MMAO Catalyst System

Progress in Olefin Polymerization Catalysts and Polyolefin Polyolefin Materials T. Shiono, K. Nomura and M. Terano (Editors) © 2006 Elsevier B.V. All rights reserved.

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10 Preparation of Ethylene/Polyhedral Oligomeric Silsesquioxane(POSS) Copolymers with rflc-Et(Ind)2ZrCl2/MMAO Catalyst System Dong-ho Leea*, Keun-byoung Yoona, Myung-sung Junga, Jin-ki Sungb and SeokKyunNotf "Department of Polymer Science, Kyungpook National University, Daegu 702-701, Korea e-mail; [email protected] b R&D Center, Korea Petrochemical Inc., Ulsan 680-110, Korea "School of Chemical Engineering and Technology, Yeongnam University, Gyeongsan, 712-749, Korea

Abstract To prepare the organic/inorganic hybrid eopolymer, the norbornene derivatives of polyhedral oligomeric silsequioxane(POSS) such as norbomenylethyl-POSS (N-POSS) and dimethyl(norbornenylethyl)silyloxy-POSS(N-Si-POSS) had been copolymerized with ethylene by catalyst system of rae-Et(Ind)2ZrCl2/modIfied methylaluminoxane(MMAO). The catalytic activity decreased with the feed amount of POSS comonomer, and it was found that the reactivity of N-Si-POSS was larger than that of N-POSS. The unreacted POSS monomer was removed completely by washing the copolymerization product with n-hexane. The melting point of eopolymer decreased with POSS content while the thermal stability, especially oxidation stability of eopolymer was improved in a great extent relative to conventional polyethylene.

1. INTRODUCTION In the past decade, the researcher's interest has been widely attracted to the possibility of preparing the hybrid materials and nanocomposites having the

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inorganic cage molecules constituted by a silicon-oxygen framework. These molecules, named polyhedral oligomeric silsesquioxane(POSS), were noted by the general formula (RSiOi,s)n where R is hydrogen or an organic group[l]. POSS is surely an attractive material as it can be easily functionalized by chemically altering the R substituent groups, thus having the potentiality of undergoing copolymerization and grafting reactions[2,3]. The presence of the thermally robust POSS moiety was found to drastically modify the polymer thermal properties supplying the greater thermal stabilily to polymer matrix and also allowing the tailoring of polymer glass transition temperature by tuning the POSS concentration^], A very few works have been reported so far as regards the preparation of nanocomposites with polymerizable POSS and olefins[5,6]. In this paper, we report an efficient synthetic route for the preparation of ethylene/POSS copolymers and the enhancements of physical properties of copolymer.

2. EXPERIMENTAL Materials. The a«sa-metallocene of rae-Et(Ind)2ZrCl2(Strem Chemicals, U.S.A.) was purchased and used as received. Modified methylaluminoxane (MMA0-3A: 6,7 wt% Al, Tosoh Finechemical Corp., Japan) was used without further purification. The norbornenylethyl-POSS(N-POSS) and dimethyl(norbomenylethyl)silyloxy-POSS(N-Si-POSS) were purchased from Aldrich and used as received. Ethylene(E, Korea Petrochem. Ind. Co., Korea) was used after passing through the columns of CaSO4 and P2O5, and toluene(Duksan Chemical Co., Korea) was purified after refluxing with sodium-benzophenone complex. Polymerization procedure. All operations were carried out under nitrogen atmosphere. In a 300 ml glass reactor were introduced sequentially the proper amounts of toluene, POSS solution, MMAO solution and then the system was saturated with E, With a continuous flow of E, the polymerization was initiated by injecting the toluene solution of metallocene catalyst and continued for lh. Polymer characterization. The composition of copolymer was analyzed with carbon-13 nuclear magnetic resonance spectroscopy(13C-NMR, Varian, Unity, 300MHz) in bromobenzene-ds at 135 °C. Gel permeation chromatography(GPC, Waters, Alliance GPC/V2000) was performed in trichlorobenzene at 150 °C to measure the molecular weight and molecular weight distribution. The thermal properties of the obtained polymer were measured by means of differential scanning calorimetry(DSC, DuPont TA 2000) at 20 °C/min with 2nd run and thermal gravimetric analysis(TGA, Shimadzu TGA-50) at 10 °C/min.

