Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
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Scandium-Catalyzed Syndiospecific Polymerization of HalideSubstituted Styrenes and Their Copolymerization with Styrene Fang Guo,† Na Jiao,† Lei Jiang,† Yang Li,*,† and Zhaomin Hou*,†,‡ †
State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China ‡ Organometallic Chemistry Laboratory and Center for Sustainable Resource Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *
ABSTRACT: The polymerization of halide-substituted styrenes (XSt) and their copolymerization with styrene by half-sandwich scandium catalysts with and without a THF ligand have been examined. The THF-free aminobenzyl scandium complex (C5Me4SiMe3)Sc(CH2C6H4NMe2-o)2 (2) in combination with 1 equiv of [Ph3C][B(C6F5)4] showed high activity and excellent syndiotacticity for the polymerization of the halide-substituted styrenes (XSt = p-, o-, and m-ClSt, p-FSt, p-BrSt, and p-ISt). Despite the presence of the electron-withdrawing halogen substituent on the aromatic ring, the polymerization activity reached up to 105 g of polymer molSc−1 h−1 with syndiotacticity (rrrr) up to 99%. This constitutes the first example of the syndiospecific polymerization of halogenated styrenes with high activity and high stereoselectivity. Moreover, the syndiospecific copolymerization of XSt with styrene has also been achieved by this catalyst, which afforded the corresponding syndiotactic copolymers with high molecular weight and a controllable amount of the halogenated styrene units. In contrast, the THF-containing trimethylsilylmethyl scandium complex (C5Me4SiMe3)Sc(CH2SiMe3)2(THF) (1) showed high activity and high syndiotacticity for the polymerization of m-ClSt but gave atactic polymers in the polymerization of p- and o-ClSt, p-BrSt, and p-ISt, while a mixture of syndiotactic and atactic polymers was yielded in the case of p-FSt. The copolymerization of XSt with styrene by complex 1 was also achieved, but the stereoselectivity and comonomer distribution sequences in the resulting copolymers were significantly different from those obtained by using complex 2.
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titanium-based catalysts Ti(Benzyl)4/MAO,9 Ti(OMenthol)4/ MAO,10 (CpCH2CH2O)TiCl2/MAO,11 and (CpCH2CH2OCH3)TiCl2/MAO11 were reported to give atactic polymers in the polymerization of p-, o-, and m-ClSt and p-BrSt, although they showed high activity and syndiotactic selectivity for the polymerization of styrene and alkyl-substituted styrenes. The polymerization of p-ClSt and p-FSt by CpTiCl3/MAO yielded a mixture of atactic and syndiotactic polymers with rather low catalytic activity (414 331 406 329 420 683 38 46 49 4 16 37 6 505 62
− −g −g 96 −h −g −g >99 n.d.i >99 90 95 n.d.i >99 >99 >99
0.2 8.1 3.5 31.9 n.d. 4.5 4.5 5.0 n.d. 11.1 2.4 2.1 n.d. 3.4 4.5 9.4
1.40 2.20 2.47 1.48 n.d. 4.95 1.10 2.00 n.d. 2.20 2.49 3.69 n.d. 2.01 3.27 2.60
88 123 126 77 n.d. 122 − − − 90 − − − − − −
− − − 206 n.d. − − 323 356 211 316 349 315 323 323 323
395 388 397 395 n.d 387 354 386 392 382 397 397 386 386 385 385
g
Reaction condition: [Sc] 19 μmol; [Ph3C][B(C6F5)4] 19 μmol; [XSt]/[Sc] = 500; 25 °C; unless otherwise noted. bIn kg of polymer molSc−1 h−1. Determined by 13C NMR. dDetermined by GPC in THF at 30 °C (for atactic polymer) and o-dichlorobenzene at 145 °C (for syndiotactic polymer) against polystyrene standard. eDetermined by DSC. fDetermined by TGA defined by the onset point of 5% degradation. gAtactic polymer. h A mixture of syndiotactic polymer (40%) and atactic polymer (60%) separated by acetone extraction. iNot determined because of very low solubility in organic solvents. j[p-ClSt]/[Sc] = 1000. a c
Figure 1. 13C NMR spectra (A) and DSC curves (B) of syndiotactic halogenated polystyrenes prepared by complex 2.
