(ArO)TiR3 complexes for highly syndiospecific styrene polymerization

Article abstract of DOI:10.1016/j.molcata.2013.11.032

A kind of (ArO)TiR₃ (ArO = 2,6-(R′)₂-4- methylphenolate, R′ = Me, ^tBu, CH₂SPh, CH ₂N(ⁱPr)₂, CH₂NPh₂) complexes was synthesized and characterized. X-ray analysis of 6a and 7 shows that neither CH₂SPh (6a) nor CH₂N(ⁱPr) ₂ group (7) coordinates to titanium. Large bond angles C(Ar)-O(1)-Ti(1) angel (152.9(2) in 6a and 175.33 in 7 indicate that the bond have partial sp-hybridized character. Upon treatment with modified methylaluminoxane (MMAO), the titanium complexes exhibit significant thermal stability, and prove useful as styrene syndiotactic polymerization catalysts. Comparisons between different complexes on the styrene polymerization were discussed. Steric instead of electronic properties at 2,6-positions of phenol affect the polymerization activity. (2-tert-Butyl)-4-methylphenoxytitanium(IV) chloride 5e was established the most efficient one. High activity (1.11 × 10⁵ g sPS/mol(Ti) h) was achieved when styrene polymerization was carried out in the presence of 5e/MMAO at 130 C for 2 h.

Full text of DOI:10.1016/j.molcata.2013.11.032

Journal of Molecular Catalysis A: Chemical 383–384 (2014) 77–82
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
(ArO)TiR3 complexes for highly syndiospecific styrene polymerization
Chongjie Xua, Zhou Chenb, Gang Jib, Xiu-Li Sunb,∗, Jun-Fang Lib, Yong Tangb
a School of Chemistry and Chemical Engineering, Suzhou University, Suzhou 215123, China
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Lu, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
A kind of (ArO)TiR3 (ArO = 2,6-(Rꢀ)2-4-methylphenolate, Rꢀ = Me, tBu, CH2SPh, CH2N(iPr)2, CH2NPh2) com-
plexes was synthesized and characterized. X-ray analysis of 6a and 7 shows that neither CH2SPh (6a)
nor CH2N(iPr)2 group (7) coordinates to titanium. Large bond angles C(Ar)-O(1)-Ti(1) angel (152.9(2)◦
in 6a and 175.33◦ in 7 indicate that the bond have partial sp-hybridized character. Upon treatment with
modified methylaluminoxane (MMAO), the titanium complexes exhibit significant thermal stability, and
prove useful as styrene syndiotactic polymerization catalysts. Comparisons between different complexes
on the styrene polymerization were discussed. Steric instead of electronic properties at 2,6-positions of
phenol affect the polymerization activity. (2-tert-Butyl)-4-methylphenoxytitanium(IV) chloride 5e was
established the most efficient one. High activity (1.11 × 105 g sPS/mol(Ti) h) was achieved when styrene
polymerization was carried out in the presence of 5e/MMAO at 130 ◦C for 2 h.
Article history:
Received 31 July 2013
Received in revised form
24 November 2013
Accepted 28 November 2013
Available online 6 December 2013
Keywords:
Phenolate
Titanium(IV) complexes
High temperature
Polystyrene
© 2013 Elsevier B.V. All rights reserved.
Syndiotactic
1. Introduction
of PS/(mol(Ti) mol styrene h). The steric bulkiness in tetraden-
tate ligands has great influence on the polymerization and the
Syndiotactic polystyrene (sPS) is a kind of important engi-
neering plastic because of its high crystallinity, high modulus
of elasticity, low dielectric constant, and excellent resistance to
heat and chemicals [1]. CpTiCl3 and its derivatives have been
proved to be excellent catalyst since their discovery in the mid
1980s [2]. Moreover, tremendous efforts [3–10] have been made
to develop new catalytic systems. Of them, chelate ligands or ␩5-
cyclopentadienyl derivatives were usually designed to modulate
the properties of active species. For example, Okuda disclosed that
bis(phenolato)titanium complexes ({(OC6H2-4-Me-6-tBu)2S}TiX2,
X = Cl, OiPr, C5Me5) were highly active to polymerize styrene
syndiospecifically [6a]. FI catalyst based on phenoxy-imine/MAO
was reported to form highly syndiotactic polystyrene [6d]. In
addition, rare metal complexes were also documented for this poly-
merization. For example, (Z5-2,5-t-Bu2C4H2N)Sc(CH2C6H4NMe2-
O)2/[Ph3C][B(C6F5)4] developed by Hou and coworkers afforded
high molecular weight syndiotactic polystyrene in high activ-
ity [9c]. Mono(indenyl)-Sc-dialkyl/[Ph3C][B(C6F5)4] developed by
Chen et al. was extremely efficient in producing syndiopolystyrene
[9f]. Recently, a monomeric pentamethylcyclopentadienyltitana-
tranes bearing more than two pairs of methyl substituents on
the side arms of triethanolamines was reported to catalyze the
styrene polymerization at 110 ◦C for 10 min in an activity of 31.7 g
activity drastically decreased in the case of Cp*Ti(OCH2CH2)3N
(Cp* = pentamethylcyclopentadienyl) being used [8b].
