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boron trifluoride etherate system
Applied Catalysis A: General 275 (2004) 271–277 www.elsevier.com/locate/apcata
Polymerization of norbornene in the presence of ethene over tetrakis (triphenylphosphine)nickel/boron trifluoride etherate system G. Myagmarsuren, O. Yong Jeong1, Son-Ki Ihm* National Research Laboratory for Environmental Catalysis, Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea Received 25 March 2004; received in revised form 9 July 2004; accepted 24 July 2004 Available online 11 September 2004
Abstract The polymerization of norbornene in the presence of ethene using the Ni(PPh3)4/BF3OEt2 catalyst system has been investigated. This Ni(PPh3)4/BF3OEt2 system is highly active for the polymerization of norbornene in the presence of ethene. The products of the reaction are norbornene oligomers with a vinylic end group, having an average polymerization degree of 9. These products are of interest as polymerizable macromonomers with high glass transition phase in thermoplastic elastomers. Ethene decreases the cocatalyst amount necessary for substantial catalytic activity compared to that needed for homopolymerization of norbornene. The nickel hydrides are likely to be catalytically active species. Ethene transformation may serve as a testing reaction for the copolymerization of norbornene with ethene in some cases. Products were characterized by NMR, IR, mass spectroscopy, GPC and GC methods. # 2004 Elsevier B.V. All rights reserved. Keywords: Boron trifluoride; Ethene; Nickel; Norbornene; Polymerization
1. Introduction The norbornene addition polymer with 2,3-enchainity displays a characteristic rigid random coil conformation, shows restricted rotation about the main chain, and exhibits strong thermal stability, excellent dielectric properties, optical transparency and unusual transport properties [1,2]. Therefore, this norbornene addition polymer and its derivatives are attractive materials for the manufacture of microelectronic and optical devices. However, norbornene addition polymers have some disadvantages, such as poor processability and lower solubility in common organic solvents. One way to improve the processability of polynorbornene is copolymerization of norbornene with ethene. Because metallocene catalyzed addition copolymer* Corresponding author. Tel.: +82 42 869 3915; fax: +82 42 869 5955. E-mail address: [email protected] (S.-K. Ihm). 1 Present address: Samsung Cheil Industries Inc., 332-2 Gochun-dong, Euiwang-shi, Gyoungki-do 437-711, Republic of Korea. 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.040
ization of cyclic olefins with ethene provides more options for tailormaking the properties of the copolymers, this field was investigated and patented by Mitsui Sekka and Hoechst. These two companies then started to work together on metallocene catalyzed cycloolefin copolymers (COC), with norbornene–ethene copolymers as their first target. These norbornene–ethylene copolymers were recently introduced on the market under the tradenames APEL and TOPAS [3]. The metallocene catalysts are known to possess single active species and therefore to give polyolefins with narrow distributions of molecular weight and chemical composition. Little is known about the copolymerization of norbornene and ethene using central and late transition metal catalysts. Heitz and coworkers [4,5] found that the homogeneous chromium(III)-based catalysts of the type [Cp*CrMeCl]2/MAO with different kinds of Cp ligands were active in copolymerization of norbornene with ethene, while bis(benzoylacetonate)cobalt(II), Co(PhC4H4O2)2, and bis(trifluoroacetylacetonate)cobalt(II), Co(C5F7O2)2, in combination with methylalumoxane (MAO) gave low
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molecular weight norbornene macromonomers terminated with a vinylic end group. Using [(CH3CN)4Pd][BF4]2 as catalyst, Haselwander et al. [6] found that the vinylic polymerization of norbornene in nitromethane in the presence of ethene results in polynorbornene with a narrow molecular weight distribution. In this case no incorporation of ethene could be detected and no chain termination or chain transfer reactions were observed. Goodall et al. [7] reported that the polymerization of norbornene in 1,2dichloroethane in the presence of a-olefins with [(h3crotyl)(cycloocta-1,5-diene)]nickel hexafluorophosphate, [1-CH3C3H5Ni(COD)]PF6, as catalyst yields polymers with an olefinic double bond as terminal group resulting from chain termination and with chain transfer due to b-hydride elimination. This reaction allows one to control the molecular weight of polynorbornene in a relatively narrow range. In the case of norbornene-type polymers, if the chain of addition polymerized cycloolefin addition units is not so long, the vinylic end group affords a polymerizable macromonomer or oligomer having from about 4 to 50 (preferably from 4 to 20) norbornene-type repeating units. We have recently shown that boron trifluoride etherate, BF3OEt2, might be successfully used as a cocatalyst towards tetrakis(triphenylphosphine)nickel, Ni(PPh3)4, for the polymerization of norbornene [8]. In this work, our interest focuses on polymerization of norbornene in the presence of ethene using the Ni(PPh3)4/BF3OEt2 catalyst system.
2. Experimental All manipulations of air sensitive compounds were carried out under a stream of dry nitrogen using standard inert techniques. 2.1. Materials Ni(Acac)2 (95%), PPh3 (99%), TEA (93%) and norbornene (99%) were supplied by Aldrich. BF3OEt2 (Aldrich, 99%) was freshly distilled over calcium hydride prior to use. Toluene, hexane and diethyl ether were purified according to standard procedures. Ni(PPh3)4 was synthesized by the procedure described by Wilke et al. [9]. 2.2. Polymerization Polymerizations were carried out in a 50 ml glass reactor equipped with a magnetic stirrer. The reactor was first purged in vacuum and filled with ethene and then was filled with norbornene as a solution in toluene. The solution was kept at each desired temperature for 15 min and then the toluene solution of the Ni(PPh3)4 was added. Polymerizations were initiated by the injection of boron compound. After stirring for a time needed, the polymers that formed were precipitated in acidified ethanol, separated by filtration, washed with an excess amount of ethanol and dried in
vacuum. In standard runs the amount of nickel precursor was 1.25 105 mol, the NB:Ni ratio was 8940, the norbornene feed was 60 mol%, the ethene pressure was 1 atm, the reaction time was 30 min and the total reaction volume was 20 ml, unless mentioned otherwise. 2.3. Characterization of polymers NMR spectra were recorded at 120 8C on a Bruker AMX-500 spectrometer with frequencies of 500 MHz for 1 H NMR and 125 MHz for 13C NMR. Each polymer sample was dissolved in toluene-d8 up to a concentration of 10 wt.% in NMR tubes (5 mm o.d.). TMS (tetramethyl silane) was used as the internal standard. The IR spectra were recorded using a KBr pellet technique with a Nicolet Fourier transform infrared (FT-IR) spectrometer. Gas chromatography (GC) analysis were performed on a HP 6890 instrument equipped with a HP-1 column and FID. Gel permeation chromatography (GPC) analysis were carried out on a PL-GPC 210 instrument with 10 mm MIXED-B columns (300 mm 7.5 mm) using 1,2,4-trichlorobenzene solvent at 145 8C and polystyrene standard. Mass spectra (MS) were recorded on a VCT Autospec Ultima instrument with GI 70 eV and DIP.
