attributed to Ti species. In contrast, the PP sample by the hybrid system with TEA (run 3) gave one melting peak at about 160 °C, but the sample with MAO (run 4) showed multiple melting peaks. Beside the primary peak at 160 °C, there were other two shoulder peaks at about 120 and 130 °C, respectively, which could be attributed to PP produced by the metallocene active sites. The metallocene catalyst was inactive with TEA. However, both TiCl4 and metallocene could be activated by MAO. As shown in runs 4-6 in Table 1, the hybrid catalyst activity increased with the increased MAO level, which was accompanied by the gradual increase of the melting peak from 100 to 140 °C, attributed to an increased portion of polypropylene prepared by the metallocene catalyst, as shown in Figure 2. The two evident melting peaks at about 120 and 130 °C suggested that the metallocene in the hybrid system had multiple active sites. It was demonstrated that the supporting metallocene catalyst could change the nature of the single-site type and lead

Figure 2. DSC heating curves of polypropylene samples prepared by hybrid catalyst with different amounts of MAO.

to multiple sites.25-27 The PP molecular weight decreased with the amount of MAO, due to the chain transfer to MAO molecules. The PP isotacticity prepared with MAO was always lower than 90%. Preparation of iPP/EPR Reactor Alloy with Hybrid Catalyst. In preparation of iPP/EPR reactor alloys with the hybrid catalyst system by reactor granule technology, MAO was used to activate the metallocene catalyst which could readily produce random EP copolymer in the copolymerization stage. When MAO was introduced in the homopolymerization stage (e.g., runs 4-6), the polypropylene product had low isotacticity (lower than 90%) and severe agglomeration, as shown in Table 1. It was thus not recommended to introduce MAO in the homopolymerization stage. To tackle this problem, TEA was used as the single cocatalyst in the homopolymerization stage in this work. The metallocene catalyst was inactive due to the absence of MAO. The polypropylene product thus prepared showed high isotacticity without agglomeration (e.g., run 3). Liquid propylene was vented out at the end of the homopolymerization stage, and MAO toluene solution was immediately introduced and mixed with the polypropylene particles to activate the metallocene sites inside the particles. The subsequent copolymerization of ethylene and propylene was catalyzed by both Ti and metallocene species. This process was thus composed of three stages: propylene homopolymerization, metallocene activation, and ethylene-propylene copolymerization. It was successful at avoiding the drawbacks caused by MAO in the propylene homopolymerization and at taking advantage of the metallocene catalyst for the ethylene and propylene copolymerization. Table 2 summarizes the results of the PP/EPR reactor alloys prepared with the metallocene/Z-N hybrid catalyst system. In all the runs, the copolymerization stage was carried out in toluene solution. Run 7 employed TEA as the only cocatalyst, and the copolymerization stage was immediately carried out after the homopolymerization stage. Therefore, in this run, the

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Table 2. Synthesis of PP/EPR Reactor Alloy with Metallocene/Z-N Hybrid Catalyst System

run

MAO/Zr (mole ratio)c

ethylene in feed (mol %)

7a 8a 9a 10a 11a 12b

0 1000 2000 3000 2000 1000

50 50 50 50 30 50

time (min)

productivity (kg of EPR/g of catal · h)

activity of hybrid catalystd

EPR content (wt %)

28 15 8 30 8 7

7.8 3.33 3.60 3.54 1.76 -

11.73 7.72 8.22 8.36 4.09 -

66.5 41.6 28.9 60.0 16.6 -

XS (wt %)

random component fraction in copolymer (wt %)

Mwf (×104 g/mol)

PDIf

11.0 19.0 16.3 39.6 12.9 91.2

16.5 45.7 56.4 66.0 77.5 91.2

21.14 10.18 8.28 6.46 3.77

6.84 5.35 4.21 3.13 2.50

e

a Homopolymerization conditions of runs 7-11 were identical to that of run 3 in Table 1. The activation stage was conducted with a defined amount of MAO dissolved in toluene at 70 °C for 1 h. Copolymerization conditions: T ) 50 °C and pressure ) 6 atm. b Copolymerization catalyzed by homogeneous metallocene catalyst (rac-Et(Ind)2ZrCl2) in toluene solution. Copolymerization conditions: T ) 50 °C and pressure ) 6 atm. c MAO was used in the metallocene activation stage for runs 8-11. d In 106 g of EPR /mol of M · h; M ) Ti + Zr. e Fraction soluble in xylene at room temperature. f Mw and polydispersity index (PDI) of copolymer contained in PP/EPR reactor alloys.