Phenol derivatives were usually used as anionic ancillary lig-
ands to replace Cl− in half titanocenes, and the cooperative
effect of Cp and phenolate was believed to affect the syn-
diospecific styrene polymerization activity [11]. So far, cationic
[CpꢀTi(polymer)(styrene)]+ (Cpꢀ = cyclopentadienyl and its deriva-
tives) was assumed to play an essential role for the styrene
polymerization. We [12] recently found that simple phenolate
titanium(IV) complexes ArOTiX3 (X = NMe2, Cl, CH2Ph) could pro-
mote the styrene syndiotactic polymerization smoothly at above
100 ◦C, and the properties of phenolate have distinctive influence
on the catalytic activity. In the presence of MMAO, the complexes
exhibit good activity and excellent thermostability for the syn-
diotactic polymerization of styrene. Steric instead of electronic
properties at 2,6-positions of phenol affect the polymerization
activity. (2-tert-Butyl)-4-methyl-phenoxytitanium(IV) trichlorides
5e was established the most efficient one. sPS was obtained in an
activity of 1.11 × 105 g sPS/mol(Ti) h even when the polymerization
proceeded at 130 ◦C for 2 h. We report the results in detail.
2. Experimental
2.1. Materials
∗
All manipulations were carried out under argon or nitro-
gen atmosphere using standard Schlenk or glove box techniques.
Corresponding author. Tel.: +86 21 54925078.
E-mail address: xlsun@sioc.ac.cn (X.-L. Sun).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.11.032

78
C. Xu et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 77–82
Toluene, THF, hexane and dichloromethane were treated with sol-
vent purification MB SPS-800 prior to use. Styrene was dried over
CaH2 under stirring for 48 h and distilled before use. The purified
styrene was stored in the refrigerator under nitrogen and used
within a week. Modified methylaluminoxane (MMAO-3A) was pur-
chased from Akzo Chemical as 1.88 M heptane solution.
Molecular weight (Mw and Mn) and molecular weight distribu-
tion (Mw/Mn) of sPS were determined with a PL-220 GPC at 150 ◦C
(using polystyrene calibration, 1,2,4-trichlorobenzene as the sol-
vent at a flow rate of 1.0 mL/min). 1H NMR and 13C NMR spectra
were recorded on a Varian XL-300 MHz and Varian XL-400 MHz
spectrometer. Mass spectra were obtained using a HP5959A spec-
trometer. Elemental analysis was performed by the Analytical
Laboratory of Shanghai Institute of Organic Chemistry (CAS). 13C
NMR data for polymers were obtained using CDCl3 as the sol-
vent at 25 ◦C. X-ray crystallographic data were collected using a
SMART APEXII X-ray diffractometer. Compounds 4d–e were pur-
chased from ACROS, and 4f was purchased from TCI. Compounds 2,
[13] 3, [13] and 5c–e [14a] were prepared according to literature.
2.5 M in hexane, 3.3 mmol) over a 5 min period at −78 ◦C. The
solution was allowed to warm to room temperature and stirred
for 2 h. The resulting solution was added dropwise within 15 min
to TiCl4(THF)2 (1.08 g, 3.3 mmol) in toluene (20 mL) at −78 ◦C with
stirring. The mixture was warmed to room temperature and stirred
overnight. The dark-red solution was filtered, concentrated and
cooled to −30 ◦C to give dark-red solid. Yield: 1.4 g (72%). 1H NMR
(400 MHz, CDCl3): ı (ppm) = 7.40–7.38 (m, 4H), 7.27–7.23 (m, 6H),
6.96 (s, 2H), 4.47 (brs, 4H), 4.44 (s, 4H), 2.24 (s, 3H), 1.97 (brs, 4H).
13C NMR (100 MHz, CDCl3): ı (ppm) = 134.6, 133.9, 130.6, 130.1,
129.8, 128.9, 127.9, 125.9, 76.3, 36.4, 25.5, 20.9. Anal. Calcd for
C25H26Cl3O2S2Ti (576.83): C 52.05, H 4.54; Found: C 51.77, H 4.62.
[2,6-Bis((diisopropylamino)methyl)-4-methylphenolate
Ti(IV)Cl3(THF)] (5b). A same procedure as that for the prepa-
ration of 5a was used. Yield dark-red solid, 2.90 g (61%). 1H NMR
(300 MHz, CDCl3): ı 7.29 (s, 2H), 4.38 (s, 4H), 4.10 (s, 4H), 3.09
(pent, J = 6.5 Hz, 4H), 2.36 (s, 3H), 2.08 (m, 4H), 1.05 (d, J = 6.3 Hz,
24H). 13C NMR (75 MHz, CDCl3): 13C NMR (100 MHz, CDCl3): ı
(ppm) = 162.9, 128.0, 126.4, 124.9, 76.3,49.9, 45.2, 26.8, 22.1, 21.4.