3. Results and discussion Ni(PPh3)4 and BF3OEt2 are separately not active either for norbornene or for ethene transformation. Thus only the combination of Ni(PPh3)4 with BF3OEt2 generates an active catalyst, which led to polynorbornene with an olefinic double bond as terminal group resulting from chain termination and chain transfer by b-hydride elimination. The 13C NMR spectrum of polynorbornene in Fig. 1a presents four groups of resonances in the region from 25 to 55 ppm. The peaks in the region of 25–34 ppm are nonbridging CH2 groups (carbons 5 and 6). The resonances present between 34 and 36.5 ppm represent a bridge CH2 group (carbon 7). The resonances between 37 and 42 ppm are bridgehead CH groups (carbons 1 and 4) and the resonance peaks between 45 and 55 ppm are backbones connecting CH groups (carbons 2 and 3). In the 13C NMR spectrum, the vinylic end group appears with a methyne resonance at 142.0 ppm and a methylene signal at 114.2 ppm in the intensity ratio 1:1. The 1H NMR spectrum of polynorbornene (Fig. 1b) exhibits a broad multiplet at 0.8–2.8 ppm (maxima at 1.1, 1.5 and 2.3 ppm) for the cycloaliphatic hydrogen atoms and multiplets at 4.9 and 5.8 ppm. One may assign the multiplet at 4.9 ppm to the =CH2 (methylene) unit and the multiplet at 5.7–5.8 to the –CH= (methyne) unit. The intensity ratio =CH2:–CH= was found to be 2:1. The comparison of the IR spectra of the products and polynorbornene obtained in the absence of the ethene allows one to identify the additional vibrations of the vinylic end group at 1636, 995 and 908 cm1 due to stretching and
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Table 1 Molecular weights of macromonomers obtained over Ni(PPh3)4/BF3OEt2 catalyst system No.
Pn (NMR)
Mn (NMR
Mn (GPC)
Mw (GPC)
Mw/Mn
1 2 3 4
9 13 15 17
847 1224 1412 1600
620 1490 1640 1830
1290 4950 8330 7580
2.08 3.32 5.08 4.14
aliphatic and methylene hydrogen and carbon atoms signal intensities: Pn ¼
IH2 =IH1 0:5 5
(1a)
Pn ¼
IC2 7IC1
(1b)
Mn ¼ ðPn 94:16 þ 27:05Þ
Fig. 1. NMR spectra of polynorbornene:
13
C NMR (a) and 1H NMR (b).
wagging vibrations. These data indicate that the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system occurred without ring opening and via a vinyl-type polymerization giving exclusively 2,3-enchained repeating units, where the chain termination occurs by b-hydride elimination after insertion of ethene into metal–carbon bond of growing chain and where the active species is likely to be [Ni]–H. Signals attributable to the vinylic end group in 1H and 13C NMR spectra are well resolved as a result of the low molecular weight of the products. For this reason, they can be successfully used for the determination of the average degree of polymerization (Pn) and the number-average molecular weight (Mn) of the polynorbornenes from the intensity ratios of the resonances of the cycloaliphatic and vinylic hydrogen or carbon atoms according to Eqs. (1) and (2) [5]. In these equations I1H, I2H and I1C, I2C denote cyclo-
(2)
The number-average molecular weight of macromonomers, calculated by means of the intensity ratio, has been confirmed by GPC measurements (Table 1). For the catalyst system Ni(PPh3)4/BF3OEt2, the yield and average polymerization degree of polynorbornene with a terminal vinyl group, as well as the catalyst activity, depend significantly on the reaction parameters applied to the polymerization. To investigate the effect of the amount of cocatalyst, a set of polymerizations was carried out with the B/Ni ratios from 50 to 500 at 25 8C. Results are presented in Table 2. The product yield showed the maximum value at B/Ni = 400. The average polymerization degree increased from Pn = 9 at B/ Ni = 50 to Pn = 17 at B/Ni = 500. Similar curves of activity versus cocatalyst/metal ratio for norbornene homopolymerization were reported in the literature [10,11] and the increase of activity with increasing cocatalyst/metal ratio was explained by an equilibrium formation of the active complex from inactive precatalyst and cocatalyst [10], i.e. by an increasing number of active species [11]. The interaction of Ni(PPh3)4 with BF3OEt2 in toluene at ratios B/ Ni = 2–160 resulted in the oxidation of Ni(0) to Ni(I) and the subsequent elimination of organophosphorus ligands from the coordination sphere of the transition metal until the formation of colloidal nickel [12].
Table 2 Effect of cocatalyst/nickel ratio on the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system B/Ni ratio
NB/Ni ratio
Catalyst (105 mol)
Time (h)
Yield (g)
Activity (kg NB/(mol Nih))
Pn
50 100 200 300 400 500
8940 8940 8940 8940 8940 8940
1.25 1.25 1.25 1.25 1.25 1.25
0.5 0.5 0.5 0.5 0.5 0.5
3.41 5.92 8.10 9.10 9.20 9.15
545.6 947.2 1296.0 1456.0 1472.0 1464.0
9 13 15 15 16 17
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Table 3 Effect of reaction temperature on the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system Temperature (8C)
NB/Ni ratio
Catalyst (105 mol)
Time (h)
Yield (g)
Activity (kg NB/(mol Nih))
Pn
25 35 45 55 65 75
8940 8940 8940 8940 8940 8940
1.25 1.25 1.25 1.25 1.25 1.25
0.5 0.5 0.5 0.5 0.5 0.5
9.20 10.44 10.73 10.61 10.50 9.16
1472.0 1670.4 1716.8 1697.4 1680.0 1465.5
16 9 9 9 9 9
Experimental conditions: B/Ni = 400, T = 25 8C, 10.52 g of NB, toluene, PEt = 1 atm, total volume 20 ml.
