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Polymer 52 (2011) 318e325

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Polymer

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Bridge length effect of new dinuclear constrained geometry catalystson controlling the polymerization behaviors of ethylene/styrene copolymerization

Thi Dieu Huyen Nguyen a, Thi Le Thanh Nguyen a, Seok Kyun Noh a,*, Won Seok Lyoo b

a School of Display and Chemical Engineering, Yeungnam University, 214-1 Daedong, Gyeongsan, Gyeongbuk 712-749, Republic of Koreab School of Textiles, Yeungnam University, 214-1 Daedong, Gyeongsan, Gyeongbuk 712-749, Republic of Korea

a r t i c l e i n f o

Article history:Received 29 September 2010Received in revised form24 November 2010Accepted 29 November 2010Available online 4 December 2010

Keywords:Bridge length effectDinuclear constrained geometry catalystEthylene copolymerization

* Corresponding author. Tel.: þ82 53 810 2526; faxE-mail address: sknoh@ynu.ac.kr (S.K. Noh).

0032-3861/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.polymer.2010.11.049

a b s t r a c t

According to the observable evidence from 1H and 13C nuclear magnetic resonance and mass spec-trometry, new dinuclear constrained geometry catalysts (DCGCs) with a structure of [{Ti(h5:h1-(C9H5)Si(CH3)2NtBu)Cl2(CH2)n}2(C6H4)] [n ¼ 0 (10), n ¼ 1 (11), n ¼ 2 (12)] were synthesized successfully. Copo-lymerization of ethylene and styrene were tested by using three new DCGCs and Dow CGC. The catalystactivity, the molecular weight (MW) and styrene content of the copolymers were sharply improved asthe bridge structure was transformed from para-phenyl (10) to para-xylyl (11) and para-dieth-ylenephenyl (12). The activity of 11 and 12 was about four to five times greater than that of 10 regardlessof the polymerization conditions. In addition, the capability to form high MW polymers increased in theorder of Dow CGC z 10 < 11 < 12. The styrene contents in copolymers generated by 11 and 12 werehigher than those of 10.

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1. Introduction

Since applying metallocene catalyst in olefin polymerizationbecame more usual, a variety of dinuclear metallocene compounds[1e15], which contain two mechanically linked metallocene units,have been prepared and tested their polymerization properties.These systems may provide a new possibility to improve thecatalytic properties of metallocene ascribed by the cooperativeelectronic and chemical interaction between two active sites[11e23]. Beside of the properties of well-defined mononuclearmetallocenes, the natures of the bridging unit such as the length,the flexibility or the rigidity were the key effective features to leadto the unique catalytic behaviors of the dinuclear metallocenesreferences. Mülhaupt first studied the propylene polymerizationusing phenylene-bridged dinuclear zirconocene in 1993, andshowed that the molecular weights of polypropylenes obtainedwith these catalysts were smaller than those made by the mono-nuclear one because of the electronic and cooperative interaction ofthe two adjacent zirconium center [3]. In 1996, Green demon-strated that the activities and molecular weights of polymers werevaried according to the various dimethylsilyl-bridged dinuclearmetallocenes used [4]. In 2002, Mark’s group reported that thecopolymers of ethylene and a-olefin obtained by ethylene bridgeddinuclear zirconium CGC had the higher monomer incorporation

: þ82 53 810 4651.

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than those obtained by mononuclear CGC presumably due to thenuclearity effects caused by the close spatial proximity betweentwo active sites of dinuclear CGC [13,14]. Our previous studies alsodemonstrated that the presence of a long and flexible poly-methylene bridge between two active sites facilitated both thepolymerization activity and the comonomer response of DCGC[15,16]. Recently, it was found that the presence of a phenyl ring indinuclear metallocene catalyst was even more effective inimproving the polymerization activity. This result was able to beinterpreted that the bridge unit contributed not only to limit thefree movement of the active species but also to reduce thefrequency of the intramolecular deactivation [17e23]. Sun’s groupshowed that the catalytic activities of 4,40-bis(methylene)biphe-nylene-bridged dinuclear metallocene were more than three timesand twice higher than that of the phenyldimethylene bridgeddinuclear metallocene and the corresponding mononuclear,respectively, in ethylene polymerization [19]. In addition, dinuclearcompound produced polyethylene with broad molecular weightdistribution. Consequently, both the electronic and steric effects ofthe active site induced by the bridge unit of the dinuclear metal-locene can be considered as the major element to be responsible forthe final polymerization behaviors of the catalyst.

Althoughmany studies have investigated the effect of the bridgestructure on dinuclear metallocenes, few have reported specificallyon the effect of bridge length on the DCGC characteristics free fromthe steric issue. Herein, we therefore describe the polymerizationproperties of three DCGCs having a distinguished bridge length of

T.D.H. Nguyen et al. / Polymer 52 (2011) 318e325 319

para-phenyl (10), para-xylyl (11) and para-diethylenephenyl (12), inorder to examine the role of the bridge length of dinuclear metal-locene (Scheme 1).

