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p-tert-butylstyrene by Ph2Zn–metallocene–MAO systems
European Polymer Journal 37 (2001) 1001±1006
Copolymerization of styrene by diphenylzinc-additive systems I. Copolymerization of styrene/p-tert-butylstyrene by Ph2Zn± metallocene±MAO systems Franco M. Rabagliati a,*, M onica A. Perez a, Marcelo A. Soto a, Antxon Martõnez de Ilarduya b, Sebasti an Mu~ noz-Guerra b a b
Grupo Polõmeros, Departamento Ciencias Quõmicas, Facultad Quõmica y Biologõa, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile Departament d'Enginyeria Quimica, Universidat Polit ecnica de Catalunya, ETS d'Enginyeria Industrial de Barcelona, Diagonal 647, Barcelona 08028, Spain Received 9 February 2000; received in revised form 30 August 2000; accepted 11 September 2000
Abstract Copolymerization of styrene (S) and p-tert-butylstyrene (p-But S) was carried out at 60°C using diphenylzinc± metallocene±MAO systems. Metallocenes bisindenylzirconium dichloride, bis(n-butylcyclopentadienyl) titanium dichloride and cyclopentadienyltitanium trichloride were tried, the latter giving the largest conversion. Copolymers obtained with this system were analyzed by NMR and showed to be syndiotactic in nature and slightly richer in p-But S than the initial feed from which they were formed. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Styrene; p-tert-Butylstyrene; Metallocene catalysts; Diphenylzinc; Tacticity
1. Introduction The synthesis of syndiotactic polystyrene (s-PS) and its copolymers through the scienti®c and patent literature has been recently reviewed [1]. Ishihara et al., reported for the ®rst time the preparation of this polymer using homogeneous catalysts including titanium complexes and methylaluminoxane [2,3]. By that time Pellecchia et al. also reported on the syndiotactic polymerization of styrene (S) [4,5]. Copolymerization of S with a variety of comonomers has been studied by dierent authors. Soga et al. [6] copolymerized S with ring-substituted S (p-CH3 S, p-ClS, p-BrS and m-ClS) using a Ti(OMen)4 ±methylaluminoxane (MAO) catalyst. They determined the comonomer reactivity ratios and related them with the Hammett *
Corresponding author. Fax: +56-2-681-2108. E-mail address: [email protected] (F.M. Rabagliati).
equation. They established that copolymerization proceeded via a coordinated cationic mechanism. In a recent paper [7] it was reported that S/p-methylstyrene copolymers prepared with Ti(OEt)4 ±MAO catalytic system are enriched in p-MeS with respect to the feed composition. Such observation is in concordance with the known fact that the reactivity of ring-substituted S monomers tends to increase with the electron releasing strength of the substituent [3,6,8]. Zambelli and Pellecchia [9] remarked that homopolymerization of S and of ring-substituted S largely depends on the substituent group attached to the aromatic ring. Electron-releasing groups lead to an increase of the polymerization rate, while electron-withdrawing groups produce the opposite eect. They reported experimental results of the copolymerization of S (M1 ) with p-methylstyrene (M2 ) and for S (M1 ) with p-chlorostyrene (M2 ). Reactivity ratio values of r1 0:49, r2 1:5 and r1 20, r2 0:37, respectively, were established, in agreement with the assessment that syndiotactic-speci®c
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 2 0 2 - 0
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active species are cationic complexes of metals of group IV. In previous works on S polymerization using combined systems including diphenylzinc (Ph2 Zn), a metallocene, and MAO, we established that syndiotactic polymer were obtained when using titanocenes while zirconocenes produced mostly atactic polystyrene (aPS). A certain amount of s-PS could be separated however from the crude polymer in a quantity that depended largely on the constitution of the zirconocene. Nevertheless the recovered s-PS fraction was always less than 20% of the total obtained polystyrene [10±14]. The objective of this study is to add additional information to our previous ®ndings regarding stereoregular polymerization of S using initiator systems in which diphenylzinc is combined with a metallocene and MAO. In addition, we are interested in checking how the properties of s-PS are aected and improved when S is copolymerized with a para-substituted S such as p-But S. The present paper reports on experimental results of S/p-But S copolymerization using dierent Ph2 Zn± metallocene±MAO systems. The composition and tacticity of the resulting copolymers for the most favorable case have been examined by NMR.
