- Email: [email protected]
Copolymerization of ethylene and non-conjugated diene using homogenous catalyst
EUROPEAN POLYMER JOURNAL
European Polymer Journal 40 (2004) 2583–2589
www.elsevier.com/locate/europolj
Copolymerization of ethylene and non-conjugated diene using homogenous catalyst Maria de Fa´tima V. Marques *, Danielle Ramos, Juliana D. Rego Instituto de Macromole´culas Professora Eloisa Mano/UFRJ, CP 68525, 21941-590 RJ, Brazil Received 8 October 2003; received in revised form 13 May 2004; accepted 2 July 2004 Available online 1 September 2004
Abstract Ethylene and different amounts of 1,7-octadiene were copolymerized using the metallocene catalyst system ethylidene-bis(fluorenyl) zirconium dichloride and methylaluminoxane (MAO) at both 50 and 90 C. The catalyst activity has slightly increased with the addition of low amounts of the diene in relation to the homopolymerization of ethylene. The obtained polymers were characterized according to their melting temperature (Tm) and crystallinity degree (xc) by differential scanning calorimetry (DSC). Weight-average molecular weight (Mw) and polydispersity were determined by gel permeation chromatography (GPC). Diene contents in the copolymer were obtained through the FTIR spectroscopy. The results indicated that at polymerization temperature of 90 C, crosslinking bonds in the obtained copolymers were low, differently from what was observed at 50 C. The diene content in the copolymer achieved more than 3 mol% and the comonomer conversion was around 15%. Moreover, the obtained copolymers have Mw around 100,000 and large polydispersity. 2004 Published by Elsevier Ltd. Keywords: Ethylene copolymer; Diene comonomer; Metallocene catalyst
1. Introduction The synthesis of polyethylene having well defined functional groups randomly distributed along the polymer chain by the use of coordination catalysts is an emerging area of interest in polymer chemistry. Such functional groups introduce some hydrophilicity in an originally hydrophobic polyolefin, as well as enable the synthesis of novel graft copolymers having a wide range of applications. Particularly, the copolymerization of ethylene and a,x-dienes is of great commercial interest * Corresponding author. Tel.: +55 212 562 7224; fax: +55 212 270 1317. E-mail address: [email protected] (M.d.F.V. Marques).
0014-3057/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2004.07.008
due to the possibility of producing distinct polymer structures, including the formation of side chains containing terminal unsaturation, allowing the further introduction of functional groups. Metallocene catalysts combined with methylaluminoxane can be employed in the copolymerization of ethylene and diene with different structures as comonomers, resulting in high activities. These homogeneous systems can be much more efficient than the traditional heterogeneous catalysts to incorporate high amounts of diene comonomer in the polyethylene chain without causing catalyst deactivation [1]. On the other hand, the pendant double bond on the polymer side chain formed after the insertion of one of the equally reactive double bonds in a symmetric diene can generate different structures on the
2584
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
polymer backbone, such as cycles, branches and crosslinks. As an example, in ethylene-1,3-butadiene copolymerization comonomer can undergo enchainment either by 1,4-cis, 1,4-trans or by 1,2-insertion due to the conjugated nature of the double bonds. Besides, over 55% of butadiene in ethylene copolymer obtained through the catalyst system (n-Bu-Cp)2ZrCl2/MAO was found to have undergone cyclopolymerization. On the other hand, ethylene-1,5-hexadiene copolymer was reported to occur through cyclopolymerization, resulting in cyclopentane units along the polymer backbone [2,3]. Recently, Naga and Imanishi [4] have studied the copolymerization of ethylene and 1,7-octadiene at 40 C with various zirconocene catalysts. They have observed that the ligand structure of the catalysts strongly affected the propagation mode of the diene, varying the amounts of 1,2-addition, cyclization and crosslinking in the polymer. Moreover, in another work [5], the authors have reported the results of 1,7-octadiene homopolymerization and found that the cyclization selectivity has decreased with lowering the polymerization temperature and drastically decreased with increasing monomer concentration. Some examples of unsaturated polyethylene synthesis with the catalyst system Cp2ZrCl2/MAO through the addition of diene comonomers, acting as intermediate polymers for functionalized polyolefins were published. The obtained materials were low-molecular weight polymers and could be employed only as interfacial modifiers to enhance adhesion and compatibility of polyolefins with other polar materials [6]. Soga and co-worker [7] have reported the synthesis of an alternating copolymer of ethylene-1,9-decadiene, presuming that the side reactions such as cyclization could be completely suppressed due to the consecutive insertion of vinyl double bonds in diene. As mentioned, the modification of polyolefin synthesis with diene comonomers is usually accompanied by undesired side reactions, such as gel formation through the development of crosslinked bonds in the polymer chain during the polymerization. Recently, several bridged half-metallocene complexes were evaluated in ethylene–octadiene copolymerization [8]. It was shown that one of the examined catalysts presented a positive comonomer effect, suggesting that the bulky comonomer has a better access to that active site. Nevertheless, the molecular weights and polydispersity of all the obtained copolymers could not be determined due to the insolubility of the resins, which could derive from the crosslinking octadiene component in the polymer. Other study [6] showed that the amount of crosslinked bonds could be controlled by the appropriate choice of the experimental synthesis conditions such as polymerization temperature, as well as the diene structure. In that work 1,7-octadiene, which has been shown to be difficult
to overcome cyclic polymerization, usually incorporates to the polymer chain through only one double bond. Therefore, linear a,x-diene such as 1,7-octadiene can produce ethylene copolymer virtually without crosslinking or cyclic structures if the reaction temperature is sufficiently high. Moreover, low quantities of 1,7-octadiene decreased significantly polymer crystallinity. The metallocene complexes based on ansa-bis(fluorenyl) zirconium when activated by MAO are very active catalyst systems for ethylene polymerization, despite the highly steric hindrance imposed by such bulky, although flat, fluorenyl rings. The cause for these elevated activities seems to be the special electronic property of fluorenyl and, more important, the ring-slippage mechanism that occurs between fluorenyl ring and the transition metal of the metallocene complex. Besides, the bridge holding both rings in the bis(fluorenyl) complex favors the molecule stability during the polymerization, since it lowers the ligand abstraction during the ring slippage reaction [9]. As a consequence, the ansametalocene catalyst systems based on bis(fluorenyl) ligands are relatively stable at high temperatures and produce polyethylene with high average molecular weight. These catalyst systems are not extensively studied yet in ethylene-diene copolymerization. At the present work, we have investigated the influence of the addition of different amounts of diene comonomer in ethylene polymerization, using this voluminous catalyst at different polymerization temperatures and times. The diene content in the copolymer chain was determined considering that 1,7-octadiene was incorporated through 1,2insertion, leading to one vinyl double bond as a pendant group in the polymer chain. The melting temperature and molecular weight of the obtained polymers were also the measured parameters.