2. Experimental

2.1. Materials

Toluene and tetrahydrofuran (THF) were distilled from sodium-benzophenone ketyl prior to use. Diethyl ether and hexane werepurified by using MBRAUN MB-SPS-800 series. 1,4-dichloroben-zene, a,a0-dichloro-p-xylene, titanium chloride (Aldrich Co. USA),1,4-bis(2-chloroethyl)benzene (TCI Tokyo Chemical Industry), n-BuLi (2.5 M solution in hexane, Aldrich Co. USA), and indene(Aldrich Co. USA) were used without further purification.[TiCl3(THF)3] was prepared by literature methods [24]. Dichlor-odimethyl silane and tert-butyl amine purchased from Aldrich Co.USA were used after distilling from calcium hydride. Ethylene(polymer grade) was purified by passage through a column ofmolecular sieve (4 Å) and Drierite (8mesh). MMAO (type 4, 6.4 wt.%Al, Akzo, USA) was used without further purification.

2.2. Methods

2.2.1. General considerationsAll reactions were carried out under a dry, oxygen-free atmo-

sphere using standard Schlenk techniques with a double manifold

Scheme 1. Structure of DCGC 10, 11 and 12.

vacuum line. Nitrogen gas was purified by passage througha column of molecular sieve (4 Å) and Drierite (8mesh).

2.3. Measurements

1H NMR and 13C NMR spectra were recorded on a VNS-300 NMRspectrometer at >300 MHz, using CDCl3 as a solvent at 25 �C. Massspectra were measured on an Autospec-Ultima E with high-reso-lution condition using CHCl3 as a solvent for liquid samples. Thestyrene contents were calculated based on the 1H NMR assays ofthe polymer which were conducted in 1,1,2,2-tetrachloroethane-d2at 110 �C [25,26]. Differential scanning calorimetry (DSC) wascarried out with a Pyris Diamond DSC under nitrogen atmosphere[conditions: heating from 20 �C to 300 �C (10 �C/min), cooling from300 �C to 20 �C (10 �C/min)]. The second heating cycle was used forcollecting the DSC thermogram data at a ramping rate of 10 �C/min.The viscosity averaged MW (Mv) of the polymer was measured indecahydronaphthalene at 135 �C by a modified Ubbelohde-typeviscometer according to ASTM-4020. MW and MW distributionwere measured by GPC (PLGPC220) at 170 �C using 1,2,4-tri-chlorobenzene. The MW was calculated by a standard procedurebased on the calibration with standard polystyrene samples.

2.4. Synthesis

2.4.1. Preparation of [(C9H6)2(C6H4)] (1)A solution of Indene lithium (IndLi) salt (1.72 g,14.21mmol)/THF

(40 ml) was cooled to �78 �C. 1,4-Dichlorobenzene (0.84 g,5.68 mmol)/THF (20ml) was dropped into solution of IndLi salt. Thereactor was warmed to room temperature, then heated up 60 �C in48 h. THF was volatilized in vacuo at least 4 h. Product was sepa-rated by filter through celite with ether. The light yellow solutionwas removed solvent to get light yellow powder. (1.69 g, 98.25%). 1HNMR (300 MHz, CDCl3, 25 �C, d, ppm): 7.45 (d, 2H, C9H7), 7.40(d, 2H, C9H7), 7.28 (t, 2H, C9H7), 7.17 (t, 2H, C9H7), 6.86 (s, 4H, C6H4),6.55 (s, 2H, C9H7), 3.38 (s, 4H, C9H7).

2.4.2. Preparation of [{(C9H6)(CH2)}2(C6H4)] (2)The compound was synthesized from a,a0-dichloro-p-xylene

(3.27 g, 18.68 mmol) using the same reaction conditions andprocedures as for 1. A light yellow compound was obtained (5.81 g,93%). 1H NMR (300 MHz, CDCl3, 25 �C, d, ppm): 7.49 (d, 2H, C9H7),7.32 (d, 2H, C9H7), 7.30 (t, 2H, C9H7), 7.29 (t, 2H, C9H7), 7.21 (s, 4H,C6H4), 6.16 (s, 2H, C9H7), 3.91 (s, 4H, CH2), 3.38 (s, 4H, C9H7).

2.4.3. Preparation of [{(C9H6)(CH2)2}2(C6H4)] (3)The compound was synthesized from 1,4-bis(2-chloroethyl)

benzene (1.29 g, 6.35 mmol) using the same reaction conditionsand procedures as for 1. A light yellow compound was obtained(2.15 g, 93.4%). 1H NMR (300MHz, CDCl3, 25 �C, d, ppm): 7.46 (d, 2H,C9H7), 7.38 (d, 2H, C9H7), 7.32 (t, 2H, C9H7), 7.26 (t, 2H, C9H7), 7.13(s, 4H, C6H4), 6.22 (s, 2H, C9H7), 3.32 (s, 4H, C9H7), 2.86 (s, 4H, CH2 ),2.81 (s, 4H, CH2).

2.4.4. Preparation of [{(CH3)2SiCl(C9H6)}2(C6H4)] (4)The solution of n-BuLi 2.5 M (5.55 ml, 13.88 mmol) was dropped

directly in solution of [(C9H7)2(C6H4)] (1.69 g, 5.55 mmol)/THF(60 ml) at �78 �C. The solution was stirred at room temperature in1 h and heated up 60 �C for 12 h, then cooled to room temperaturebefore removed THF. The salt was purified by filtering with hexaneand drying under vacuum pressure (1.55 g, 88%).

