VINYL POLYMERIZATION OF NORBORNENE CATALYZED BY [2-(2- BENZIMIDAZOLYL)-6-((1-ARYLIMINOETHYL)PYRIDYL)]NICKEL CHLORIDE/MAO SYSTEM

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1 Chinese Journal of Polymer Science Vol. 26, No. 5, (2008), Chinese Journal of Polymer Science 2008 World Scientific VINYL POLYMERIZATION OF NORBORNENE CATALYZED BY [2-(2- BENZIMIDAZOLYL)-6-((1-ARYLIMINOETHYL)PYRIDYL)]NICKEL CHLORIDE/MAO SYSTEM Qi-song Shi a, b a, b* and Wei-wei Zuo a Department of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing , China b Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing , China Abstract The catalytic system of [2-(2-benzimidazolyl)-6-((1-aryliminoethyl)pyridyl)]nickel chloride/mao (methylaluminoxane) was found to be good active for vinyl polymerization of norbornene and provided polymers with relative narrow molecular distributions. Various reaction parameters, such as the ratios of nickel precursor to MAO or monomer norbornene, and the nature of the ligands in complexes were carefully investigated to realize their effects on the catalytic activities, polymer molecular weight and molecular weight distributions. Keywords: Nickel complex; 2-(2-Benzimidazolyl)-6-(1-aryliminoethyl)pyridyl; Norbornene; Vinyl polymerization of norbornene. INTRODUCTION Vinyl polymerization of norbornene has attracted much attention in recent years [1 3] due to the unique properties of the resultant polynorbornene (PNB) such as thermostablity, optical, dielectric and photoresist properties [4 6]. Chinese chemists working on nickel complexes [7 10] found that sometimes those polynorbornenes could be totally solvable and therefore their molecular weights could be measured successfully [8 10]. Usually, polymers should be characterized with their molecular weights and distributions for controlling their properties. However, [1, 3] the previous work reported that partly unsolvable polymers were obtained in vinyl polymerization of norbornene. Following these findings, there are numerous reports focusing on new nickel complexes for their vinyl polymerization of norbornene [11 18]. Those motivations have led to the discovery of new nickel complexes as catalysts and the understanding of polymerization mechanism [19] as well as the properties of polynorbornene [20]. However, from the view of chemistry, it is a forever-young topic to explore new catalysts. Recently, new nickel complexes ligated by 2-(2-benzimidazolyl)-6-(1-aryliminoethyl)pyridines were reported for their highly activities towards ethylene reactivity [21, 22]. Extensively, we would like to investigate their activity in norbornene polymerization, and indeed good activities were obtained under suitable reaction conditions. Comparing with the early reports of resultant polynorbornenes with broad molecular weight distributions, the current system provides more desirable polynorbornenes with narrow molecular weight distributions. Herein the detailed results are reported with discussing the effects of reaction conditions and ligand environments on the catalytic activities and polymer properties. * Corresponding author: Wei-wei Zuo ( 左伟伟 ), zuoweiwei@iccas.ac.cn Oral lecture presented at the Asian Polyolefin Workshop 2007, 2007, Hangzhou, China Received February 27, 2008; Revised April 9, 2008; Accepted April 11, 2008

