Synthesis of low-molecular-weight poly-α-olefins using silicon-bridged zirconocene catalyst for lubricant basestock

2.1 Materials

Ph₂Si(Cp)(9-Flu)ZrCl₂ was purchased from J&K Co.. Methylaluminoxane (MAO, 1.4 mol L⁻¹ in toluene) was purchased from Albemarle. 1-Hexene, 1-octene, 1-decene, 1-dodecene and 1-tetradecene were obtained from J&K Co., dried over CaH₂ and subsequently distilled under N₂. 1-Butene was obtained from Tianjin Summit Specialty Gases and purified through a deoxygenating catalyst and 4-Å molecular sieves prior to use. Toluene was dried by refluxing over Na and distilling under N₂.

2.2 Oligomerization procedure

The oligomerization was performed in a 0.1-L glass reactor (Lab Crest) with graduation. After evacuation and flushing with N₂ (three times), the α-olefin, solutions of MAO and Ph₂Si(Cp)(9-Flu)ZrCl₂ in toluene were injected sequentially into the reactor, which was sealed and heated to a desired temperature and maintained for 3 h. The resulting oligomer was washed sequentially with NaOH solution and water. The organic phase was dried (Na₂SO₄) and filtered. The toluene and unreacted α-olefin were distilled off to obtain an oily oligomer.

The co-oligomerization of 1-decene and 1-butene was performed in the same glass reactor. After evacuation and flushing with N₂ (three times) and then 1-butene gas, the reactor temperature was decreased to −10 °C and maintained at that temperature. Then 1-butene was continuously charged into the cold reactor and liquefied. When the desired volume of 1-butene was reached, the 1-decene, solutions of MAO and Ph₂Si(Cp)(9-Flu)ZrCl₂ in toluene were injected sequentially into the reactor, which was sealed and heated to a desired temperature and maintained until the reactor pressure had decreased to −0.05 MPa. The resulting oligomer was treated by using method mentioned above.

2.3 Oligomer characterization

KV was determined in a classical manner, using an Ubbelohde viscometer, according to ASTM D 445. VI was calculated from the KV data measured at 40 and 100 °C according to ASTM D 2770. The molecular weight (MW) and molecular weight distribution (MWD) of the oligomers were determined using an Agilent GPC 220 gel permeation chromatograph equipped with two MIXED-B columns. The columns were calibrated with standard polystyrenes of narrow molar mass distribution. ¹H and ¹³C NMR spectra of the oligomers dissolved in CDCl₃ were recorded using a Bruker AVANCE III 400 spectrometer at operating frequencies of 400 and 100 MHz, respectively.

3.1 Reaction performance

The oligomerizations of α-olefins were catalyzed by Ph₂Si(Cp)(9-Flu)ZrCl₂ in the presence of MAO, and the influence of the α-olefin on the catalyst activity and the properties of the oligomers resulted were presented in Table 1.

The oligomerization activity of α-olefins increased upon increasing the chain size of the α-olefins from 1-hexene to 1-decene, and then decreased upon further increasing the chain size of the α-olefins from 1-decene to 1-tetradecene. The high conversions in excess of 90% of olefin were presented on the oligomerization of 1-hexene, 1-octene and 1-decene. In fact, the oligomerization degree decreased with increasing the chain size of the α-olefins from 1-hexene to 1-tetradecene (see Table 1), so the oligomerization activities of the α-olefins decreased with increasing the chain size of the α-olefins. The oligomerization activity is affected by three factors, the lateral size of the oligomer chain, the chain size of the monomer, and the viscosity of the oligomer. First, a long lateral oligomer chain opens the π-ligands wider than a shorter one, making the insertion of the monomer easier (Grumel et al., 2001). Second, it is typical that the insertion of a larger monomer into an oligomer chain is more difficult than that of a smaller one, because of steric hindrance. Third, an oligomer of high viscosity would inhibit the transfer of the α-olefin to decrease the activity (Fan et al., 2000). A combination of these effects would likely account for the observed trends in the activity of the polymerization (Kim et al., 2000). The effect of the viscosity can be neglected here because of the similar viscosities of the oligomers. Upon proceeding from 1-decene to 1-tetradecene, the second factor presumably plays a major role, resulting decreased activity. When 1-butene as co-monomer was added in 1-decene, the activity and conversion of α-olefins decreased obviously. The third factor might play a major role due to obviously increased viscosity.