2. Experimental Copolymerizations were carried out under argon atmosphere in a 100 cm3 Schlenk tube equipped with a magnetic stirrer. Solvent toluene (20 cm3 ), MAO solution, Ph2 Zn, and metallocene toluene solution, were sequentially charged by syringe under argon pressure. Polymerization was initiated by injecting simultaneously the required amount of S and the second comonomer. The reactions were kept at 60°C under stirring for the required length of time. Polymerization was ®nished by adding a mixture of hydrochloric acid and methanol. The polymers, coagulated in the acidi®ed methanol, were recovered by ®ltration after washing several times with methanol, and dried in vacuum at 60°C. Viscosities were measured either in chloroform or in o-dichlorobenzene depending on the solubility of the polymer and intrinsic viscosities were determined by the one-point method [15]. For a-PS, viscosity measurements were carried out in chloroform at 25°C and viscosity-average molecular weights, Mv , were calculated according to equation, g 1:12 10 4 Mv0:73 [16], which is reported to be valid for the 7±150 104 molecular weight range. For s-PS fractions, viscosities were measured in o-dichlorobenzene at 135°C. DSC analyses were performed by using a Rheometrics Scienti®c DSC apparatus with samples placed under a nitrogen atmosphere. 3±4 mg samples were heated at a rate of 10°C/min, and after cooling to room tempera-
ture, reheated at the same rate. The reported Tg and Tm were those obtained in the second scan. NMR spectra were recorded on a Bruker AMX-300 spectrometer at 70°C, operating at 300.1 and 75.5 MHz for 1 H and 13 C respectively. The polymers and copolymers were dissolved in deuterated benzene (C6 D6 , 5% w/ v). A total of 64 and 4000 scans with 16 and 32 K data points and with a relaxation delay of 1 and 2 s were collected for 1 H and 13 C respectively. Chemical shifts were calibrated to tetramethylsilane (TMS) used as internal reference. 3. Results and discussion In order to polymerize S, we have used initiator systems that include a metallocene, with or without diphenylzinc, and, in which activation is achieved by MAO [10±14]. The presence of Ph2 Zn, in the initiator system, increased the conversion to polymer [11,13]. Furthermore, conversion depends on the metallocene/ Ph2 Zn molar ratio, where the ratio 1:1 produced the best conversion in which either Ind2 ZrCl2 [11] or Cp2 TiCl2 [13] were used. Table 1 summarizes the comparative values for S polymerization using either the metallocene±MAO or the Ph2 Zn±metallocene±MAO systems. For both type of metallocenes the presence of Ph2 Zn increases the conversion to polymer, but, is more signi®cant for titanocenes than for zirconocenes. Until now, we have not been able to establish the exact role of Ph2 Zn. However, according to our results, when using Ph2 Zn±titanocene±MAO systems, we postulate that Ph2 Zn favors the reduction of Ti4 to Ti3 [12], as it is known that complexes of Ti(III) are the active species responsible of formation of syndiotactic polystyrene [17,18]. Copolymerization experiments were performed using initiator Ph2 Zn±metallocene±MAO systems, for metallocene being Ind2 ZrCl2 , (n-BuCp)2 TiCl2 , and CpTiCl3 and matched with S and p-But S homopolymerizations carried out under the same conditions. Table 2 reports the experimental results for copolymerization of S with p-But S at various initial feed S/pBut S ratios and also for the homopolymerization of each individual comonomer. Accordingly to these results, every assayed initiator system appears to be able to induce the homopolymerization of S and of p-But S as well as their copolymerization. The attained conversion, and the nature and thermal behavior of the resulting polymer were found to depend on both the type of metallocene and the initial composition S/p-But S employed. From results of Table 2, it becomes clear that the capacity of Ph2 Zn±metallocene±MAO initiator systems to induce S/p-But S copolymerization increases in the order Ind2 ZrCl2 < n-BuCp2 TiCl2 < CpTiCl3 .
F.M. Rabagliati et al. / European Polymer Journal 37 (2001) 1001±1006
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Table 1 S polymerization using metallocene±MAO and Ph2 Zn±metallocene±MAO systems in toluene, after 48 h at 60°Ca Metallocene
Metallocene±MAO
Ph2 Zn±metallocene±MAO
Conversion (%)
Tacticity
Conversion (%)
Tacticity
1.0 2.5 7.8 1.6 3.1 8.2 1.8 11.0 30.5b
Atactic Atactic Atactic Atactic Atactic Atactic Syndiotactic Syndiotactic Syndiotactic
1.1 2.7 8.4 2.1 3.3 8.5 6.7 17.4 42.0b
Atactic Atactic Atactic Atactic Atactic Atactic Syndiotactic Syndiotactic Syndiotactic
Cp2 ZrCl2 (i-BuCp)2 ZrCl2 Ind2 ZrCl2 (H4 -Ind)2 ZrCl2 Et(Ind)2 ZrCl2 i-Pr(Flu)(Cp)ZrCl2 Cp2 TiCl2 (n-BuCp)2 TiCl2 CpTiCl3
a Polymerization conditions: total volume 40 ml, [S] 2.0 mol/l, [MAO] 0.33 mol/l; [metallocene] 4.010 locene±MAO systems; [Ph2 Zn] [metallocene] 2:0 10 4 mol/l for Ph2 Zn±metallocene±MAO systems. b After 6 h polymerization period.
