6. Olefin polymerization by zirconium enolatoimine complexes
6.2.4. Polymerization of 1-olefins
6.2.4. Polymerization of 1-olefins
Propylene and 1-hexene polymerization were studied with catalyst precursor 1b which exhibited the highest activity in ethylene polymerization. With 5 µmol of 1b, 1000 equivalent
MAO at 25 °C and 2 bar propylene pressure over a polymerization time of 2 hours, 5.2 g of viscous, sticky and colourless low-molecular weight polypropylene were obtained (Mw = 1.4 × 104 g mol-1, Mw/Mn = 2.1, Tg= -13 °C). This corresponds to an average activity of 12 000 mol(propylene) mol(Zr)-1 h-1. High temperature 13C NMR spectroscopy reveals the polymer to be atactic. The material is largely regioregular, with a small portion of observable regioirregular sequences (cf. Addendum, Figure S23 to S24 and Table S16 for NMR spectra and full assignments). 1H and 13C NMR spectroscopy (Figure S21 and S22) reveal vinylidene endgroups (=CH1H2 at δ = 4.76 and 4.83 ppm, CH3 resonace at δ = 1.8 ppm). While the CH2 resonance is observed at 111.6 ppm, the quaternary carbon atom (C2) resonance expected at δ = 144.6 pm was not observable despite addition of [Cr(acac)3] as relaxation aid. The formation of atactic polymer has also been observed for salicylaldiminato Zr complexes.132
Exposure of 1b/MAO (5 µmol/ 500 equiv) to neat 1-hexene at ambient temperature for 10 min afforded 2 g of atactic poly(1-hexene), corresponding to an average activity of 28 500 mol(C6H12) mol(Zr)-1 h-1. A molecular weight of Mw = 4 700 g/mol and a molecular weight distribution Mw/Mn of 2.3 were determined against polyethylene standards.
6.3. Summary and conclusion
A series of zirconium complexes [κ2-N,O-{2,6-R2C6H3N=C(CH3)C(H)=C(O)CF3]2ZrCl2 of enolatoimines with an electron-withdrawing substituted alkoxy moiety, bearing different substituents in the 2,6-positions of the N-aryl moiety were obtained by reaction of the corresponding ketoenamine with [ZrCl2(CH2Ph)2]. The steric bulk of the isopropyl analogous (R= iPr) is reflected both in the solution and solid state structure. Activation of the dichloro complexes with MAO affords active catalysts for the polymerization of ethylene to high molecular weight linear polyethylene and for the polymerization of propylene and 1-hexene to
fashion. Polymer molecular weight and catalyst productivities are strongly affected by the nature of the aryl substituents. An increasing steric bulk appears to retard activation of the precursors and/or chain growth, and even more pronounced chain transfer.
6.4. Experimental section
6.4.1. Materials and general considerations
All manipulations of air- and/or water-sensitive compounds were carried out under an inert atmosphere using standard glove box or Schlenk techniques. All glassware was flame-dried under vacuum before use. Toluene and benzene-d6 were distilled from sodium under argon.
Dichloromethane-d2 was distilled from CaH2. THF-d8 was distilled from Na/K alloy. 1,1,1-trifluoro-2,4-pentanedione were obtained from ABCR. ZrCl4, 2-methylaniline (purum p.a.,
≥99.5%) and 2,6-diisopropylaniline (90% purity, technical) were received from Aldrich.
[Zr(CH2Ph)4] and [Cl2Zr(CH2Ph)2*THF] were supplied by MCAT (Konstanz, Germany).
Ketoenamines (a-c) were prepared according to published procedures.105 Methylalumoxane (MAO), purchased from Crompton as a 10 wt.-% solution in toluene, was evaporated to dryness at room temperature in vacuo, and stored as a solid white powder. 1-Hexene was purchased from Aldrich, stirred over molecular sieves for several days, degassed by three freeze-pump thaw cycles, vacuum distilled, and stored under nitrogen prior to use.
NMR spectra were recorded on a Varian Unity INOVA 400 spectrometer. 1H and 13C NMR chemical shifts were referenced to the solvent signal. The assignment of chemical shifts is based on 1H-, 1H, 1H-gCOSY, {1H}13C-, DEPT135, 1H, 13C-gHMQC, 1H, 13C-gHMBC NMR experiments. High-temperature NMR spectroscopy of polypropylene was performed in 1,1,2,2-tetrachlorethane-d2 at 130 °C. A small amount of [Cr(acac)3] was added as relaxation aid. 13C NMR resonances of polypropylene were assigned according to [133].