2. Experimental part 2.1. Material Ethylene and nitrogen were employed after passing ˚ and cupper catthrough columns of molecular sieve 3 A alyst. The metallocene Et(Flu)2ZrCl2 and MAO as 10% toluene solution was purchased by Crompton GmbH and used without further treatment. Toluene was distilled under sodium and benzophenone in nitrogen atmosphere. 1,7-Octadiene PA was purchased by Merck and treated with molecular sieve. 2.2. Polymerization Reactions were carried out in a 500 ml Bu¨chi glass autoclave using nitrogen as inert gas, coupled with mechanical stirrer and equipped with a flowmeter (model 5850D from Brooks Instruments Div.), with
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
scale up to 1000 ml/min of ethylene flow. Hundred milliliters of toluene, 1,7-octadiene and methylaluminoxane (MAO) at a [Al]/[Zr] ratio of 2000 were added in the reactor in this order. Afterwards, ethylene was admitted up to the reaction pressure of 2 bars. After saturation, the catalyst solution was injected under pressure, starting the polymerization. The reaction was maintained at constant temperature and pressure for either 30 min or 1 h, being quenched by the addition of acidified ethanol (5% HCl). 2.3. Polymer characterization The obtained polymers were characterized in relation to their melting temperature (Tm) and crystallinity degree (xc) with a differential scanning calorimeter from Perkin–Elmer model DSC7. The analysis was carried out between 50 and 250 C at a heating rate of 10 C/ min. Polymer crystallinity degree was calculated by comparison between the enthalpy value obtained in the second heating period and the theoretical value for 100% crystalline polyethylene (293 J/g). Weight-average molecular weight (Mw) and polydispersity (Mw/Mn) were determined by gel permeation chromatography (GPC) with a Waters 150CV plus instrument at 135 C using a set of Styragel columns HT6E, HT3, HT4 from Waters, calibrated with monodisperse polystyrene standards and trichlorobenzene as eluent. 13C-NMR spectrum was recorded and measured on a Varian model Mercury Spectrometer, operating at 300 MHz. Polymer was analyzed in a 1,2,4-trichlorobenzene/benzene-d6 solution (9:1 v/v). The spectrum was assigned according to the literature [3]. Infrared spectroscopy with Fourier transform (FTIR) was employed to determine the degree of the copolymer double bonds, admitting the absence of cyclic or crosslinking structures. Copolymer pressed films were prepared and analyzed at the Perkin–Elmer model 1720X spectrophotometer, obtaining the values of the absorbance ratios at 910 and 720 cm 1 (A910/
2585
A720). Diene molar percentage incorporated in the copolymer was calculated according to the literature [2,10].
3. Results and discussion Ethylene-diene copolymerization was performed with the catalyst system Et(Flu)2ZrCl2/MAO in toluene at both 50 and 90 C with different reaction times, and amounts of added 1,7-octadiene varying from 0 to 0.33 molar, at ethylene pressure of 2 bar. The results of catalyst activity are presented in Table 1. According to the results of experiments 2 and 3, it can be verified that the increase in the temperature for ethylene homopolymerization from 50 to 90 C has decreased the catalyst activity, which is possibly related to the much lower ethylene concentration at the highest temperature, under the same pressure. At these temperatures the polymers were insoluble in the reaction medium, indicating that they presented high molecular weights. On the other hand, in ethylene-1,7-octadiene copolymerization with 0.33 M of diene the increase of the reaction time from 30 to 60 min has comparatively low influence on the catalyst activity (contrary to the behavior of ethylene homopolymerization) or on the diene content incorporated in the copolymer (see Table 2), as it can be seen from the experiments 5 and 6. Furthermore, it is important to observe through the experiments 6 to 9, that the increase on the 1,7-octadiene concentration had relatively low influence on the catalyst activity at polymerization temperature of 90 C (Fig. 1). A slightly positive comonomer effect could be noticed. Moreover, comparing the catalyst activities obtained in the present work (in the order of 1100 kpol/mol Zr h––1 h of polymerization) for ethylene–octadiene polymerization with those achieved by Naga et al. [3] with different metallocene systems, we have observed
Table 1 Ethylene and 1,7-octadiene copolymerization with the catalyst system Et(Flu)2ZrCl2/MAOa Experiment
Polymerization T (C)
Reaction time (min)
Octadiene in the feed (ml)
[Octadiene] in the feed (M)
Yield (g)
a (kgPol/[Zr]h)
1 2 3 4 5 6 7 8 9
50 50 90 50 90 90 90 90 90
30 60 60 30 30 60 60 60 60
0 0 0 5.00 5.00 5.00 3.75 2.50 1.25
0 0 0 0.33 0.33 0.33 0.25 0.17 0.08
5.2 8.0 4.6 0.2 3.1 5.2 3.8 6.0 7.0
208 160 92 8 124 103 76 120 140
a
Hundred milliliters of toluene; [Zr] = 50 lM; [Al]/[Zr] = 2000; ethylene pressure = 2 bar; Tp = polymerization temperature.