4
mol/l for metal-
Table 2 S/p-But S copolymerization initiated by various Ph2 Zn±metallocene±MAOa Ind2 ZrCl2 b
(n-BuCp)2 TiCl2 b
CpTiCl3 c
Feed S/ But S (mol/ mol)
Conversion (%)
jgj (dl/g)
Tg (°C)
Conversion (%)
jgj (dl/g)
Tg (°C)
Conversiond (%)
jgje (dl/g)
Tg f (°C)
S only 95/5 75/25 60/40 50/50 25/75 But S only
8.4 n.p. 5.0 4.7 6.3 13.3 19.6
0.50 n.p. 0.52 0.58 0.52 0.28 0.29
102.6 n.p. 116.9 123.2 126.8 134.0 139.2
17.4 n.p. 12.6 8.3 7.5 12.0 12.4
0.15 n.p. 0.34 0.36 0.39 0.46 0.80
100.6 n.p. 116.8 122.3 127.4 133.3 135.4
42.0 54.1 68.2 n.p. 70.2 n.p. 94.0
0.20 0.22 0.23 n.p. 0.23 n.p. 0.22
100.7 103.0 118.9 n.p. 129.6 n.p. 141.6
d
e
f
d
e
f
n.p.: not performed. a Polymerization carried in toluene at 60°C under the following conditions: solvent toluene, total volume 40 ml, S p-But S 2:0 mol/l, [MAO] 0.33 mol/l, [Ph2 Zn] [metallocene] 2:0 10 4 mol/l. b Reaction time 48 h. c Reaction time 6 h. d Conversion based on initial feed. e Measured in chloroform at 25°C or in o-dichlorobenzene ( ) at 135°C. f Measured by DSC.
Furthermore, values reported in Table 2 correspond to a 48 h period of reaction for the ®rst two systems and only to a 6 h period for the Ph2 Zn±CpTiCl3 ±MAO system, emphasizing the higher activity of CpTiCl3 when compared with Ind2 ZrCl2 and (n-BuCp)2 TiCl2 . Zambelli et al. [19] have established that syndiotactic S polymerization, using titanocene±MAO systems, initiates through coordination of the S monomer to metal active species. Furthermore, they demonstrated that propagation result from poly-insertion of a S monomer via secondary insertion. Taking account these achievement and in concordance of our previous results [10±14] and the present ones, we think that syndiotactic S polymerization catalized by our Ph2 Zn±CpTiCl3 ±MAO systems, conversely with Zambelli and other authors, proceed through coordination of S monomer to the active species
followed by the insertion of the monomer into the growing chain. We have already considered that in these polymerizations, propagation goes throughout an ionic pathway [12,14]. Homopolymer polystyrene obtained with the concourse of either Ph2 Zn±(n±BuCp)2 TiCl2 ±MAO or Ph2 Zn±CpTiCl3 ±MAO initiator systems showed melting temperature signals in the range of s-PS (Fig. 1). Their contents in s-PS, estimated as the boiling butanone insoluble fraction, turned to be 91.7% and 99.8% respectively. Conversely, S polymerization using the Ph2 Zn±Ind2 ZrCl2 ±MAO initiator system produced mainly amorphous polymer, as deduced from the absence of any melting peak in its DSC trace (Fig. 1). Anyhow, the obtained crude polymer contained 15.6% of butanone-insoluble fraction which, according to previous work, may be identi®ed as s-PS [21].
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F.M. Rabagliati et al. / European Polymer Journal 37 (2001) 1001±1006
Fig. 1. DSC thermograms of crude PS obtained using Ph2 Zn± metallocene±MAO initiator systems, in toluene at 60°C. For metallocenes Ind2 ZrCl2 and (n-BuCp)2 TiCl2 48 h polymerization. For CpTiCl3 , 6 h polymerization. Second heating at 10°C/ min (taken from Ref. [20]).
All the obtained products showed only one Tg signal in their DSC thermograms which steadily increased with the increasing of the p-But S content in the initial feed. Firstly, such results are indicative that a single sort of material is obtained, a homopolymer or a copolymer accordingly with the composition of the starting reaction mixture. Secondly, the increase of Tg values with the content in p-But S is consistent with an increase in chain stiness due to the steric hindrance caused by the bulky tert-butyl group. Similar results have been reported for syndiotactic copolymers of S and p-methylstyrene, where the Tg was found to increase with the content of the substituted component [22]. The homopolymers and copolymers obtained with Ph2 Zn±CpTiCl3 ±MAO system, which is that one giving the largest conversion of the three combinations used in this work, were examined in detail by NMR and their compositions determined by this method. Fig. 2 shows the 1 H-NMR spectra of all the polymers together with the spectrum of a polystyrene homopolymer obtained by free radical polymerization which is known to be fully atactic. Results indicate that both S and p-But S homopolymers obtained with metallocene are essentially syndiotactic. Signals corresponding to CH2 groups appear as well resolved triplets because the two protons of syndiotactic CH2 diads, tetrads, or hexads are magnetically equivalents. The copolymers also seem to be eminently syndiotactic for any composition. The fact that signals do not appear well resolved in these cases except for the extreme copolymer 95/5 with a very low content in p-But S, is attributed to sequence distribution eects. 13 C-NMR spectra, which are shown in Fig. 3, con®rmed the syndiotacticity of the two homopolymers as well as of the copolymers. In this case, the signal cen-
Fig. 2. 1 H-NMR spectra in C6 D6 at 70°C of S/p-But S copolymers, obtained using Ph2 Zn±CpTiCl3 ±MAO system in toluene after 6 h at 60°C.
