Synthesis of polypropylene nanocomposites using graphite oxide-intercalated Ziegler–Natta catalyst

Accepted Manuscript Synthesis of Polypropylene Nanocomposites Using Graphite Oxide-Intercalated Ziegler-Natta Catalyst Jin-Yong Dong, Yuan Liu PII:

S0022-328X(15)00246-6

DOI:

10.1016/j.jorganchem.2015.04.034

Reference:

JOM 19020

To appear in:

Journal of Organometallic Chemistry

Received Date: 31 December 2014 Revised Date:

17 March 2015

Accepted Date: 5 April 2015

Please cite this article as: J.-Y. Dong, Y. Liu, Synthesis of Polypropylene Nanocomposites Using Graphite Oxide-Intercalated Ziegler-Natta Catalyst, Journal of Organometallic Chemistry (2015), doi: 10.1016/j.jorganchem.2015.04.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Highlights:


Preparation of PP/exfoliated graphite oxide nanocomposites using in situ polymerization technique is summarized; Intercalation of Ziegler-Natta catalyst into graphite oxide is accompanied by graphene

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oxide reduction;

Highly efficient propylene polymerization leads to PP/graphene oxide nanocompoistes

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with high electrical conductivity.

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ACCEPTED MANUSCRIPT Graphical Abstract PP/exfoliated graphite oxide nanocomposites were prepared by in situ polymerization technique. A pre-treatment of graphite oxide with n-BuMgCl resulted in reduction of

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graphene oxide, which eventually led to the formation of PP/graphene oxide nanocompoistes

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with high electrical conductivity.

ACCEPTED MANUSCRIPT For Journal of Organometallic Chemistry

Synthesis of Polypropylene Nanocomposites Using Graphite

CAS Key Laboratory of Engineering Plastics, Institute of Chemistry,

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1

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Jin-Yong Dong*1, Yuan Liu1,2

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Oxide-Intercalated Ziegler-Natta Catalyst

Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China

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2

* Corresponding author. Tel.: +861082611905; fax: +861082611905; E-mail address:

[email protected].

ACCEPTED MANUSCRIPT Abstract This paper summarizes our research in the preparation of isotactic polypropylene (PP) nanocomposites containing exfoliated graphite oxide (GO) using in situ polymerization

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technique with Ziegler-Natta catalyst intercalated in GO. Intercalation of Ziegler-Natta catalyst in GO was conducted by treating GO with a Grignard reagent of n-BuMgCl, resulting in anchoring of -Mg-Cl species on individual GO sheets. Successive treating

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-Mg-Cl-intercalated GO with TiCl4 rendered the generation of structurally simple yet highly

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effective Ziegler-Natta catalyst species on the substrate of GO sheets, which initiated with a high efficiency iso-specific propylene polymerization, affording nanocomposites of PP containing well-dispersed GO sheets. Further studies on the treatment of GO with n-BuMgCl revealed that with an excess amount of the Grignard reagent GO would be largely removed of

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its functional groups amidst -Mg-Cl species anchoring, which led to the formation of reduced GO (rGO)-intercalated Ziegler-Natta catalyst. Efficient propylene polymerization inside the rGO in turn prompted the dispersion of electrically conductive rGO sheets in the matrix of PP,

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the nanocomposites of which were discovered with rather high electrical conductivities with a

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low percolation threshold (~0.2 vol.%). Keywords: Propylene; Graphite oxide; Grignard reagent; Nanocomposite; In situ polymerization

ACCEPTED MANUSCRIPT 1. Introduction Nanotechnology is widely considered as a major area that will make great technological progress in the 21st century. In materials category, polymer nanocomposites, hailed as a

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‘radical alternative to conventional filled polymers or polymer blends’[1] and ‘a revolutionary new class of materials that have demonstrated vastly improved properties compared to those of conventional composites’[2], are experiencing an unprecedented rapid development. In

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general, polymer nanocomposites can be accessed through two routes: direct polymer/nano