2586
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
Table 2 Polyethylene and ethylene-1,7-octadiene copolymers characteristics with Et(Flu)2ZrCl2/MAO Experiment
Tm (C)
xc (%)
Mw · 10
1 2 3 4 5 6 7 8 9
132 134 133 74 110 107 113 115 121
43.0 45.9 56.9 8.9 9.8 7.7 14.4 24.0 29.2
1603a 421 290 185 80 92 113 90 190
3
(g/mol)
Mw/Mn
Octadiene in the copolymer (mol%)
Octadiene conversion (%)
n.d 2.4 2.7 2.4 7.0 7.1 10.9 10.6 3.6
– – – n.d 3.05 3.14 2.16 1.06 0.59
– – – n.d 11.0 16.8 11.1 13.3 21.0
Tm = melting temperature; Xc = crystallinity degree; Mw = weight-average molecular weight; Mw/Mn = polydispersity. a Extrapolated value from the GPC calibration curve.
180 160
a(Kgpol/[M]h)
140 120 100 80 60 40 20 0 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
1.7-Octadiene in the feed [M]
Fig. 1. Catalyst activity versus 1,7-octadiene concentration in the feed.
0.14
Exp 3
0.12
Exp 6 Rp*103 (mol/ls)
0.10
Exp 7 Exp 8
0.08
Exp 9 0.06 0.04 0.02 0.00 0
10
20
30
40
50
60
Tempo (h)
Fig. 2. Polymerization rate profile of ethylene in the copolymerization with octadiene with the catalyst Et(Flu)2ZrCl2/MAO.
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
that the metallocene Et(Flu)2ZrCl2 have exceeded the activities of most the bridged and unbridged metallocene catalysts employed. Fig. 2 shows the polymerization rate profile of ethylene in the copolymerization with different concentrations of octadiene. As expected, the ethylene homopolymerization profile (experiment 3) has a sharp decrease in the polymerization rate (Rp) especially in the beginning of the reaction. On the other hand, the curves of the experiments 6–9, with 1,7-octadiene, differ considerably. The initiation rate of polymerization (zero time) has decreased in the same order of the increase in comonomer amount added to the polymerization system. In contrast, an increase of ethylene consumption with time, after few minutes of reaction, was observed for almost all copolymerization reactions, with the rate at the end of the reaction reaching values higher than that of ethylene homopolymerization. An explanation for that behavior would be the fact that, despite the addition of rising amounts of a much less reactive monomer such as 1,7-octadiene, the produced copolymers were more soluble in the reaction medium than polyethylene, decreasing the diffusion limitation of monomers to the catalyst sites of polymerization through the polymer particle. The copolymer characterization was performed and the results of gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and infrared spectroscopy for quantitative analysis (FTIR) are
2587
showed in Table 2. Fig. 3 shows the 13C-NMR spectrum expanded in the region between 22 and 48 ppm, of the ethylene-1,7-octadiene copolymer obtained in experiment 7. The signals marked with a circle in Fig. 3 are those of the cyclic structure in ethylene-1,7-octadiene copolymer [3]. In spite of the bulky bis-fluorenyl structure of the catalyst and the employed high polymerization temperature, it can be noticed that, according to this assignment, a low portion of the diene is indeed incorporated as 1,3disubstituted cycloheptane units, preferentially in conformation cis (signal at 27.8 ppm from cis and 28.7 ppm from trans conformation). FTIR technique was employed to determine the amount of double bonds in ethylene–diene copolymers, assuming that the presence of unsaturation is attributed only to the C@C bond at polymer side chains as a result of the 1,7-octadiene incorporation. The analysis of polyethylene did not show absorption band at 910 cm 1. Infrared spectroscopy is an extremely sensible method for low concentration of double bonds, exceeding the sensibility of nuclear magnetic resonance spectroscopy analysis [2]. Polyethylene produced by this catalyst system presented a very high molecular weight and a relatively low value of crystallinity degree. According to the results of experiments 2 and 3, it can be verified that the increase of the polymerization temperature caused an increase of polymer crystallinity
Fig. 3. Ethylene-1,7-octadiene copolymer (experiment 7) obtained with catalyst Et(Flu)2ZrCl2/MAO.
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589 300
70
140
250
60
120
Mw
100
40
150 30
Tm (°C)
50
Mw/Mn
Mwx10-3
Mw/Mn
200
Tm (°C)
80
Xc (%)
60 40
100
20
50
20
10
0
0
0
0.5
0
0
0.5
1
1.5
2
2.5
3
1
1.5
2
2.5
3
100 90 80 70 60 50 40 30 20 10 0 3.5
Xc (%)
2588
Octadiene in the copolymer (mol %)
3.5
Octadiene in the copolymer (mol %)
Fig. 5. Tm and Xc versus Diene content in the copolymer.