The similar MWs of the oligomers formed from α-olefins were presented in Table 1, suggesting that small influence of side chain size of α-olefins on the oligomerization, although the poly-1-decene had the highest MW. Likewise, Brüll concluded that the MWs of various poly-1-olefins obtained using Me₂Si(2-methylbenz[e]indenyl)₂ZrCl₂ catalyst was independent of the α-olefin chain length (Brüll et al., 2000). The MW of oligomers increased with addition of 1-butene as co-monomer to 1-decene, and further increased with the increasing molar ratios of 1-butene to 1-decene. In the 1-decene-1-butene cooligomer, the oligomerization degree of 1-decene was similar with oligomer of 1-decene itself. The result means that the addition of 1-butene does not decrease the oligomerization of 1-decene itself, although the 1-butene already is inserted in the chain of the cooligomer. The grafting of poly-1-butene chain to poly-1-decene chain led to increase of the molecular weight. The normal distribution of polydispersities (M_w/M_n) of the oligomers produced from α-olefins varied from 2.06 to 2.25, presented in the MWD curves (see Table 1 and Fig. S2 in Supporting Information). Such narrow mass distributions were predicted by Schulz–Flory statistics for polymers arising from identical centers with a fixed rate of chain propagation and chain termination (Brintzinger et al., 1995).

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.07.003.

The similar MWs of the oligomers formed from α-olefins were presented in Table 1, suggesting that small influence of side chain size of α-olefins on the oligomerization, although the poly-1-decene had the highest MW. Likewise, Brüll concluded that the MWs of various poly-1-olefins obtained using Me₂Si(2-methylbenz[e]indenyl)₂ZrCl₂ catalyst was independent of the α-olefin chain length (Brüll et al., 2000). The MW of oligomers increased with addition of 1-butene as co-monomer to 1-decene, and further increased with the increasing molar ratios of 1-butene to 1-decene. In the 1-decene-1-butene cooligomer, the oligomerization degree of 1-decene was similar with oligomer of 1-decene itself. The result means that the addition of 1-butene does not decrease the oligomerization of 1-decene itself, although the 1-butene already is inserted in the chain of the cooligomer. The grafting of poly-1-butene chain to poly-1-decene chain led to increase of the molecular weight. The normal distribution of polydispersities (M_w/M_n) of the oligomers produced from α-olefins varied from 2.06 to 2.25, presented in the MWD curves (see Table 1 and Fig. S2 in Supporting Information). Such narrow mass distributions were predicted by Schulz–Flory statistics for polymers arising from identical centers with a fixed rate of chain propagation and chain termination (Brintzinger et al., 1995).

The KV and VI are critical factors affecting the potential utility of an oligomer as a lubricant basestock, and the VI is a more important factor than the KV when developing a lubricant (Ray et al., 2012). Table 1 revealed the influence of the structure of the α-olefins on the KV of the resulted oligomer. Similar viscosities were presented on the oligomers formed from various α-olefins, and poly-1-decene had the highest viscosity. This result was in consonance with changed trend of molecular weight of poly-α-olefins. The VI of the oligomer increased upon increasing the lateral size of the monomer from 1-hexene to 1-decene, but decrease thereafter. The VI is related to the branch ratio, and it increases with decreasing branch ratio (Wu, 1989), and linear alkanes have the lowest possible branch ratios and tend to have high VI (Denis, 1984). α-Olefin with large chain size could decrease the branch ratio due to the α-olefin molecular located at chain-end of oligomer increased length of the main chain. On the contrary, the long lateral size of oligomer would increase the branch ratio. So a combination of the above factors led to the decrease of VI of oligomer formed from 1-dodecene and tetradecene. The branch ratio of cooligomers formed from 1-decene and 1-butene obviously decreased with the increasing main chain length and decreasing the lateral size of the cooligomer, so increased VI of the cooligomer was presented and further increased with increasing molar ratio of 1-butene to 1-decene.