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filler combination and in situ monomer polymerization on the surfaces of the nano fillers [3]. The former is usually carried out by solution or melt blending. The latter, however, is incorporated into the polymer formation process and thus free of those energy and environmental concerns associated with solution and melt polymer blending processes, a

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highly desired advantage that will be more prominent during materials mass production. In particular, the in situ approach is interesting for fabricating polyolefins [polypropylene (PP), polyethylene (PE), et c.] - the largest entity of thermoplastic polymer – nanocomposites, for

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the chemically inert, often crystalline polymers with very low surface energies are usually

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thermodynamically prevented from being exhaustively dispersed by nano fillers with very high surface energies[4]. However, by embedding olefin polymerization catalyst species immediately on the surfaces of nano fillers’ nano entities (e.g. nano silicate layer for clay, individual nano tube for carbon nanotube, graphene oxide sheet for graphite oxide, et c.) and allowing polyolefins chains to freshly grow on the nano filler substrate, the in situ polymerization approach takes a detour to successfully avoid the thermodynamic barrier in polyolefin nanocomposites fabrication, which, in practice, has been adequately proved of its

ACCEPTED MANUSCRIPT effectiveness[5]. In this context, for polyolefin nanocomposites preparation, the in situ polymerization approach is not only “green” but essential. Graphite oxide (GO) is a layered carbon nanomaterial produced by the oxidation of natural

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graphite flakes, which by ultrasonication treatment in aqueous solution will exfoliate to individual sheets of nanoscale thickness [6], having a great potential to improve the mechanical and barrier and even electrical- and thermal- conductive properties of polymers

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[7]. However, despite the potential advantages, the synthesis of polymer nanocomposites

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bearing dispersed GO sheets was enormously challenging, especially when the polymer is selected from nonpolar polymer category typically represented by a polyolefin like PP and PE [8].

We focus our attention on in situ polymerization approach to polyolefin nanocomposites.

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Following the successes of PP and PE nanocomposites containing exfoliated montmorillonite [9] and further that of PP nanocomposites with multi-walled carbon nanotubes as nano filler [10], we have tried to test such a technique in the preparation of polyolefin/GO

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nanocomposites in hoping to further boost the promising approach in polyolefin

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nanocomposite preparations [11]. For that, we summarize in this paper our research in the preparation of isotactic PP nanocomposites containing exfoliated GO using in situ polymerization technique with GO-intercalated Ziegler-Natta catalyst.

2. Overall synthetic scheme We fabricate PP/GO nanocomposites containing well exfoliated and dispersed GO sheets by in situ polymerization technique based on a reaction scheme illustrated in Figure 1. First, a

ACCEPTED MANUSCRIPT GO-intercalated Ziegler-Natta catalyst is synthesized by the reaction of a Grignard reagent, n-BuMgCl, with GO in THF followed by excess TiCl4 treatment to generate Mg/Ti catalyst species on individual GO sheet surfaces. The thus-obtained GO-intercalated Ziegler-Natta

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catalyst thenceforth initiates efficient and iso-specific propylene polymerization that concurrently makes the PP matrix and renders exfoliation and dispersion of GO and results in

O

HO O

HO O OH

OH

OH

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the formation of PP/GO nanocomposites.

ClMgO R O RMgCl R OH

ClMgO

OMgCl

OMgCl OMgCl R

OMgCl

R OMgCl OMgCl

RMgCl/GO

GO

TiCl4

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Cl ClCl Cl Cl Cl Cl Cl Ti Cl Cl Ti Ti Cl Cl Ti Cl Cl Cl Cl Cl Cl Mg Mg ClMgO OMgClO O C3H6 R R R R AlEt3 OMgCl O O OMgCl Mg Mg Cl Cl Cl Cl Cl Cl Ti Cl Ti Cl Cl Ti Cl Ti Cl ClCl Cl Cl Cl Cl Cl

TiCl4/(RMgCl/GO)

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PP/GO nanocomposites

Fig. 1. Synthetic scheme for PP/GO nanocomposites.