BF3 is known as a phosphine scavenger [13] and the elimination of organophosphorus ligands from the coordination sphere of nickel with BF3 is an equilibrium process. Therefore, the results obtained may be explained by both the formation of an increasing number of active species through the elimination of the organophosphorous ligands from the coordination sphere of nickel and the increased ionic character of active species at higher B/Ni ratios. The increased ionic character of active species resulted in the increased chain propagation rate, i.e. in increased average polymerization degree (molecular weight). This is in agreement with data reported in [10]. At the same time, it is worth noting that the substantial catalytic activity might be achieved at lower B/Ni ratios of 200 compared to that for the polymerization of norbornene in the absence of ethene, where a remarkable activity had been recorded only at B/Ni = 400 [8]. This contrast indicates that ethene not only acts as a chain transfer agent, but also shows a promoting effect in the polymerization of norbornene. This promoting effect of ethene is a subject for special study. A series of polymerization runs were performed at different temperatures between 25 and 75 8C at a B/Ni = 400 ratio. As one may see from Table 3, the polymer yield is not substantially influenced by increasing temperature and reached a shallow maximum at 45 8C. Here, again the promoting effect of ethene is noteworthy. For the homopolymerization of norbornene in the absence of ethane, the temperature dependence of catalyst activity is significant and shows a maximum at 65 8C [8]. At the same time, a strong drop of average polymerization degree from Pn = 16 at 25 8C to Pn = 9 was observed starting from 35 8C. The probable explanation for this phenomenon is that, at temperatures higher than 25 8C, insertion of ethene into the Ni–C bond of active species becomes more favorable
than that of norbornene unit and a subsequent b-hydride elimination leads to chain termination. The effects of reaction time on the conversion, activity and intrinsic viscosity are summarized in Fig. 1 and Table 4. Polymer yield increases with time; two distinct periods can be observed in the polymerization process. In the first period, about 5–10 min, the polymer yield increased rapidly. In the second period, the polymer yield increased very slightly and then leveled off, due to a significant decrease of the monomer content in the reaction mixture. An almost quantitative conversion was reached after 30 min (Fig. 2). At the reaction time of 5 min, the activity was 6280 kg NB/(mol Nih). A more detailed study is desirable to optimize the effect of reaction parameters on the yield of polymers. The average polymerization degree was independent of reaction time and remained equal to 9. The variation of norbornene feed from 10 to 60 mol% at 65 8C and B/Ni = 400 revealed increased product yield, but only the average polymerization degree Pn = 9 was observed (Table 5). At higher norbornene feeds of 80 mol%, no ethene insertion has been detected by NMR. Runs at different ethene pressures at 65 8C and B/Ni = 400 showed that the increase of ethene pressure up to 5 atm led to decreased product yield, while the average polymerization degree remained unchanged at Pn = 9 (Table 6). The formation of lower molecular mass oligomers was also possible. It was demonstrated for the standard polymerization run carried out at B/Ni = 400 and 65 8C for 30 min. The polymers that formed were precipitated and separated by filtration. After evaporation of the solvent and unreacted norbornene from the mother liquor in vacuum, a viscous product (approximately 0.1 g) was obtained. The 1H NMR spectrum of this product showed Pn = 5. The directinjection mass-spectrum of this product registered in the m/z
Table 4 Effect of reaction time on the polymerization of norbornene over Ni(PPh3)4/BF3OEt2 catalyst system Time (min)
NB/Ni ration
Catalyst (105 mol)
Yield (g)
Conversion (%)a
Activity (kg NB/(mol Nih))
Pn
5 10 15 20 30
8940 8940 8940 8940 8940
1.25 1.25 1.25 1.25 1.25
6.54 8.41 9.57 10.20 10.50
60.2 77.5 88.2 94.0 96.8
6280 4037 3062 2448 1680
9 9 9 9 9
a
The conversion was calculated relative to 10.85 g of oligomer with Pn expected from 10.52 g NB. Experimental conditions: B/Ni = 400, T = 65 8C.
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Fig. 2. Effect of reaction time on the polymerization of norbornene over Ni(PPh3)4/BF3OEt2 catalyst system (B:Ni = 400, 65 8C, PEt = 1 atm, 10.52 g of NB, total volume 20 ml).
Table 5 Effect of monomer/nickel ratio on the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system NB feed (mol%)
NB/Ni ratio
Catalyst (105 mol)
Time (h)
Yield (g)
Activity (kg NB/(mol Nih))
Pn
10 20 40 60 80
1490 2980 5960 8940 11920
1.25 1.25 1.25 1.25 1.25
0.5 0.5 0.5 0.5 0.5
0.20 0.89 5.44 10.50 12.65
32.0 142.4 870.4 1680.0 2024.0
9 9 9 9 –
Experimental conditions: B/Ni = 400, T = 65 8C, 10.52 g of NB.