Differential scanning calorimetry (DSC) was performed on a Netzsch Phoenix 204 F1 at a heating/cooling rate of 10 K min-1. DSC data reported are from second heating cycles.
Polymer crystallinities were calculated based on a melt enthalpy of 293 J g-1 for 100%
crystalline polyethylene. Gel permeation chromatography (GPC) was carried out in 1,2,4-trichlorobenzene at 160 °C at a flow rate of 1 mL min-1 on a Polymer Laboratories 220 instrument equipped with Olexis columns with differential refractive index, viscosity and light scattering (15° and 90°) detectors. Data reported were determined via linear PE calibration (Mw
< 30 000 g/mol), universal calibration (30 000 g/mol < Mw < 100 000 g/mol), and triple detection (Mw > 100 000 g/mol) employing the PL GPC-220 software algorithm. As the instrument records light scattering at only two angles, data analysis involves an iteration for the calculation of molecular weights and form factors for each measured interval. The instrument was calibrated with narrow polystyrene and polyethylene standards. Data given is referenced to linear polyethylene. Elemental analyses were performed up to 950°C on an Elementar Vario EL.
6.4.2. Synthesis of the ketoenamine ligands
To a stirred solution of 1,1,1-trifluoro-2,4-pentanedione (3.0 g, 2.36 ml, 19.5 mmol) and p-toluenesulfonic acid (catalytic amount, around 5 mg) in dry toluene (48 ml) was added the aniline component (23.4 mmol). The reaction mixture was refluxed at a temperature of 110 °C over night on a soxhlet extractor filled with molecular sieve in order to remove water. The solution was concentrated in vacuum, and the crude product was purified by column chromatography (d: hexane : Et2O = 7:1; e: hexane : Et2O = 5:1). The pure compound e was obtained as yellow liquid (yield: 2.19 g, 9.0 mmol, 46%). In the case of compound d, recrystallization of the resulting solid in ethanol afforded d as white crystals (yield: 1.29 g, 4.1 mmol, 21%).
4-(2,6-Diisopropylphenylamino)-1,1,1-trifluoro-pent-3-en-2-one (d)
1H NMR (400 MHz, C6D6, 25 °C): δ/ppm = 12.60 (br, 1H, NH), 7.10 (t, 3JHH = 8 Hz, 1H, para H to NH), 6.95 (d, 3JHH = 8 Hz, 2H, meta H to NH), 5.53 (s, 1H, vinylic HC=C), 2.77 (sept, 3JHH = 8 Hz, 2H, iPr CH), 1.24 (s, 3H, CH3), 0.98 (d, 3JHH = 7 Hz, 6H, iPrCH3), 0.91 (d, 3JHH = 8 Hz, 6H, iPrCH3). 13C NMR (100 MHz, C6D6, 25 °C): δ/ppm = 177.23 (q, 2JCF = 32.7 Hz, CF3CO), 170.36 (vinylic C), 145.67 (ortho C), 132.36 (ipso C), 129.41 (para C), 124.10 (meta C), 118.56 (q, 1JCF = 288.2 Hz, CF3), 89.76 (q, 4JCF = 1.2 Hz, vinylic CH), 28.86 (iPr CH), 24.19 (iPr CH3), 22.39 (iPr CH3), 18.97 (CH3). 19F NMR (375 MHz, C6D6, 25°C): δ/ppm = -76.53 (s, CF3CO).
Anal. calcd. (%) for C17H22F3NO: C, 65.16; H, 7.08; N, 4.47; Found: C, 65.17; H, 7.17; N, 4.53.
MS (m/z, %): 314.3 (100.0, [M+H]+).
4-(2-methylphenylamino)-1,1,1-trifluoro-pent-3-en-2-one (e)
NH O F3C
1 2
3 4 5 6
1H NMR (400 MHz, C6D6, 25 °C): δ/ppm = 12.50 (br, 1H, NH), 6.98-6.86 (m, 3H, aromatic H), 6.58 (d, 3JHH = 7.2 Hz, 1H, aromatic H 3), 5.44 (s, 1H, vinylic HC=C), 1.85 (s, 3H, aromatic CH3), 1.39 (s, 3H, CH3). 13C NMR (100 MHz, C6D6, 25°C): δ/ppm = 176.65 (q, 2JCF = 31.5 Hz, C=O), 168.70 (vinylic C), 136.03 (s, C 1), 133.76 (s, C 2), 131.44 (C 3), 127.92 (C 4), 126.80 (C 5), 126.25 (C 6), 118.50 (q, 1JCF = 287.4 Hz, CF3), 90.62 (vinylic CH), 19.32 (s, CH3), 17.41 (aromatic CH3). 19F NMR (376 MHz, C6D6, 25 °C): δ/ppm = -76.75 (s, CF3CO). MS (m/z, %):
243.9 (100.0, [M+H]+). No elemental analysis was applied due to the liquid consistency of the compound.