3. Effect of Grignard reagent treatment on structure evolution of GO GO was prepared from natural graphite by the Hummers method. The as-prepared powdery GO contained 57.84 wt% of C, 1.82 wt% of H by Elemental Analysis measurement. As high as 40.44 wt% of O could be calculated based on the contents of C and H in GO, indicating

ACCEPTED MANUSCRIPT that the degree of oxidation was very high. Morphology of GO was studied with Scanning Electron Microscopy (SEM) (Figure 2a). Obviously, layered structure of GO was retained. To further characterize the structure of GO, TEM analysis of cast film samples at the

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concentration of 1.0 mg/mL in THF was conducted. The TEM image of GO (Figure 2b) indicates that GO was fully exfoliated to nano sheets with micrometer-long wrinkles by ultrasonic treatment, exhibiting clearly a flake-like shape of the individual graphene oxide

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sheets. The inset is the measured electron diffraction pattern of the nano sheets which shows

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many diffraction dots, suggesting these graphene oxide sheets are highly crystalline.

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a

Fig. 2. (a) SEM image and (b) TEM image (with inset electron diffraction pattern) of GO

Grignard reagent (e.g. n-BuMgCl, EtMgCl) had been reported to be able to react with hydroxyl groups of thermally treated SiO2 forming RMgCl-modified SiO2 [11], which was taken as a support to further react with TiCl4 for the synthesis of SiO2-supported Ziegler-Natta catalyst. However, in the midst of GO reacting with n-BuMgCl the Grignard reagent, we

ACCEPTED MANUSCRIPT found, intriguingly, that the amount of n-BuMgCl used affects substantially the structure of GO that is derived. In our experiment, three dosages of the Grignard reagent, n-BuMgCl, were used to treat GO in THF, which, in reference to the total amount of O functional groups,

thoroughly characterized by

13

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were 1, 3, and 6 equivalents. The products (rGO-Mg-1~3), along with pristine GO, were C MAS NMR, XPS, Raman spectroscopy, WAXD, and

powdery electrical conductivity measurement. Clearly, n-BuMgCl causes the reduction of GO. 13

C MAS NMR spectrum of GO before and after

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For instance, shown in Figure 3 is the

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n-BuMgCl treatment. Both the C-OH (70 ppm) and C-O-C (60 ppm) resonances become significantly weakened at the treatment of n-BuMgCl at 1 and 3 equiv. Increasing n-BuMgCl to 6 equiv., both resonances are hardly detectable, and the sp2 -carbon resonance continuously

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up-field shifting with n-BuMgCl becomes dominant in the NMR spectrum.

sp2 Carbon

C-OH

C-O-C

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a

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b

ppm

c

d 200

150

100

50

Fig. 3. 13C MAS NMR spectra of (a) GO, (b) GO treated with 1 equiv. of n-BuMgCl, rGO-Mg-1, (c) GO treated with 3 equiv. of n-BuMgCl, rGO-Mg-2, and (d) GO treated with 6 equiv. of n-BuMgCl, rGO-Mg-3. 4. Synthesis of the GO-intercalated Ziegler-Natta catalyst The most extensively reduced GO (rGO-Mg-3) with the highest conductivity was then

ACCEPTED MANUSCRIPT continued to proceed with TiCl4 immobilization. The reaction was carried out in pure TiCl4 at 120oC under nitrogen atmosphere. The product was exhaustively washed with anhydrous hexane. The impregnated Ti content was determined to be 0.88 wt.% using a

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spectrophotometer. Other characterizations of the catalytically active Ti-bearing GO (rGO-Mg-Ti-3) were focused on a structural impact of TiCl4 treatment on GO, and the results are summarized in Figure 4. Matter-of-factly, except for absorbing TiCl4 on its sheets surfaces,

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the structure of GO was hardly affected by this inflictive process. As evidenced in Figure 4, 13

C MAS NMR spectrum (Figure

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the sp2 -carbon resonance stands as the sole signal in the

4-a), with a noticeably down-field-shifted peak at 129 ppm (from 119 ppm) probably due to the immobilization of TiCl4 generating many peripheral Cl atoms. Raman spectrum (Figure 4-b) gives a similar D/G intensity ratio at 2.0 (versus 2.3 for rGO-Mg-3). Interestingly,

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besides showing the majority of C atoms are sp2-hybridized, the C1s XPS spectrum (Figure 4-c) is detected with C-O, C-O-Mg, and even the π → π* shake-up satellite peak, a characteristic of aromatic or conjugated systems. And WAXD (Figure 4-d) showing no (002)

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diffraction peak indicates the reduced GO remained exfoliated.