A solution of Me2SiCl2 (1.48 ml, 12.25 mmol) in ether (40 ml)was cooled to �78 �C and treated dropwise over 15e20 min witha solution of {[Li(C9H6)]2(C6H4)} (1.55 g, 4.9 mmol) in ether (25 ml).The mixture was quickly warmed at room temperature (RT) and

T.D.H. Nguyen et al. / Polymer 52 (2011) 318e325320

a yellow suspension solution appeared. The reaction was thenstirred at RT for 6 h. The yellow compoundwas extractedwith etherto afford a yellow sticky product (1.83 g, 76.47%). 1H NMR (300MHz,CDCl3, 25 �C, d, ppm): 7.58 (d, 2H, C9H6), 7.48 (d, 2H, C9H6), 7.28(t, 2H, C9H6), 7.21 (t, 2H, C9H6), 7.00 (s, 4H, C6H4), 6.66 (s, 2H, C9H6),3.74 (s, 2H, C9H6), 0.24 (s, 6H, SieCH3), 0.18 (s, 6H, SieCH3).

2.4.5. Preparation of [{(CH3)2SiCl(C9H6)(CH2)}2(C6H4)] (5)The compound was synthesized from [{(C9H6)(CH2)}2(C6H4)],

(2) using the same reaction conditions and procedures as for 4. Alight yellow compound was obtained (79.27%). 1H NMR (300 MHz,CDCl3, 25 �C, d, ppm): 7.54 (d, 2H, C9H6), 7.34 (d, 2H, C9H6), 7.23(t, 2H, C9H6), 7.21 (t, 2H, C9H6), 7.18 (s, 4H, C6H4), 6.23 (s, 2H, C9H6),3.91 (s, 4H, CH2), 3.62 (s, 2H, C9H6), 0.17 (s, 12H, SieCH3).

2.4.6. Preparation of [{(CH3)2SiCl(C9H6)(CH2)2}2(C6H4)] (6)The compound was synthesized from [{(C9H6)(CH2)2}2(C6H4)],

(3) using the same reaction conditions and procedures as for 4. Theyellow sticky product was obtained (79.96%). 1H NMR (300 MHz,CDCl3, 25 �C, d, ppm): 7.59 (d, 2H, C9H6), 7.50 (d, 2H, C9H6), 7.36(t, 2H, C9H6), 7.28 (t, 2H, C9H6), 7.16 (s, 4H, C6H4), 6.37 (s, 2H, C9H6),3.73 (s, 2H, C9H6), 3.01 (m, 4H, CH2), 2.99 (m, 4H, CH2), 0.20 (s, 12H,SieCH3).

2.4.7. Preparation of [{(NHtBu)(CH3)2Si(C9H6)}2(C6H4)] (7)A solution of 4 (1.83 g, 3.74 mmol) in THF (60 ml) was treated

with tert-butylamine (1.97 ml, 18.69 mmol) via a syringe slowly atbelow 0 �C. As the reactor was warmed to RT, more suspensionwasformed. The mixture was then heated to 60 �C and stirred over-night. After THF was removed in vacuo for 4 h, hexanewas added toextract the product. Volatiles were removed in vacuo for 4 h toafford an orange-yellow sticky product. (1.54 g, 73.08%). 1H NMR(300 MHz, CDCl3, 25 �C, d, ppm): 7.57 (d, 2H, C9H6), 7.48 (d, 2H,C9H6), 7.28 (t, 2H, C9H6), 7.22 (t, 2H, C9H6), 7.14 (s, 4H, C6H4), 6.66(s, 2H, C9H6), 3.57 (s, 2H, C9H6), 1.17 (s, 18H, t-Bu), 0.68 (s, 2H, NH),�0.09 (d, 6H, SieCH3), �0.14 (d, 6H, SieCH3).

2.4.8. Preparation of [{(NHtBu)(CH3)2Si(C9H6)(CH2)}2(C6H4)] (8)The compound was synthesized from 5 (6.34 g, 12.2 mmol)

using the same reaction conditions and procedures as for 7. Anorange-yellow sticky product was obtained (6.2 g, 85.7%). 1H NMR(300 MHz, CDCl3, 25 �C, d, ppm): 7.33 (d, 2H, C9H6), 7.26 (t, 2H,C9H6), 7.21 (t, 2H, C9H6), 7.14 (s, 4H, C6H4), 6.27 (s, 2H, C9H6), 3.92(s, 4H, CH2), 3.47 (s, 2H, C9H6), 1.14 (s, 18H, t-Bu), 0.86 (s, 2H, NH),�0.06 (d, 6H, SieCH3), �0.15 (d, 6H, SieCH3).

2.4.9. Preparation of [{(NHtBu)(CH3)2Si(C9H6)(CH2)2}2(C6H4)] (9)The compound was synthesized from 6 using the same reaction

conditions and procedures as for 7. The orange-yellow stickyproduct was obtained (95.47%). 1H NMR (300 MHz, CDCl3, 25 �C, d,ppm): 7.59 (d, 2H, C9H6), 7.55 (d, 2H, C9H6), 7.36 (t, 2H, C9H6), 7.28(t, 2H, C9H6), 7.16 (s, 4H, C6H4), 6.37 (s, 2H, C9H6), 3.68 (s, 2H, C9H6),3.01 (m, 4H, CH2), 2.98 (m, 4H, CH2), 1.14 (s, 18H, t-Bu), 0.63 (s, 2H,NH), �0.06 (d, 6H, SieCH3), �0.12 (d, 6H, SieCH3).