Fig. 3. 13 C-NMR spectra in C6 D6 at 70°C of S/p-But S copolymers, obtained using Ph2 Zn±CpTiCl3 ±MAO system in toluene after 6 h at 60°C.
tered at 45 ppm and corresponding to the main chain CH2 , appears as a broad multiplet in the a-PS spectrum. This is obviously due to the eect of dierent isotactic, syndiotactic, and heterotactic triads, pentads or heptads present in the polymer. The same may be said for the signal appearing around 147 ppm and assigned to the aromatic-C1 carbon atom. In S and p-But S homopolymers obtained using our Ph2 Zn±CpTiCl3 ±MAO initiator system, these signals appear as well de®ned single peaks con®rming the syndiotacticity of the polymer chain in both cases. The same can be concluded from the spectra of the copolymers where such signals are also
F.M. Rabagliati et al. / European Polymer Journal 37 (2001) 1001±1006
1005
Table 3 Copolymer compositions calculated from NMR data Copolymer
Initial feed t
95/5 75/25 50/50 a b
Crude copolymer
S/p-Bu S (mol/mol)
1
H-NMRa S/p-But S (mol/mol)
13 C-NMRb S/p-But S (mol/mol)
95.0/5.0 75.0/25.0 50.0/50.0
93.2/6.8 69.7/30.3 42.2/57.8
94.1/5.8 68.7/31.3 42.0/58.0
Obtained by integration of the main chain CH and CH2 and the side chain CH3 proton signals. Obtained by integration of the main chain CH carbon signals of S and p-But S units.
exempted of complexity. It should be noticed, that in agreement with 1 H-NMR observations, signals corresponding to the main chain CH and CH2 , and to the quarternary aromatic carbons as well, show small splittings attributable to sequence eects. Further work addressed to the determination of the comonomer sequence distribution in these copolymers is in progress. Copolymer compositions were calculated by integration of the 1 H-NMR signals corresponding to CH and CH2 main chain units and CH3 units of tert-butyl side groups attached to p-But S units. For comparison, copolymer composition were also estimated from 13 CNMR data. In this case main chain CH of S and p-But S units were used for calculations. Table 3, shows the copolymer compositions calculated from 1 H-NMR and from 13 C-NMR spectra. The results are practically the same for both methods and reveal that copolymers are slightly enriched in p-But S respect to the initial feed composition. This is a largely expected result in concordance with previous work of dierent authors on the copolymerization of S with p-methylstyrene using other coordination initiator systems [3,6,8,9]. Identical eects have been also reported for copolymers made of S with p-n-butylstyrene [23].
4. Conclusions (1) Ph2 Zn±metallocene±MAO systems, including Ind2 ZrCl2 , (n-BuCp)2 TiCl2 or CpTiCl3 induce S/p-But S copolymerization. (2) The resulting P(S/p-But S) copolymers are syndiotactic in nature when including a titanocene such as (n-BuCp)2 TiCl2 or CpTiCl3 , but are atactic when using Ind2 ZrCl2 . (3) p-But S polymerization with these systems, results in an improved conversion to polymer compared to S polymerization, in accordance with an electron-donating eect of the p-But -substituent group. Furthermore, the copolymer S/p-But S obtained with these initiator systems shows compositions richer in p-But S compared to the initial feed from which they originated. (4) The present results, together with our previous results, [10±14] suggest that the Ph2 Zn±metallocene±
MAO systems are rather complex in which S polymerization initiates through coordination of monomer to the active species, and then polymerization propagates through a cationic pathway.
Acknowledgements Financial support from Departamento de Investigaciones Cientõ®cas y Tecnol ogicas, Universidad de Santiago de Chile, DICYT-USACH, Grant 05-9741RC, and from Fondo Nacional de Desarrollo Cientõ®co y Tecnol ogico, FONDECYT, Grant 198-1135 are gratefully acknowledged. We thank Witco, Polymerchemikalien and Kunstharze, Bergkamen, Germany, for donation of methylaluminoxane. We also thank Miss Ana M. Cavieres for DSC measurements and her skillful assistance in experimental work.
References [1] P o R, Cardi N. Prog Polym Sci 1996;21:47. [2] Ishihara N, Seimiya T, Kuramoto M, Uoi M. Macromolecules 1986;19:2464. [3] Ishihara N, Kuramoto M, Uoi M. Macromolecules 1988;21:3356. [4] Pellecchia C, Longo P, Grassi A, Ammendola P, Zambelli A. Makromol Chem, Rapid Commun 1987;8:277. [5] Ammendola P, Pellecchia C, Longo P, Zambelli A. Gazz Chim Ital 1987;117:65. [6] Soga K, Nakatani H, Monoi T. Macromolecules 1990; 23:953. [7] Cardi N, P o R, Abbondanza L, Abis L, Conti G. Macromol Symp 1996;102:123. [8] Grassi A, Longo P, Proto A, Zambelli A. Macromolecules 1989;22:104. [9] Zambelli A, Pellecchia C. In: Soga K, Terano M, editors. Studies in surface science and catalysis, vol. 89. Amsterdam: Elsevier; 1994. p. 209. [10] Rabagliati FM, Quijada R, Cuevas MV, Terraza C. Polym Bull 1996;37:13. [11] Rabagliati FM, Terraza CA, Quijada R. Int J Polym Mater 1996;34:163. [12] Rabagliati FM, Perez M, Cancino R, Quijada R. Polym Int 1999;48:681.