6.4.3. Synthesis of dichloro complexes
A solution of ketoenamine (0.1 mmol) in toluene was added dropwise to a toluenic solution of [Cl2Zr(CH2Ph)2*THF] (0.05 mmol) at - 30°C. A colour change was observed immediately. By evaporation of the solvent in vacuo, analytically pure material was obtained.
Dichloro-bis[к2-N,O-4-phenylimino-1,1,1-trifluoropent-2-en-2-olato]zirconium(IV) (1a)
1H NMR (400 MHz, THF-d8, 25°C): δ/ppm= 7.39 (t, 3JHH= 7.8 Hz, 1 H, para H), 7.27-7.22 (m, 2H, meta H), 6.99 (d, 3JHH= 7.2 Hz, 2H, ortho H), 5.88 (s, 1H, vinylic CH), 1.93 (s, 3H, CH3).
13C NMR (100 MHz, THF-d8, 25°C): δ/ppm= 177.43 (imine C), 155.66 (q, 2JCF= 35 Hz, CF3CO), 147.28 (ipso C), 130.24 (para C), 127.59 (meta C), 124.38 (ortho C), 119.58 (q, 1JCF= 279 Hz, CF3), 105.14 (vinylic C), 24.48 (CH3). 19F NMR (376 MHz, THF-d8, 25°C): δ/ppm= -72.64 (s, CF3CO). Anal. calcd. (%) for C22H18Cl2F6N2O2Zr: C, 42.72; H, 2.93; N, 4.53; Found:
C, 42.62; H, 3.03; N, 4.54.
Dichloro-bis[к2 -N,O-4-(2,6-difluorophenylimino)-1,1,1-trifluoropent-2-en-2-olato]zirconium(IV) (1b)
1H NMR (400 MHz, CD2Cl2, 25°C): δ/ppm= 7.36-7.27 (m, 1H, para H), 7.05-6.97 (m, 2H, meta H), 6.03 (s, 1H, vinylic CH), 2.01 (s, 3H, CH3). 13C NMR (100 MHz, CD2Cl2, 25°C): δ/ppm=
180.93 (imine C), 156.87 (q, 2JCF= 36 Hz, CF3CO), 155.56 (d, 1JCF= 253 Hz, ortho C), 129.19 (para C), 123.66 (ipso C), 118.82 (q, 1JCF= 277 Hz, CF3), 112.85 (t, 2JCF= 19 Hz, meta C), 105.23 (vinylic C), 25.42 (CH3). 19F NMR (376 MHz, CD2Cl2, 25°C): δ/ppm= -74.24 (s, 3F, CF3CO), -118.09 (s, 2F, aryl F). Anal. calcd. (%) for C22H14Cl2F10N2O2Zr: C, 38.27; H, 2.04; N, 4.06; Found: C, 38.37; H, 2.07; N, 4.07.
Dichloro-bis[к2 -N,O-4-(2,6-dimethylphenylimino)-1,1,1-trifluoropent-2-en-2-olato]zirconium(IV) (1c)
1H NMR (400 MHz, C6D6, 25°C): δ/ppm= 6.92-6.89 (br, 3H, aryl H), 5.73 (s, 1H, vinylic CH), 2.22 (s, 6H, aryl CH3), 1.07 (s, 3H, CH3). 13C NMR (100 MHz, C6D6, 25°C): δ/ppm= 177.87 (imine C), 157.79 (q, 2JCF= 35 Hz, CF3CO), 146.39 (ipso C), 131.47 (ortho C), 129.72 (para C), 127.78 (meta C), 120.00 (q, 1JCF= 278 Hz, CF3), 105.04 (q, 3JCF= 3.1 Hz, vinylic C), 23.59 (CH3), 19.10 (aryl CH3). 19F NMR (376 MHz, C6D6, 25°C): δ/ppm= -73.74 (s, CF3CO). Anal.
calcd. (%) for C26H26Cl2F6N2O2Zr: C, 46.29; H, 3.88; N, 4.15; Found: C, 46.37; H, 3.99; N, 4.10.