range up to 600 exhibits, among others, peaks at m/z 499 and 471, 405 and 377, 311 and 280, 216 and 190 with a base peak at m/z 95. The base peak at m/z 95 arises from the norbornene cation (Fig. 3). Peaks at m/z 499 and 471, 405 and 377, 311 and 280, 216 and 190 represent macromonomers with Pn = 5, 4, 3, and 2 and their fragmentation by loss of ethane, respectively. It should be noted that the amount of products in the mother liquor is substantially small in respect to the filter-separated product. Broad molecular weight distributions, indicated in Table 1, also might be explained by the formation of a mixture of oligomers with divergent Pn values. It is well known that metallocenes, in combination with MAO, are extremely active catalysts for the polymerization of ethene [14]. Transformation of ethene in the presence of Ni(PPh3)4/BF3OEt2 catalyst system at standard run condi-
tions led to its dimerization; the product consisted of 48.11% trans-2-butene, 27.19% cis-2-butene, and 24.7 1-butene. In fact, Sen et al. [15] showed that the cationic complex [(CH3CN)4Pd][BF4]2 catalyzes the oligomerization of ethene to give oligomers with Pn = 2–5. To the best of our knowledge, there is no data in the literature regarding ethene transformation by means of the cationic complex [1CH3C3H5Ni(COD)]PF6 [7], nor about any catalyst systems based on Co(II) compounds and MAO [5]. However, the cationic allylnickel(II) complexes, [2-RC3H4Ni(COD)]PF6, are valuable catalysts for the oligomerization of ethene [16,17]. When ethene is copolymerized with norbornene, all these catalysts lead to macromonomers with a vinylic end group due to b-hydride elimination after insertion of an ethene monomer unit. On the other hand, [Cp*CrMeCl]2/ MAO systems afforded a highly linear, ultra-high molecular
Table 6 Effect of ethene pressure on the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system Ethene pressure (atm)
NB/Ni ratio
Catalyst (105 mol)
Time (h)
Yield (g)
Activity (kg NB/(mol Nih))
Pn
1 2 3 4 5
8940 8940 8940 8940 8940
1.25 1.25 1.25 1.25 1.25
0.5 0.5 0.5 0.5 0.5
10.50 5.85 3.36 1.20 0.39
1680.0 936.0 537.0 192.0 62.4
9 9 9 9 9
Experimental conditions: B/Ni = 400, T = 65 8C, 10.52 g of NB.
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Fig. 3. MS spectrum of the product separated from mother liquor.
weight polyethylene [4]. These Cr(III)-based catalyst systems are active in the copolymerization of norbornene with ethene to give copolymers with a high a-olefin content. On the strength of these data, one might suggest that the transformation of ethene by means of a certain catalyst may serve as a testing reaction for the copolymerization of norbornene with ethene: if the catalyst leads to low molecular oligomers of ethene, the product of copolymerization is a macromonomer with a vinylic end group; if the catalyst is active in the polymerization of ethene, one can expect a copolymer of norbornene and ethene. In the experiments with Ni(PPh3)4/BF3OEt2 catalyst system, the product of intermolecular interaction between BF3 and the trace amounts of water in solvent can serve as a proton source for the initiation of the catalytic cycle. Then the proton concentration should depend on the equilibrium concentration of the ionic form of a molecular complex BF3H2O, Eq. (3): BF3 OEt2 þ H2 O ! BF3 H2 O þ OEt2 ! OEt2 þ Hþ ½BF3 OH
(3)
This assumption is in agreement with the data on the reaction between BF3OEt2 and trace amounts of water in deuterotoluene. The metathesis reaction between the molecular complex BF3H2O and its ionic form H+[BF3OH] produces H+, as is evident from the appearance of a broad signal at 12.0 ppm. The oxidation state of nickel in hydride species cannot be identified at present. However, zerovalent Ni(PPh3)4
is inactive and one-electron oxidation of initial Ni(0) complex to Ni(I) with BF3 (electron transfer) seems to be faster than oxidative addition of Brønsted acid to Ni(0) centers.
4. Conclusion The system Ni(PPh3)4/BF3OEt2 is highly active for the polymerization of norbornene in the presence of ethene. Catalyst activity up to 6250 kg/(mol Nih) has been obtained. The products of polymerization are norbornene oligomers with a vinylic end group, having an average polymerization degree of 9. These products are of interest as polymerizable macromonomers with high glass transition phase in thermoplastic elastomers. The nickel hydrides are likely to be catalytically active species. Ethene showed a promoting effect to decrease the cocatalyst amount necessary for substantial catalytic activity compared to homopolymerization of norbornene. The ethene transformation may serve as a testing reaction for the copolymerization of norbornene with ethene in some cases.
Acknowledgements One of the authors, G. Myamarsuren, gratefully acknowledges the KOFST for the International Scholar Exchange Fellowship. This work was also supported by the National Research Laboratory Project and the BK 21 Project.
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References [1] T.F.A. Haselwander, W. Heitz, S.A. Krugel, J.H. Wendorff, Macromol. Chem. Phys. 197 (1996) 3435. [2] T.F.A. Haselwander, W. Heitz, S.A. Krugel, J.H. Wendorff, Macromolecules 30 (1997) 5345. [3] Asian Chem. News 30 (1995) 23. [4] U. Peukert, W. Heitz, Macromol. Rapid Commun. 19 (1998) 159. [5] F.P. Alt, W. Heitz, Acta Polym. 49 (1998) 477. [6] T.F.A. Haselwander, W. Heitz, M. Maskos, Macromol. Rapid Commun. 18 (1997) 689. [7] L. Goodall, G.M. Benedikt, L.H. Mcintosh III, D.A. Barnes, US Patent 5468819 (1995), to B.F. Goodrich Co., USA. [8] G. Myagmarsuren, O.-Y. Jeong, S.-K. Ihm, Appl. Catal. A: Gen. 255 (2003) 203.
277
[9] G. Wilke, E.W. Muller, M. Kroner, Angew. Chem. 73 (1961) 33. [10] C. Janiak, P.G. Lassahn, Polym. Bull. 47 (2002) 539. [11] C. Zhao, M.R. Ribeiro, M.F. Portela, S. Pereira, T. Nunes, Eur. Polym. J. 37 (2001) 45. [12] V.S. Tkach, V.A. Gruznykh, G. Myagmarsuren, L.B. Belykh, V.V. Saraev, F.K. Shmidt, Russ. J. Coord. Chem. 20 (1994) 584–587. [13] H.S. Booth, D.R. Martin, Boron Trifluoride and its Derivatives, Wiley, New York, 1946. [14] W. Kaminsky, Catal. Today 62 (2000) 23. [15] A. Sen, T.-W. Lai, R.R. Thomas, J. Organomet. Chem. 358 (1988) 567. [16] J.P. Gehrke, R. Taube, E. Balbolov, K. Kurtev, J. Organomet. Chem. 304 (1986) 4. [17] J.J. Brunet, A. Sivade, I. Tkatchenko, J. Mol. Catal. 50 (1989) 291.