2.4.10. Preparation of [{Ti(h5:h1-C9H5Si(CH3)2NtBu)Cl2}2(C6H4)] (10)In this 2-stage step, tetralithium salt was created in the first

stage. A solution of 7 (1.54 g, 2.74 mmol) in hexane (40 ml) wastreated with five equivalents of n-BuLi (2.5 M hexane solution,5.47 ml, 13.68 mmol) at �78 �C drop by drop, and then slowlywarmed to RTand heated to 60 �C. After stirring for at least 12 h, thesolution of tetralithium salt was cooled down to RT, the salt waswashed with hexane and the volatiles were removed at reducedpressure overnight to afford an orange salt (1.49 g, 93%). Thecomplex 10 was created in the second stage. TiCl3(THF)3 (1.38 g,

3.75 mmol)/THF (25 ml) was cooled to�78 �C and cannula transferwas used to dropwise the solution of tetralithium salt (1 g,1.7 mmol) to TiCl3(THF)3. After the reaction was warmed to RT, thecolor of the solution changed very quickly from dark brown to blackand the reaction was continued for 3 h more. Then, AgCl (0.54 g,3.75 mmol) was added and the silver mirror precipitated imme-diately. After stirring for 1h at RT, THFwas removed in vacuo for 4 h.Toluene was added to the residue and stirred for a while. Afterbeing filtered and having the toluene removed, the residue wasrecrystallized with mix of toluene and hexane, washed withhexane. Recrystallization of the resulting solution gave the productas a dark brown-yellow solid (0.41 g, 30.1%). 1H NMR (300 MHz,CDCl3, 25 �C, d, ppm): 7.72 (t, 2H, C9H5), 7.40 (t, 2H, C9H5), 7.28(d, 2H, C9H5), 7.22 (d, 2H, C9H5), 7.14 (s, 4H, C6H4), 6.57 (s, 2H, C9H5),1.17 (s, 18H, t-Bu), 0.92 (d, 6H, SieCH3), 0.70 (d, 6H, SieCH3). 13CNMR (75.46 MHz, CDCl3, 25 �C, d, ppm): 138.00 (C6H4), 136.07(C9H5), 134.87 (C9H5), 129.23 (C9H5), 128.36 (C9H5), 127.53 (C9H5),125.42 (C9H5), 120.06 (C6H4), 98.52 (C9H5), 63.44 (C, t-Bu), 32.41(CH3, t-Bu), 3.56 (SieCH3), 1.13 (SieCH3). High-resolution massspectrum: [Pþ] C36H44N2Ti2Cl4Si2, m/z ¼ 800 (Mþ), 763 (Mþ e Cl),728 (Mþ e Cl2), 689 (Mþ e Cl3).

2.4.11. Preparation of [{Ti(h5:h1-C9H5Si(CH3)2NtBu)Cl2(CH2)}2(C6H4)](11)

The compound was synthesized from 8 using the same reactionconditions and procedures as for 10. Recrystallization of theresulting solution gave the product as a dark brown-yellow solid(31.5%). 1H NMR (300MHz, CDCl3, 25 �C, d, ppm): 7.71 (d, 2H, C9H5),7.58 (d, 2H, C9H5), 7.38 (t, 2H, C9H5), 7.28 (t, 2H, C9H5), 7.10 (s, 4H,C6H4), 6.35 (s, 2H, C9H5), 4.35 (q, 4H, CH2), 1.35 (s, 18H, t-Bu), 0.86(d, 6H, SieCH3), 0.63 (d, 6H, SieCH3). 13C NMR (75.46 MHz, CDCl3,25 �C, d, ppm): 138.01 (C6H4), 136.38 (C9H5), 135.18 (C9H5), 129.19(C9H5), 128.90 (C9H5), 128.38 (C9H5), 128.08 (C9H5), 124.75 (C6H4),97.43 (C9H5), 63.48 (C, t-Bu), 34.80 (CH2), 32.54 (CH3, t-Bu), 3.52(SieCH3), 1.22 (SieCH3). High-resolution mass spectrum: [Pþ]C38H48N2Ti2Cl4Si2, m/z ¼ 828 (Mþ), 790 (Mþ e Cl), 758 (Mþ e Cl2),720 (Mþ e Cl3).

2.4.12. Preparation of {[Ti(h5:h1-C9H5Si(CH3)2NtBu)Cl2(CH2)2]2(C6H4)}(12)

The compound was synthesized from 9 using the same reactionconditions and procedures as for 10. The dark brown-yellow solidwas obtained (35.1%). 1H NMR (300MHz, CDCl3, 25 �C, d, ppm): 7.71(d, 2H, C9H5), 7.55 (d, 2H, C9H5), 7.41 (t, 2H, C9H5), 7.28 (t, 2H, C9H5),7.16 (s, 4H, C6H4), 6.30 (s, 2H, C9H5), 3.33 (m, 4H, CH2eC6H4), 3.03(m, 4H, CH2eC9H5), 1.36 (s, 18H, t-Bu), 0.92 (d, 6H, SieCH3), 0.66(d, 6H, SieCH3). 13C NMR (75.46 MHz, CDCl3, 25 �C, d, ppm): 138.92(C6H4), 135.77 (C9H5), 135.00 (C9H5), 129.06 (C9H5), 128.42 (C9H5),127.91 (C9H5), 127.42 (C9H5), 124.43 (C6H4), 96.83 (C9H5), 63.11 (C,t-Bu), 35.67 (CH2), 32.23 (CH3, t-Bu), 31.37 (CH2), 3.42 (SieCH3), 1.11(SieCH3). High-resolution mass spectrum: [Pþ] C40H52N2Ti2Cl4Si2,m/z ¼ 852 (Mþ), 815 (Mþ e Cl), 786 (Mþ e Cl2), 711 (Mþ e Cl3).