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[13] Rabagliati FM, Perez M, Ayal HA, Terraza CA, Quijada R. Polym Bull 1997;39:693. [14] Rabagliati FM, Perez MA, Quijada R. Polym Bull 1998; 41:441. [15] Solomon OF, Gotesman W. Makromol Chem 1967;104:177. [16] Brandrup J, Immergut EH. Polymer handbook. 2nd ed. New York: Wiley Interscience; 1975. p. iv-17. [17] Zambelli A, Pellecchia C, Oliva L, Longo P, Grassi A. Makromol Chem 1991;192:223. [18] Zambelli A, Pellecchia C, Oliva L. Makromol Chem, Macromol Symp 1991;48/49:297.
[19] Longo P, Proto A, Zambelli A. Macromol Chem Phys 1995;196:3015. [20] Rabagliati FM, Perez MA, Cancino RA, Soto MA, Rodrõguez FJ, Le on AG, Ayal HA, Quijada R. Bol Soc Chil Quim 2000;45:219. [21] Chien JCW, Salajka Z. J Polym Sci, A: Polym Chem 1991;29:1243. [22] Nakatani H, Nitta K-h, Soga K, Takata T. Polymer 1997;38:4751. [23] Thomann R, Sernetz FG, Steinmann S, M ulhaupt R, Kressler J. Macromolecules 1997;30:8401.
Copolymerization of styrene by diphenylzinc-additive systems I. Copolymerization of styrene/p-tert-butylstyrene by Ph2Zn± metallocene±MAO systems Franco M. Rabagliati a,*, M onica A. Perez a, Marcelo A. Soto a, Antxon Martõnez de Ilarduya b, Sebasti an Mu~ noz-Guerra b a b
Grupo Polõmeros, Departamento Ciencias Quõmicas, Facultad Quõmica y Biologõa, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile Departament d'Enginyeria Quimica, Universidat Polit ecnica de Catalunya, ETS d'Enginyeria Industrial de Barcelona, Diagonal 647, Barcelona 08028, Spain Received 9 February 2000; received in revised form 30 August 2000; accepted 11 September 2000
Abstract Copolymerization of styrene (S) and p-tert-butylstyrene (p-But S) was carried out at 60°C using diphenylzinc± metallocene±MAO systems. Metallocenes bisindenylzirconium dichloride, bis(n-butylcyclopentadienyl) titanium dichloride and cyclopentadienyltitanium trichloride were tried, the latter giving the largest conversion. Copolymers obtained with this system were analyzed by NMR and showed to be syndiotactic in nature and slightly richer in p-But S than the initial feed from which they were formed. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Styrene; p-tert-Butylstyrene; Metallocene catalysts; Diphenylzinc; Tacticity
1. Introduction The synthesis of syndiotactic polystyrene (s-PS) and its copolymers through the scienti®c and patent literature has been recently reviewed [1]. Ishihara et al., reported for the ®rst time the preparation of this polymer using homogeneous catalysts including titanium complexes and methylaluminoxane [2,3]. By that time Pellecchia et al. also reported on the syndiotactic polymerization of styrene (S) [4,5]. Copolymerization of S with a variety of comonomers has been studied by dierent authors. Soga et al. [6] copolymerized S with ring-substituted S (p-CH3 S, p-ClS, p-BrS and m-ClS) using a Ti(OMen)4 ±methylaluminoxane (MAO) catalyst. They determined the comonomer reactivity ratios and related them with the Hammett *
Corresponding author. Fax: +56-2-681-2108. E-mail address: [email protected] (F.M. Rabagliati).
equation. They established that copolymerization proceeded via a coordinated cationic mechanism. In a recent paper [7] it was reported that S/p-methylstyrene copolymers prepared with Ti(OEt)4 ±MAO catalytic system are enriched in p-MeS with respect to the feed composition. Such observation is in concordance with the known fact that the reactivity of ring-substituted S monomers tends to increase with the electron releasing strength of the substituent [3,6,8]. Zambelli and Pellecchia [9] remarked that homopolymerization of S and of ring-substituted S largely depends on the substituent group attached to the aromatic ring. Electron-releasing groups lead to an increase of the polymerization rate, while electron-withdrawing groups produce the opposite eect. They reported experimental results of the copolymerization of S (M1 ) with p-methylstyrene (M2 ) and for S (M1 ) with p-chlorostyrene (M2 ). Reactivity ratio values of r1 0:49, r2 1:5 and r1 20, r2 0:37, respectively, were established, in agreement with the assessment that syndiotactic-speci®c
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 2 0 2 - 0
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F.M. Rabagliati et al. / European Polymer Journal 37 (2001) 1001±1006
active species are cationic complexes of metals of group IV. In previous works on S polymerization using combined systems including diphenylzinc (Ph2 Zn), a metallocene, and MAO, we established that syndiotactic polymer were obtained when using titanocenes while zirconocenes produced mostly atactic polystyrene (aPS). A certain amount of s-PS could be separated however from the crude polymer in a quantity that depended largely on the constitution of the zirconocene. Nevertheless the recovered s-PS fraction was always less than 20% of the total obtained polystyrene [10±14]. The objective of this study is to add additional information to our previous ®ndings regarding stereoregular polymerization of S using initiator systems in which diphenylzinc is combined with a metallocene and MAO. In addition, we are interested in checking how the properties of s-PS are aected and improved when S is copolymerized with a para-substituted S such as p-But S. The present paper reports on experimental results of S/p-But S copolymerization using dierent Ph2 Zn± metallocene±MAO systems. The composition and tacticity of the resulting copolymers for the most favorable case have been examined by NMR.