2 562 Q.S. Shi and W.W. Zuo EXPERIMENTAL Materials Toluene was dried over sodium and distilled under nitrogen. Methylaluminoxane (MAO) was purchased from Albemarle as a 1.46 mol/l solution in toluene and used as received. [2-(2-Benzimidazolyl)-6-((1- aryliminoethyl)pyridyl)] nickel chloride complexes (Scheme 1, a f) was synthesized and characterized according to the literatures [21, 22]. Norbornene was purified by distillation over potassium and used as a solution in toluene. All work involving air and moisture-sensitive compounds was carried out under an atmosphere of nitrogen by using standard Schlenk techniques. a b c d e f R Me Me Me R 1 Me Et Me Et Scheme 1 Nickel complexes a f Measurements IR spectra were recorded on a Perkin Elmer system 2000 FT-IR spectrometer. Molecular weights and molecular weight distribution were determined by a Polymer Laboratory GPC PL220 system at 135 C in 1,2,3- trichlorobenzene with narrow polystyrene as the distribution standards. Typical Polymerization Procedure In a typical procedure, the complex a (5 μmol) was dissolved in a Schlenk tube in ml of dry, degassed toluene under nitrogen and 3.40 ml solution of norbornene (29 mol/l) in dry, degassed toluene was added via syringe through a rubber septum. The polymerization was initiated by addition of 1.46 mol/l solution of methylaluminoxane (3.60 ml, 5.00 mmol) in toluene (MAO) via syringe. Thirty minutes later, the polymerization was terminated by pouring into 200 ml acidic methanol (methanol:hcl conc. = 95:5). The polymers was isolated by filtration, washed with methanol, and dried in vacuum at 100 C until its weight was constant. Unless otherwise stated, the total reaction volume of vinyl polymerization of norbornene was kept at 20 ml. This was achieved by adding different amount of toluene if necessary. RESULTS AND DISCUSSION Characterization of PNB The active species were produced in situ in toluene solution by activating the nickel complexes with cocatalyst methylaluminoxane (MAO) in the presence of norbornene. All the polymers obtained have been characterized by IR spectroscopy. The absence of the peaks of cm 1 corresponding to C=C double bond in IR spectra, could ensure that the polymerization took place exclusively in a vinyl addition manner rather than the ring-opening metathesis polymerization style [9, 23]. The resultant polymers were soluble in chlorobenzene at room temperature. No indication of stereoregularity was observed as revealed by the amorphous morphology of the products. Polymerization Activities of Different Nickel Complexes To compare the effect of the ligands environment on the catalytic activity, norbornene polymerization with

3 Vinyl Polymerization of Norbornene Catalyzed by Nickel Catalysts 563 nickel analogue a f/mao systems were investigated. The results on catalytic polymerization are presented in Table 1. Variation of the R 1 group at the ortho-positions of the imino-n aryl ring resulted in strong influence on the catalytic activities. It was observed that the increase of steric hindrance of the R 1 group led to decreased activity (Entries 1 3 and 4 6, Table 1). The complexes a and d which contain less bulky substituents methyl groups showed better activity for norbornene polymerization. The reason may be that less bulky substituents has provided more open space around the metal center, which would facilitate the coordination and insertion of the norbornene monomer and therefore, result in higher catalytic activity. Table 1. Activities of complexes a f for the polymerization of norbornene Entry Complex Activity (g PNB/(mol Ni h) ) M w ( 10 5 ) M w /M n 1 a b c d e f Condition: 5 μmol cat., 18 C, 30 min, toluene, total volume 20 ml; x(m)/x(ni) = 20000; x(al)/x(ni) = 1000 Changing R substituent does not show clear influences on activity and polymer properties (Entries 1 3 versus 4 6, Table 1). This is contrast to the observation in ethylene polymerization [21, 22], where the isopropylcontaining complexes usually show lower activities. Effects of Polymerization Conditions on the Catalytic Activity of Complex a To systematically investigate the effects of reaction parameters on vinyl-polymerization of norbornene, the catalyst precursor a/mao was typically investigated via the variation of the reaction parameters such as the Al/Ni ratio, monomer/catalyst ratio (M/Ni) and reaction volume. The results are shown in Tables 2 4. The obtained polymer molecular weight and molecular weight distribution, as well as the catalyst activity depended on the reaction parameters. Table 2. Influence of the MAO amount on the activity (5μmol a) Entry Al/Ni Activity (g PNB/(mol Ni h)) M w ( 10 5 ) M w /M n Condition: 18 C, 30 min, toluene, total volume 20 ml; x(m)/x(ni) = Table 3. Influence of the norbornene concentration on the activity (5μmol a) Entry M/Ni Activity (g PNB/(mol Ni h) ) M w ( 10 5 ) M w /M n Condition: 18ºC, toluene, Total volume 20 ml, 30 min; x(al)/x(ni) = 1000 Table 4. Influence of the reaction volume (5 μmol a) Entry V (ml) Activity (g PNB/(mol Ni h) ) Conditions: 18 C, Toluene, 30 min; Al/Ni = 1000; x(m)/x(ni) = 10000