Polymerization of norbornene in the presence of ethene over tetrakis (triphenylphosphine)nickel/boron trifluoride etherate system G. Myagmarsuren, O. Yong Jeong1, Son-Ki Ihm* National Research Laboratory for Environmental Catalysis, Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea Received 25 March 2004; received in revised form 9 July 2004; accepted 24 July 2004 Available online 11 September 2004
Abstract The polymerization of norbornene in the presence of ethene using the Ni(PPh3)4/BF3OEt2 catalyst system has been investigated. This Ni(PPh3)4/BF3OEt2 system is highly active for the polymerization of norbornene in the presence of ethene. The products of the reaction are norbornene oligomers with a vinylic end group, having an average polymerization degree of 9. These products are of interest as polymerizable macromonomers with high glass transition phase in thermoplastic elastomers. Ethene decreases the cocatalyst amount necessary for substantial catalytic activity compared to that needed for homopolymerization of norbornene. The nickel hydrides are likely to be catalytically active species. Ethene transformation may serve as a testing reaction for the copolymerization of norbornene with ethene in some cases. Products were characterized by NMR, IR, mass spectroscopy, GPC and GC methods. # 2004 Elsevier B.V. All rights reserved. Keywords: Boron trifluoride; Ethene; Nickel; Norbornene; Polymerization
1. Introduction The norbornene addition polymer with 2,3-enchainity displays a characteristic rigid random coil conformation, shows restricted rotation about the main chain, and exhibits strong thermal stability, excellent dielectric properties, optical transparency and unusual transport properties [1,2]. Therefore, this norbornene addition polymer and its derivatives are attractive materials for the manufacture of microelectronic and optical devices. However, norbornene addition polymers have some disadvantages, such as poor processability and lower solubility in common organic solvents. One way to improve the processability of polynorbornene is copolymerization of norbornene with ethene. Because metallocene catalyzed addition copolymer* Corresponding author. Tel.: +82 42 869 3915; fax: +82 42 869 5955. E-mail address: [email protected] (S.-K. Ihm). 1 Present address: Samsung Cheil Industries Inc., 332-2 Gochun-dong, Euiwang-shi, Gyoungki-do 437-711, Republic of Korea. 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.040
ization of cyclic olefins with ethene provides more options for tailormaking the properties of the copolymers, this field was investigated and patented by Mitsui Sekka and Hoechst. These two companies then started to work together on metallocene catalyzed cycloolefin copolymers (COC), with norbornene–ethene copolymers as their first target. These norbornene–ethylene copolymers were recently introduced on the market under the tradenames APEL and TOPAS [3]. The metallocene catalysts are known to possess single active species and therefore to give polyolefins with narrow distributions of molecular weight and chemical composition. Little is known about the copolymerization of norbornene and ethene using central and late transition metal catalysts. Heitz and coworkers [4,5] found that the homogeneous chromium(III)-based catalysts of the type [Cp*CrMeCl]2/MAO with different kinds of Cp ligands were active in copolymerization of norbornene with ethene, while bis(benzoylacetonate)cobalt(II), Co(PhC4H4O2)2, and bis(trifluoroacetylacetonate)cobalt(II), Co(C5F7O2)2, in combination with methylalumoxane (MAO) gave low
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molecular weight norbornene macromonomers terminated with a vinylic end group. Using [(CH3CN)4Pd][BF4]2 as catalyst, Haselwander et al. [6] found that the vinylic polymerization of norbornene in nitromethane in the presence of ethene results in polynorbornene with a narrow molecular weight distribution. In this case no incorporation of ethene could be detected and no chain termination or chain transfer reactions were observed. Goodall et al. [7] reported that the polymerization of norbornene in 1,2dichloroethane in the presence of a-olefins with [(h3crotyl)(cycloocta-1,5-diene)]nickel hexafluorophosphate, [1-CH3C3H5Ni(COD)]PF6, as catalyst yields polymers with an olefinic double bond as terminal group resulting from chain termination and with chain transfer due to b-hydride elimination. This reaction allows one to control the molecular weight of polynorbornene in a relatively narrow range. In the case of norbornene-type polymers, if the chain of addition polymerized cycloolefin addition units is not so long, the vinylic end group affords a polymerizable macromonomer or oligomer having from about 4 to 50 (preferably from 4 to 20) norbornene-type repeating units. We have recently shown that boron trifluoride etherate, BF3OEt2, might be successfully used as a cocatalyst towards tetrakis(triphenylphosphine)nickel, Ni(PPh3)4, for the polymerization of norbornene [8]. In this work, our interest focuses on polymerization of norbornene in the presence of ethene using the Ni(PPh3)4/BF3OEt2 catalyst system.
2. Experimental All manipulations of air sensitive compounds were carried out under a stream of dry nitrogen using standard inert techniques. 2.1. Materials Ni(Acac)2 (95%), PPh3 (99%), TEA (93%) and norbornene (99%) were supplied by Aldrich. BF3OEt2 (Aldrich, 99%) was freshly distilled over calcium hydride prior to use. Toluene, hexane and diethyl ether were purified according to standard procedures. Ni(PPh3)4 was synthesized by the procedure described by Wilke et al. [9]. 2.2. Polymerization Polymerizations were carried out in a 50 ml glass reactor equipped with a magnetic stirrer. The reactor was first purged in vacuum and filled with ethene and then was filled with norbornene as a solution in toluene. The solution was kept at each desired temperature for 15 min and then the toluene solution of the Ni(PPh3)4 was added. Polymerizations were initiated by the injection of boron compound. After stirring for a time needed, the polymers that formed were precipitated in acidified ethanol, separated by filtration, washed with an excess amount of ethanol and dried in
vacuum. In standard runs the amount of nickel precursor was 1.25 105 mol, the NB:Ni ratio was 8940, the norbornene feed was 60 mol%, the ethene pressure was 1 atm, the reaction time was 30 min and the total reaction volume was 20 ml, unless mentioned otherwise. 2.3. Characterization of polymers NMR spectra were recorded at 120 8C on a Bruker AMX-500 spectrometer with frequencies of 500 MHz for 1 H NMR and 125 MHz for 13C NMR. Each polymer sample was dissolved in toluene-d8 up to a concentration of 10 wt.% in NMR tubes (5 mm o.d.). TMS (tetramethyl silane) was used as the internal standard. The IR spectra were recorded using a KBr pellet technique with a Nicolet Fourier transform infrared (FT-IR) spectrometer. Gas chromatography (GC) analysis were performed on a HP 6890 instrument equipped with a HP-1 column and FID. Gel permeation chromatography (GPC) analysis were carried out on a PL-GPC 210 instrument with 10 mm MIXED-B columns (300 mm 7.5 mm) using 1,2,4-trichlorobenzene solvent at 145 8C and polystyrene standard. Mass spectra (MS) were recorded on a VCT Autospec Ultima instrument with GI 70 eV and DIP.