Figure 3. DSC heating curves of PP/EPR alloys prepared with different MAO/Zr ratios: (a) 0 (run 7), (b) 1000 (run 8), (c) 2000 (run 9), (d) 3000 (run 10), and (e) homogeneous catalyst (run 12).

Figure 4. IR spectra of PP/EPR alloys prepared with different MAO/Zr ratios: (a) 0 (run 7), (b) 1000 (run 8), (c) 2000 (run 9), (d) 3000 (run 10), and (e) homogeneous catalyst (run 12).

metallocene catalyst was inactive due to the absence of MAO. EPR was produced by the Ti species. In runs 8-11, TEA acted as the cocatalyst for the propylene homopolymerization, and MAO was introduced prior to the copolymerization. The system was mixed for about 1 h to activate the metallocene sites residing in the amorphous phase of polypropylene particles. Run 12 was a control experiment with the copolymerization catalyzed with the homogeneous metallocene catalyst. The content of random copolymers in the alloy was characterized by the fraction of sample soluble in xylene at room temperature, and the copolymer (EPR) content of alloy was calculated from the yield of alloy in the copolymerization stage and that of polypropylene in the homopolymerization stage. The random copolymer contents of the net Z-N and net metallocene catalysts were obtained from runs 7 and 12, which were conducted with the Z-N catalyst and the metallocene catalyst in copolymerization, respectively. The effects of MAO/Zr ratio on the copolymerization activity of the hybrid catalyst system and on the properties of the obtained polymers are evident in Table 2 (runs 7-10). Run 7 exhibited the highest activity. This might be because of absence of the activation stage in run 7, while 1 h activation was applied in all the other runs. The activity of Ti species decreased significantly during the period of activation. In runs 8-10, MAO was introduced in the activation stage. The complexation of MAO with Ti species might decrease the activity of the Z-N catalyst in the subsequent copolymerization. The activity of the hybrid catalyst system increased from 7.72 × 106 to 8.36 × 106 g of EPR/(mol of M · h) with the increased MAO/Zr ratio from 1000 to 3000. The metallocene active sites were dormant in the absence of MAO and were located inside the PP particles during the propylene homopolymerization. The PP homopolymerization was conducted with the Ti active site only in the

hybrid catalyst system. In the metallocene activation stage, MAO penetrated the amorphous phase of the PP matrix, and accessed and activated the metallocene active sites. Resistance existed when the MAO molecules diffused into the amorphous phase. Therefore, it was difficult for the metallocene sites to be completely activated at a low concentration of MAO. The more MAO used, the more metallocene active sites in the amorphous phase activated. This accounted for the activity increase in the hybrid catalyst system. The copolymer sample in run 7 was solely produced by the Z-N catalyst due to the absence of MAO. It contained only 16.5% random component, which was the lowest among all the runs. However, the highest content (i.e., 91.2%) was achieved in run 12, which was conducted by the net metallocene catalyst. This result suggests that the copolymers produced by the Z-N catalyst were mainly blocky under the current conditions, but those by the metallocene catalyst were random. The copolymerization was simultaneously catalyzed by both Z-N and metallocene active sites in runs 8-10. The metallocene activity increased with increasing the MAO/Zr ratio from 1000 to 3000, and thus the metallocene copolymer content increased. Figure 3 shows the DSC profiles of the PP/EPR reactor alloys produced with a variation of the MAO/Zr ratio. Besides the melting peak of isotactic PP at about 160 °C, the additional peaks at the lower temperature region were observed and could be attributed to the ethylene-propylene copolymers. The ethylene-propylene copolymer produced with MAO/Zr ) 0 (run 7) gave one melting peak ranging from 100 to 130 °C, and the copolymer by the homogeneous metallocene catalyst (run 12) had the melting peak lower than 100 °C. It is clear that the melting peaks appearing at >100 °C were attributed to the Ziegler-Natta active sites, while those at 5) rocking vibrations. The doublet at 720-740 cm-1 suggests the presence of a crystalline