Anal. Calcd for C25H44Cl3N2O2Ti (558.86): C 53.73, H 7.94, N 5.01;
Found: C 52.98, H 8.18, N 4.73.
2.2. Synthesis of ligands and complexes
[2-tert-Butyl-4-methylphenolate Ti(IV)Cl3(THF)] (5e). To a solu-
tion of TiCl4 (0.52 mL, 4.7 mmol) in toluene (20 mL) at −78 ◦C was
added dropwise a solution of 4f (0.77 g, 4.7 mmol) in toluene/THF
(8 mL/1 mL) over 10 min, and the resulting mixture was allowed
to warm to 65 ◦C and stirred for 8 h. After removing the solvent
under reduced pressure, the brown-red solid was collected and
dried in vacuo to give dark-red solid. Yield: 0.74 g (97%) 1H NMR
(400 MHz, CDCl3): ı (ppm) = 7.60 (d, J = 8.0 Hz, 1H), 7.10–7.04 (m,
2H), 4.54 (t, J = 6.8 Hz, 4H), 2.37 (s, 3H), 2.12 (pent, J = 2.8 Hz, 4H),
1.53 (s, 9H). 13C NMR (75 MHz, CDCl3): ı (ppm) = 168.7, 137.6,
136.3, 128.2, 127.1, 126.6, 76.0, 35.0, 30.4, 25.7, 21.4. Anal. Calcd for
2,6-Bis(Phenylthiomethyl)-4-methylphenol (4a). Compound 3
(2.03 g, 10 mmol) in THF (30 mL) was added dropwise to a solution
of benzenethiol (2.10 mL, 20 mmol) and triethylamine (2.80 mL,
20 mmol) in THF (30 mL). After stirring for about 10 h, the solvent
was removed under vacuum. The residue was purified by column
chromatography on silica gel to give the product as a pale-yellow
oil (2.8 g, 80%). 1H NMR (300 MHz, CDCl3): ı (ppm) = 7.35–7.19
(m, 10H), 6.79 (s, 2H), 6.45 (s, 1H), 4.14 (s, 4H), 2.11(s, 3H). 13C
NMR (75 MHz, CDCl3): ı (ppm) = 150.4, 134.9, 130.1, 129.7, 129.1,
128.4, 126.2, 123.1, 34.3, 19.9. HRMS (EI): calcd for C21H20OS2 (M+)
352.0956; found 352.0956.
C15H23Cl3O2Ti (389.57): C 46.25, H 5.95; Found: C 45.86, H 6.30.
2,6-Bis((Diphenylamino)methyl)-4-methylphenol (4b). Com-
pound 3 (5.13 g, 25 mmol) in methanol (60 mL) was added dropwise
to a solution of diphenylamine (8.46 g, 50 mmol) and triethyl-
amine (7.00 mL, 50 mmol) in methanol (30 mL). After stirring 24 h
at room temperature, the precipitated product was filtered and
washed with methanol. White solid was obtained by recrystalliza-
tion from ethyl acetate/petroleum (1/10, v/v). Yield: 7.40 g (63%)
1H NMR (300 MHz, CDCl3): ı (ppm) = 8.90 (s, 1H), 7.27–7.22 (m,
8H), 7.09–7.07 (m, 8H), 7.02–6.97 (m, 4H), 6.87 (s, 2H), 4.87 (s,
4H), 2.13 (s, 3H). 13C NMR (100 MHz, CDCl3): ı (ppm) = 151.7, 148.6,
129.2, 128.7, 127.4, 123.4, 122.5, 121.8, 54.5, 20.8. IR (KBr, cm−1):
3036, 2930, 2064, 2496, 1589, 1490, 1448, 1366, 1246, 1223, 1195,
1153, 1098, 1085, 1074, 1034, 993, 865, 763, 746, 725, 698. MS (EI):
m/z = 470 (M+), 169 (Ph2N). HRMS (EI): calcd for C33H30N2O (M+)
470.2358; found 470.2358.
[2,6-Bis(phenylthiomethyl)-4-methyl-phenolate
Ti(IV)(NMe2)3] (6a). To a solution of Ti(NMe2)4 (0.32 g, 1.42 mmol)
in toluene (5 mL) at −78 ◦C was added dropwise a solution of 4a
(0.50 g, 1.42 mmol) in toluene (8 mL) over 5 min, and the resulting
mixture was allowed to warm up to room temperature and stirred
for 3 h. After removing the solvent under reduced pressure, red
solid was obtained and recrystallized in hexane at −30 ◦C to
give red crystal. Yield: 0.70 g (93%). 1H NMR (400 MHz, C6D6): ı
(ppm) = 7.32–7.30 (m, 4H), 7.04–6.92 (m, 8H), 4.24 (s, 4H), 3.12
(s, 18H), 2.04 (s, 3H). 13C NMR (100 MHz, C6D6): ı (ppm) = 159.2,
138.7, 130.6, 129.3, 129.1, 128.9, 125.8, 125.4, 44.6, 34.3, 20.6.