Fig. 4. Mw and Mw/Mn versus Diene content in the copolymer.
to 0.33 M, tended to decrease polymer molecular weight, while it broadened polydispersity (Fig. 4). At the same time, the increase of 1,7-octadiene content in the copolymer caused an almost linear decrease of melting temperature and crystallinity degree (Fig. 5). The presence of low amounts of cycloheptane in the copolymer did not interfere on the crystalline structure of the material in a sense that, by the increase of diene concentration, the melting temperature decreased almost linearly with the increase of 1-hexenyl branches in the polymer. Moreover, the increase in the polymerization temperature caused an increase of both polymer melting temperature Tm and polydispersity. On the other hand, Mw has decreased and crystallinity degree did not varied considerably. The reactivity ratio of the monomers were calculated according to Fineman and Ross [11] (Fig. 6), resulting in the values r1 = 185.3 for ethylene and r2 = 0.0887 for octadiene. This shows unsurprisingly the large difference in reactivity between both monomers. Furthermore, there is any chance to obtain alternating monomer distribution in the polymer chain, since r1 · r2 resulted higher than unit. According to these results, the sequence length distribution [11] of both monomers in the chain of copoly-
degree and polydispersity, since the molecular weight has decreased as a consequence of the more occurrence of chain transfer reactions. It was not possible to prepare the polymer film of the material obtained in experiment 4, once it showed to be partially infusible. We have concluded that the copolymer obtained at this polymerization condition was partially crosslinked and merely its soluble fraction was analyzed through GPC. Additionally, this could also explain the very low melting temperature obtained for this polymer. On the other hand, the copolymers obtained in the experiments 5–8 (with the amounts of diene in the feed from 0.17 to 0.33 M) showed broad polydispersities. It is possible that small amounts of branches and crosslinked bonds were formed, leading to higher Mw/Mn, since the copolymers were soluble in the reaction medium at 90 C. Despite this, the molecular weight of the copolymers was still very high, around 90 · 103 g/mol. The comonomer conversion was maintained at an acceptable range, making it plausible to consider that the monomer/comonomer ratio in the reaction medium has not varied. Moreover, it was verified that the increase of 1,7-octadiene concentration in the reaction medium from 0.08 0.4 0.2
H(1-h)/h
0 -0.2
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
-0.4 -0.6
y = -185.25x + 0.0887 R2 = 0.9658
-0.8 -1 -1.2 2
H /h
Fig. 6. Reactivity ratios for ethylene and octadiene copolymerization (H = [M1]/[M2]; h = d[M1]/d[M2]).
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
2589
2.5
W1(n)%
2 1.5 1 0.5 0 1
7
13
19
25
31
37
43
49
(a)
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139
Sequence length (n) 80 70
W2(n)%
60 50 40 30 20 10 0 1
(b)
2
3
4
5
6
7
8
9
10
11
Sequence length (n)
Fig. 7. Sequence length distribution: (a) ethylene (W1(n)) and (b) 1,7-octadiene (W2(n)) in the copolymer chain (3.14% molar of diene).
mer containing around 3% molar of 1,7-octadiene (produced at experiment 6) was estimated (Fig. 7). It can be seen that ethylene units (M1) form a large range of sequences with different sizes, especially very long sequences due to the low comonomer amount, and only 2% of sequences contains alternated M1M2 units. On the other hand, more than 70% of 1,7-octadiene is found as isolated units, although the diene forms also sequences of until 3 mers in the copolymer chain. 4. Conclusions High molecular weight ethylene-1,7-octadiene copolymers were produced with high catalyst activity through the metallocene system Et(Fluorenyl)2ZrCl2/ MAO. The comonomer octadiene was effectively incorporated into the polyethylene chain, mostly as isolated units with vinyl pendant groups, showing a considerable effect on the polyethylene characteristics, such as decrease in melting temperature and increase in polymer polydispersity. Acknowledgment The authors are grateful to Fundacao Universitaria Jose Bonifacio-FUJB, Fundacao Carlos Chagas Filho
de Amparo a Pesquisa do Estado do RJ-FAPERJ, Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico-CNPq, and Crompton.
References [1] Pietika¨inen P, Seppa¨la¨ JV, Ahjopalo L, Pietila¨ L. Eur Polym J 2000;36:183–92. [2] Yanjarappa MJ, Sivaram S. Recent developments in the synthesis of functional poly(olefin)s. Prog Polym Sci 2002;27:1347–98. [3] Naga N, Imanishi Y. Macromol Chem Phys 2002;203: 771–7. [4] Naga N, Imanishi Y. Macromol Chem Phys 2002;203: 2155–62. [5] Naga N, Shiono T, Ikeda T. Macromol Chem Phys 1999;200:1466–72. [6] Pietika¨inen P, Va¨a¨na¨nem T, Seppa¨la¨ JV. Eur Polym J 1999;35:1047–55. [7] Iozumi T, Soga K. J Polym Sci, Part A: Polym Chem 2000;38:1844–50. [8] Alt HG, Weis AG, Reb A, Ernst R. Inorg Chim Acta 2003;343:253–74. [9] Schertl P, Alt HG. J Organometall Chem 1999;582:328–37. [10] Pietika¨inen P, Starck P, Seppa¨ la¨ JV. Eur Polym J 1998;37:2379–89. [11] Odian G, editor. Principles of polymerization. New York: John Wiley Sons; 1991.
