M(□5-C5Me5)TEA [M=Ti, Zr, Hf] 메탈로센 촉매와 공촉매인 Methylaluminoxane에 의한 에틸렌 중합

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한국과학기술원 화학공학과

ETHYLENE POLYMERIZATION OVER M(□5-C5Me5)TEA [M=Ti, Zr, Hf] METALLOCENE CATALYSTS WITH METHYLALUMINOXANE

AS A COCATALYST

Ki-Soo Lee, O-Yong Jeong and Son-Ki Ihm

Dept. of Chem. Eng., Korea Advanced Institute of Science and Technology

Introduction

Since the discovery of Ziegler-Natta catalyst, continual and sometimes drastic improvement has been made to the catalyst system for olefin polymerization. Sinn and Kaminsky's discovery of new catalytic systems based on the combination of metallocene with alkylaluminoxane started a new era in polyolefins. The chemistry of metallocenes is a widespread and rapidly progressing discipline[1,2]. The interest in modifying early transition metal complexes with main group elements as substituents is supported by the potential application of these complexes in various processes of olefin conversion and due to their further synthetic advances[3,4]. Our research interest is focused on the new type of ancillary ligand with group IV metals. Ethylene polymerizations and copolymerization of ethylene with 1-hexene were carried out over these metallocene complexes with methylaluminoxane(MAO) as a cocatalyst to investigate the catalytic behaviors of each metallocene catalyst and the properties of polyethylene synthesized.

Experimental

Materials

All procedures were carried out under nitrogen using Schlenk techniques. Polymerization-grade ethylene and nitrogen were further purified before feeding to the reactor by passing them through columns packed with oxygen scavenger (Ficher RIDOX) and molecular sieve 5A (Aldrich), respectively. 1-Hexene was purified by distillation under reduced pressure over CaH2. Toluene used in the reaction and catalyst handling was dried by refluxing through a distillation column over sodium/benzophenone under dry nitrogen. Some of Cp*M(N(CH2CH2O)3) [M = Ti(a), Zr(b) and Hf(c)] complexes were donated by Do et al.[5]. Other materials were used without further purification.

a                  b                 c

Scheme. Cp*M(N(CH2CH2O)3) [M = Ti(a), Zr(b) and Hf(c)] complexes

Polymerization and polymer characterizations

Ethylene polymerization was carried out in a 500ml glass reactor with mechanical stirrer. The reactor was filled with toluene and part of the methylaluminoxane (MAO) needed. After thermostating, nitrogen was removed under vacuum and the reaction mixture saturated with ethylene. 8.61mmole metallocene solution was then injected into the reactor. Ethylene pressure was kept constant during the polymerization by a pressure regulator. The ethylene flow rate was measured by a mass flow controller. All the runs were stopped by venting off excess ethylene and adding acidic ethanol. The precipitated polymer was washed with the plenty of ethanol and dried at 80 oC for 6 hours under vacuum to a constant weight. NMR spectra of polymers were recorded on a Bruker AMX 500 FT-NMR at 80oC using benzene-d6/1,2,4-tri-chlorobenzene(1/3, v/v) as a solvent. The differential thermal analysis was carried out with a calorimeter (DuPont Instruments, Thermal Analyst 2000) under a nitrogen atmosphere at a heating rate of 10oC/min. The results of the second scan were reported to eliminate difference due to the thermal history of the sample. The intrinsic viscosity of the polymers was determined in decalin at 135oC with Ubbelohde viscometer and the value of Mv was calculated from conventional method[6].

Results and Discussion

Figure 1. shows the polymerization rate as a function of time over three catalysts(a, b and c) with MAO. For a catalyst, the maximum rate of polymerization, Rp,m , is reached rather quickly after introducing the catalyst followed by a rapid decay to about one-third of the rate. The reason why a catalyst suffered rapid deactivation might be due to the instability of titanium complex with aluminum compound. But initial ethylene consumption rate in b catalyst system was very low, but the catalytic activity of b catalyst in ethylene polymerization increased with polymerization time and sustained for some hours.	The results on the ethylene polymerization over a, b and c metallocene complexes with MAO as a cocatalyst are given in Table 1. The catalytic activity for ethylene polymerization at 30 min. reaction time decreased in the order of b > a >> c.  The molecular weight and melting temperature of polymer product over a catalyst were much higher than those over Ti or Hf complexes.