Anal. Calcd for C27H37N3OS2Ti (531.19): C 61.00, H 7.02, N 7.90;
Found: C 60.78, H 7.02, N 7.54.
2,6-Bis((diphenylamino)methyl)-4-methylphenolate
Ti(IV)(NMe2)3 (6b). The same procedure as that for the prepara-
tion of 6a was used. Yellow crystal 0.49 g (yield: 76%). 1H NMR
(300 MHz, C6D6): ı (ppm) = 7.39 (s, 2H), 7.28–7.08 (m, 17H),
6.85–6.82 (m, 3H), 5.26 (s, 4H), 3.00 (s, 18H), 1.99 (s, 3H). 13C NMR
(100 MHz, C6D6): ı (ppm) = 158.0, 148.9, 129.6, 129.1, 126.6, 126.4,
121.6, 120.9, 53.2, 44.5, 21.4. Anal. Calcd for C39H47N5OTi (649.69):
C 72.10, H 7.29, N 10.78; Found: C 72.16, H 7.56, N 10.59.
[2,6-di-tert-Butyl-4-methylphenolate Ti(IV)(NMe2)3 (6c). The
same procedure as that for the preparation of 6a was used. Yield yel-
low crystal, 0.37 g (92.6%). 1H NMR (400 MHz, C6D6): ı (ppm) = 7.21
(s, 2H), 3.08 (s, 18H), 2.31 (s, 3H), 1.57 (s, 18H). 13C NMR (100 MHz,
C6D6): ı (ppm) = 162.3, 139.6, 127.7, 125.9, 45.4, 35.2, 30.5, 21.5.
Anal. Calcd for C21H41N3OTi (399.44): C 63.15, H 10.35, N 10.52;
Found: C 62.60, H 10.10, N 9.87.
2,6-Bis((Diisopropylamino)methyl)-4-methylphenol
(4c).
Compound 3 (2.03 g, 10 mmol) in THF (30 mL) was added drop-
wise to a solution of diisopropylamine (7.00 mL, 50 mmol) and
triethylamine (4.20 mL, 30 mmol) in THF (30 mL). The solution
was refluxed for 10 h and then cooled to room temperature.
The triethylammonium salts were filtered and the solvent was
removed under vacuum. The residue was purified by column
chromatography on silica gel to give the product as a pale-yellow
solid. Yield: 2.8 g (84%). 1H NMR (300 MHz, CDCl3): ı (ppm) = 6.96
(s, 2H), 3.71 (s, 4H), 3.09 (m, J = 6.6 Hz, 4H), 2.25 (s, 3H), 1.08–1.06
(d, J = 6.0 Hz, 24H). 13C NMR (75 MHz, CDCl3): ı (ppm) = 153.5,
127.1, 126.8, 125.0, 47.8, 45.1, 20.7, 20.1. IR (cm−1): 2958, 2933,
2872, 2810, 1612, 1467, 1383, 1296, 1258, 1225, 1187, 1161, 1078,
1028, 985, 864, 840, 787, 747. MS (EI): m/z = 334 (M+). HRMS (EI):
calcd for C21H38N2O (M+) 334.2984; found 334.2984.
[2,6-Bis((diisopropylamino)methyl)-4-methylphenolate
Ti(IV)Bn3] (7). To a solution of TiBn4 (0.43 g, 1.0 mmol) in toluene
(10 mL) at −78 ◦C was added dropwise a solution of 4c (0.32 g,
1.0 mmol) in toluene (10 mL) over 10 min. The resulting mixture
was allowed to warm to room temperature and stirred for 3 h. The
[2,6-Bis(phenylthiomethyl)-4-methyl-
phenolate]Ti(IV)Cl3(THF) (5a). To a stirred solution of 4a (1.16 g,
3.3 mmol) in toluene (10 mL) was added n-BuLi solution (1.32 mL,

C. Xu et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 77–82
79
Scheme 1. Synthesis of ligands 4.
dark-red solution was concentrated. The precipitated dark-red
solid was filtered and recrystallized from hexane/toluene (30/1) at
−30 ◦C. Yield 0.46 g (71%). 1H NMR (300 MHz, C6D6): ı (ppm) = 7.81
(s, 2H), 7.20–7.14 (m, 6H), 7.02–6.90 (m, 9H), 4.08 (s, 4H), 3.14
(pent, J = 6.3 Hz, 4H), 2.74 (s, 6H), 2.51 (s, 3H), 1.10 (d, J = 6.3 Hz,
24H). 13C NMR (100 MHz, C6D6): ı (ppm) = 163.28, 142.20, 132.58,
131.73, 129.71, 129.33, 126.73, 124.85, 87.66, 49.05, 45.16, 21.92,
21.17. Anal. Calcd for C42H58N2OTi (654.790): C 77.04, H 8.93, N
4.28; Found: C 76.73, H 8.53, N 4.16.