2.5. Polymerization

Ethylene/styrene copolymerizations were carried out in a dry300-ml glass reactor, sealed with a rubber septum and cycled twotimes between vacuum and nitrogen to remove the oxygen. Afternitrogen evacuation, the reactor was saturated with a continuousflow of ethylene at atmospheric pressure (1.0 atm) and reactiontemperature. Then, proper amounts of toluene, MMAO and styrenemonomer were injected into the flask. The polymerization wasinitiated by injection of the prepared catalyst solution in toluene.After a measured time interval, the polymerization was quenchedby the addition of acidified methanol containing 10% HCl. The

T.D.H. Nguyen et al. / Polymer 52 (2011) 318e325 321

polymer was collected by filtration, washed with excess methanol,and dried under vacuum overnight to a constant weight.

3. Results and discussion

3.1. Synthesis and characterization

The three DCGCs, distinguished by their bridge structure ofpara-phenyl, para-xylyl, and para-diethylenephenyl were preparedby the procedure shown in Scheme 2. The complexes 10, 11, and 12were prepared by the reaction of TiCl3(THF)3 with the corre-sponding tetralithium salt of ligand in THF at �78 �C followed byoxidation with AgCl [27e29]. These catalysts can be separated asa reddish brown solid via recrystallization in a mixed solvent oftoluene and hexane with a moderate yield (w35%). All theproduced catalysts were easily contaminated through decomposi-tion by moisture or air exposure.

The dinuclear metallocenes catalysts 10, 11, and 12 were char-acterized by 1H and 13C nuclear magnetic resonance (NMR) andmass spectrometry. The 1H NMR spectra of the complexes 10, 11,and 12 were conveniently used to identify the assigned DCGCstructure. All of these outcomes were in accord with the reportedresults of DCGC with polymethylene bridges [15,16]. The remainingassignments are summarized in the Experimental section. In theproton NMR spectra of these compounds, the four resonancesexhibited between 7.2 and 7.6 ppm were assigned as the fourprotons of the six-member ring in the indenyl fragment. The singlepeak at 7.1 ppm indicated the four protons of phenyl bridge. The

Li +

(CH 2 ) n (CH 2 ) n

Si Si

H 3 C

H 3 C Cl

CH 3

CH 3 Cl

2eq Me 2 SiCl 2

2eq t -BuNH 2

(CH 2 ) n (CH 2 ) n

Si

H 3 C

H 3 C

NH

Si

CH 3

CH 3

NH

+ 2eq

(CH 2 ) n (CH 2 ) n

+

+

2eq n -BuLi +

4eq n-BuLi +

+ 2 eq TiCl 3 (THF) 3

Cl (CH 2 ) n (CH 2 ) n Cl

2 eq AgCl

(CH 2 ) n (CH 2 ) n

Si

H 3 C

H 3 C

N

Si

CH 3

CH 3

N

Ti

Ti

Cl

Cl

Cl

Cl

1 (n = 0)

2 (n = 1)

3 (n = 2)

4 (n = 0)

5 (n = 1)

6 (n = 2)

7 (n = 0)

8 (n = 1)

9 (n = 2)

10 (n = 0)

11 (n = 1)

12 (n = 2)

Scheme 2. Preparation route of the catalysts 10, 11, and 12.

peak located at about 6.3 ppm as a singlet was due to one proton ofthe five-member ringside at the indenyl group (Fig. 1). In the 1HNMR catalyst spectra, two positions of chemical shift changed at0.92 ppm and 0.66 ppm comparing to amine ligand spectra (insteadof�0.14 ppm and �0.09 ppm). It indicated that the existence of themetallated dinuclear complexes as these chemical shifts of twomethyl groups at silicon moved to the high field. In addition, thechemical shifts of the two protons of the methylene (CH2) in DCGC11 between the indenyl and phenyl groups presented as twostrongly coupling doublets at 4.35 ppm because these two protonsbecame chemical shift inequivalent due to titanium coordination tothe indenyl ring. For DCGC 12, the resonances of the twomethylenegroups between the indenyl and phenyl groups appeared as twomultiplets at 3.03 and 3.33 ppm. The 13C NMR spectra of thecomplexes also exhibited these features well. Chemical shifts from120 ppm to 140 ppm indicated the aromatic carbons of the phenyland indenyl groups. The peak at 32 ppm was assigned to 3 methylcarbons at the t-butyl group and the peaks near 1 and 3 ppmindicated 2 methyl carbons connected at the silicon atom. Theresonance signals at 63 ppmwhich are attributed to the quaternarycarbon of the tert-butyl group connected at the coordinatednitrogen demonstrated the right structure of catalyst. In contrast,the chemical shift of the carbon bridgehead of the indenyl ringmoved toward the high field to present at around 98 ppm due tothe metal coordination.

Because these catalysts were extremely sensitive to moistureand air exposure, successful elemental analysis could not beobtained. Therefore, mass spectrometry was used to confirm theirformulation. The mass spectra of these compounds exhibited notonly molecular ions of catalysts 10 (800),11 (828) and 12 (855), butalso the fragment masses of (Mþ e 35) and (Mþ e 70) generatedfrom the exclusion of one and two chlorines, respectively.

3.2. Copolymerization

Since CGC proved to be powerful catalyst when activated withmethylaluminoxane (MAO) for the copolymerization of ethyleneand a-olefin [30e32], the dinuclear complexes 10, 11 and 12 wereused, along with Dow CGC as the typical mononuclear CGC forcomparison, for the copolymerization of ethylene and styrene to

Fig. 1. 1H NMR (a) and 13C NMR (b) spectrum catalyst 11.