3.2 Influence of reaction conditions

Table 2 revealed the influences of the reaction temperature and Al/Zr molar ratio on the catalytic activity and the properties of the oligomer resulted. The catalytic activity increased upon increasing the temperature from 40 to 60 °C, but decreased thereafter. A higher temperature increased the propagation rate of α-olefins, but it also increased the deactivation rate of the active sites. A combination of these effects likely accounted for the observed temperature dependence of the catalytic activity. The MW decreased upon increasing the reaction temperature, indicating that chain propagation was favored at the lower temperatures, whereas chain termination occurred readily at higher temperature because it facilitated β-hydride elimination and termination through transfer to the co-catalyst (Fan et al., 2000). The KVs of the oligomers at 40 °C decreased upon the increasing temperature, in accordance with the trend in the MW. However, the KVs of the oligomers at 100 °C increased with increasing temperature from 40 to 60 °C, and then decreased thereafter, so the oligomer produced at 60 °C had the highest VI. Thus, the structures of the oligomers produced at different reaction temperatures were different, because the VIs were affected by both the branch ratio and the chain length.

The highest catalytic activity arose at an Al/Zr molar ratio of 300, decreasing thereafter. MAO can activate Zr sites to form appropriate activated state, but an excess of MAO can interfere with the formation of the active Zr species through the over-reduction of Zr species (Shao et al., 2014). The decreased trend of MWs was presented with increasing the Al/Zr molar ratio, because termination through transfer to the co-catalyst was favored at higher Al/Zr molar ratio. Similarly, the decreased trend of KVs and VIs of the oligomers was presented upon increasing the Al/Zr ratio, in accordance with the trend in the MWs. This changed trend was similar with our previous work, which used (Me₄Cp)₂ZrCl₂ as catalyst for co-oligomerization of 1-butene with 1-dodecene (Shao et al., 2014).

3.3 Microstructure of oligomers

The ¹³C NMR and ¹H NMR spectroscopy were used to characterize the microstructure of the oligomers. The ¹³C NMR spectra of the oligomer produced from 1-decene and mixture of 1-decene and 1-butene were presented in Fig. 1, and the ¹³C NMR spectra of the other oligomers were presented in Fig. S3 of the Supporting Information. The assignments of the signals were showed in Table S1 of the Supporting Information.

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Fig. 1 ¹³C NMR Spectra of the oligomer formed from 1-decene (a) and mixture of 1-decene and 1-butene (b).

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The ¹³C NMR and ¹H NMR spectroscopy were used to characterize the microstructure of the oligomers. The ¹³C NMR spectra of the oligomer produced from 1-decene and mixture of 1-decene and 1-butene were presented in Fig. 1, and the ¹³C NMR spectra of the other oligomers were presented in Fig. S3 of the Supporting Information. The assignments of the signals were showed in Table S1 of the Supporting Information.

According to Fig. 1(a), regular oligomerization of head-to-tail 1,2-insertion of 1-decene was presented, and signals at about 36.1 and 46.3 ppm, corresponding to oligomerization of head-to-head and tail-to-tail, was very weak (Huang et al., 2005). The oligomers of 1-decene existed as a mixture, so the multi-peaks at 34.05 ppm for T(X) and 40.7 ppm for S_αα(XX) were presented. Similar results were presented on the oligomers formed from 1-hexene, 1-octene, 1-dodecene and 1-tetradecene. According to Fig. 1(b), the signals for the atoms labeled S_αα(BB), S_αα(BX) and S_αα(XX) suggested that oligomerization occurred between pairs of 1-butene units, between paris of 1-decene units, and between individual 1-decene and 1-butene units.

¹H NMR spectroscopy was used to analyze the unsaturated bonds and end groups. The ¹H NMR spectra of the oligomers formed from α-olefins and mixture of 1-decene with 1-butene revealed the presence of different types of double bonds (Table 3 and Fig. S4). Four types of carbon-carbon double bond were presented. The vinylidene signals were a pair of doublets at 4.73 and 4.68 ppm, which were assigned to the terminal olefin 1 produced through β-hydride elimination or rearrangement followed by β-hydride elimination, terminating the chain propagation after 1,2-insertion of the monomer (scheme 1) (Babu et al., 1994). Two groups of signals at 4.97 and 5.81 ppm were attributed to the terminal olefin 4 produced from β-alkyl elimination after 1,2-insertion of the monomer. The minor vinylidene signals at 5.13 ppm were assigned to the trisubstituted olefins 2, 3 and/or 8, which produced through β-hydride elimination after rearrangement (Grumel et al., 2001). Notably, the major vinylene signals at 5.36 ppm, attributing to R₁CH = CHR₂ (olefin 5, 6 and/or 7), were observed on the oligomers produced from α-olefin and mixture of 1-decene and 1-butene. These olefins produced through the oligomerization of head-to-head 2,1-misinsertion enchained units of α-olefin or 1-butene/1-decene, followed by β-hydride elimination, or β-hydride elimination after rearrangement. The saturated end-groups of the oligomer 9 formed from α-olefin were produced from the initial chain side of the oligomer produced through CH₃Zr-catalyzed oligomerization and chain transfer to the co-catalyst (MAO) (Rossi et al., 1995). Summarized possible reaction pathways for the oligomerization of α-olefins were presented in scheme 1.