Fig. 1. Molecular structure of complex 6a (hydrogen atoms were omitted
for clarity). Selected bond lengths (A) and angles (deg): Ti(1)-N(1) = 1.901(3),
Ti(1)-N(2) = 1.867(3), Ti(1)-N(3) = 1.892(3), N(2)-Ti(1)-O(1) = 112.20(13), N(2)-Ti(1)-
N(3) = 106.48(14), O(1)-Ti(1)-N(3) = 112.86(12), N(2)-Ti(1)-N(1) = 107.87(14), O(1)-
Ti(1)-N(1) = 110.51(12), N(3)-Ti(1)-N(1) = 106.61(14), C(18)-C(20)-S(1) = 108.4(2),
C(14)-C(21)-S(2) = 108.0(2).
˚
2.3. General procedure of styrene polymerization
Mixture of desired amount of MMAO and catalyst in toluene
(1.88 M, 1 mL) was stirred at room temperature for 15 min. Then
styrene was added and the resulting mixture was vigorously stirred
at desired temperature. After a desired time, the polymerization
was quenched by the fast introduction of acidified ethanol (200 mL,
10% HCl), and the resulting mixture was stirred for 12 h. The poly-
mer was filtered off, washed several times with ethanol, and dried
under vacuum until constant weight. sPS and aPS were separated
by extracting the polymer with 2-butanone for 12 h.
All compounds were well characterized by 1H NMR, 13C NMR
and elemental analysis. In the 1H NMR spectra of the complexes 5a,
5b, 6a, 6b and 7, single signal appeared at above 4.00 ppm, which
are assigned as the methylene attached to the phenol group and
suggested that both amine groups and thioether groups are chem-
ically equal. Fortunately, the structure of complexes 6a and 7 were
further confirmed by X-ray analysis [15] (Figs. 1 and 2). As shown in
Fig. 1, complex 6a adopts a distorted tetrahedral geometry around
Ti(IV). Both phenylthioether groups extend outward. The distances
˚
˚
between Ti–S(1) and Ti–S(2) are 4.199 A and 4.274 A respectively,
which are longer than the corresponding sum of van der Waals
3. Results and discussion
˚
radius (3.69 A) [16]. The Ti1-O1 bond length in complex 6a is longer
3.1. Synthesis of ligands and complexes
˚
˚
than that found for 5c [1.880(2) A vs 1.750(2) A] [14a]. Correspond-
˚
˚
ingly, O1-C22 is shortened (1.344(4) A vs 1.390(2) A). The angle of
C22-O1-Ti1 [152.9(2)◦] indicated that the lone-pair at the oxygen
atom back bonding into the unoccupied Ti(3d) or phenyl ␲ system.
The geometry structure of 7 is similar to that of 6a (Fig. 2, and
Table 1). With the substitution of NMe2 groups by CH2Ph groups,
Compound
2 was accessible from p-cresol and aqueous
formaldehyde (37.0–40.0%) [13]. After treatment with SOCl2, 2 was
readily transformed into 3, which could be used without further
purification. In the presence of Et3N, the reaction between 3 and
amine or benzenethiol generated 4 in 63–85% yield (Scheme 1).
Counter-ion usually has distinctive influence on the properties
of complexes [11], and we tried to synthesize complexes 5–7 and
investigate their performance in styrene polymerization. Titanium
(IV) complexes 5–7 were readily available from the correspond-
ing phenol in good yields (Scheme 2). However, in the cases of
2,6-(bis-aminomethyl)-4-methylphenol being used, a mixture of
monophenolate and biphenolate titanium complexes was always
obtained. The pure 5b, 6b, and 7 were isolated by repeated recrys-
tallization.
˚
Ti1-O1 bond length is shortened to 1.7777(17) A. Bond O1-C22 is
lengthened (1.368(3) A). The coordinative unsaturation around the
˚
metal center is compensated by the coordination of the ipso car-
bon of the benzyl group. For example, Ti-C14-C13 bond angle and
Ti-C7-C6 bond angle are 87.69(17)◦ and 105.73(18)◦ respectively,
2
suggesting that the benzyl groups may be involved in ␩ bind-
ing interactions with the metal atom. The third benzyl group was
Scheme 2. Synthesis of complexes 5–7.
Fig. 2. Molecular structure of complex 7 (hydrogen atoms were omitted for clarity).