2. Experimental Copolymerizations were carried out under argon atmosphere in a 100 cm3 Schlenk tube equipped with a magnetic stirrer. Solvent toluene (20 cm3 ), MAO solution, Ph2 Zn, and metallocene toluene solution, were sequentially charged by syringe under argon pressure. Polymerization was initiated by injecting simultaneously the required amount of S and the second comonomer. The reactions were kept at 60°C under stirring for the required length of time. Polymerization was ®nished by adding a mixture of hydrochloric acid and methanol. The polymers, coagulated in the acidi®ed methanol, were recovered by ®ltration after washing several times with methanol, and dried in vacuum at 60°C. Viscosities were measured either in chloroform or in o-dichlorobenzene depending on the solubility of the polymer and intrinsic viscosities were determined by the one-point method [15]. For a-PS, viscosity measurements were carried out in chloroform at 25°C and viscosity-average molecular weights, Mv , were calculated according to equation, g 1:12 10 4 Mv0:73 [16], which is reported to be valid for the 7±150 104 molecular weight range. For s-PS fractions, viscosities were measured in o-dichlorobenzene at 135°C. DSC analyses were performed by using a Rheometrics Scienti®c DSC apparatus with samples placed under a nitrogen atmosphere. 3±4 mg samples were heated at a rate of 10°C/min, and after cooling to room tempera-
ture, reheated at the same rate. The reported Tg and Tm were those obtained in the second scan. NMR spectra were recorded on a Bruker AMX-300 spectrometer at 70°C, operating at 300.1 and 75.5 MHz for 1 H and 13 C respectively. The polymers and copolymers were dissolved in deuterated benzene (C6 D6 , 5% w/ v). A total of 64 and 4000 scans with 16 and 32 K data points and with a relaxation delay of 1 and 2 s were collected for 1 H and 13 C respectively. Chemical shifts were calibrated to tetramethylsilane (TMS) used as internal reference. 3. Results and discussion In order to polymerize S, we have used initiator systems that include a metallocene, with or without diphenylzinc, and, in which activation is achieved by MAO [10±14]. The presence of Ph2 Zn, in the initiator system, increased the conversion to polymer [11,13]. Furthermore, conversion depends on the metallocene/ Ph2 Zn molar ratio, where the ratio 1:1 produced the best conversion in which either Ind2 ZrCl2 [11] or Cp2 TiCl2 [13] were used. Table 1 summarizes the comparative values for S polymerization using either the metallocene±MAO or the Ph2 Zn±metallocene±MAO systems. For both type of metallocenes the presence of Ph2 Zn increases the conversion to polymer, but, is more signi®cant for titanocenes than for zirconocenes. Until now, we have not been able to establish the exact role of Ph2 Zn. However, according to our results, when using Ph2 Zn±titanocene±MAO systems, we postulate that Ph2 Zn favors the reduction of Ti4 to Ti3 [12], as it is known that complexes of Ti(III) are the active species responsible of formation of syndiotactic polystyrene [17,18]. Copolymerization experiments were performed using initiator Ph2 Zn±metallocene±MAO systems, for metallocene being Ind2 ZrCl2 , (n-BuCp)2 TiCl2 , and CpTiCl3 and matched with S and p-But S homopolymerizations carried out under the same conditions. Table 2 reports the experimental results for copolymerization of S with p-But S at various initial feed S/pBut S ratios and also for the homopolymerization of each individual comonomer. Accordingly to these results, every assayed initiator system appears to be able to induce the homopolymerization of S and of p-But S as well as their copolymerization. The attained conversion, and the nature and thermal behavior of the resulting polymer were found to depend on both the type of metallocene and the initial composition S/p-But S employed. From results of Table 2, it becomes clear that the capacity of Ph2 Zn±metallocene±MAO initiator systems to induce S/p-But S copolymerization increases in the order Ind2 ZrCl2 < n-BuCp2 TiCl2 < CpTiCl3 .
F.M. Rabagliati et al. / European Polymer Journal 37 (2001) 1001±1006
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Table 1 S polymerization using metallocene±MAO and Ph2 Zn±metallocene±MAO systems in toluene, after 48 h at 60°Ca Metallocene
Metallocene±MAO
Ph2 Zn±metallocene±MAO
Conversion (%)
Tacticity
Conversion (%)
Tacticity
1.0 2.5 7.8 1.6 3.1 8.2 1.8 11.0 30.5b
Atactic Atactic Atactic Atactic Atactic Atactic Syndiotactic Syndiotactic Syndiotactic
1.1 2.7 8.4 2.1 3.3 8.5 6.7 17.4 42.0b
Atactic Atactic Atactic Atactic Atactic Atactic Syndiotactic Syndiotactic Syndiotactic
Cp2 ZrCl2 (i-BuCp)2 ZrCl2 Ind2 ZrCl2 (H4 -Ind)2 ZrCl2 Et(Ind)2 ZrCl2 i-Pr(Flu)(Cp)ZrCl2 Cp2 TiCl2 (n-BuCp)2 TiCl2 CpTiCl3
a Polymerization conditions: total volume 40 ml, [S] 2.0 mol/l, [MAO] 0.33 mol/l; [metallocene] 4.010 locene±MAO systems; [Ph2 Zn] [metallocene] 2:0 10 4 mol/l for Ph2 Zn±metallocene±MAO systems. b After 6 h polymerization period.