Dichloro-bis[к2 -N,O-4-(2,6-diisopropylhenylimino)-1,1,1-trifluoropent-2-en-2-olato]zirconium(IV) (1d)
1H NMR (400 MHz, CD2Cl2, 25°C): δ/ppm= 7.06-6.99 (m, 1H, para H), 6.99-6.95 (d, 3JHH= 6.4 Hz, 1H, meta H),, 6.95-6.90 (d, 3JHH= 7.6, 1H, meta H), 5.84 (s, 1H, vinylic CH), 2.86 (sept,
3JHH= 6.6 Hz, 1H, iPr CH), 2.73 (sept, 3JHH= 6.8 Hz, 1H, iPr CH), 1.69 (s, 3H, CH3), 1.08 (d,
3JHH= 6.4 Hz, 3H, iPr CH3), 0.95 (d, 3JHH= 6.4 Hz, 3H, iPr CH3), 0.84 (d, 3JHH= 6.8 Hz, 1H, iPr CH3), 0.78 (d, 3JHH= 6.8 Hz, 1H, iPr CH3). 13C NMR (100 MHz, CD2Cl2, 25°C): δ/ppm= 177.89 (imine C), 156.16 (q, 2JCF= 35.3 Hz, CF3CO), 146.18 (ipso C), 142.10 (ortho C), 140.96 (ortho C), 128.13 (para C), 125.26 (meta C), 124.70 (meta C), 119.30 (q, 1JCF= 277.7 Hz, CF3), 103.73 (vinylic C), 26.65 (CH3), 28.45 (iPr CH), 28.37 (iPr CH), 25.33 (iPr CH3), 24.98 (iPr CH3), 24.80 (iPr CH3), 24.07 (iPr CH3). 19F NMR (376 MHz, CD2Cl2, 25°C): δ/ppm= -74.58 (s, 3F, CF3CO). Anal. calcd. (%) for C34H42Cl2F6N2O2Zr: C, 51.90; H, 5.38; N, 3.56; Found: C, 52.03;
H, 5.40; N, 3.58.
6.4.4. Synthesis of dibenzyl complexes
A solution of 1c (13.3 mg, 0.05 mmol) in toluene (5 mL) was added dropwise at -30 °C to a solution of [Zr(CH2C6H5)4] (11.4 mg, 0.025 mmol) in toluene (5 mL). The yellow solution turned to orange-reddish and was stirred for 1h. The volatiles were removed under vacuum. The crude product was recrystallized from pentane at -30 °C.
Dibenzyl-bis[κκκκ2 -N,O-4-(2,6-difluorophenylimino)-1,1,1-trifluoropent-2-en-2-olato]zirconium(IV) (2b)
1H NMR (400 MHz, Tol-d8, 25°C): δ/ppm = 7.06 (vt, 3JHH = 7.0 Hz, 2H, meta CH of benzyl), 6.88 (t, 3JHH = 7.2 Hz, 1H, para CH of benzyl), 6.80 (d, 3JHH = 7.6 Hz, 2H, ortho CH of benzyl), 6.40 (m, 3H, aryl), 5.18 (s, 1H, vinylic CH=C), 2.12 (s, 2H, CH2), 1.45 (s, 3H, CH3). 13C NMR (101 MHz, Tol-d8, 25°C): δ/ppm = 176.8 (imine C), 155.7 (d, 1JCF = 251.8 Hz, ortho C), 155.6 (q, 2JCF = 34.3 Hz, CF3CO), 154.4 (br, ipso C), 143.5 (ipso C of benzyl), 129.5 (ortho C of benzyl), 129.3 (meta C of benzyl), 127.2 (m, para C), 123.3 (para C of benzyl), 119.8 (q, 1JCF = 279.6 Hz, CF3), 112.2 (br, meta C), 103.2 (vinylic C=CH), 68.0 (CH2 of benzyl), 24.8 (CH3). 19F NMR (376 MHz, Tol-d8, 25°C): δ/ppm = -73.73 (s, 3F, CF3CO), 114.27 (s, 1F, aryl F) and -116.93 (s, 1F, aryl F).
6.4.5. Ethylene or propylene polymerization
Ethylene and propylene, respectively, polymerization was carried out under atmospheric pressure in a thermostated 500 mL double-mantle glass reactor equipped with a mechanical stirrer. Toluene (200 mL) was introduced into the nitrogen-purged reactor and stirred vigorously (500 rpm). The toluene was kept at the desired polymerization temperature, and then the ethylene gas feed was started. After 15 min, polymerization was initiated by the addition of a
reactor, such that the total volume of toluene in the reactor was 250 mL. After the desired reaction time, ethanol (5 mL) was added to terminate the polymerization reaction, and the ethylene gas feed was stopped. The reaction mixture was added to acidified methanol (1 mL of concentrated HCl in 500 mL of methanol). Solid polyethylene was recovered by filtration, washed with methanol, and dried at 50 °C for 24 h in a vacuum oven.