European Polymer Journal 40 (2004) 2583–2589
www.elsevier.com/locate/europolj
Copolymerization of ethylene and non-conjugated diene using homogenous catalyst Maria de Fa´tima V. Marques *, Danielle Ramos, Juliana D. Rego Instituto de Macromole´culas Professora Eloisa Mano/UFRJ, CP 68525, 21941-590 RJ, Brazil Received 8 October 2003; received in revised form 13 May 2004; accepted 2 July 2004 Available online 1 September 2004
Abstract Ethylene and different amounts of 1,7-octadiene were copolymerized using the metallocene catalyst system ethylidene-bis(fluorenyl) zirconium dichloride and methylaluminoxane (MAO) at both 50 and 90 C. The catalyst activity has slightly increased with the addition of low amounts of the diene in relation to the homopolymerization of ethylene. The obtained polymers were characterized according to their melting temperature (Tm) and crystallinity degree (xc) by differential scanning calorimetry (DSC). Weight-average molecular weight (Mw) and polydispersity were determined by gel permeation chromatography (GPC). Diene contents in the copolymer were obtained through the FTIR spectroscopy. The results indicated that at polymerization temperature of 90 C, crosslinking bonds in the obtained copolymers were low, differently from what was observed at 50 C. The diene content in the copolymer achieved more than 3 mol% and the comonomer conversion was around 15%. Moreover, the obtained copolymers have Mw around 100,000 and large polydispersity. 2004 Published by Elsevier Ltd. Keywords: Ethylene copolymer; Diene comonomer; Metallocene catalyst
1. Introduction The synthesis of polyethylene having well defined functional groups randomly distributed along the polymer chain by the use of coordination catalysts is an emerging area of interest in polymer chemistry. Such functional groups introduce some hydrophilicity in an originally hydrophobic polyolefin, as well as enable the synthesis of novel graft copolymers having a wide range of applications. Particularly, the copolymerization of ethylene and a,x-dienes is of great commercial interest * Corresponding author. Tel.: +55 212 562 7224; fax: +55 212 270 1317. E-mail address: [email protected] (M.d.F.V. Marques).
0014-3057/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2004.07.008
due to the possibility of producing distinct polymer structures, including the formation of side chains containing terminal unsaturation, allowing the further introduction of functional groups. Metallocene catalysts combined with methylaluminoxane can be employed in the copolymerization of ethylene and diene with different structures as comonomers, resulting in high activities. These homogeneous systems can be much more efficient than the traditional heterogeneous catalysts to incorporate high amounts of diene comonomer in the polyethylene chain without causing catalyst deactivation [1]. On the other hand, the pendant double bond on the polymer side chain formed after the insertion of one of the equally reactive double bonds in a symmetric diene can generate different structures on the
2584
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
polymer backbone, such as cycles, branches and crosslinks. As an example, in ethylene-1,3-butadiene copolymerization comonomer can undergo enchainment either by 1,4-cis, 1,4-trans or by 1,2-insertion due to the conjugated nature of the double bonds. Besides, over 55% of butadiene in ethylene copolymer obtained through the catalyst system (n-Bu-Cp)2ZrCl2/MAO was found to have undergone cyclopolymerization. On the other hand, ethylene-1,5-hexadiene copolymer was reported to occur through cyclopolymerization, resulting in cyclopentane units along the polymer backbone [2,3]. Recently, Naga and Imanishi [4] have studied the copolymerization of ethylene and 1,7-octadiene at 40 C with various zirconocene catalysts. They have observed that the ligand structure of the catalysts strongly affected the propagation mode of the diene, varying the amounts of 1,2-addition, cyclization and crosslinking in the polymer. Moreover, in another work [5], the authors have reported the results of 1,7-octadiene homopolymerization and found that the cyclization selectivity has decreased with lowering the polymerization temperature and drastically decreased with increasing monomer concentration. Some examples of unsaturated polyethylene synthesis with the catalyst system Cp2ZrCl2/MAO through the addition of diene comonomers, acting as intermediate polymers for functionalized polyolefins were published. The obtained materials were low-molecular weight polymers and could be employed only as interfacial modifiers to enhance adhesion and compatibility of polyolefins with other polar materials [6]. Soga and co-worker [7] have reported the synthesis of an alternating copolymer of ethylene-1,9-decadiene, presuming that the side reactions such as cyclization could be completely suppressed due to the consecutive insertion of vinyl double bonds in diene. As mentioned, the modification of polyolefin synthesis with diene comonomers is usually accompanied by undesired side reactions, such as gel formation through the development of crosslinked bonds in the polymer chain during the polymerization. Recently, several bridged half-metallocene complexes were evaluated in ethylene–octadiene copolymerization [8]. It was shown that one of the examined catalysts presented a positive comonomer effect, suggesting that the bulky comonomer has a better access to that active site. Nevertheless, the molecular weights and polydispersity of all the obtained copolymers could not be determined due to the insolubility of the resins, which could derive from the crosslinking octadiene component in the polymer. Other study [6] showed that the amount of crosslinked bonds could be controlled by the appropriate choice of the experimental synthesis conditions such as polymerization temperature, as well as the diene structure. In that work 1,7-octadiene, which has been shown to be difficult
to overcome cyclic polymerization, usually incorporates to the polymer chain through only one double bond. Therefore, linear a,x-diene such as 1,7-octadiene can produce ethylene copolymer virtually without crosslinking or cyclic structures if the reaction temperature is sufficiently high. Moreover, low quantities of 1,7-octadiene decreased significantly polymer crystallinity. The metallocene complexes based on ansa-bis(fluorenyl) zirconium when activated by MAO are very active catalyst systems for ethylene polymerization, despite the highly steric hindrance imposed by such bulky, although flat, fluorenyl rings. The cause for these elevated activities seems to be the special electronic property of fluorenyl and, more important, the ring-slippage mechanism that occurs between fluorenyl ring and the transition metal of the metallocene complex. Besides, the bridge holding both rings in the bis(fluorenyl) complex favors the molecule stability during the polymerization, since it lowers the ligand abstraction during the ring slippage reaction [9]. As a consequence, the ansametalocene catalyst systems based on bis(fluorenyl) ligands are relatively stable at high temperatures and produce polyethylene with high average molecular weight. These catalyst systems are not extensively studied yet in ethylene-diene copolymerization. At the present work, we have investigated the influence of the addition of different amounts of diene comonomer in ethylene polymerization, using this voluminous catalyst at different polymerization temperatures and times. The diene content in the copolymer chain was determined considering that 1,7-octadiene was incorporated through 1,2insertion, leading to one vinyl double bond as a pendant group in the polymer chain. The melting temperature and molecular weight of the obtained polymers were also the measured parameters.