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polymerization activity (>4.14 × 105 g of polymer molSc−1 h−1) and higher molecular weight (Mn = 8.1 × 104) were observed, but the resulting poly(p-ClSt) was still an atactic polymer (Table 1, run 2). Similarly, the polymerization of ochlorostyrene (o-ClSt), p-bromostyrene (p-BrSt), and piodostyrene (p-ISt) also gave the corresponding atactic polymers (Table 1, runs 3, 6, and 7), while the polymerization of p-fluorostyrene (p-FSt) produced a mixture of syndiotactic (40%) and atactic (60%) polymers (Table 1, run 5).36 These results are generally similar to what was observed previously in the case of titanium-based catalysts,9−12 and the poor stereoselectivity is obviously due to the influence of the electron-withdrawing halogen substituents. Interestingly, the
RESULTS AND DISCUSSION Homopolymerization of Halide-Substituted Styrenes. At fir s t , t h e c om b i n a t i o n o f [ ( C 5 Me 4 SiMe 3 ) S c(CH2SiMe3)2(THF)] (1) with [Ph3C][B(C6F5)4] was chosen as a catalyst for the polymerization of p-chlorostyrene (p-ClSt) because this catalyst showed high activity and high syndiotacticity for the polymerization of styrene.15 The polymerization took place at room temperature in toluene, yielding atactic poly(p-ClSt) with low molecular weight (Mn = 2 × 103). Unsaturated chain ends were observed in the 1H NMR spectrum (SFigure 1, Supporting Information), suggesting that β-H elimination could be a chain-termination reaction. When using chlorobenzene (PhCl) as a solvent, a much higher C
DOI: 10.1021/acs.macromol.7b01668 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Scandium-Catalyzed Copolymerization of para-Halide-Substituted Styrenes (p-XSt) with Styrene (St)a
run
[Sc]
X
XSt/[Sc]
St/[Sc]
time (min)
yield (g)
activityb
XSt contc (mol %)
Mnd (×104)
Mw/Mnd
Tge (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
1 1 1 2 2 2 1 1 1 2 2 2 2 1 1 1 2 2 2 1 1 1 2 2 2
F F F F F F Cl Cl Cl Cl Cl Cl Cl Br Br Br Br Br Br I I I I I I
50 100 150 100 180 400 50 100 150 100 150 160 180 50 100 150 100 150 180 50 100 150 100 150 180
150 100 50 100 20 20 150 100 50 100 50 40 20 150 100 50 100 50 20 150 100 50 100 50 20
2 3 3 2 60 180 3 5 10 3 30 60 60 3 3 15 2 240 240 3 5 30 3 240 240
0.38 0.38 0.35 0.22 0.06 0.21 0.36 0.38 0.49 0.23 0.16 0.16 0.11 0.37 0.50 0.49 0.23 0.16 0.10 0.42 0.52 0.59 0.31 0.30 0.24
600 400 368 347 3 4 379 240 155 242 17 8 6 389 526 103 363 2 1 484 379 73 326 4 3
25 49 72 17 49 79 16 42 74 15 35 49 62 17 47 72 14 32 59 22 44 70 21 49 70
3.3 4.4 2.7 6.6 1.1 1.6 2.5 3.1 2.2 6.7 2.7 2.3 1.7 1.7 2.7 1.5 7.5 1.5 1.1 1.3 0.9 1.1 4.2 6.5 3.4
1.73 1.64 2.63 2.26 2.94 2.69 1.98 1.88 2.49 2.00 2.17 2.02 2.46 2.78 1.97 2.02 2.62 3.66 3.03 2.26 2.68 2.31 3.21 1.29 1.69
− − − − − − − 104 103 − − − − − 67 72 100 86 85 − − − − − −
Tme (°C) 264, 261, 250, 269 252, 314 254 239 − 246 223 315 312 239 − − 241 210 317 241 202 − 251 − −
311 314 309 271
Reaction condition: [Sc] 19 μmol; [Ph3C][B(C6F5)4] 19 μmol; chlorobenzene 2 mL; 25 °C. bIn kg of polymer molSc−1 h−1. cDetermined by 1H NMR. dDetermined by GPC in THF at 30 °C against polystyrene standard. eDetermined by DSC. a
polymerization of m-chlorostyrene (m-ClSt) by 1/[Ph3C][B(C6F5)4] afforded the corresponding syndiotactic polymer with high molecular weight (Mn = 31.9 × 104) and high syndiotacticity (rrrr = 96%) (Table 1, run 4). The high stereoselectivity in the polymerization of m-ClSt might possibly be attributed to the relatively high electron density at the ipso carbon atom of the styrene unit caused by the chlorine substituent at the meta position. To have a stronger interaction between the catalyst metal center and the styrene unit in XSt and thereby achieve high syndiotacticity,11,37−39 we then examined the THF-free aminobenzyl complex [(C5Me4SiMe3)Sc(CH2C6H4NMe2-o)2] (2) because 2 may generate a base-free, more electropositive metal center in the polymerization after migration of the aminobenzyl unit from Sc to a XSt monomer. To our delight, the polymerization of p-, o-, and m-ClSt, p-FSt, p-BrSt, and p-ISt by 2/[Ph3C][B(C6F5)4] all afforded the corresponding syndiotactic halogenated polystyrenes with high molecular weight (Mn = (2.1−11.1) × 104) and high syndiotacticity (rrrr up to >99%) regardless of the type and position of halide group on the aromatic ring (Table 1, runs 8−16). All of these polymer products showed high melting point (211−356 °C), reflecting their high stereoregularity. When the polymerization of p-ClSt was carried out in bulk, a higher activity (105 g of polymer molSc−1 h−1) was observed, while the syndiotacticity remained high (Table 1, run 15). Increasing the monomer loading led to