6.4.6. 1-Hexene polymerization
In the glove box, 500 eq. MAO (145 mg) was added to 5g 1-hexene in a Schlenk flask.
To this solution was added a dichloromethane solution of 5 µmol of the catalyst precursor 1b (3.4 mg), and the reaction was allowed to proceed with continuous stirring for the desired period of time. The polymerization was quenched with 5 mL of acidified (HCl) methanol. The polymer was extracted with hexane and dried under vacuum.
6.4.7. X-ray crystal structure determinations
The data collection was performed at 100 K on a STOE IPDS-II diffractometer equipped with a graphite-monochromated radiation source (λ = 0.71073 Å) and an image plate detection system. A crystal mounted on a fine glass fibre with silicon grease was employed. The selection, integration and averaging procedure of the measured reflex intensities, the determination of the unit cell dimensions by a least-squares fit of the 2Θ values, data reduction, LP-correction and space group determination were performed using the X-Area software package of the diffractometer. A semiempirical absorption correction was performed. The structure was solved by direct methods (SHELXS-97), completed with difference Fourier syntheses, and refined with full-matrix least-squares using SHELXL-97 minimizing w(Fo2 – Fc2)2. Weighted R factor (wR) and the goodness of fit S are based on F2; the conventional R factor (R) is based on F. All non-hydrogen atoms were refined with anisotropic displacement parameters. All scattering factors
and anomalous dispersion factors are provided by the SHELXL-97 program. The hydrogen atom positions were located in the difference Fourier map and refined isotropically.
Table 6-3. Details of the crystal structure determination of d.
formula C17 H22 F3 N O
Table 6-4. Details of the crystal structure determination of 1a.
formula C22 H18 Cl2 F6 N2 O2 Zr
Fw 618.50
cryst. size, mm 0.4 x 0.383 x 0.35
space group C2/c
a, Å 9.7961(6)
b, Å 14.0461(8)
c, Å 18.3236(12)
α, deg. 90.00
β, deg. 95.011(5)
γ, deg. 90.00
V, Å3 2511.6(3)
Z 4
δcalc, g cm-3 1.636
T, K 100
µ, mm-1 0.717
F(000) 1232
Θmax, deg. 28.06
No. of rflns measd. 18366
No. of unique rflns 3015
No. of rflns I > 2σ(I) 2643
R1, I > 2σ(I) a 0.0394
R1, all data 0.0473
wR2a 0.0736
diff. Fourier peak min./ max., e Å-3 -0.50/ 0.45
a R1 = Σ||F0-Fc||/ Σ|F0|, wR2 = [Σ(w(F02
-Fc2
)2)/Σ(w(F02
)2)]1/2
Table 6-5. Details of the crystal structure determination of 1b.
formula C22 H14 Cl2 F10 N2 O2 Zr
Fw 690.47
cryst. size, mm 0.38 x 0.36 x 0.35
space group C2/c
a, Å 10.313(9)
b, Å 13.317(1)
c, Å 19.161(2)
α, deg. 90.00
β, deg. 95.468(9)
γ, deg. 90.00
V, Å3 2619.8(5)
Z 4
δcalc, g cm-3 1.751
T, K 100
µ, mm-1 0.719
F(000) 1360
Θmax, deg. 26.84
No. of rflns measd. 17115
No. of unique rflns 2789
No. of rflns I > 2σ(I) 2428
R1, I > 2σ(I) a 0.0301
R1, all data 0.0386
wR2a 0.072
diff. Fourier peak min./ max., e Å-3 -0.81/ 0.36
a R1 = Σ||F0-Fc||/ Σ|F0|, wR2 = [Σ(w(F02
-Fc2
)2)/Σ(w(F02
)2)]1/2
Table 6-6. Details of the crystal structure determination of 1d.
formula C34 H42 Cl2 F6 N2 O2 Zr
Fw 786.82
cryst. size, mm 0.3 x 0.23 x 0.15
space group P-1
a, Å 11.652(3)
b, Å 11.781(2)
c, Å 13.684(2)
α, deg. 74.216(12)
β, deg. 82.723(17)
γ, deg. 86.135(17)
V, Å3 1791.9(6)
Z 2
δcalc, g cm-3 1.458
T, K 100
µ, mm-1 0.52
F(000) 808
Θmax, deg. 27.72
No. of rflns measd. 28369
No. of unique rflns 8067
No. of rflns I > 2σ(I) 6610
R1, I > 2σ(I) a 0.0364
R1, all data 0.0516
wR2a 0.0949
diff. Fourier peak min./ max., e Å-3 -0.69/ 0.74
a R1 = Σ||F0-Fc||/ Σ|F0|, wR2 = [Σ(w(F02
-Fc2
)2)/Σ(w(F02
)2)]1/2
Chapter 7
Synthesis and characterization of copolymers from ethylene and cyclopentene by an ortho-fluorinated
enolatoimine Ti complex
Abstract: The synthesis and characterization of ethylene/cyclopentene copolymers is reported.