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Fig. 4. Characterizaiton of rGO-Mg-3. (a) 13C MAS NMR spectrum; (b) Raman spectrum; (c) C 1S XPS spectrum; (d) WAXD pattern.

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5. In situ propylene polymerization and Synthesis of PP/GO nanocomposites The reduced GO-supported Mg-Ti (rGO-Mg-Ti-3) was then combined with AlEt3 to

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catalyze propylene polymerization in slurry conditions. The results are summarized in Table 1. In general, the combination of rGO-Mg-Ti-3 and AlEt3 affords a fairly high efficiency and iso-specificity catalyst system for propylene polymerization. Thus by varying polymerization conditions such as propylene pressures (monomer concentrations) and polymerization durations, a series of PP polymers containing 0.09-10.2 volume per cent (vol.%) of reduced GO (actually graphene sheets) had been materialized. To explore the dispersion quality of the graphene sheets in these composite materials, SEM and TEM techniques were employed and

ACCEPTED MANUSCRIPT applied to 190oC hot-press-processed samples. As expected, the carbon sheets are found well dispersed in the PP matrix. Exemplarily shown in Figure 5 is TEM image of a composite sample containing 10.2 vol.% of reduced GO sheets. The bright edges of the dispersed sheets

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due to relatively long time (over 15 s) exposures to electron beam disclose almost individual dispersions with very few occasions of multi-layer stacking.

no.

Cat. Feed

T

P

o

Yield t

Mw c)

filler content

b)

PDI

Tm d)

ΔHm d)

(oC)

(J/g)

(g)

(vol. %)

(×104 g/mol)

11.7

0.09

33.1

6.6

160.9

81.0

4.1

0.3

44.1

5.2

161.3

82.6

3.5

0.6

45.4

8.4

161.7

82.8

( C)

(MPa)

(min)

1

0.025

60

0.5

60

2

0.029

60

0.5

10

3

0.050

40

0.5

10

4

0.053

60

0.3

30

2.0

1.2

63.7

5.5

161.8

77.3

5

0.051

60

0.5

8

0.6

4.1

48.4

9.1

162.0

79.4

6

0.056

40

0.1

20

0.3

10.2

n.d.

n.d.

161.2

50.0

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(g)

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a

Polymerization conditions a)

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Run

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Table 1 Results of in situ propylene polymerization catalyzed by rGO-Mg-Ti-3

General conditions: Slurry polymerization, 50 mL hexane as solvent, 2.0 mL TEA (1.8 M) as cocatalyst, b

The graphene sheets contents in PP composites are expressed by volume fraction ν

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[TEA]/[DDS] = 20/1.

(vol %) transformed from mass fraction ω (wt. %) via the following equation: ν = w ρp /[ w ρp +(1- w ) ρg ], where v and w are the volume and mass fractions of graphene sheets. ρp and ρg represent the density of the PP matrix and graphene nanosheets, which can be taken as 0.9 g/cm3 and 2.2 g/cm3, respectively. c Determined by GPC. d Melting temperature and thermal enthalpy determined by DSC.

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Fig. 5. TEM image of a composite sample containing 10.2 vol.% of reduced GO sheets. 6. Electrical conductivity of PP/GO nanocomposites

The conducting feature of graphene motivated the conductivity measurement of the PP nanocomposites even though GO was reported to have a much lower electrical conductivity

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than graphene. The polymerized samples were compressed to be films with thickness around 0.3-0.5mm at 190oC for electrical conductivity measurement. Under the processing temperature, GO cannot be thermally reduced to graphene. An in-plane direct current

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electrical conductivity was measured at ambient temperature for each specimen and recorded