polyethylene (PE) block. When PE crystallinity was small, the band at 730 cm-1 of the doublet was reduced to a shoulder of the band at 720 cm-1. As shown in Figure 4, the IR spectrum of run 7 showed an obvious doublet at 720-740 cm-1, indicating that the Z-N catalyst produced a great deal of blocky copolymer with crystalline PE segments. In contrast, the IR spectrum of run 12 (homogeneous metallocene catalyst) showed a single absorption band at 720-740 cm-1 without a shoulder peak, indicating that the copolymer produced by the metallocene catalyst was mainly random with little crystallinity. The doublet at 720-740 cm-1 appeared in the spectra of runs 7-10. The band at 730 cm-1 of the doublet gradually reduced from run 7 to run 10 with the increase of MAO/Zr, because the amount of crystalline PE block decreased due to the increased fraction of metallocene component in the total copolymer with the increased MAO/Zr. The reactor alloys were extracted in boiling heptane for 24 h for the study of copolymer chain microstructural properties. The heptane soluble parts of runs 7-10 and 12 were analyzed by GPC. As shown in Figure 5 and Table 2, both the weightaverage molecular weight and polydispersity of the copolymers decreased from run 7 to run 10 as the MAO/Zr ratio increased. It was known that the run 7 sample was solely produced by Z-N active sites in the hybrid catalyst system due to the absence of MAO. Run 12 was conducted with homogeneous metallocene and exhibited the lowest weight-average molecular weight and polydispersity. This suggested that the Z-N active sites in the hybrid system produced the copolymer chains having high molecular weight and broad distribution, and that the metallocene sites produced the copolymer chains having low molecular weight and narrow distribution. Increasing the MAO/Zr ratio resulted in an increase in the metallocene-copolymer content of the total copolymer due to the increased activity of metallocene catalyst, thus lowering both the molecular weight and polydispersity of the copolymers. To compare chain microstructural properties of the copolymers produced by Z-N, metallocene, and their hybrid catalyst system, the heptane soluble parts of runs 7, 8, and 12 were analyzed by 13C NMR as shown in Figure 6. Table 3 presents the comonomer content, average sequence length, and triad

Table 3. Comonomer Content, Average Sequence Length, and Triad Sequence Distributions of Copolymers Contained in PP/EPR Reactor Alloys run

P (%)

E (%)

NPa

NEa

PPP (%)

PPE (%)

EPE (%)

PEP (%)

EEP (%)

EEE (%)

7 8 12

11.30 17.89 36.60

88.70 82.11 63.40

1.24 1.43 1.70

9.77 6.56 2.59

0.87 1.01 8.31

5.83 6.64 13.26

7.78 10.04 15.03

2.22 3.79 12.22

13.07 17.53 23.30

70.23 60.99 27.87

a

NP and NE are the number-average sequence lengths of propylene and ethylene, respectively.

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Figure 8. IR spectra of EPR/PP alloys prepared with different compositions. Ethylene in feed: (a) 30% (run 11) and (b) 50% (run 9).

sequence distribution of the copolymers estimated from 13C NMR spectra. Figure 6 shows that the Sδδ peak of run 7 was much stronger than those of runs 8 and 12, suggesting that there were more EEE sequences in the run 7 sample. As shown in Table 3, the propylene content was only 11.30% in run 7. The NP value was 1.24 and NE was 9.77. The EEE content in the triad sequence reached 70.23%, and the other triad sequences were very low. The results suggested that the copolymers produced by the Z-N catalyst had a lot of long ethylene sequences which tended to form crystalline domains. In run 12, the Sδδ peak was the weakest, and the Tβδ, Tδδ, Sββ, SRγ, SRδ, Sβδ, and Tββ peaks were the strongest among the three samples, suggesting that there were fewer EEE sequences and more other triad sequences. The 13C NMR spectrum of run 12 was a typical ethylene-propylene random copolymer. The propylene content was 36.60%. NP and NE were 1.70 and 2.59, respectively. The triad sequence distributions were very homogeneous. The EEE content was only 27.87%, and all the other triad sequences were higher than those in run 7. The metallocene catalyst appeared

to be more effective than the Z-N catalyst in incorporating propylene comonomer. As shown in Figure 6, the Sδδ peak of run 8 was weaker, and the Tβδ, Tδδ, Sββ, SRγ, SRδ, Sβδ, and Tββ peaks of run 8 were stronger than those of run 7. The propylene content in run 8 was higher than in run 7, NP was higher, NE was lower, and the EEE triad sequence was lower, but all the other triad contents were higher. The metallocene/Z-N hybrid catalyst system showed higher ability in incorporating R-olefin comonomer than the net Z-N catalyst due to the simultaneous action of metallocene and Z-N active sites in the hybrid system. The effect of comonomer composition on the copolymerization activity of the hybrid catalyst system was also investigated (runs 9 and 11). The activity of run 9 with 50% ethylene in the feed was much higher than that of run 11 with 30% ethylene, attributed to the higher reactivity of ethylene than propylene in the copolymerization. Reducing ethylene in the feed limited the production of blocky copolymers of long ethylene sequence. The random copolymer component content in run 11 was much higher than that in run 9. Figure 7 shows the DSC profiles of the alloys produced with various gas compositions. It was observed that the melting peaks shifted to the lower temperature region and became weak when ethylene in the feed was reduced from 50% to 30%. Figure 8 shows the infrared absorption spectra of runs 9 and 11. In the case of run 9 with 50% ethylene in the feed, the doublet at 720-740 cm-1 attributed to the highly crystalline PE segments was clearly observed, but the band at 730 cm-1 of the doublet became very weak as the ethylene fraction was reduced from 50% to 30%. Particle Morphology. The morphological replication of the catalyst particles was observed as the PP particles prepared by the hybrid catalyst system were spherical and porous as shown in Figure 9. The PP/EPR reactor alloy with 41.6 wt % copolymer prepared by the hybrid catalyst maintained the spherical shape of the PP particles. In comparison of Figure 7b with Figure 7d,