10. Preparation ofEthylene/POSS Copolymers

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3. RESULTS AND DISCUSSION 3.1 Copolymerization ofethylene with N-Si-POSS or N-POSS The oraa-metallocene/MAO catalysts systems can be used for copolymerization of ethylene(E) with norbornene(N) to generate cyclic olefin copolymers with N concentration up to 30 mol%[7]. In stead of Ns POSS derivatives of NSi-POSS and N-POSS were used in the E copolymerization initiated with racEt(Ind)2ZrCl2/MMAO catalyst system as summarized in Table 1. Table 1. Copolymerization of E and POSS Comonomer Feed(mol/LxlO j ) POSS Activity* Copolymer PDIC ([E]/[POSS]) (mol%f 1002 33.9 E/N-Si-POSS-1 0.73 6.3 5/0.8 E/N-Si-POSS-2 5/1.6 931 22.4 6.2 1.17 0.22 E/N-FOSS-1 5/0.8 1944 11.7 2.3 E/N-POSS-2 2.1 5/1.6 31 0.37 8.7 PEd 5/2315 10,2 2.8 Polymerization conditions; [Zr]=1.9 xio"s mol/L, [Al]/[Zr]=3000, 50 mL toluene, 1 atm ethylene, reaction time : lh. a Activity in [kg polymer/(mol catalyst-h)], b As determined by 13C NMR. e As determined by GPC. d Reaction time : 15 min.

With the addition of POSS, the catalytic activity decreased. As expected, the POSS content of copolymer increased with feed amount of POSS. The composition of copolymer was obtained from 13C-NMR spectrum of Fig. 1. C H 2, CH

A

-E/N-Si-POSS-1 E /N -S i-P O S S -1 -E/N-POSS-1 E /N -P O S S -1

y

x

H 2C

H 3C R

O S iO Si O

R

OR O Si Si O R

-

O

O

Si Si O R O

Si

CH 2

b, c

CH 3

b b

Si O Si O

I' !.

R

R

40

a

C 7cbb

c

O

35

30

I

a

1

Cyclopentyl c t g group of POSS Of

i 25

20

15

7

6

5

4

3

L o g (M w ) Log(MJ

1.13C-NMR spectrum of E/N-Si-POSS-1 copolymers Fig. 1.

Fig. 2. GPC curves of E/POSS copolymer

At constant feed mole ratio, the POSS content of E/N-Si-POSS copolymer was larger than that of E/N-POSS, which indicated that N-Si-POSS was more reactive than N-POSS in the copolymerization with E. The polydispersity index (PDI) of E/N-Si-POSS copolymer was much higher than that of E/N-POSS copolymer as well as PE homopolymer. In other word,

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the molecular weight distribution of E/N-Si-POSS copolymer was found to be much wider than that of E/N-POSS copolymer as shown in Fig. 2, And the E/N-Si-POSS copolymer exhibited a bimodal GPC curve. It could be speculated that the reason of bimodal distribution might be due to the silyloxy, Si-O- bridge bond of N-Si-POSS comonomer which can give some effects on the formation of catalytic active site. The more detailed study on the bimodal molecular weight distribution of E/N-Si-POSS copolymer is on the progress. 3.2. Purification ofE/POSS copolymer For the exact physical characterization of polymeric material, the purification procedure of polymer product is critical in removal of the unreacted monomers and impurity. Because POSS is soluble in n-hexane, the polymerization mixture obtained by precipitating with methanol was washed with a plenty of n-hexane to remove the unreacted POSS comonomer from E/POSS copolymer. After washing with n-hexane, the solid product was analyzed with GPC as shown in Fig. 3. \ TCB(trichlorobenzene) TCB(trichlorobenzene)

N-Si-POSS antioxidantt antioxidan

I E/N-Si-POSS-1 copolymer E/N-Si-POSS-1

POSS S/MA-POSS copolymer 0

10

20

30

Retention Time(min) Retention

Fig. 3. GPC chromatograms of E/N-Si-POSS-1 and S/MA-POSS copolymers

For E/N-Si-POSS-1 copolymer after treating with n-hexane, no peak of N-SiPOSS comonomer was observed indicating that unreacted POSS monomer was removed completely by washing with n-hexane. On the other hand, a peak of POSS comonomer residue was observed in the commercial product (Hybrid Plastics, U.S.A.) of styrene(S)/methacrylpropylisobutyl-POSS(MA-POSS) copolymer. No information for the purification method of S/MA-POSS copolymer had been received from Hybrid Plastics after being asked by e-mail.