Table 2Results of ethylene/styrene copolymerizationa at styrene concentration [S] ¼ 0.4 M.

Tpb Catalyst Activityc Sd Mv (�10�3)e

40 Dow CGC 190.6 9.5 44.610 92.3 11.3 87.011 293.1 14.2 342.012 394.8 16.5 461.0

70 Dow CGC 240.5 10.4 42.610 102.8 11.3 47.711 385.5 16.0 121.012 473.9 16.5 135.0

a Conditions: [cat] ¼ 20 mmol/l, [MMAO]/[cat] ¼ 2000, styrene concentration[S] ¼ 0.4(mol/l), 100 ml toluene ethylene 1 atm, 2 h.

b Polymerization temperature (�C).c Activity (kg-polymer mol�1 h�1).d Styrene content in copolymer calculated by 1HNMR spectra of E/S copolymer (%).e The viscosity averaged molecular weight (g/mol).

T.D.H. Nguyen et al. / Polymer 52 (2011) 318e325322

identify the catalytic characteristics. The results are summarized inTable 1 and Table 2.

3.3. Polymerization activity

From the standpoint of the catalyst activity, several importantpoints should be noted for characterizing DCGC in terms of thenature of the bridge. Firstly, the catalytic activity increaseddramatically with lengthening of the bridging ligand. At a styreneconcentration of 1.3 mol/l at 70 �C, the catalyst activity increased inthe order of 10 (168.7 kg of polymer/mol of Ti.h.atm) < Dow CGC(290.5 kg of polymer/mol of Ti.h.atm) < 11 (739.9 kg of polymer/mol of Ti.h.atm) < 12 (791.0 kg of polymer/mol of Ti.h.atm), whichillustrated that DCGCs 11 and 12 with para-xylene and para-diethylenephenyl bridges, respectively, showed four to five timesgreater activity than DCGC 10 with para-phenyl bridge and aboutthree times higher activity than Dow CGC. This basic tendency wasobserved consistently even in a variety of changing reactioncondition such as styrene concentration and polymerizationtemperature, as shown in Fig. 2. This result supports the critical roleplayed by the length of the linkage unit between two active sites inenhancing the DCGC activity.

To compare the approximated length of the bridge between twoindenyl groups, the well-known software ChemBio 3D Ultra 11.0was applied because of its simplicity. The separation distance ofapproximately 5.4 Å (para-phenyl) between the two indenyl frag-ments of the catalyst 10may not have permitted a satisfactory roomfor monomer coordination that lead to the low activity. In contrast,the equivalent distances of 7.3 Å and 9.8 Å for the catalysts 11 and12, respectively, should provide sufficient space for copolymeriza-tion to enhance the activity. The length of the styrene monomer ofabout 6.6 Å implies that the DCGC bridge length should be at least6.6 Å in order to provide an appropriate active site volume forstyrene for moving in and out and thus increase the activity. Aminimum of five methylene molecules is required for poly-methylene-bridged dinuclear zirconocene to exhibit greateractivity than that of mononuclear zirconocene [33]. The activities ofthe dinuclear zirconocenes with three methylenes were lower thanthose of the mononuclear one. Interestingly, the stretched lengthsof five and three methylenes, at 7.3 Å and 4.9 Å, respectively, aresimilar to the lengths of para-xylyl (7.3 Å) and para-phenyl (5.4 Å),which further supports the mechanism presented above.

Regarding the activity gap between the three DCGCs, theaverage activity of DCGC 11was about four-fold greater than that of10, while that of DCGC 12 was only 20e30% greater than that ofDCGC 11, even though the 2.5 Å length difference between para-diethylenephenyl and para-xylyl is actually more than that (of1.9 Å) between para-phenyl and para-xylyl. This outcome suggests

Table 1Results of ethylene/styrene copolymerizationa at styrene concentration [S] ¼ 1.3 M.

Tpb Catalyst Activityc Sd Mw (�10�3)e MWDe

40 Dow CGC 253.7 19.9 75.0 2.4010 101.7 20.0 62.8 2.4211 481.0 22.4 135.5 2.8212 594.6 22.3 213.0 2.56

70 Dow CGC 290.5 25.2 38.3 1.7210 168.7 26.0 28.4 2.2511 739.9 32.5 56.0 3.3712 791.0 32.5 105.0 4.01

a Conditions: [cat] ¼ 20 mmol/l, [MMAO]/[cat] ¼ 2000, styrene concentration[S] ¼ 1.3(mol/l), 100 ml toluene, ethylene 1atm, 2 h.

b Polymerization temperature (�C).c Activity (kg-polymer mol�1 h�1).d Styrene content in copolymer calculated by 1HNMR spectra of E/S copolymer (%).e GPC data in 1,2,4-trichorobenzene(TCB) vs polystyrene standards at 170 �C.

the existence of a critical bridge length in dinuclear metallocenesthat determines the catalyst performance, which is actually anobvious merit of dinuclear metallocene compared to the normalmononuclear metallocene system. These experimental resultsprovided further confirmation that the critical DCGC bridge lengthis around 6.6 Å, which is the length of the styrene monomer. It islikely that the effect of the bridge length gradually diminishes toreach a plateau as the length exceeds the critical length.