Figure 9. Surface morphologies of particles prepared by the hybrid catalyst system. (a, b) Polypropylene particles; (c, d) PP/EPR reactor alloy containing 41.6 wt % copolymer (run 8).

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it could be observed that EPR produced in the copolymerization stage filled in the pores of PP particles. A final point worth mentioning is that the objective of this work is to produce PP/EPR reactor alloys having good PP particle morphology and EPR elastomeric property through the combined action of Z-N and metallocene catalysts. In the laboratory scale, we focused on the fundamental issues. We removed residual propylene monomer after the homopolymerization and isolated PP particles, because we would like to know the development in each stage. However, in industrial practice, there is no need to remove propylene: the residual monomer could be used in the copolymerization stage with ethylene. Industrial processes are continuous. Propylene homopolymerization and EP copolymerization are often implemented in two reactors in series. The metallocene activation can readily be done in between. Conclusions The metallocene/Ziegler-Natta (rac-Et(Ind)2ZrCl2/TiCl4/ MgCl2) hybrid catalyst system was prepared by treating a conventional Ziegler-Natta catalyst with rac-Et(Ind)2ZrCl2. The hybrid catalyst system was applied in reactor granule technology to produce PP/EPR reactor alloys. The whole process was composed of three stages: propylene homopolymerization, metallocene activation, and ethylene and propylene copolymerization. It was found that the prepared metallocene/ZieglerNatta hybrid system exhibited the characteristics of both metallocene and Ziegler-Natta catalysts. In the homopolymerization stage, polypropylenes synthesized by the hybrid catalyst with TEA as single cocatalyst showed high isotacticity without agglomeration. In the copolymerization stage, the hybrid catalyst showed better ability in incorporating R-olefin than the net Z-N catalyst due to the action of metallocene active sites. The effects of the MAO/Zr ratio and monomer composition on the copolymerization activity and the copolymer properties in the alloys were investigated. The DSC and IR analyses suggested that EPR in the alloy became random with increased MAO/Zr ratio due to the increased metallocene activity. In addition, reducing the ethylene content in the feed decreased the activity and promoted the production of random copolymers. The PP particles prepared by the hybrid catalyst system were spherical and porous, and the PP/EPR reactor alloy with 41.6 wt % copolymer maintained the spherical shape of the PP particles. Acknowledgment The work is supported by the National Basic Research Program of China (No. 2005CB623804), the National Science Foundation for Distinguished Overseas Young Scholars (No. 20428605), and the National Science Foundation (No. 20476090). Literature Cited (1) Zhu, H.; Monrabal, B.; Han, C. C.; Wang, D. Phase Structure and Crystallization Behavior of Polypropylene in Reactor Alloys: Insights from Both Inter- and Intramolecular Compositional Heterogeneity. Macromolecules 2008, 41, 826. (2) Urdampilleta, I.; Gonza´lez, A.; Iruin, J. J.; Cal, J. C. D. L.; Asua, J. M. Origins of Product Heterogeneity in the Spheripol High Impact Polypropylene Process. Ind. Eng. Chem. Res. 2006, 45, 4178. (3) Grein, C.; Bernreitner, K.; Hauer, A.; Gahleitner, M. Impact Modified Isotatic Polypropylene with Controlled Rubber Intrinsic Viscosities: Some New Aspects About Morphology and Fracture. J. Appl. Polym. Sci. 2003, 87, 1702. (4) Lo´pez-Manchado, M. A.; Kenny, J. M. Analysis of the Effects of the Polymerization Route of Ethylene-Propylene-Diene Rubbers (EPDM) on the Properties of Polypropylene-EPDM Blends. J. Appl. Polym. Sci. 2002, 85, 25.

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ReceiVed for reView April 9, 2009 ReVised manuscript receiVed July 19, 2009 Accepted July 21, 2009 IE900579H