Living polymerization of ethylene and cyclopentene with a catalyst derived from ortho-fluorinated bis(ketoenamine)titanium catalyst [(o-F2C6H3N=CMeCHC=CF3O)2TiCl2](1) yields copolymers with exclusive cis-1,2 cyclopentane units, high molecular weights and low polydispersities. The effects of cyclopentene concentration, ethylene pressure and reaction temperature on cyclopentene incorporation were determined; under appropriate reaction conditions up to 50 mol % cyclopentene were incorporated to afford a perfectly alternating iso-enriched copolymer ([rr]~ 0.51). The glass transition temperatures of the polymers were found to range from -26 °C (19 mol % cyclopentene) to 21 °C (50 mol % cyclopentene).
7.1. Introduction
Over the last two decades, the development of homogeneous catalysts for polymerization has rendered possible the synthesis of a broad range of macromolecules with tailored structures.
Catalysts are now accessible that allow control of polymer stereochemistry, molecular weight and composition.12a,39,134 However, it still remains challenging to synthesize ordered copolymers with e.g. block or alternating sequences, particularly in olefin polymerization. Living polymerization provides new routes to block copolymers by the sequential addition of monomer.94a One of the most appealing block copolymer structures are those with ‘‘hard’’ or semicrystalline end blocks (e.g. PE, iPP, sPP) and amorphous midblocks (e.g. aPP, poly (E-co-P), LLPE); triblock copolymers of this type have been shown to behave as thermoplastic elastomers.50,59,105,135
A generic route to alternating copolymers is polymerization with catalysts which are unable of homopolymerization of either monomers, but capable of cross-propagation.136 This method is rather rare owing to the little number of complexes known to feature such polymerization properties. An alternative for the synthesis of alternating polyolefins is the utilization of a catalyst where the rate of incorporation of one monomer is significantly greater than that of the other monomer. Increasing the concentration of the less readily polymerized monomer, alternating polymer is obtained in some cases provided that the consecutive insertion of the less favourable monomer is hindered or extremely slow.
Cycloolefin copolymers have been targeted as a new class of engineering plastics.137 According to the type of cycloolefin, the composition and sequence distribution of comonomers in the chain, polymers with various microstructures can be prepared. Random ethylene–
cycloolefin copolymers are amorphous glasses with a wide range of glass transition temperatures, from low to elevated temperatures. Therefore, these materials have promising characteristics as elastomers. Moreover, cycloolefin copolymers are of interest due to the
combination of thermal stability, good transparency, high refractive index, high stiffness or softness, chemical resistance and good processability.137a Although a number of cyclopentene-ethylene copolymerizations using several metallocene catalysts were performed recently, these copolymers have a mixed microstructure of 1,2- and 1,3-enchained cyclopentene units together with ethylene-ethylene and cyclopentene-cyclopentene monomer sequences.138
Recently, Coates and co-workers prepared and assigned the isotactic alternating ethylene–cyclopentene copolymer by an indirect route involving stereoselective ring-opening metathesis followed by hydrogenation; the observation of a melting point of 185 °C corroborates Natta’s assignment of this copolymer as the erythro-diisotactic alternating E–cP copolymer.139 More recently, highly alternating copolymers with isotactic 1,2-cyclopentene incorporation have been obtained employing a half-titanocene catalyst [Me2Si(Ind)(NtBu)]TiCl2. (Mn vs PS = 3.4 x 104 g mol-1, Mw/Mn = 1.9, TOF= 39 kg mol(Ti)-1 h-1).140
Given the established living nature of ethylene polymerization with fluorinated imineenolato-based titanium catalyst (chapter 4),105 the synthesis of alternating ethylene/cyclopentene copolymers with this catalytic system was studied.