The second point to be considered regarding the activitydifference between DCGCs 11 and 12 is the potential difference inthe electronic effect between the para-xylyl and para-dieth-ylenephenyl groups. The active site with greater electron densitygenerally exhibits more pronounced activity because a greaterelectron density is able to stabilize the electron-deficient active sitemore effectively. Accordingly, the para-diethylenephenyl bridge ofDCGC 12 was assumed to deliver greater electron density to theelectron-deficient titanium cationic species than the para-xylylbridge of DCGC 11. Considering the distance from the methylene tothe active site, the contribution of the increased electron densityinduced by the single methylene unit to the catalytic activity mayhave been a secondary factor compared to that of the bridge lengtheffect.

An important featureof theDCGCactivity is that thecompositionofthemonomer feedwas the primary influence on the catalytic activity.Irrespective of the catalyst type or the polymerization temperature,the activities with styrene concentration ([S]) [S] ¼ 1.3 mol/l weresignificantly higher than thosewith [S]¼ 0.4mol/l.WithDCGC 12, theactivitieswith theconditionof [S]¼1.3mol/l at 70 �Candat40 �Cwere1.7 and 1.5 times, respectively, larger than those with the condition of

Fig. 2. The correlation between activity and the catalysts.

T.D.H. Nguyen et al. / Polymer 52 (2011) 318e325 323

[S] ¼ 0.4 mol/l at the same temperatures. However, the decrease inpolymerization temperature from 70 �C to 40 �C at the same styreneconcentration enhanced the activity by 20e30%. The high styreneconcentration eased the reversible deactivation of the active site toa dormant species by additional coordination of the phenyl ring’sp-electron system [28e30]. However, in this study the correlation ofthe activity of DCGC and styrene concentration was consideredunlikely to comply with this tendency.

3.4. Molecular weight (MW)

The variations of molecular weight (MW) with the type ofcatalyst are plotted in Fig. 3 at constant temperature and styreneconcentration. Because GPC was not able to be used to measureMWs of some copolymer samples formed at [S] ¼ 0.4 M, theviscosity average molecular weights (Mv) were measured to obtainMWs of those polymers instead. Among the noticeable trends, theDCGC structure primarily affected the MW of the copolymer. Thecapability to form a high MW polymer increased in the order ofDow CGC z 10 < 11 < 12 either [S] ¼ 1.3 M or [S] ¼ 0.4 M. Mw aswell as Mv values of copolymers made by DCGCs 11 and 12 weregreater at least 2 and 3 times, respectively, than those made byeither DCGC 10 or Dow CGC. Beside the difference of MWs, themolecular weight distribution (MWD) of copolymers obtained byDCGC 11 and 12 became broader than those obtained by DCGC 10 orDow CGC. It is a general feature that the polymers made by thedinuclear metallocenes exhibit broader distribution of MW [19].This large difference in the MWs and MWDs of the polymers mayhave been caused by the bridge length difference in the dinuclearmetallocene. Some previous studies showed thatMWvariationwasnot sensitive to the catalyst structure, and that polymerization

Fig. 3. The correlation of molecular weight of Ethylene/Styrene copolymer with thecatalysts: (a) styrene concentration [S] ¼ 1.3 M; (b) styrene concentration [S] ¼ 0.4 M.

conditions such as temperature and concentration were moreimportant factors in determining the MW of the polymer [15e18].However, the present study results clearly demonstrated thepredictive power of the DCGC structure, as determined by thebridge length, in characterizing the polymer properties.

In another surprising result, DCGC 12, with the longest para-diethylenephenyl linkage, generated copolymers with the highestMW, followed by DCGC 11 with the second longest para-xylyl one.In cases of polymethylene-bridged dinuclear metallocenes, thecatalyst with a longer bridge formed a lower MW polymer [15,16].Accordingly, the influence of the bridge length with the rigidproperty of the phenyl-containing bridge group on MW was actu-ally opposite to that with the flexible property of the poly-methylene bridge.We considered that themore facile interaction ofthe two active sites through the short flexible linkage wasresponsible for the formation of a long polymer chain due to thedisturbed termination via b-H elimination. The para-dieth-ylenephenyl bridgemay have been long enough to permit sufficientand untouchable room between the two active sites. Similarly, the7.3 Å long bridge of para-xylyl, which is slightly longer than 6.6 Å,may have experienced some steric hindrance that may have beena minor factor in increasing the polymer MW. Consequently, theeffect of the steric interference of the two active sites of DCGCs 11and 12 in reducing the b-H elimination frequency should not beconsidered the principal factor to explain the formation of a highMW polymer.

This introduces the question of what is the secondary cause ofthis result. Exclusion of the steric issue surely leaves the electronicfactor behind. In terms of the electronic effect of DCGC, the bridgeof para-diethylenephenyl of DCGC 12 was anticipated to generatethe most stable active site among the three DCGCs due to thegreater electron supply arising from the existence of twomethyleneunits at the active site. This explanation is in good agreement withaction of DCGC 12, followed by DCGC 11, in forming a longerpolymer since the more stable active site affords a higher MWpolymer in metal-catalyzed polymerization systems. Anotherreason may have been the combined effect of the rate of coordi-nation and the b-H elimination because the degree of polymeri-zation is determined by the relation between the rate ofpropagation and that of termination. This suggested that catalyst 12may have not only a faster rate of coordination but also easier b-Helimination compared with catalysts 10 and 11. The final degree ofpolymerization derived from the relation of the two factors of DCGC12 was actually greater than those of DCGCs 10 and 11. As a finalpoint, DCGC 10 could be used to fabricate copolymers with similar

Fig. 4. The correlation between styrene content of Ethylene/Styrene copolymer andthe catalysts.