Table 1. Ethylene polymerization by using various metallocene catalysts.

Catalysts	Al/M mole ratio	Tp[oC]	Activitya)	Mv(x103)b)	Tm[oC]	□Hf[J/g]
a	1,000	40	1430	17.7	127.3	134
a	2,000	40	1890	17.3	126.7	144
a	4,000	40	3300	10.4	129.8	104
a	6,000	40	3700	8.1	127.6	120
b	1,000	40	900	91.8	139.1	94
b	2,000	40	1020	178.6	139.2	157
b	4,000	40	1170	181.1	141.9	120
b	6,000	40	1300	180.3	140.2	95
c	1,000	40	90	56.3	127.8	149
c	2,000	40	140	43.9	133.0	176
c	4,000	40	190	37.5	132.9	159
c	6,000	40	210	23.4	134.6	177

Cocatalyst : MAO(methylaluminoxane), Polymerization conditions : [Me]=8.61mmol, Ethylene press.= 5 atm, time=30min., 10ml toluene, a) Activity unit : Kg PE/(mol Ti x hr), b) Viscosity average molecular weight measured by Ubbelohde viscometer

The activity of ethylene polymerization increased with Al/M ratio as expected. The activity of catalyst a seemed to be influenced more by the mole ratio of Al/M ratio than catalysts b and c.  In the case of catalyst b, the molecular weight(Mv) did significantly increase with addition of cocatalyst(MAO). Comparison of catalyst b with catalysts a and c showed that the melting temperature(Tm) and molecular weight(Mv) of polyethylene were much higher.

The three metallocene catalysts(a, b and c) were activated with MAO and used to copolymerize ethylene and 1-hexene. Only catalyst a(Ti) resulted in a good incorporation of 1-hexene into the polyethylene main chain, while b(Zr) and c(Hf) catalysts were not active.  The copolymerization results over catalyst a were given in Table 2. Obtained copolymers showed elastic properties without any clear endotherm (no melting temp.). In Figure 2, 13C NMR spectra of copolymers over Ti complex catalyst showed that the main chains of copolymers had no 1-hexene block. From the collective method of assignment[7], 1-hexene content in copolymer was estimated to be over 25mole%.

Summary

All the catalysts had fairly good activities in ethylene polymerization in the order Cp*Ti(TEA) > Cp*Zr(TEA) >> Cp*Hf(TEA). The catalytic activity of Cp*Zr(TEA) with MAO in ethylene polymerization increased with polymerization  time  and  the  molecular weight of polymer product was much higher. Cp*Ti(TEA) with MAO catalyst resulted in a good incorporation of 1-hexene into the polyethylene main chain.

Table 2. The results of copolymerization of ethylene with 1-hexene over catalyst a with MAO as cocatalyst.

Catalyst	monomer	mole ratio	Yield[g]	Activity [kg-Pol/mol Ti atm hr]	Comonomer content
a	C6/C2	2	2.17	25.2	25.8
a	C6/C2	4	1.75	20.3	39.5

Polymerization condition; Tp=40oC, ethylene press.=5atm, Al/Ti =2,000, tp=2hr in 100cc toluene

Reference

[1] H. Sinn, W. Kaminsky, and R. Woldt, Angew. Chem. Int. Ed. Engl., 19(5), 390(1980).

[2] J.H. Yim, K.J. Chu, K.W. Choi, and S.K. Ihm, European Polym. J., 32, 1381(1996).

[3] N. Ishihara, M. Kuranotom, and M. Uoi, Macromolecules, 21, 3356(1988).

[4] R. Leino, H.J.G. Luttikhedde, P. Lehmus, C.-E. Wilen, R. Sjoholm, A. Lehtonen, J. V. Sepp□l□, and J. H. Nasman, Macromolecules, 30, 3477(1997).

[5] Y. Kim, E. Hong, M.H. Lee, J. Kim, Y. Han, and Y. Do, Organometallics, 18, 36(1999).

[6] J. H. Elliot, J. Appl. Polym. Sci., 19 29476(1970).

[7] J.C. Randall. JMS-REV. Makromolek. Chem. Phys. C29 (2&3), 201(1989).