2. Experimental part 2.1. Material Ethylene and nitrogen were employed after passing ˚ and cupper catthrough columns of molecular sieve 3 A alyst. The metallocene Et(Flu)2ZrCl2 and MAO as 10% toluene solution was purchased by Crompton GmbH and used without further treatment. Toluene was distilled under sodium and benzophenone in nitrogen atmosphere. 1,7-Octadiene PA was purchased by Merck and treated with molecular sieve. 2.2. Polymerization Reactions were carried out in a 500 ml Bu¨chi glass autoclave using nitrogen as inert gas, coupled with mechanical stirrer and equipped with a flowmeter (model 5850D from Brooks Instruments Div.), with
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
scale up to 1000 ml/min of ethylene flow. Hundred milliliters of toluene, 1,7-octadiene and methylaluminoxane (MAO) at a [Al]/[Zr] ratio of 2000 were added in the reactor in this order. Afterwards, ethylene was admitted up to the reaction pressure of 2 bars. After saturation, the catalyst solution was injected under pressure, starting the polymerization. The reaction was maintained at constant temperature and pressure for either 30 min or 1 h, being quenched by the addition of acidified ethanol (5% HCl). 2.3. Polymer characterization The obtained polymers were characterized in relation to their melting temperature (Tm) and crystallinity degree (xc) with a differential scanning calorimeter from Perkin–Elmer model DSC7. The analysis was carried out between 50 and 250 C at a heating rate of 10 C/ min. Polymer crystallinity degree was calculated by comparison between the enthalpy value obtained in the second heating period and the theoretical value for 100% crystalline polyethylene (293 J/g). Weight-average molecular weight (Mw) and polydispersity (Mw/Mn) were determined by gel permeation chromatography (GPC) with a Waters 150CV plus instrument at 135 C using a set of Styragel columns HT6E, HT3, HT4 from Waters, calibrated with monodisperse polystyrene standards and trichlorobenzene as eluent. 13C-NMR spectrum was recorded and measured on a Varian model Mercury Spectrometer, operating at 300 MHz. Polymer was analyzed in a 1,2,4-trichlorobenzene/benzene-d6 solution (9:1 v/v). The spectrum was assigned according to the literature [3]. Infrared spectroscopy with Fourier transform (FTIR) was employed to determine the degree of the copolymer double bonds, admitting the absence of cyclic or crosslinking structures. Copolymer pressed films were prepared and analyzed at the Perkin–Elmer model 1720X spectrophotometer, obtaining the values of the absorbance ratios at 910 and 720 cm 1 (A910/
2585
A720). Diene molar percentage incorporated in the copolymer was calculated according to the literature [2,10].
3. Results and discussion Ethylene-diene copolymerization was performed with the catalyst system Et(Flu)2ZrCl2/MAO in toluene at both 50 and 90 C with different reaction times, and amounts of added 1,7-octadiene varying from 0 to 0.33 molar, at ethylene pressure of 2 bar. The results of catalyst activity are presented in Table 1. According to the results of experiments 2 and 3, it can be verified that the increase in the temperature for ethylene homopolymerization from 50 to 90 C has decreased the catalyst activity, which is possibly related to the much lower ethylene concentration at the highest temperature, under the same pressure. At these temperatures the polymers were insoluble in the reaction medium, indicating that they presented high molecular weights. On the other hand, in ethylene-1,7-octadiene copolymerization with 0.33 M of diene the increase of the reaction time from 30 to 60 min has comparatively low influence on the catalyst activity (contrary to the behavior of ethylene homopolymerization) or on the diene content incorporated in the copolymer (see Table 2), as it can be seen from the experiments 5 and 6. Furthermore, it is important to observe through the experiments 6 to 9, that the increase on the 1,7-octadiene concentration had relatively low influence on the catalyst activity at polymerization temperature of 90 C (Fig. 1). A slightly positive comonomer effect could be noticed. Moreover, comparing the catalyst activities obtained in the present work (in the order of 1100 kpol/mol Zr h––1 h of polymerization) for ethylene–octadiene polymerization with those achieved by Naga et al. [3] with different metallocene systems, we have observed
Table 1 Ethylene and 1,7-octadiene copolymerization with the catalyst system Et(Flu)2ZrCl2/MAOa Experiment
Polymerization T (C)
Reaction time (min)
Octadiene in the feed (ml)
[Octadiene] in the feed (M)
Yield (g)
a (kgPol/[Zr]h)
1 2 3 4 5 6 7 8 9
50 50 90 50 90 90 90 90 90
30 60 60 30 30 60 60 60 60
0 0 0 5.00 5.00 5.00 3.75 2.50 1.25
0 0 0 0.33 0.33 0.33 0.25 0.17 0.08
5.2 8.0 4.6 0.2 3.1 5.2 3.8 6.0 7.0
208 160 92 8 124 103 76 120 140
a
Hundred milliliters of toluene; [Zr] = 50 lM; [Al]/[Zr] = 2000; ethylene pressure = 2 bar; Tp = polymerization temperature.