3. Results and discussion Ni(PPh3)4 and BF3OEt2 are separately not active either for norbornene or for ethene transformation. Thus only the combination of Ni(PPh3)4 with BF3OEt2 generates an active catalyst, which led to polynorbornene with an olefinic double bond as terminal group resulting from chain termination and chain transfer by b-hydride elimination. The 13C NMR spectrum of polynorbornene in Fig. 1a presents four groups of resonances in the region from 25 to 55 ppm. The peaks in the region of 25–34 ppm are nonbridging CH2 groups (carbons 5 and 6). The resonances present between 34 and 36.5 ppm represent a bridge CH2 group (carbon 7). The resonances between 37 and 42 ppm are bridgehead CH groups (carbons 1 and 4) and the resonance peaks between 45 and 55 ppm are backbones connecting CH groups (carbons 2 and 3). In the 13C NMR spectrum, the vinylic end group appears with a methyne resonance at 142.0 ppm and a methylene signal at 114.2 ppm in the intensity ratio 1:1. The 1H NMR spectrum of polynorbornene (Fig. 1b) exhibits a broad multiplet at 0.8–2.8 ppm (maxima at 1.1, 1.5 and 2.3 ppm) for the cycloaliphatic hydrogen atoms and multiplets at 4.9 and 5.8 ppm. One may assign the multiplet at 4.9 ppm to the =CH2 (methylene) unit and the multiplet at 5.7–5.8 to the –CH= (methyne) unit. The intensity ratio =CH2:–CH= was found to be 2:1. The comparison of the IR spectra of the products and polynorbornene obtained in the absence of the ethene allows one to identify the additional vibrations of the vinylic end group at 1636, 995 and 908 cm1 due to stretching and
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Table 1 Molecular weights of macromonomers obtained over Ni(PPh3)4/BF3OEt2 catalyst system No.
Pn (NMR)
Mn (NMR
Mn (GPC)
Mw (GPC)
Mw/Mn
1 2 3 4
9 13 15 17
847 1224 1412 1600
620 1490 1640 1830
1290 4950 8330 7580
2.08 3.32 5.08 4.14
aliphatic and methylene hydrogen and carbon atoms signal intensities: Pn ¼
IH2 =IH1 0:5 5
(1a)
Pn ¼
IC2 7IC1
(1b)
Mn ¼ ðPn 94:16 þ 27:05Þ
Fig. 1. NMR spectra of polynorbornene:
13
C NMR (a) and 1H NMR (b).
wagging vibrations. These data indicate that the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system occurred without ring opening and via a vinyl-type polymerization giving exclusively 2,3-enchained repeating units, where the chain termination occurs by b-hydride elimination after insertion of ethene into metal–carbon bond of growing chain and where the active species is likely to be [Ni]–H. Signals attributable to the vinylic end group in 1H and 13C NMR spectra are well resolved as a result of the low molecular weight of the products. For this reason, they can be successfully used for the determination of the average degree of polymerization (Pn) and the number-average molecular weight (Mn) of the polynorbornenes from the intensity ratios of the resonances of the cycloaliphatic and vinylic hydrogen or carbon atoms according to Eqs. (1) and (2) [5]. In these equations I1H, I2H and I1C, I2C denote cyclo-
(2)
The number-average molecular weight of macromonomers, calculated by means of the intensity ratio, has been confirmed by GPC measurements (Table 1). For the catalyst system Ni(PPh3)4/BF3OEt2, the yield and average polymerization degree of polynorbornene with a terminal vinyl group, as well as the catalyst activity, depend significantly on the reaction parameters applied to the polymerization. To investigate the effect of the amount of cocatalyst, a set of polymerizations was carried out with the B/Ni ratios from 50 to 500 at 25 8C. Results are presented in Table 2. The product yield showed the maximum value at B/Ni = 400. The average polymerization degree increased from Pn = 9 at B/ Ni = 50 to Pn = 17 at B/Ni = 500. Similar curves of activity versus cocatalyst/metal ratio for norbornene homopolymerization were reported in the literature [10,11] and the increase of activity with increasing cocatalyst/metal ratio was explained by an equilibrium formation of the active complex from inactive precatalyst and cocatalyst [10], i.e. by an increasing number of active species [11]. The interaction of Ni(PPh3)4 with BF3OEt2 in toluene at ratios B/ Ni = 2–160 resulted in the oxidation of Ni(0) to Ni(I) and the subsequent elimination of organophosphorus ligands from the coordination sphere of the transition metal until the formation of colloidal nickel [12].
Table 2 Effect of cocatalyst/nickel ratio on the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system B/Ni ratio
NB/Ni ratio
Catalyst (105 mol)
Time (h)
Yield (g)
Activity (kg NB/(mol Nih))
Pn
50 100 200 300 400 500
8940 8940 8940 8940 8940 8940
1.25 1.25 1.25 1.25 1.25 1.25
0.5 0.5 0.5 0.5 0.5 0.5
3.41 5.92 8.10 9.10 9.20 9.15
545.6 947.2 1296.0 1456.0 1472.0 1464.0
9 13 15 15 16 17
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Table 3 Effect of reaction temperature on the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system Temperature (8C)
NB/Ni ratio
Catalyst (105 mol)
Time (h)
Yield (g)
Activity (kg NB/(mol Nih))
Pn
25 35 45 55 65 75
8940 8940 8940 8940 8940 8940
1.25 1.25 1.25 1.25 1.25 1.25
0.5 0.5 0.5 0.5 0.5 0.5
9.20 10.44 10.73 10.61 10.50 9.16
1472.0 1670.4 1716.8 1697.4 1680.0 1465.5
16 9 9 9 9 9
Experimental conditions: B/Ni = 400, T = 25 8C, 10.52 g of NB, toluene, PEt = 1 atm, total volume 20 ml.
