7. Synthesis and characterization of copolymers from ethylene and cyclopentene by an
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.