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for establishing a relationship between graphene sheets volume per cent loading and composite electrical conductivity. PP is a known insulating material, such an electrical measurement though as inappropriate as it is giving a conductivity value as low as <10-13 S·m-1 (Figure 6-A). With 0.09 vol.% of the reduced graphene sheets present, the composite is measured with an electrical conductivity of 2.51×10-8 S·m-1. Further increasing the loading to 0.20 vol. %, the obtained conductivity, 2.88×10-6 S·m-1, has well surpassed the antistatic criterion value (10-6 S·m-1) for thin films. Over the loading span from 0.12 vol.% to 1.2 vol.%,

ACCEPTED MANUSCRIPT a rapid, 6 orders of magnitude increase of electrical conductivity is observed, which is followed by a more gradual increment that, nonetheless, still gives such high electrical conductivities as 3.92 S·m-1 at 1.2 vol.%, 28.5 S·m-1 at 4.1 vol.%, and 163.1 S·m-1 at 10.2

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vol.%, respectively (Figure 6-A). A bond percolation model is adopted to describe the overall conductivity behavior of the PP/graphene nanocomposites [7a].The conductivity, σc, above the percolation threshold is treated with a power law: σc=σf[(φ-φc)/(1-φc)]t, where σf is the

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conductivity of the graphene sheets, φ the graphene sheets volume fraction, φc the percolation

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threshold and t the “universal critical exponent”. Plot of logarithmic σc vs. logarithmic (φ-φc) (Figure 6-B) using the electrical conductivity data at different graphene sheets loadings renders the estimation of t to be 2.38±0.21, σf 10-1.98±0.11 S·m-1, and φc 0.2 vol.%. The low percolation threshold (0.2 vol.%), together with the high absolute electrical conductivities, is

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deemed to be the result of the overall structural superiorities of the in situ-prepared nanocomposites that excel not only in graphene sheets dispersion but in sp2 -carbon network’s

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completeness in the graphene sheets themselves.

Fig. 6. Electrical conductivity of the PP/graphene composites as a function of graphene sheets volume fraction. (A) Composite conductivity, σc (S m-1), is logarithmized, and plotted

ACCEPTED MANUSCRIPT against filler volume fraction, φ; (B) Logσc plotted against log(φ-φc), where φc is the percolation threshold. Inset in A: the four-probe setup for DC conductivity measurement. 7. Experimental 7.1.Treatment of GO with n-BuMgCl

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The treatment of GO with n-BuMgCl was carried out in anhydrous THF under nitrogen atmosphere. In a typical reaction (preparation of rGO-Mg-3), 0.21 mol of n-BuMgCl in THF

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was added dropwise into a 300 mL anhydrous THF suspension containing 1.3 g GO. No sonication was applied to the suspension at any time. After 48 h of reaction at refluxing

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temperature (80 oC), the excess Grignard reagent was filtered off and the solid was washed in turn with anhydrous THF and hexane for three times. The powdery product was dried under vacuum at 60 oC for 12 h to give 1.7 g of rGO-Mg-3. rGO-Mg-1 and rGO-Mg-2 were prepared following the same procedure except that the amounts of n-BuMgCl used were

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0.034 and 0.1 mol, respectively.

7.2. Preparation of GO-intercalated Ziegler-Natta catalyst

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The reaction of rGO-Mg with TiCl4 to prepare rGO-Mg-Ti was carried out in pure TiCl4 at 120 oC. The following procedure was applicable to any of the three rGO-Mg-Ti samples’

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preparation: 1.2 g of rGO-Mg (rGO-Mg-1~3) was added into 50 mL TiCl4 at ambient temperature. With stirring, the reaction temperature was brought up to 120 oC. In 4 h, the reaction was deemed completed, and then the reactants were filtered to remove the excess TiCl4. After repeated washing with dry hexane until the filtrate became colorless, the remaining solid was collected and vacuum-dried at 60 oC for 12 h. 7.3. In situ propylene polymerization Polymerization of propylene with rGO-Mg-Ti-3 to prepare PP/graphene composites was