2586
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
Table 2 Polyethylene and ethylene-1,7-octadiene copolymers characteristics with Et(Flu)2ZrCl2/MAO Experiment
Tm (C)
xc (%)
Mw · 10
1 2 3 4 5 6 7 8 9
132 134 133 74 110 107 113 115 121
43.0 45.9 56.9 8.9 9.8 7.7 14.4 24.0 29.2
1603a 421 290 185 80 92 113 90 190
3
(g/mol)
Mw/Mn
Octadiene in the copolymer (mol%)
Octadiene conversion (%)
n.d 2.4 2.7 2.4 7.0 7.1 10.9 10.6 3.6
– – – n.d 3.05 3.14 2.16 1.06 0.59
– – – n.d 11.0 16.8 11.1 13.3 21.0
Tm = melting temperature; Xc = crystallinity degree; Mw = weight-average molecular weight; Mw/Mn = polydispersity. a Extrapolated value from the GPC calibration curve.
180 160
a(Kgpol/[M]h)
140 120 100 80 60 40 20 0 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
1.7-Octadiene in the feed [M]
Fig. 1. Catalyst activity versus 1,7-octadiene concentration in the feed.
0.14
Exp 3
0.12
Exp 6 Rp*103 (mol/ls)
0.10
Exp 7 Exp 8
0.08
Exp 9 0.06 0.04 0.02 0.00 0
10
20
30
40
50
60
Tempo (h)
Fig. 2. Polymerization rate profile of ethylene in the copolymerization with octadiene with the catalyst Et(Flu)2ZrCl2/MAO.
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
that the metallocene Et(Flu)2ZrCl2 have exceeded the activities of most the bridged and unbridged metallocene catalysts employed. Fig. 2 shows the polymerization rate profile of ethylene in the copolymerization with different concentrations of octadiene. As expected, the ethylene homopolymerization profile (experiment 3) has a sharp decrease in the polymerization rate (Rp) especially in the beginning of the reaction. On the other hand, the curves of the experiments 6–9, with 1,7-octadiene, differ considerably. The initiation rate of polymerization (zero time) has decreased in the same order of the increase in comonomer amount added to the polymerization system. In contrast, an increase of ethylene consumption with time, after few minutes of reaction, was observed for almost all copolymerization reactions, with the rate at the end of the reaction reaching values higher than that of ethylene homopolymerization. An explanation for that behavior would be the fact that, despite the addition of rising amounts of a much less reactive monomer such as 1,7-octadiene, the produced copolymers were more soluble in the reaction medium than polyethylene, decreasing the diffusion limitation of monomers to the catalyst sites of polymerization through the polymer particle. The copolymer characterization was performed and the results of gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and infrared spectroscopy for quantitative analysis (FTIR) are
2587
showed in Table 2. Fig. 3 shows the 13C-NMR spectrum expanded in the region between 22 and 48 ppm, of the ethylene-1,7-octadiene copolymer obtained in experiment 7. The signals marked with a circle in Fig. 3 are those of the cyclic structure in ethylene-1,7-octadiene copolymer [3]. In spite of the bulky bis-fluorenyl structure of the catalyst and the employed high polymerization temperature, it can be noticed that, according to this assignment, a low portion of the diene is indeed incorporated as 1,3disubstituted cycloheptane units, preferentially in conformation cis (signal at 27.8 ppm from cis and 28.7 ppm from trans conformation). FTIR technique was employed to determine the amount of double bonds in ethylene–diene copolymers, assuming that the presence of unsaturation is attributed only to the C@C bond at polymer side chains as a result of the 1,7-octadiene incorporation. The analysis of polyethylene did not show absorption band at 910 cm 1. Infrared spectroscopy is an extremely sensible method for low concentration of double bonds, exceeding the sensibility of nuclear magnetic resonance spectroscopy analysis [2]. Polyethylene produced by this catalyst system presented a very high molecular weight and a relatively low value of crystallinity degree. According to the results of experiments 2 and 3, it can be verified that the increase of the polymerization temperature caused an increase of polymer crystallinity
Fig. 3. Ethylene-1,7-octadiene copolymer (experiment 7) obtained with catalyst Et(Flu)2ZrCl2/MAO.
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589 300
70
140
250
60
120
Mw
100
40
150 30
Tm (°C)
50
Mw/Mn
Mwx10-3
Mw/Mn
200
Tm (°C)
80
Xc (%)
60 40
100
20
50
20
10
0
0
0
0.5
0
0
0.5
1
1.5
2
2.5
3
1
1.5
2
2.5
3
100 90 80 70 60 50 40 30 20 10 0 3.5
Xc (%)
2588
Octadiene in the copolymer (mol %)
3.5
Octadiene in the copolymer (mol %)
Fig. 5. Tm and Xc versus Diene content in the copolymer.