7.2. Results and discussion
7.2.1. Cyclopentene/ethylene copolymerization
TiCl2 2 N
O F3C
F
F
+
MAO
1
Figure 7-1. Copolymerization of ethylene and cyclopentene using 1/MAO (polymer is
Copolymerization of cyclopentene and ethylene was carried out using 1 with MAO under a variety of conditions to yield ethylene/cyclopentene copolymers (Figure 7-1, Table 7-1).
Under 1 bar of ethylene, complex 1 copolymerizes cyclopentene (0.67 M) with ethylene to a random amorphous copolymer with a cyclopentene content of 14 mol % over 1h at 25 °C (Table 7-1, Entry 1). The number-average molecular weight Mn is ca. 2.6 x 105 g/mol. Furthermore, the polydispersity is extremely narrow (Mw/Mn = 1.03), consistent with the aforementioned living character of polymerization with 1. The melting point (Tm = 131 °C) is slightly lower than that of the polyethylene obtained in the absence of cyclopentene under otherwise identical conditions (139 °C) as a consequence of the incorporation of cyclopentene. Complex 1 displays a remarkable catalytic activity of 105 g mol (Ti)-1h-1 and a high degree of incorporation at low cyclopentene concentration. However, the yield of polymer decreased with decreasing ethylene concentration and increasing cyclopentene incorporation. (Table 7-1, entries 5-7, 9). This is due to the higher insertion rate of ethylene than that of cyclopentene. Overall, molecular weight distributions of the cyclopentene/ethylene copolymers (Mw/Mn = 1.03-1.43) are much narrower than conventional polymers that were obtained by homogeneous catalysts such as metallocenes.138,140,141
Table 7-1: Results of ethylene/cyclopentene copolymerization experiments with 1.[a] polyethylene standard. e 20 µmol of catalyst 1, 250 mL glass reactor. f Determined by GPC vs. polystyrene standards. g 20 µmol of [{F5C6N=CHC6H2(tBu)2O}2TiCl2], 250 mL glass reactor.
All polymers studied contain isolated cyclopentene units exclusively, in accordance with the rate of cyclopentene homopolymerization being very low. In addition, the cyclopentene units are incorporated solely via cis-1,2-enchainment between 0 and 40 °C as determined by 13C NMR. 142 It is known that cyclopentene-ethylene copolymers derived from metallocene catalysts show a significant amount of 1,3-enchainment of cyclopentene.138a,139 Using metallocene catalysts, the polymerization must be carried out with appreciable ethylene concentration in order to observe perfect 1,2-enchainment of cyclopentene units in the copolymers; under these conditions, cyclopentene incorporation is low.139 Cyclopentene incorporation, again determined by 13C NMR, correlates with the molar fraction of the comomer in the reaction mixture. In 13C NMR spectrum (Figure 7-2) of the polymer synthesized by 1 at a relatively high ethylene
29.1-30.1 ppm are due to the ethylene-ethylene sequences.
Figure 7-2. 13C NMR spectrum (100 MHz) of the cyclopentene-ethylene copolymer synthesized by 1/MAO (Table 7-1, entry 1).
The cyclopentene content increased up to 50 mol % by decreasing the ethylene pressure.
When the ethylene pressure was 0.11 bar and the concentration of cyclopentene is increased to 7.90 mol L-1, perfectly alternating cyclopentene-ethylene copolymer was formed (Figure 7-3;
Table 7-1, entry 9). Under identical reaction conditions in a comparative experiment, poly(ethylene-co-cyclopentene) with 29 mol % incorporation rate of cyclopentene was formed with a fluorinated bis(phenoxyimine)titanium catalyst [{F5C6N=CHC6H2(tBu)2O}2TiCl2]. By comparison, an alternating copolymer (47 %) is obtained with the phenoxyimine system only at much higher cyclopentene/ethylene ratios, with a low activity of 30 g mol-1 h-1 (Mn vs PS = 2.1 x 104 g mol-1, Mw/Mn = 1.34). 143 On the basis of the microstructural assignments of these polymers (vide supra), the alternating copolymers made prepared by 1/MAO are slightly isotactic, which leads to a melting point of 149 °C. The Tm of linear PE prepared with 1/MAO is
around 139°C. Increasing the cyclopentene portion in the polymer gave rise firstly to a disappearance of a sharp melting peak, determined by DSC, and at higher cyclopentene incorporation (XCPE = 36 mol %)subsequently a recurrence of a melting peak at >110 °C. The higher the cyclopentene content, the higher the melting point which is due to the slight preference of isotacticity of the cyclopentene-ethylene units. The Tg increased as well by increasing the incorporation of cyclopentene.
Figure 7-3: 13C NMR spectrum (100 MHz) of alternating cyclopentene-ethylene copolymer synthesized by 1/MAO (Table 7-1, entry 9).