4
mol/l for metal-
Table 2 S/p-But S copolymerization initiated by various Ph2 Zn±metallocene±MAOa Ind2 ZrCl2 b
(n-BuCp)2 TiCl2 b
CpTiCl3 c
Feed S/ But S (mol/ mol)
Conversion (%)
jgj (dl/g)
Tg (°C)
Conversion (%)
jgj (dl/g)
Tg (°C)
Conversiond (%)
jgje (dl/g)
Tg f (°C)
S only 95/5 75/25 60/40 50/50 25/75 But S only
8.4 n.p. 5.0 4.7 6.3 13.3 19.6
0.50 n.p. 0.52 0.58 0.52 0.28 0.29
102.6 n.p. 116.9 123.2 126.8 134.0 139.2
17.4 n.p. 12.6 8.3 7.5 12.0 12.4
0.15 n.p. 0.34 0.36 0.39 0.46 0.80
100.6 n.p. 116.8 122.3 127.4 133.3 135.4
42.0 54.1 68.2 n.p. 70.2 n.p. 94.0
0.20 0.22 0.23 n.p. 0.23 n.p. 0.22
100.7 103.0 118.9 n.p. 129.6 n.p. 141.6
d
e
f
d
e
f
n.p.: not performed. a Polymerization carried in toluene at 60°C under the following conditions: solvent toluene, total volume 40 ml, S p-But S 2:0 mol/l, [MAO] 0.33 mol/l, [Ph2 Zn] [metallocene] 2:0 10 4 mol/l. b Reaction time 48 h. c Reaction time 6 h. d Conversion based on initial feed. e Measured in chloroform at 25°C or in o-dichlorobenzene ( ) at 135°C. f Measured by DSC.
Furthermore, values reported in Table 2 correspond to a 48 h period of reaction for the ®rst two systems and only to a 6 h period for the Ph2 Zn±CpTiCl3 ±MAO system, emphasizing the higher activity of CpTiCl3 when compared with Ind2 ZrCl2 and (n-BuCp)2 TiCl2 . Zambelli et al. [19] have established that syndiotactic S polymerization, using titanocene±MAO systems, initiates through coordination of the S monomer to metal active species. Furthermore, they demonstrated that propagation result from poly-insertion of a S monomer via secondary insertion. Taking account these achievement and in concordance of our previous results [10±14] and the present ones, we think that syndiotactic S polymerization catalized by our Ph2 Zn±CpTiCl3 ±MAO systems, conversely with Zambelli and other authors, proceed through coordination of S monomer to the active species
followed by the insertion of the monomer into the growing chain. We have already considered that in these polymerizations, propagation goes throughout an ionic pathway [12,14]. Homopolymer polystyrene obtained with the concourse of either Ph2 Zn±(n±BuCp)2 TiCl2 ±MAO or Ph2 Zn±CpTiCl3 ±MAO initiator systems showed melting temperature signals in the range of s-PS (Fig. 1). Their contents in s-PS, estimated as the boiling butanone insoluble fraction, turned to be 91.7% and 99.8% respectively. Conversely, S polymerization using the Ph2 Zn±Ind2 ZrCl2 ±MAO initiator system produced mainly amorphous polymer, as deduced from the absence of any melting peak in its DSC trace (Fig. 1). Anyhow, the obtained crude polymer contained 15.6% of butanone-insoluble fraction which, according to previous work, may be identi®ed as s-PS [21].
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F.M. Rabagliati et al. / European Polymer Journal 37 (2001) 1001±1006
Fig. 1. DSC thermograms of crude PS obtained using Ph2 Zn± metallocene±MAO initiator systems, in toluene at 60°C. For metallocenes Ind2 ZrCl2 and (n-BuCp)2 TiCl2 48 h polymerization. For CpTiCl3 , 6 h polymerization. Second heating at 10°C/ min (taken from Ref. [20]).