80
C. Xu et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 77–82
Table 1
of atactic polystyrene in the blank experiments increased in the
order of PhCH3 < PhCl < PhBr < CH2Cl2 (from 0.29 g to 4.39 g, entries
1–4, Table 2). Considering that replacing 2,6-dimethyl with steric
CH2NPh2 group might benefit the syndioselectivity, 6b (in 1.0 mL
toluene) was first introduced to catalyze the styrene polymeriza-
tion at 110 ◦C for 1 h, and an activity of 1.33 × 105 g sPS/mol(Ti) h
was achieved (Table 2). Introduction 6b either in PhCl or in PhBr to
the mixture of styrene and MMAO led to the reduction of sPS activ-
ity (entry 1 vs 2, 3). The presence of 6b seems to be able to retard the
formation of aPS, since the amount of atactic polystyrene obtained
in the presence of 6b was smaller than that produced in the blank
experiments in all cases. On the basis of these results, it was con-
cluded that 6b/MMAO affords sPS exclusively. Different from the
above mentioned, atactic polystyrene was produced solely when
6b in CH2Cl2 was added to styrene polymerization system. Increas-
ing the amount of CH2Cl2 led to the improvement of aPS yield. This
result is similar to that reported in literature [18]. The yield of aPS
increased with a rise in the dielectric constant of the organic sol-
vent, probably because of the different dissociation level of the ion
pair [18].
The polymerization conditions strongly influenced activity
(Table 3). For example, decreasing the Al/Ti molar ratio from 4000
to 2000 deteriorated the polymerization and halved the activ-
ity to 0.62 × 105 g sPS/mol(Ti) h when styrene polymerization was
carried out at 110 ◦C for 1 h. Different from the reported exam-
ples [1–10], 6b showed good stability at high temperatures. For
instance, prolonging the polymerization time to 90 min at 110 ◦C
resulted in a slightly increased activity (1.45 × 105 g sPS/mol(Ti) h),
and good activity could be reached at elevated temperature as well
(entries 3–5). At 130 ◦C, 6b can promote the styrene polymerization
for at least 60 min to give an activity of 1.09 × 105 g sPS/mol(Ti) h at
a Al/Ti ratio of 4000 (entry 5). The deactivation and encapsulation
of active species were likely the reason for the activity reduction
under this condition. Catalyst aging before styrene polymerization
favored the improvement of activities. For example, in the case of
6b being introduced directly without pretreating with MMAO, the
activity was 0.87 × 105 g sPS/mol(Ti) h (entry 3 vs 6). As Ti(III) com-
plexes were generally supposed to be the real catalytic species to
catalyze the styrene polymerization, the reason for catalyst aging
is probably to reduce Ti(IV) species with MMAO into Ti(III).[19] It is
apparent that the initial amount of catalyst not only influence the
activity of sPS, but also has an effect on the yield of aPS. 10 ␮mol
6b gave the highest activity of sPS. Comparing to 10 or 15 ␮mol 6b
in the polymerization system, slightly higher amount of aPS was
separated when 5 ␮mol 6b was used (entry 7 vs 3, 8). This result
further elucidated that the presence of 6b hindered aPS formation.
Either increasing Al/Ti or improving the polymerization tem-
perature led to reduction of Mn of sPS (entries 1–3, 5), suggesting
that chain transfer to alkyl aluminum is probably one of the chain
˚
Selected bond lengths (A) and angles (deg) of complexes 6a and 7.
Entry
6a
7
O(1)-C(22)
O(1)-Ti(1)
C(22)-O(1)-Ti(1)
C(6)-C(7)-Ti(1)
C(13)-C(14)-Ti(1)
C(20)-C(21)-Ti(1)
1.344(4)
1.880(2)
1.368(3)
1.7777(17)
152.9(2)◦
175.33(16)
105.73(18)
87.69(17)
114.49(19)
involved in ␩1-bound to the metal (Ti-C21-C20 angle, 114.49(19)◦)
[17]. The coordination mode of the three benzyl groups in solid
state is different from that in solution. 1H NMR of 7 reveals that only
one single signal appears at 2.74 ppm (−CH2Ph), which means that
the three benzyl groups were chemically equal. 13C NMR of 7 also
confirms the speculation since only one single signal at 87.66 ppm
appears for three CH2 fragments in benzyl groups. C(22)-O(1)-
Ti(1) angel becomes 175.33◦, which means that the existence of a
d-p ␲ bond between isolated electron pairs at the oxygen atom and
the unoccupied Ti(3d) or phenyl ␲ system makes the bond have
partial sp-hybridized character. Though we have tried to deter-
mine the molecular structure of both 5a and 5b, several attempts
to develop single crystals failed.
3.2. Styrene polymerization
These titanium complexes proved to be good catalysts for
styrene polymerization in the presence of MMAO. Even when the
styrene polymerization was carried out at above 100 ◦C for several
hours, moderate to high activities could be achieved with excellent
syndioselectivity.