BF3 is known as a phosphine scavenger [13] and the elimination of organophosphorus ligands from the coordination sphere of nickel with BF3 is an equilibrium process. Therefore, the results obtained may be explained by both the formation of an increasing number of active species through the elimination of the organophosphorous ligands from the coordination sphere of nickel and the increased ionic character of active species at higher B/Ni ratios. The increased ionic character of active species resulted in the increased chain propagation rate, i.e. in increased average polymerization degree (molecular weight). This is in agreement with data reported in [10]. At the same time, it is worth noting that the substantial catalytic activity might be achieved at lower B/Ni ratios of 200 compared to that for the polymerization of norbornene in the absence of ethene, where a remarkable activity had been recorded only at B/Ni = 400 [8]. This contrast indicates that ethene not only acts as a chain transfer agent, but also shows a promoting effect in the polymerization of norbornene. This promoting effect of ethene is a subject for special study. A series of polymerization runs were performed at different temperatures between 25 and 75 8C at a B/Ni = 400 ratio. As one may see from Table 3, the polymer yield is not substantially influenced by increasing temperature and reached a shallow maximum at 45 8C. Here, again the promoting effect of ethene is noteworthy. For the homopolymerization of norbornene in the absence of ethane, the temperature dependence of catalyst activity is significant and shows a maximum at 65 8C [8]. At the same time, a strong drop of average polymerization degree from Pn = 16 at 25 8C to Pn = 9 was observed starting from 35 8C. The probable explanation for this phenomenon is that, at temperatures higher than 25 8C, insertion of ethene into the Ni–C bond of active species becomes more favorable
than that of norbornene unit and a subsequent b-hydride elimination leads to chain termination. The effects of reaction time on the conversion, activity and intrinsic viscosity are summarized in Fig. 1 and Table 4. Polymer yield increases with time; two distinct periods can be observed in the polymerization process. In the first period, about 5–10 min, the polymer yield increased rapidly. In the second period, the polymer yield increased very slightly and then leveled off, due to a significant decrease of the monomer content in the reaction mixture. An almost quantitative conversion was reached after 30 min (Fig. 2). At the reaction time of 5 min, the activity was 6280 kg NB/(mol Nih). A more detailed study is desirable to optimize the effect of reaction parameters on the yield of polymers. The average polymerization degree was independent of reaction time and remained equal to 9. The variation of norbornene feed from 10 to 60 mol% at 65 8C and B/Ni = 400 revealed increased product yield, but only the average polymerization degree Pn = 9 was observed (Table 5). At higher norbornene feeds of 80 mol%, no ethene insertion has been detected by NMR. Runs at different ethene pressures at 65 8C and B/Ni = 400 showed that the increase of ethene pressure up to 5 atm led to decreased product yield, while the average polymerization degree remained unchanged at Pn = 9 (Table 6). The formation of lower molecular mass oligomers was also possible. It was demonstrated for the standard polymerization run carried out at B/Ni = 400 and 65 8C for 30 min. The polymers that formed were precipitated and separated by filtration. After evaporation of the solvent and unreacted norbornene from the mother liquor in vacuum, a viscous product (approximately 0.1 g) was obtained. The 1H NMR spectrum of this product showed Pn = 5. The directinjection mass-spectrum of this product registered in the m/z
Table 4 Effect of reaction time on the polymerization of norbornene over Ni(PPh3)4/BF3OEt2 catalyst system Time (min)
NB/Ni ration
Catalyst (105 mol)
Yield (g)
Conversion (%)a
Activity (kg NB/(mol Nih))
Pn
5 10 15 20 30
8940 8940 8940 8940 8940
1.25 1.25 1.25 1.25 1.25
6.54 8.41 9.57 10.20 10.50
60.2 77.5 88.2 94.0 96.8
6280 4037 3062 2448 1680
9 9 9 9 9
a
The conversion was calculated relative to 10.85 g of oligomer with Pn expected from 10.52 g NB. Experimental conditions: B/Ni = 400, T = 65 8C.
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Fig. 2. Effect of reaction time on the polymerization of norbornene over Ni(PPh3)4/BF3OEt2 catalyst system (B:Ni = 400, 65 8C, PEt = 1 atm, 10.52 g of NB, total volume 20 ml).
Table 5 Effect of monomer/nickel ratio on the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system NB feed (mol%)
NB/Ni ratio
Catalyst (105 mol)
Time (h)
Yield (g)
Activity (kg NB/(mol Nih))
Pn
10 20 40 60 80
1490 2980 5960 8940 11920
1.25 1.25 1.25 1.25 1.25
0.5 0.5 0.5 0.5 0.5
0.20 0.89 5.44 10.50 12.65
32.0 142.4 870.4 1680.0 2024.0
9 9 9 9 –
Experimental conditions: B/Ni = 400, T = 65 8C, 10.52 g of NB.