the formation of higher molecular weight syndiotactic poly(pClSt) (Table 1, run 16). The 13C NMR of spectra of poly(XSt)s (X = p-F, p-Cl, p-Br, and m-Cl) prepared by 2/[Ph3C][B(C6F5)4] are shown in Figure 1A. The sharp singlets at 142.6 ppm for poly(p-ClSt) and 146.1 ppm for poly(m-ClSt) are attributed to the rrrr pentads (Figure 1A, runs 8 and 10). There were no visible stereoerror resonances, suggesting that these polymers are highly stereoregular (rrrr > 99%). The 13C NMR spectra of poly(p-FSt) (rrrr = 90%) and poly(p-BrSt) (rrrr = 95%) showed a sharp singlet at 139.9 and 143.1 ppm with some weak stereoerror resonances at 140.1−140.9 ppm (Figure 1A, run 11) and 143.3−143.8 ppm (Figure 1A, run 12), respectively. Poly(o-ClSt) and poly(p-ISt) prepared by complex 2 showed very poor solubility in organic solvents such as THF, chloroform, 1,2,4-trichlorobenzene, and 1,1,2,2-tetrachloroethane and did not give an informative NMR spectrum, in contrast with the highly soluble atactic poly(o-ClSt) and poly(p-ISt) prepared by complex 1. The FT-IR spectrum of poly(o-ClSt) prepared by complex 2 showed obvious crystalline phase peaks originating from a syndiotactic chain at 1311, 1299, 976, and 582 cm−1 (SFigure 18).40,41,12 Similarly, the FT-IR spectrum of poly(p-ISt) exhibited peaks at 1510, 1275, and 977 cm−1 (SFigure 19).40,41,12 All of the XSt homopolymers prepared by complex 2 exhibited semicrystalline morphology as shown by the DSC measurement (Figure 1B). The melting D
DOI: 10.1021/acs.macromol.7b01668 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. 13C NMR spectra (A) and DSC curves (B) of p-FSt−St copolymers prepared by complexes 1 (blue) and 2 (red).
Figure 3. 13C NMR spectra (A) and DSC curves (B) of p-ClSt−St copolymers prepared by complexes 1 (blue) and 2 (red).
signals of p-FSt−St sequences in the 13C NMR spectrum (Figure 2A, run 2) and the appearance of two Tms (Figure 2B, runs 1−3) originating from both syndiotactic polystyrene (sPS) blocks and syndiotactic poly(p-FSt) blocks. The copolymerization of p-FSt with styrene by complex 2 also afforded p-FSt−St copolymers. The composition of the copolymer products could be controlled in a wide range (17− 79 mol %) simply by changing the monomer feed ratio (Table 2, runs 4−6). The p-FSt content in the copolymers prepared by 2 was much lower than the initial p-FSt/St feed ratio, and the microstructures were significantly different from those prepared by complex 1. Relatively strong signals of the p-FSt−St joint sequences in the 13C NMR spectra of the p-FSt−St copolymers prepared by 2 were observed with varying peak shapes depending on the pFSt contents (Figure 2A, runs 4−6). When the p-FSt content was relatively low (17 mol %), the isolated p-FSt units (peak 8) and syndiotactic St−St sequences (peak 3) were observed (Figure 2A, run 4). Similarly, the isolated St units (peak 7) and
points of syndiotactic poly(p-FSt), poly(p-ClSt), poly(p-BrSt), poly(p-ISt), and poly(o-ClSt) (315−356 °C) are much higher than that of the unsubstituted syndiotactic polystyrene (sPS) (Tm = 270 °C). Syndiotactic poly(m-ClSt) showed a Tg at 90 °C and a Tm at 211 °C. All of the syndiotactic poly(XSt)s showed good thermal stability, with the decomposition temperatures (Td) defined by the onset point of 5% degradation being over 382 °C, which are higher than that of sPS (Td = 378 °C). Copolymerization of Halide-Substituted Styrenes with Styrene. The copolymerization of para-halogenated styrenes (p-XSt) with styrene (St) was then examined by using complexes 1 and 2. Some representative results are summarized in Table 2.42 The copolymerization of p-FSt with styrene by 1/ [Ph3C][B(C6F5)4] afforded the corresponding p-FSt−St copolymers with high activity (105 g of polymer molSc−1 h−1) (Table 2, runs 1−3).36 The composition of the copolymers coincided with the comonomer feed ratio. The copolymers may contain block microstructures as shown by the limited joint E
DOI: 10.1021/acs.macromol.7b01668 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. 13C NMR spectra (A) and DSC curves (B) of p-BrSt−St copolymers prepared by complexes 1 (blue) and 2 (red).