ACCEPTED MANUSCRIPT conducted with slurry process. In a typical polymerization reaction (entry 1 in Table 1), to a 450 mL Parr stainless steel autoclave reactor equipped with a mechanical stirrer was added 50 mL of hexane, which was followed by feeding propylene under a constant pressure of 0.5

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MPa. Heated to 60 oC, the reactor was charged with 0.025 g rGO-Mg-Ti-3, and then stirring was turned on at a constant rate. Initiation of the reaction was realized after charging 2.0 mL of AlEt3 (TEA)/heptane solution (1.8 M) and 2.0 mL dimethoxydiphenylsilane (DDS)/heptane (0.089 M) into the reactor via syringe. After 30 min, the reaction was quenched by

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solution

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20 mL of acidified ethanol (containing 10% HCl). The polymer product was collected by filtration, repeatedly washed with ethanol and distilled water, and dried under vacuum at 60 o

C for 24 h, 11.7 g of polymer product was obtained as a gray powder.

7.4. Characterization 13

C-NMR spectra were acquired on a Bruker Avance III-400 spectrometer (10

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Solid-State

KHz 13C, 400 MHz 1H) using a home-built 4 mm MAS probe at a spinning speed of 10 kHz. All chemical shifts were referenced to tetramethylsilane (TMS) using adamantine as an

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external reference. Scanning electron micrographs (SEM) of PP nanocomposites were

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obtained using a JEOL-S4300 and S4800 cold field emission scanning electron microscopy. Ultra-thin sections of the nanocomposites were subjected to transmission electron microscopy (TEM) examination using Jeol JEM 2200FS and Jeol JEM 2011 at an accelerating voltage of 200 kV. The crystallization and melting behaviors of PP nanocomposites were characterized using a Perkin-Elmer DSC-7 differential scanning calorimeter under nitrogen atmosphere. Molecular weight and molecular weight distribution of PP polymers were determined by gel permeation chromatography (GPC) using a Waters Alliance GPC 2000 instrument equipped

ACCEPTED MANUSCRIPT with a refractive index (RI) detector and a set of m-Styragel HT columns of 106, 105, 104, and 103 pore sizes in series. Electrical conductivity of composite samples was measured by Keithley 4200 using a standard four probe method at room temperature. Films with

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thicknesses around 0.3-0.5 mm were prepared by hot-compression of the polymerized products at 190 oC. Equation (1) was used to calculate the electrical resistivity (ρ):

∆Vwt iL

(1)

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ρ=

where ∆Vis the voltage drop over the center of the sample, wis the sample width, tis the

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sample thickness, iis the current, and Lis the length over which ∆Vis measured. Equation (2) was used to calculate the electrical conductivity (σc)

8. Conclusion

1

ρ

(2)

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σc=

In summary, isotactic polypropylene nanocomposites containing exfoliated graphite oxide

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has been prepared using in situ polymerization technique. Intercalation of Ziegler-Natta

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catalyst in GO was conducted by treating GO with a Grignard reagent of n-BuMgCl followed successively by TiCl4 complexation. Treatment of GO with n-BuMgCl the Grignard reagent entails triple effects on GO structure simultaneously: Anchoring Mg-Cl species on the graphene oxide sheet surfaces, enlarging sheet-sheet inter-distances, and reducing the highly functionalized (oxidized) sheets back to sp2 -carbon-dominant graphene sheets. With some excess amount of n-BuMgCl (e.g. 6 equiv. of n-BuMgCl to O functionalities in GO), the three effects can be reconciled to render a Mg-Cl-functionalized graphene material that is ready to

ACCEPTED MANUSCRIPT immobilize TiCl4 and further to undergo in situ olefin polymerization with concomitant graphene sheets exfoliation and dispersion to prepare polyolefin/graphene nanocomposites. Featured

by

well-restored

sp2

-carbon-networked

graphene

sheets,

PP/grapheme

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nanocomposites thus prepared possess a rather low electrical percolation threshold (~0.2 vol.%) and show high electrical conductivities.

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Acknowledgements

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We are grateful to financial support for this work by the National Science Foundation of China (Grant nos. 51003105, 21374121, 20874104, 51373178, and 51103163).

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