4 564 Q.S. Shi and W.W. Zuo MAO used as cocatalyst was essential for the polymerization of norbornene catalyzed by [2-(2- benzimidazolyl)-6-((1-aryliminoethyl)pyridyl)]nickel chloride. The role of MAO is to form the active sites and probably create an empty site for the coordination and insertion of the monomers. Variation of the molar ratio of MAO/Cat (Al/Ni ratio) showed considerable effects on catalytic activity (Table 2). The optimized Al/Ni molar ratio for the activity was 1000, and the activity decreased for both lower and higher molar ratios. The catalyst activities were in the range of g PNB/(mol Ni h) to g PNB/(mol Ni h), and the molecular weights were in the range of to Noteworthy, the molecular weight distributions ( ) of the resultant polymers were much narrower than our previously observed ones in nickel systems [8 10]. Increase in the monomer concentration (reaction volume and catalyst amount were kept constant), which serve as the increase in the norbornene/ni ratio, was found to be very significant for the catalytic activity as well as for the molecular weight and molecular weight distribution of resultant polynorbornene (Table 3). The high activity of g PNB/(mol Ni h) and the M w value of were obtained when the polymerization was performed at the M/Ni ratio of 20000:1 (entry 3 in Table 3), whereas the activity of g PNB/(mol Ni h) and the M w value of were obtained at the M/Ni ratio of 5000:1 (entry 1 in Table 3). This observation can be easily understood, just like in the regular olefin polymerization, where higher concentration of monomer gives higher catalytic activity and high-order polyolefins. The molecular weight distributions of the resultant polymers increased slightly with increasing monomer amount, which may be due to more frequent chain transfer and termination processes. Similar phenomena were also observed in our previous nickel catalytic systems [8 10]. Thus regulating the M/Ni ratio provides an efficient way to control the M w and M w /M n of polynorbornene. Changing the total volume of the catalytic system alters the concentrations of both monomer and catalyst. Table 4 listed the result of enlarging the reaction volume through adding more toluene. The catalytic activities significantly decreased with increasing the reaction volume due to decrease in concentrations of both monomer and catalyst. CONCLUSIONS In summary, the nickel catalytic systems of a f/mao exhibited good activities in the vinyl polymerization of norbornene. Their catalytic activities, molecular weight and molecular weight distributions of the resultant polymers can be controlled by variation of the ligand and reaction parameters. The catalytic activity increased with the decrease of bulky substituents at the ortho-positions of the imino-n aryl ring, while the substituent on the benzimidazolyl group show slightly influence on the catalytic behaviors. Suitable amount of cocatalyst was required to observe good activities. Higher monomer concentration led to increased catalytic activity and higher molecular weights. Enlarging the volume of the reaction volume clearly decrease the catalytic activity. All the obtained PNBs are soluble in chlorobenzene at room temperature and showed similar IR spectra. ACKNOWLEDGEMENT We are grateful to Prof. Wen-Hua Sun (Institute of Chemistry, Chinese Academy of Sciences) for his kind suggestion and discussions. REFERENCES 1 Good, B.L., Late Transition Metal Polymerization Catalysts, ed. by Rieger, B., Baugh, L.S., Kacker, S. and Striegler, S., Wiley-VCH, Weinheim, 2003, p Shi, Q.S., Jie, S.Y., Zhang, S., Yang, H.J. and Sun, W.H., Macromol. Symp., 2007, 260: 74 3 Janiak, C. and Lassahn, P.G., J. Mol. Catal. A: Chem., 2001, 166: Grove, N.R., Kohl, P.A., Allen, S.A.B., Jayaraman, S. and Shick, R., J. Polym. Sci., Part B: Polym. Phys., 1999, 37: 3003

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