range up to 600 exhibits, among others, peaks at m/z 499 and 471, 405 and 377, 311 and 280, 216 and 190 with a base peak at m/z 95. The base peak at m/z 95 arises from the norbornene cation (Fig. 3). Peaks at m/z 499 and 471, 405 and 377, 311 and 280, 216 and 190 represent macromonomers with Pn = 5, 4, 3, and 2 and their fragmentation by loss of ethane, respectively. It should be noted that the amount of products in the mother liquor is substantially small in respect to the filter-separated product. Broad molecular weight distributions, indicated in Table 1, also might be explained by the formation of a mixture of oligomers with divergent Pn values. It is well known that metallocenes, in combination with MAO, are extremely active catalysts for the polymerization of ethene [14]. Transformation of ethene in the presence of Ni(PPh3)4/BF3OEt2 catalyst system at standard run condi-
tions led to its dimerization; the product consisted of 48.11% trans-2-butene, 27.19% cis-2-butene, and 24.7 1-butene. In fact, Sen et al. [15] showed that the cationic complex [(CH3CN)4Pd][BF4]2 catalyzes the oligomerization of ethene to give oligomers with Pn = 2–5. To the best of our knowledge, there is no data in the literature regarding ethene transformation by means of the cationic complex [1CH3C3H5Ni(COD)]PF6 [7], nor about any catalyst systems based on Co(II) compounds and MAO [5]. However, the cationic allylnickel(II) complexes, [2-RC3H4Ni(COD)]PF6, are valuable catalysts for the oligomerization of ethene [16,17]. When ethene is copolymerized with norbornene, all these catalysts lead to macromonomers with a vinylic end group due to b-hydride elimination after insertion of an ethene monomer unit. On the other hand, [Cp*CrMeCl]2/ MAO systems afforded a highly linear, ultra-high molecular
Table 6 Effect of ethene pressure on the polymerization of norbornene in the presence of ethene over Ni(PPh3)4/BF3OEt2 catalyst system Ethene pressure (atm)
NB/Ni ratio
Catalyst (105 mol)
Time (h)
Yield (g)
Activity (kg NB/(mol Nih))
Pn
1 2 3 4 5
8940 8940 8940 8940 8940
1.25 1.25 1.25 1.25 1.25
0.5 0.5 0.5 0.5 0.5
10.50 5.85 3.36 1.20 0.39
1680.0 936.0 537.0 192.0 62.4
9 9 9 9 9
Experimental conditions: B/Ni = 400, T = 65 8C, 10.52 g of NB.
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Fig. 3. MS spectrum of the product separated from mother liquor.
weight polyethylene [4]. These Cr(III)-based catalyst systems are active in the copolymerization of norbornene with ethene to give copolymers with a high a-olefin content. On the strength of these data, one might suggest that the transformation of ethene by means of a certain catalyst may serve as a testing reaction for the copolymerization of norbornene with ethene: if the catalyst leads to low molecular oligomers of ethene, the product of copolymerization is a macromonomer with a vinylic end group; if the catalyst is active in the polymerization of ethene, one can expect a copolymer of norbornene and ethene. In the experiments with Ni(PPh3)4/BF3OEt2 catalyst system, the product of intermolecular interaction between BF3 and the trace amounts of water in solvent can serve as a proton source for the initiation of the catalytic cycle. Then the proton concentration should depend on the equilibrium concentration of the ionic form of a molecular complex BF3H2O, Eq. (3): BF3 OEt2 þ H2 O ! BF3 H2 O þ OEt2 ! OEt2 þ Hþ ½BF3 OH
(3)
This assumption is in agreement with the data on the reaction between BF3OEt2 and trace amounts of water in deuterotoluene. The metathesis reaction between the molecular complex BF3H2O and its ionic form H+[BF3OH] produces H+, as is evident from the appearance of a broad signal at 12.0 ppm. The oxidation state of nickel in hydride species cannot be identified at present. However, zerovalent Ni(PPh3)4
is inactive and one-electron oxidation of initial Ni(0) complex to Ni(I) with BF3 (electron transfer) seems to be faster than oxidative addition of Brønsted acid to Ni(0) centers.
4. Conclusion The system Ni(PPh3)4/BF3OEt2 is highly active for the polymerization of norbornene in the presence of ethene. Catalyst activity up to 6250 kg/(mol Nih) has been obtained. The products of polymerization are norbornene oligomers with a vinylic end group, having an average polymerization degree of 9. These products are of interest as polymerizable macromonomers with high glass transition phase in thermoplastic elastomers. The nickel hydrides are likely to be catalytically active species. Ethene showed a promoting effect to decrease the cocatalyst amount necessary for substantial catalytic activity compared to homopolymerization of norbornene. The ethene transformation may serve as a testing reaction for the copolymerization of norbornene with ethene in some cases.
Acknowledgements One of the authors, G. Myamarsuren, gratefully acknowledges the KOFST for the International Scholar Exchange Fellowship. This work was also supported by the National Research Laboratory Project and the BK 21 Project.
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References [1] T.F.A. Haselwander, W. Heitz, S.A. Krugel, J.H. Wendorff, Macromol. Chem. Phys. 197 (1996) 3435. [2] T.F.A. Haselwander, W. Heitz, S.A. Krugel, J.H. Wendorff, Macromolecules 30 (1997) 5345. [3] Asian Chem. News 30 (1995) 23. [4] U. Peukert, W. Heitz, Macromol. Rapid Commun. 19 (1998) 159. [5] F.P. Alt, W. Heitz, Acta Polym. 49 (1998) 477. [6] T.F.A. Haselwander, W. Heitz, M. Maskos, Macromol. Rapid Commun. 18 (1997) 689. [7] L. Goodall, G.M. Benedikt, L.H. Mcintosh III, D.A. Barnes, US Patent 5468819 (1995), to B.F. Goodrich Co., USA. [8] G. Myagmarsuren, O.-Y. Jeong, S.-K. Ihm, Appl. Catal. A: Gen. 255 (2003) 203.
277
[9] G. Wilke, E.W. Muller, M. Kroner, Angew. Chem. 73 (1961) 33. [10] C. Janiak, P.G. Lassahn, Polym. Bull. 47 (2002) 539. [11] C. Zhao, M.R. Ribeiro, M.F. Portela, S. Pereira, T. Nunes, Eur. Polym. J. 37 (2001) 45. [12] V.S. Tkach, V.A. Gruznykh, G. Myagmarsuren, L.B. Belykh, V.V. Saraev, F.K. Shmidt, Russ. J. Coord. Chem. 20 (1994) 584–587. [13] H.S. Booth, D.R. Martin, Boron Trifluoride and its Derivatives, Wiley, New York, 1946. [14] W. Kaminsky, Catal. Today 62 (2000) 23. [15] A. Sen, T.-W. Lai, R.R. Thomas, J. Organomet. Chem. 358 (1988) 567. [16] J.P. Gehrke, R. Taube, E. Balbolov, K. Kurtev, J. Organomet. Chem. 304 (1986) 4. [17] J.J. Brunet, A. Sivade, I. Tkatchenko, J. Mol. Catal. 50 (1989) 291.