Initially, (mesityloxy)titanium(IV) chloride was tried in styrene
polymerization at 110 ◦C. The products were a mixture of syn-
diotactic polystyrene (sPS, MEK-insoluble fraction) and atactic
polystyrene (MEK-soluble fraction). To our delight, 13C NMR spec-
troscopy of the MEK insoluble part of the resulting polymer exhibits
a single sharp resonance at 145.2 ppm, which is a typical signal for
the phenyl ipso carbon atom of the highly syndiotactic polystyrene,
and can be attributed to the rrrr structure of the sPS [9a]. The highly
syndiotacticity was further confirmed by its melting temperature
(Tm = 268 ◦C) [2]. This result indicates a simple phenolate exhibit
thermal stability, and could promote the styrene syndiotactic poly-
merization (Table 2).
Since catalyst would be introduced into the polymerization sys-
tem as a solution in organic solvent, blank experiments were carried
out in 1 mL organic solvent in the presence of MMAO (4000 Al/Ti
ratio) at110 ◦C. The resulting polymer is soluble in methyl ethyl
ketone (MEK), suggesting only atactic polystyrene was generated.
Solvent (only 1 mL) affected the results considerably, and the yield
Table 2
Effects of solvents to styrene polymerization with 6b/MMAO.
Entrya
Solvent
Dielectric constant (ε)
Styrene polymerization with 6b/MMAO
Blank experimentf
aPS (g)
Activityb
Mn
PDId
sPS/PS (%)e
aPSe (g)
c,d
1
2
3
PhCH3
PhCl
PhBr
CH2Cl2
CH2Cl2
2.3
5.7
5.4
9.1
9.1
1.33
0.57
0.82
–
1.64
0.84
1.12
–
2.56
1.87
2.76
–
>99
>99
>99
–
0.04
0.09
0.33
6.55
16.70
0.29
0.38
0.51
4.39
4
5g
–
–
–
–
a
Condition: 6b (10 ␮mol) in solvent (1 mL), MMAO, Al/Ti = 4000, temperature of bath = 110 ◦C, t = 60 min, Vstyrene = 30 mL, preaged time = 15 min.
b
c
d
e
f
105 g sPS/mol(Ti) h.
Mn for sPS, 104 g/mol.
For sPS, determined by GPC.
As the amount of aPS was smaller than that produced in the blank experiments, it was concluded that sPS was afforded exclusively.
Blank experiment. MMAO, Al/Ti = 4000, temperature of bath = 110 ◦C, t = 60 min, Vstyrene = 30 mL.
6b (10 ␮mol) in CH2Cl2 (2 mL).
g

C. Xu et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 77–82
81
Table 3
Polymerization of styrene with 6b/MMAO.
Entrya
6b (␮mol)
Al/Ti
Time (min)
Tb (◦C)
Insoluble part
Activityc
Soluble part
aPS (g)
aPS (g)g
d,e
Mn
PDIe
sPS/PS (%)f
1
2
3
4
10
10
10
10
10
10
5
2000
3000
4000
4000
4000
4000
4000
4000
4000
60
60
60
90
60
60
60
60
60
110
110
110
110
130
110
110
110
110
0.62
0.91
1.33
1.45
1.09
0.87
1.10
0.80
2.60
1.86
1.72
1.64
1.89
0.97
0.43
1.38
1.37
0.50
2.85
2.67
2.56
2.30
2.37
4.47
2.51
2.61
1.99
>99
>99
>99
>99
>99
>99
>99
>99
>99
0.03
0.05
0.04
0.11
0.13
0.11
0.16
0.06
0.23
0.25
0.29
0.36
0.27
0.29
0.23
0.31
5
6h
7
8
15
10
9i
a
Condition: 6b in toluene (1 mL), Vstyrene = 30 mL, aging time = 15 min.
b
c
d
e
f
Temperature of bath, under 1 atm N2 atmosphere.
105 g sPS/mol(Ti) h.
104 g/mol.
Determined by GPC.
As the amount of aPS was smaller than that produced in the blank experiments, it was concluded that sPS was afforded exclusively.
Blank experiment, only MMAO solution and styrene was added.
6b (10 ␮mol in 1 mL toluene) was added directly without pretreatment with MMAO.
Catalyst is CpTiCl3.
g
h
i
termination way, and high temperature prefers chain transfer to
chain propagation. Furthermore, improving Al/Ti as well as the
polymerization temperature from 110 to 130 degree narrowed
PDI of sPS, suggesting that it takes time to form the catalytic
species, and high temperature as well as more MMAO benefit the
formation of active species. This guess is also supported by the fact
that a wide PDI was achieved when 6b was used directly without
aging. When CpTiCl3 was used in the presence of 4000 Al/Ti ratio
at 110 ◦C for 1 h, sPS was formed in 2.60 g sPS/mol(Ti) h.