Fig. 5. 13C NMR spectra of poly(ethylene-co-styrene) prepared by catalyst 11 at 70 �C, (a) [S] ¼ 1.3 M, styrene content 32.5%; (b) [S] ¼ 0.4 M, styrene content 16.0%.

T.D.H. Nguyen et al. / Polymer 52 (2011) 318e325324

MW to those achieved with Dow CGC. The close proximity of thetwo active sites may have delayed the b-H elimination process andthereby offered an opportunity for chain lengthening.

3.5. Styrene contents in copolymer

The study results revealed the efficiency of DCGC for incorpo-rating styrene into the polyethylene backbone. Among the threecatalysts 10, 11 and 12, the styrene contents in the generatedcopolymers increased in order of catalyst DowCGC< 10<11<12 atthe same styrene concentration (Fig. 4). Remarkably, DCGC 10exhibited a higher styrene reactivity than the mononuclear DowCGC, considering that the 5.4 Å long para-phenyl bridge is actuallyshorter than the styrene length of 6.6 Å in DCGC 10. This may reflectthat the nucleating effect of DCGC with short ethylene bridgeproposed by Marks [13,19] is likely to be active in dinuclear metal-locene possessing short phenyl-based rigid bridges. Despite thestrong ability of DCGC 10 to incorporate styrene on the polyethylenebackbone, a comparison between the styrene reactivity of DCGCs 11and 12 supports the inferior styrene reactivity of DCGC 10. Thestyrene contents in the copolymers generated by DCGCs 11 and 12were almost same, regardless of the polymerization conditions.However, the styrene incorporation capability differed betweenDCGCs 10 and 11. This result was attributed to the critical effect ofthe DCGC bridge length in determining the reactivity of the como-nomer. In an outstanding result, DCGC 12, with the longest para-diethylenephenyl bridge, was used to prepare copolymers with notonly the greatest MW but also the highest styrene contents.

Fig. 5 shows 13C NMR spectra (methylene and methane region)of the copolymers prepared by catalyst 11 at 70 �C. From the styrene

concentration 0.4 Me1.3 M, the reducing resonance intensities ofstrong signals at 29.7 ppm of the polyethylene sequences, theincreasing of resonance intensities at 27.5, 36.9, and 46.2 ppm (Sbd,Sad, and Tdd, respectively) of sequences of EESEE and in addition ofpeak at 25.3 ppm (Sbb) which represents the SES sequence indi-cated that produced polymers had a substantially alternatingstructure. The absence of a signal for Tbb at 41.3 ppm and for Saa at43.6 ppm shows that there is no styreneestyrene sequence in thecopolymers. This demonstrated that the dinuclear CGC in thisexperiment might be advantageous over the reported catalysts toobtain more randomly distributed poly(ethylene-co-styrene)s.

Beside that the copolymerization products are rubber-like inappearance. The melting points (Tm) of the obtained copolymerscouldnot bedetected byDSCmeasurements, nomatterhowhigh thecomonomer incorporation in the copolymers is. The melting point(Tm) is attributed to a unique blocky microstructure which offersenough consecutive sequences of comonomer units in the polymerbackbone to form a crystalline phase. Therefore, all ethylene/styrenecopolymer produced by our catalyst were amorphous.

4. Conclusion

A series of new DCGCs with the structure of [{Ti(h5:h1-(C9H5)Si(CH3)2NtBu)Cl2(CH2)n}2(C6H4)] [n ¼ 0 (10), n ¼ 1 (11), n ¼ 2 (12)]were synthesized and characterized successfully. In the presence ofmodified MAO as cocatalyst, ethylene and styrene were copoly-merized to examine specifically the effect of bridge length on thepolymerization behaviors of DCGCs, with Dow CGC being used forcomparison. Not only the catalyst activity but also the MWs andstyrene contents of the copolymers were promoted as the bridge

T.D.H. Nguyen et al. / Polymer 52 (2011) 318e325 325

structurewas extended from para-phenyl (10) to para-xylyl (11) andthen to para-diethylenephenyl (12), which demonstrated that theoverall DCGC performance could be controlled by the addition ofa long and rigid unit as the bridge ligand of DCGC. The activity of thecatalysts increased in the order of 10 < Dow < 11 < 12. Theimportant feature of the DCGC activity is that the composition of themonomer feed was the principal factor affecting the catalyticactivity. Irrespective of the catalyst type or the polymerizationtemperature, the activities with [S] ¼ 1.3 mol/l were significantlyhigher than those with [S] ¼ 0.4 mol/l. The capability to form highMW polymer increased in the order of Dow CGC z 10 < 11 < 12,which indicated that the DCGC structure, as distinguished by thebridge length, could be used as a decisive tool to characterize thepolymer length. This result provided further support for the exis-tence of a critical bridge length in dinuclear metallocenes to controltheMWof the produced polymer. In addition, the electron donatingeffect of the longer bridge may be regarded as the secondary factorpromoting the polymerization properties of DCGC. The styrenecontents in the copolymers increased in order of catalyst DowCGC< 10< 11<12 at the same styrene concentration. Remarkably,DCGC 12, with the longest para-diethylenephenyl bridge, exhibitedthe highest activity while being used to prepare copolymers withnot only the greatest MW but also the highest styrene contents.

Acknowledgements

The authors are grateful for the Korea Ministry of Knowledgeand Economy (Grant RT 104-01-04, Regional Technology Innova-tion Program).

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