Fig. 4. Mw and Mw/Mn versus Diene content in the copolymer.
to 0.33 M, tended to decrease polymer molecular weight, while it broadened polydispersity (Fig. 4). At the same time, the increase of 1,7-octadiene content in the copolymer caused an almost linear decrease of melting temperature and crystallinity degree (Fig. 5). The presence of low amounts of cycloheptane in the copolymer did not interfere on the crystalline structure of the material in a sense that, by the increase of diene concentration, the melting temperature decreased almost linearly with the increase of 1-hexenyl branches in the polymer. Moreover, the increase in the polymerization temperature caused an increase of both polymer melting temperature Tm and polydispersity. On the other hand, Mw has decreased and crystallinity degree did not varied considerably. The reactivity ratio of the monomers were calculated according to Fineman and Ross [11] (Fig. 6), resulting in the values r1 = 185.3 for ethylene and r2 = 0.0887 for octadiene. This shows unsurprisingly the large difference in reactivity between both monomers. Furthermore, there is any chance to obtain alternating monomer distribution in the polymer chain, since r1 · r2 resulted higher than unit. According to these results, the sequence length distribution [11] of both monomers in the chain of copoly-
degree and polydispersity, since the molecular weight has decreased as a consequence of the more occurrence of chain transfer reactions. It was not possible to prepare the polymer film of the material obtained in experiment 4, once it showed to be partially infusible. We have concluded that the copolymer obtained at this polymerization condition was partially crosslinked and merely its soluble fraction was analyzed through GPC. Additionally, this could also explain the very low melting temperature obtained for this polymer. On the other hand, the copolymers obtained in the experiments 5–8 (with the amounts of diene in the feed from 0.17 to 0.33 M) showed broad polydispersities. It is possible that small amounts of branches and crosslinked bonds were formed, leading to higher Mw/Mn, since the copolymers were soluble in the reaction medium at 90 C. Despite this, the molecular weight of the copolymers was still very high, around 90 · 103 g/mol. The comonomer conversion was maintained at an acceptable range, making it plausible to consider that the monomer/comonomer ratio in the reaction medium has not varied. Moreover, it was verified that the increase of 1,7-octadiene concentration in the reaction medium from 0.08 0.4 0.2
H(1-h)/h
0 -0.2
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
-0.4 -0.6
y = -185.25x + 0.0887 R2 = 0.9658
-0.8 -1 -1.2 2
H /h
Fig. 6. Reactivity ratios for ethylene and octadiene copolymerization (H = [M1]/[M2]; h = d[M1]/d[M2]).
M.d.F.V. Marques et al. / European Polymer Journal 40 (2004) 2583–2589
2589
2.5
W1(n)%
2 1.5 1 0.5 0 1
7
13
19
25
31
37
43
49
(a)
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139
Sequence length (n) 80 70
W2(n)%
60 50 40 30 20 10 0 1
(b)
2
3
4
5
6
7
8
9
10
11
Sequence length (n)
Fig. 7. Sequence length distribution: (a) ethylene (W1(n)) and (b) 1,7-octadiene (W2(n)) in the copolymer chain (3.14% molar of diene).
mer containing around 3% molar of 1,7-octadiene (produced at experiment 6) was estimated (Fig. 7). It can be seen that ethylene units (M1) form a large range of sequences with different sizes, especially very long sequences due to the low comonomer amount, and only 2% of sequences contains alternated M1M2 units. On the other hand, more than 70% of 1,7-octadiene is found as isolated units, although the diene forms also sequences of until 3 mers in the copolymer chain. 4. Conclusions High molecular weight ethylene-1,7-octadiene copolymers were produced with high catalyst activity through the metallocene system Et(Fluorenyl)2ZrCl2/ MAO. The comonomer octadiene was effectively incorporated into the polyethylene chain, mostly as isolated units with vinyl pendant groups, showing a considerable effect on the polyethylene characteristics, such as decrease in melting temperature and increase in polymer polydispersity. Acknowledgment The authors are grateful to Fundacao Universitaria Jose Bonifacio-FUJB, Fundacao Carlos Chagas Filho
de Amparo a Pesquisa do Estado do RJ-FAPERJ, Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico-CNPq, and Crompton.
References [1] Pietika¨inen P, Seppa¨la¨ JV, Ahjopalo L, Pietila¨ L. Eur Polym J 2000;36:183–92. [2] Yanjarappa MJ, Sivaram S. Recent developments in the synthesis of functional poly(olefin)s. Prog Polym Sci 2002;27:1347–98. [3] Naga N, Imanishi Y. Macromol Chem Phys 2002;203: 771–7. [4] Naga N, Imanishi Y. Macromol Chem Phys 2002;203: 2155–62. [5] Naga N, Shiono T, Ikeda T. Macromol Chem Phys 1999;200:1466–72. [6] Pietika¨inen P, Va¨a¨na¨nem T, Seppa¨la¨ JV. Eur Polym J 1999;35:1047–55. [7] Iozumi T, Soga K. J Polym Sci, Part A: Polym Chem 2000;38:1844–50. [8] Alt HG, Weis AG, Reb A, Ernst R. Inorg Chim Acta 2003;343:253–74. [9] Schertl P, Alt HG. J Organometall Chem 1999;582:328–37. [10] Pietika¨inen P, Starck P, Seppa¨ la¨ JV. Eur Polym J 1998;37:2379–89. [11] Odian G, editor. Principles of polymerization. New York: John Wiley Sons; 1991.