Figure 5. 13C NMR spectra (A) and DSC curves (B) of p-ISt−St copolymers prepared by complexes 1 (blue) and 2 (red).
atactic poly(p-ClSt) blocks as shown by the 13C NMR spectra, in consistence with the stereoselectivity of the catalyst 1 for each monomer. The 13C NMR spectrum of a p-ClSt−St copolymer with p-ClSt content = 42 mol % (Table 2, run 8) is shown in Figure 3A (run 8). Its DSC profile is given in Figure 3B (run 8), which showed a Tg at 104 °C originating from atactic poly(p-ClSt) sequences and a Tm at 239 °C originating from sPS sequences. The copolymerization of p-ClSt with styrene by using 2/[Ph3C][B(C6F5)4] afforded the corresponding copolymers containing p-ClSt−St sequences and syndiotactic St−St sequences (or syndiotactic ClSt−ClSt sequences, depending on the ClSt contents) (Table 2, runs 10−13). The 13 C NMR spectra and DSC curves of p-ClSt−St copolymers with different compositions prepared by using 2 are shown in Figures 3A and 3B (runs 10−14), respectively. A p-ClSt−St copolymer containing 15 mol % p-ClSt units showed a Tm = 246 °C originating from sPS blocks, while a p-ClSt−St
syndiotactic p-FSt−p-FSt sequences (peak 6) were observed when the p-FSt content was relatively high (79 mol %) (Figure 2A, run 6). In the copolymer containing 49 mol % of p-FSt and 51 mol % of St, the signal intensities of the p-FSt−St sequences, p-FSt−p-FSt sequences, and St−St sequences were similar (Figure 2A, run 5). The p-FSt−St copolymer containing 17 mol % p-FSt units prepared by 2 (Table 2, run 4) showed a Tm (269 °C) originating from sPS blocks (Figure 2B). The p-FSt−St copolymer containing 79 mol % p-FSt units (Table 2, run 6) showed a Tm (314 °C) originating from syndiotactic poly(pFSt) blocks. The p-FSt−styrene copolymer with a p-FSt content of 49 mol % (Table 2, run 5) showed two Tms (252 and 271 °C) possibly as a result of the rather random distributions of the p-FSt and styrene units (Figure 2B). The copolymerization of p-ClSt with styrene was also achieved by using 1/[Ph3C][B(C6F5)4] (Table 2, runs 7−9). The resulting p-ClSt−St copolymers contained sPS blocks and F
DOI: 10.1021/acs.macromol.7b01668 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Notes
copolymer containing 32 mol % St units possessed a Tm = 312 °C originating from sydiotactic poly(p-ClSt) blocks. The copolymerization of p-BrSt or p-ISt with styrene by complexes 1 and 2 afforded the corresponding copolymers (Table 2, runs 14−25). The 13C NMR spectra and DSC curves of p-BrSt−St and p-ISt−St copolymers with different compositions are shown in Figures 4 and 5, respectively. The p-BrSt−St and p-ISt−St copolymers prepared by 1 (Table 2, runs 14−16 and runs 20−22) contained sPS blocks, atactic poly(p-XSt) blocks, and p-XSt−St sequences (X = Br and I), with Tm lower than that of sPS (or without a Tm). The p-BrSt− St and p-ISt−St copolymers prepared by 2 contained sPS blocks, syndiotactic poly(p-XSt) blocks, and p-XSt−St sequences (X = Br and I). The p-BrSt−St copolymer with a p-BrSt content of 14 mol % gave a Tm at 241 °C originating from sPS blocks (Figure 4B, run 17), while the p-BrSt−St copolymer with a p-BrSt content of 59 mol % showed two Tm at 317 °C originating from syndiotactic poly(p-BrSt) blocks (Figure 4B, run 19). The p-BrSt−St copolymer with a p-BrSt content of 32 mol % gave a much lower Tm (210 °C) (Figure 4B, run 18). The p-ISt−St copolymer with a p-ISt content of 49 mol % prepared by 2 did not show a Tm (Figure 5B, run 24), while the copolymer containing 21 mol % p-ISt gave a Tm originating from sPS blocks at 251 °C (Figure 5B, run 23).