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Scheme 1 Possible reaction pathways in the oligomerization of α-olefins catalyzed by Ph₂Si(Cp)(9-Flu)ZrCl₂.

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¹H NMR spectroscopy was used to analyze the unsaturated bonds and end groups. The ¹H NMR spectra of the oligomers formed from α-olefins and mixture of 1-decene with 1-butene revealed the presence of different types of double bonds (Table 3 and Fig. S4). Four types of carbon-carbon double bond were presented. The vinylidene signals were a pair of doublets at 4.73 and 4.68 ppm, which were assigned to the terminal olefin 1 produced through β-hydride elimination or rearrangement followed by β-hydride elimination, terminating the chain propagation after 1,2-insertion of the monomer (scheme 1) (Babu et al., 1994). Two groups of signals at 4.97 and 5.81 ppm were attributed to the terminal olefin 4 produced from β-alkyl elimination after 1,2-insertion of the monomer. The minor vinylidene signals at 5.13 ppm were assigned to the trisubstituted olefins 2, 3 and/or 8, which produced through β-hydride elimination after rearrangement (Grumel et al., 2001). Notably, the major vinylene signals at 5.36 ppm, attributing to R₁CH = CHR₂ (olefin 5, 6 and/or 7), were observed on the oligomers produced from α-olefin and mixture of 1-decene and 1-butene. These olefins produced through the oligomerization of head-to-head 2,1-misinsertion enchained units of α-olefin or 1-butene/1-decene, followed by β-hydride elimination, or β-hydride elimination after rearrangement. The saturated end-groups of the oligomer 9 formed from α-olefin were produced from the initial chain side of the oligomer produced through CH₃Zr-catalyzed oligomerization and chain transfer to the co-catalyst (MAO) (Rossi et al., 1995). Summarized possible reaction pathways for the oligomerization of α-olefins were presented in scheme 1.

According to Table 3, different distributions of vinyl were presented in the oligomer formed from various α-olefins. The main products were olefins 1 and 5, and/or 6 and/or 7 in oligomers formed from 1-hexene, 1-octene and 1-dodecene. Olefins 1, 4 and 5, and/or 6 and/or 7 became main vinyl units in the oligomer formed from 1-decene, and olefins from 1 to 9 were presented in the oligomer formed from 1-tetradecene. This result implies that the chain size of α-olefins obviously affect the oligomerization pathway, so obtained oligomers consisted of different distribution of the olefins from 1 to 9 and had different properties, such as KV and VI. The difference of main vinylidene units between present work and previous work used (Me₄Cp)₂ZrCl₂ and rac-Me₂Si(1-C₅H₂-2-CH₃-4-^tBu)₂Zr(NMe₂)₂ as catalysts confirmed the influence of metallocene catalyst on oligomer structure, so properties of oligomer can be further adjusted by using different metallocene catalyst (Shao et al., 2014; Kim et al., 2000).

According to polymerization degree results, the intensities of the signals for the double bonds and saturated end groups in the NMR spectra, the relative amounts of the products from β-hydride elimination and chain transfer to the co-catalyst were calculated (Table 3). The oligomerization of α-olefin favored chain transfer to the co-catalyst, which affected by the chain size of the α-olefin. The α-olefin with short chain size was more favored chain transfer to the co-catalyst. Unfortunately, the oligomers produced from oligomerization of α-olefin consist of many complex unsaturated and saturated hydrocarbons, so it is difficult to establish a quantitative relationship between oligomer structure and temperature dependence of viscosity.