All the obtained products showed only one Tg signal in their DSC thermograms which steadily increased with the increasing of the p-But S content in the initial feed. Firstly, such results are indicative that a single sort of material is obtained, a homopolymer or a copolymer accordingly with the composition of the starting reaction mixture. Secondly, the increase of Tg values with the content in p-But S is consistent with an increase in chain stiness due to the steric hindrance caused by the bulky tert-butyl group. Similar results have been reported for syndiotactic copolymers of S and p-methylstyrene, where the Tg was found to increase with the content of the substituted component [22]. The homopolymers and copolymers obtained with Ph2 Zn±CpTiCl3 ±MAO system, which is that one giving the largest conversion of the three combinations used in this work, were examined in detail by NMR and their compositions determined by this method. Fig. 2 shows the 1 H-NMR spectra of all the polymers together with the spectrum of a polystyrene homopolymer obtained by free radical polymerization which is known to be fully atactic. Results indicate that both S and p-But S homopolymers obtained with metallocene are essentially syndiotactic. Signals corresponding to CH2 groups appear as well resolved triplets because the two protons of syndiotactic CH2 diads, tetrads, or hexads are magnetically equivalents. The copolymers also seem to be eminently syndiotactic for any composition. The fact that signals do not appear well resolved in these cases except for the extreme copolymer 95/5 with a very low content in p-But S, is attributed to sequence distribution eects. 13 C-NMR spectra, which are shown in Fig. 3, con®rmed the syndiotacticity of the two homopolymers as well as of the copolymers. In this case, the signal cen-
Fig. 2. 1 H-NMR spectra in C6 D6 at 70°C of S/p-But S copolymers, obtained using Ph2 Zn±CpTiCl3 ±MAO system in toluene after 6 h at 60°C.
Fig. 3. 13 C-NMR spectra in C6 D6 at 70°C of S/p-But S copolymers, obtained using Ph2 Zn±CpTiCl3 ±MAO system in toluene after 6 h at 60°C.
tered at 45 ppm and corresponding to the main chain CH2 , appears as a broad multiplet in the a-PS spectrum. This is obviously due to the eect of dierent isotactic, syndiotactic, and heterotactic triads, pentads or heptads present in the polymer. The same may be said for the signal appearing around 147 ppm and assigned to the aromatic-C1 carbon atom. In S and p-But S homopolymers obtained using our Ph2 Zn±CpTiCl3 ±MAO initiator system, these signals appear as well de®ned single peaks con®rming the syndiotacticity of the polymer chain in both cases. The same can be concluded from the spectra of the copolymers where such signals are also
F.M. Rabagliati et al. / European Polymer Journal 37 (2001) 1001±1006
1005
Table 3 Copolymer compositions calculated from NMR data Copolymer
Initial feed t
95/5 75/25 50/50 a b
Crude copolymer
S/p-Bu S (mol/mol)
1
H-NMRa S/p-But S (mol/mol)
13 C-NMRb S/p-But S (mol/mol)
95.0/5.0 75.0/25.0 50.0/50.0
93.2/6.8 69.7/30.3 42.2/57.8
94.1/5.8 68.7/31.3 42.0/58.0
Obtained by integration of the main chain CH and CH2 and the side chain CH3 proton signals. Obtained by integration of the main chain CH carbon signals of S and p-But S units.
exempted of complexity. It should be noticed, that in agreement with 1 H-NMR observations, signals corresponding to the main chain CH and CH2 , and to the quarternary aromatic carbons as well, show small splittings attributable to sequence eects. Further work addressed to the determination of the comonomer sequence distribution in these copolymers is in progress. Copolymer compositions were calculated by integration of the 1 H-NMR signals corresponding to CH and CH2 main chain units and CH3 units of tert-butyl side groups attached to p-But S units. For comparison, copolymer composition were also estimated from 13 CNMR data. In this case main chain CH of S and p-But S units were used for calculations. Table 3, shows the copolymer compositions calculated from 1 H-NMR and from 13 C-NMR spectra. The results are practically the same for both methods and reveal that copolymers are slightly enriched in p-But S respect to the initial feed composition. This is a largely expected result in concordance with previous work of dierent authors on the copolymerization of S with p-methylstyrene using other coordination initiator systems [3,6,8,9]. Identical eects have been also reported for copolymers made of S with p-n-butylstyrene [23].
4. Conclusions (1) Ph2 Zn±metallocene±MAO systems, including Ind2 ZrCl2 , (n-BuCp)2 TiCl2 or CpTiCl3 induce S/p-But S copolymerization. (2) The resulting P(S/p-But S) copolymers are syndiotactic in nature when including a titanocene such as (n-BuCp)2 TiCl2 or CpTiCl3 , but are atactic when using Ind2 ZrCl2 . (3) p-But S polymerization with these systems, results in an improved conversion to polymer compared to S polymerization, in accordance with an electron-donating eect of the p-But -substituent group. Furthermore, the copolymer S/p-But S obtained with these initiator systems shows compositions richer in p-But S compared to the initial feed from which they originated. (4) The present results, together with our previous results, [10±14] suggest that the Ph2 Zn±metallocene±
MAO systems are rather complex in which S polymerization initiates through coordination of monomer to the active species, and then polymerization propagates through a cationic pathway.
Acknowledgements Financial support from Departamento de Investigaciones Cientõ®cas y Tecnol ogicas, Universidad de Santiago de Chile, DICYT-USACH, Grant 05-9741RC, and from Fondo Nacional de Desarrollo Cientõ®co y Tecnol ogico, FONDECYT, Grant 198-1135 are gratefully acknowledged. We thank Witco, Polymerchemikalien and Kunstharze, Bergkamen, Germany, for donation of methylaluminoxane. We also thank Miss Ana M. Cavieres for DSC measurements and her skillful assistance in experimental work.
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