All complexes were tested active to catalyze the styrene
polymerization sydiotactically at 110 ◦C and 4000 Al/Ti ratio
(Table 4). Of them, complex 5d bearing mesityloxy group
gave the lowest activity (entry 6). Replacing 2,6-dimethyl
with 2,6-di-tert-butyl resulted in the activity improvement
from 0.96 × 105 g sPS/mol(Ti) h to 1.62 × 105 g sPS/mol(Ti) h
(entries 5, 6). The highest activity (1.95 × 105 g sPS/mol(Ti) h)
was achieved when 2-(tert-butyl)-4-methyl-phenolate titanium
trichloride 5e was used (entry 7). Complex 5a derived from
2,6-bis((phenylthio)methyl) group gave a comparable activity
with that of 5e (entry 1 vs 7). Complex 5b bearing steric 2,6-
bis((diisopropylamino)methyl)phenolate showed an even lower
activity than that of 5c. Relative to 5, both ArOTi(NMe2)3 6 and
ArOTiBn3 7 catalyzed the styrene syndiotactic polymerization
in lower yields at 110 ◦C (entries 10–12). Thus, under the same
polymerization conditions, 6c exhibited lower activity than that
of 5c (entries 5 vs 11). The activity variation probably resulted
from the higher BDEs of both Ti N bond and Ti C bond than
that of Ti Cl bond [20]. When 5e, 6b, or 7 was used without
aging, the activities decreased apparently in all cases, whereas
5e is still the most active one. The complexes were stable over
time. However, prolonging the polymerization time led to the
catalyst wrap seriously. Consequently, moderate activities were
observed when either 5a or 5e was used for 4 h at 110 ◦C (entries
2 and 8). Even when the polymerization was carried out at 130 ◦C
for 2 h, 0.80 × 105 g sPS/mol(Ti) h and 1.11 × 105 g sPS/mol(Ti) h
were achieved for 5a and 5e separately (entries 3 and 9). This is
mainly because that the viscous reaction system encapsulated
catalyst species and retarded the contact between monomer and
the species. In all cases, sPS was afforded exclusively under the
conditions, as the amount of aPS was smaller than that produced
in the blank experiments even under these conditions. However,
[2,6-(CH2NiPr2)2-4-Me-C6H3O]TiBn3/Ph3CB(C6F5)4 only produced
aPS, probably in a cationic manner [11b].
Table 4
Influence of catalysts on styrene polymerization.a
d,e
Entrya
Cat.
sPS (g)b
Activityc
Mn
PDId
1
2f
5a
5a
5a
5b
5c
5d
5e
5e
5e
6b
6c
7
0.63
2.67
1.60
0.43
0.54
0.32
0.65
2.16
2.23
0.27
0.44
0.24
0.53
0.19
0.17
1.89
0.67
0.80
1.29
1.62
0.96
1.95
0.54
1.11
0.80
1.32
0.73
1.60
0.57
0.41
1.34
1.12
0.89
1.30
1.38
2.73
1.87
1.08
1.13
0.99
1.11
1.35
1.41
0.91
1.96
3.52
2.74
2.32
2.77
3.26
2.69
2.85
3.03
2.26
3.28
2.67
2.69
2.56
2.92
2.75
Mn of the resulting sPS varied around 1 × 104 g/mol except 5d.
5d produced sPS with 2.73 × 104 g/mol, probably owing to the small
methyl groups on the phenylortho positions. In other cases, little
effect of catalyst on molecular weight of the resulting polystyrene
was observed. The PDI of the polymers varied in the range of
2.26–3.52 without distinct trend. The reason is unclear.
3g
4
5
6
7
8f
9g
10
11
12
1h
14h
15h
4. Conclusions
5e
6b
7
In conclusion, though phenol derivatives were usually used as
anionic ancillary ligands to replace Cl− in half titanocenes, we found
that simple phenolate titanium(IV) complexes ArOTiX3 (X = NMe2,
Cl, CH2Ph) could promote the styrene syndiotactic polymerization
smoothly at high temperature (110–130 ◦C) for at least 4 hours.
The properties of phenolate have distinctive influence on the cat-
alytic activity. And (2-(tert-butyl)-6-methylphenoxy)titanium(IV)
trichlorides gave the highest activity (1.95 × 105 g sPS/mol(Ti) h)
in the styrene polymerization at 110 ◦C. Catalyst aging is beneficial
to the polymerization. Since the electronic and steric nature of
a
Condition: catalyst (10 ␮mol, in 1 mL toluene), MMAO, Al/Ti = 4000, temperature
of bath = 110 ◦C, t = 20 min, Vstyrene = 30 mL, aging time = 15 min.
b
As the amount of aPS was smaller than that produced in the blank experiments,
it was concluded that sPS was afforded exclusively.
c
105 g sPS/mol(Ti) h.
d
Determined by GPC.
e
104 g/mol.
f
Polymerization time = 4 h.
g
Temperature of bath = 130 ◦C, t = 2 h.
h
No catalyst aging.

82
C. Xu et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 77–82
phenolate could be modified readily by introducing different
groups, it provides a potential way to develop thermal stable
styrene syndiospecific polymerization catalyst.
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Acknowledgements
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We are grateful for the financial support from the National Nat-
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Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molcata.2013.
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