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (No. 21674016, 21429201, 21034001, and U1508204) and by a grant-in-aid for Scientific Research (S) (No. 26220802) from JSPS.
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CONCLUSION We have achieved for the first time the syndiospecific polymerization of halogenated styrenes (XSt) as well as syndiospecific copolymerization of halogenated styrenes with styrene (St) by using a half-sandwich scandium complex such as 2. We have found that the stereoselectivity of the XSt polymerization was significantly influenced by the ligand environment around the metal center. The THF-containing complex 1 did not show significant stereoselectivity in most cases probably due to the strong coordination of the THF Lewis-base ligand. In contrast, the THF-free complex 2 showed excellent syndiotacticity for the polymerization of a wide range halogenated styrenes, regardless of the type and position of the halogen group on the aromatic ring. The copolymerization of pXSt (X = F, Cl, Br, I) with styrene by the present half-sandwich catalysts has selectively afforded the corresponding copolymers containing a controllable amount of halogenated styrene units, with the stereoslectivity being effected by the catalysts. This protocol has offered a convent and efficient route for the synthesis of a novel family of halogen-functionalized polystyrenes, which may find applications in various areas.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01668. GPC curves, NMR spectra, and DSC charts of representative polymer products (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Z.H.). *E-mail:
[email protected] (Y.L.). ORCID
Fang Guo: 0000-0001-5114-6534 Zhaomin Hou: 0000-0003-2841-5120 G
DOI: 10.1021/acs.macromol.7b01668 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
(36) The syndiotacticity of the polymerization of FSt by complex 1 seemed to be influenced by the FSt monomer concentration. See also: Jiao, N.; Guo, F.; Li, Y. Syndiotactic Homo- and Co-Polymerization of p-Fluorostyrene and Ethylene Catalyzed by Half-Sandwich Scandium Complexes. Acta Polym. Sin. 2017, DOI: 10.11777/j.issn10003304.2017.17051. (37) Luo, Y.; Luo, Y.; Qu, J.; Hou, Z. QM/MM Studies on Scandium-Catalyzed Syndiospecific Copolymerization of Styrene and Ethylene. Organometallics 2011, 30, 2908−2919. (38) Kang, X.; Yamamoto, A.; Nishiura, M.; Luo, Y.; Hou, Z. Computational Analyses of the Effect of Lewis Bases on Styrene Polymerization Catalyzed by Cationic Scandium Half-Sandwich Complexes. Organometallics 2015, 34, 5540−5548. (39) Wang, X.; Lin, F.; Qu, J.; Hou, Z.; Luo, Y. DFT Studies on Styrene Polymerization Catalyzed by Cationic Rare-Earth-Metal Complexes: Origin of Ligand-Dependent Activities. Organometallics 2016, 35, 3205−3214. (40) Guerra, G.; Dal Poggetto, F.; Iuliano, M.; Manfredi, C. FTIR Spectra and Chain Conformations in the Crystalline Forms and Clathrates of Syndiotactic poly(p-methylstyrene). Makromol. Chem. 1992, 193, 2413−2420. (41) Mauro, A. D. G. D.; Loffredo, F.; Venditto, V.; Longo, P.; Guerra, G. Polymorphic Behavior of Syndiotactic Poly(p-chlorostyrene) and Styrene/p-Chlorostyrene Cosyndiotactic Random Copolymers. Macromolecules 2003, 36, 7577−7584. (42) Catalyst 1 generally showed higher activity than that of catalyst 2 in both homopolymerization and copolymerization, probably due to the higher initiation efficiency of the Sc−CH2(SiMe3)2 species in 1 than that of the Sc−CH2C6H4NMe2-o species in 2. For a DFT studies, see: Kang, X.; Zhou, G.; Wang, X.; Qu, J.; Hou, Z.; Luo, Y. Alkyl Effects on the Chain Initiation Efficiency of Olefin Polymerization by Cationic Half-Sandwich Scandium Catalysts: A DFT Study. Organometallics 2016, 35, 913−920.
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DOI: 10.1021/acs.macromol.7b01668 Macromolecules XXXX, XXX, XXX−XXX