The molecular weight distribution was narrow (Mw/Mn = 1.22, Figure 7-4), indicative of living polymerization. Another hint of the livingness of the catalyst system is that the polymer molecular weight determined (Mn = 8 x 103 g/mol) agrees within experimental error with the theoretical value calculated from the amount of polymer obtained and catalyst precursor employed (Mn(calc) = 9 x 103 g/mol). This shows that the catalyst precursor is rapidly converted to a single type of active species, chains are initiated simultaneously, and essentially no chain
contrast, it was reported that the fraction of active chains, estimated by the polymer yield, catalyst amount and Mn of the polymer, in ethylene/cyclopentene copolymerization using a fluorinated bis(phenoxyimine)titanium catalyst is less than 0.5.143
Figure 7-4. GPC profile of poly(ethylene-co-cyclopentene) obtained with complex 1 at 25 ºC (Table 7-1, entry 7, Mn = 8×103 g mol-1, Mw/Mn = 1.22).
In terms of the influence of polymerization temperature on the polymer properties, polymerizations at 25 °C and 40 °C under otherwise exactly identical conditions have been carried out (cf. Table 7-1, entries 7-8). At higher temperature, the catalyst showed a higher activity, but the molecular weight distribution was broader. Otherwise, the copolymers obtained exhibited similar characteristics; namely a similar incorporation rate of cyclopentene and comparable molecular weight Mn.
7.3. Summary and conclusion
The catalyst system [(o-F2C6H3N=CMeCHC=CF3O)2TiCl2] 1 /MAO promotes living copolymerization of ethylene with cyclopentene. This provides access to copolymers which cover a wide range of crystallinities, and glass transition and melt temperatures, as previously demonstrated for ethylene/ cyclopentene copolymers prepared with other catalysts.140,141,143
The system studied here appears useful for the preparation of perfectly alternating copolymers. By
comparison to related N^O-chelated phenoxyimine-based catalysts, cyclopentene competes better with ethylene for incorporation, and alternating copolymer can be obtained at lower cyclopentene/ethylene ratios and with significantly high catalyst activity. Also, all metal sites are active during the formation of the alternating copolymer. The material formed is iso-enriched, what is reflected by a Tm= 149 °C.
7.4. Experimental section
7.4.1. Materials and general considerations
All manipulations of air- and/or water-sensitive compounds were carried out under an inert atmosphere using standard glove box or Schlenk techniques. All glassware was flame-dried under vacuum before use. Toluene was distilled from sodium under argon. Complex 1 and the fluorinated phenoxyimine Ti complex were prepared according to published procedures.105,143 Methylalumoxane (MAO), purchased from Crompton as a 10 wt.-% solution in toluene, was evaporated to dryness at room temperature in vacuo, and stored as a solid white powder.
High-temperature NMR spectra of copolymers were recorded on a Varian Unity INOVA 400 spectrometer in 1,1,2,2-tetrachlorethane-d2 at 130 °C. 1H and 13C NMR chemical shifts were referenced to the solvent signal. A small amount of [Cr(acac)3] was added as relaxation aid.
Differential scanning calorimetry (DSC) was performed on a Netzsch Phoenix 204 F1 at a heating/cooling rate of 10 K min-1. DSC data reported are from second heating cycles. Polymer crystallinities were calculated based on a melt enthalpy of 293 J g-1 for 100% crystalline polyethylene. Gel permeation chromatography (GPC) was carried out in 1,2,4-trichlorobenzene at 160 °C at a flow rate of 1 mL min-1on a Polymer Laboratories 220 instrument equipped with Olexis columns with differential refractive index, viscosity and light scattering (15° and 90°) detectors. Data reported were determined via linear PE calibration (Mw < 30 000 g/mol),
g/mol) employing the PL GPC-220 software algorithm. As the instrument records light scattering at only two angles, data analysis involves an iteration for the calculation of molecular weights and form factors for each measured interval. The instrument was calibrated with narrow polystyrene and polyethylene standards. Data given is referenced to linear polyethylene.
7.4.2. Synthesis of poly(ethylene-co-cyclopentene)
Ethylene-cyclopentene copolymerization was carried out under varied pressure of ethylene in a 500 mL or 250 mL glass reactor equipped with a mechanical stirrer. Toluene and cyclopentene were introduced into the nitrogen-purged reactor and stirred vigorously (250 rpm).
The mixture was kept at the desired polymerization temperature, and then the ethylene gas feed
The mixture was kept at the desired polymerization temperature, and then the ethylene gas feed