10, Preparation ofEthylene/POSS Copolymers

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3.3, Thermal properties of copofymer The melting temperaturefFm) and heat of fusion(^i^) of E/POSS eopolymers was measured by DSC and the results were given in Table 2, Table 2, Thermal Characterization of E/POSS Copolymers AHQIg) Ciystallinityf/o) TJX)

Copolymer E/N-Si-POSS-1 E*4-Si-POSS-2 E*4-POSS-1 E*J-POSS-2 PE

112 106 121 118 131

56 51 112 100 142

19 17 38 34 48

AHf (Heat of fusion of 100 % crystallized PE)=294 J/g [8] A gradual decrease in Tm and AH of the eopolymers was observed with an increase in POSS content (Table 1) of the copolymer, which clearly suggested a random copolymer structure. The degree of erystallinity which was calculated[8] from AR° also declined with the incorporation of POSS comonomer. To examine the thermal stability of E/POSS copolymers, the TGA thermograms of copolymers were obtained under air as well as under nitrogen as shown in Fig. 4.

Under N2

-PE - BN-Si-POSS-1 • BN-Si-POSS-2

— PE — - BN-SI-POSS-1 — BN-H-PQSS-2

100

200

300

TempfC)

400

500

800

TempfC)

Fig. 4. TGA thermograms of E/POSS copolymers The thermal stability of copolymer under air was much improved compared to PE. Since PE decomposition in air is through random chain scission to generate free radicals, a cross-linking mechanism around the POSS silicone core was speculated as an explanation for the improved thermal stability[6]. The

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existence of POSS nanoparticles facilitates recombination of the free radicals and raises stable temperature regime. Another possible explanation for the improvement of thermal oxidative stability is the formation of a silica layer on the surface of the polymer melt in the presence of oxygen, which serves as a barrier preventing further degradation of the underlying polymer.

4. CONCLUSION The ethylene could be copolymerized with norbomenylethyl-POSS(N-POSS) and dimethyl(norbornenylethyl)silyloxy-POSS(N-Si-POSS) by using the ansametallocene of rae-Etflnd^ZrCla catalyst with MMAO cocatalyst The catalyst activity for copolymers decreased with incorporation of POSS comolecules. By washing the copolymerization product with n-hexane, the unreacted POSS comonomer could be removed completely. The melting temperature and heat of fusion of E/POSS eopolymer dramatically decreased with addition of small amount of POSS molecule. By incorporation of POSS, thermal oxidative stability of eopolymer was much improved, especially.

5. Acknowledgements The part of this work was supported by grant (R01-2004-000-10563-0) from the Korea Science and Engineering Foundation.

6. References [1] G. Li, L. Wang, H. Ni, C. U. Pittman Jr., J. Inorg. Organomet. Polym. 11, (2001) 123. [2] F. J. Feher, T. L. Tajima, J. Am. Chem. Soc., 116, (1994) 2145. [3] F. J. Feher, K. J. Weller, J. Chem. Mater., 6, (1994) 7. [4] J. J. Schwab, J. D. Lichtenhan, Appl. Organometal, Chem., 12, (1998) 707. [5] P.T. Mather, H. G. Jeon, A. Romo-Uribe, Macromolecules, 32, (1999) 1194. [6] L. Zheng, R. J. Farris, E. B. Coughlin, Macromolecules, 34, (2001) 8034. [7] D.H. Lee, Y.Y. Choi, J.H. Lee, Y.S. Park, S.S. Woo, e-Polymers, no. 019 (2001) [8] B. W. Wunderlich, "Macromolecular Physics", Vol. 3, p.63, Academic Press, New York, 1980