Article pubs.acs.org/Macromolecules
Kinetic and Mechanistic Studies of the Polymerization of Isobutylene Catalyzed by EtAlCl2/Bis(2-chloroethyl) Ether Complex in Hexanes Sanjib Banerjee,† Badri Nath Jha,† Priyadarsi De,‡ Jack Emert,§ and Rudolf Faust*,† †
Polymer Science Program, Department of Chemistry, University of Massachusetts Lowell, One University Avenue, Lowell, Massachusetts 01854, United States ‡ Polymer Research Centre, Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, 741246, Nadia, West Bengal, India § Infineum USA, 1900 E. Linden Avenue, Linden, New Jersey 07036, United States S Supporting Information *
ABSTRACT: The kinetics and mechanism of the polymerization of isobutylene (IB) initiated by tert-butyl chloride and catalyzed by ethylaluminum dichloride (EADC)•bis(2-chloroethyl) ether (CEE) complex was studied in hexanes at 0 °C. The polymerization is first order in [IB] and in dry hexanes the slopes of the first order plots are independent of the [CEE]/ [EADC] ratio in the range 1−1.5. From the slopes the steady state concentration of propagating cations [Mn+] = 1.8 × 10−11 M was calculated, which suggests an equilibrium between dormant and active species. The olefin distribution is independent of IB concentration and conversion. At low conversion, the number average molecular weight (Mn) is proportional to the starting IB concentration. In hexanes saturated with water, the polymerization rate is 5 times higher at [CEE]/[EADC] = 1 compared to that in dry hexanes. However, at [CEE]/[EADC] = 1.5, the rates in dry or wet hexanes are identical. To account for the low concentration of active centers determined from the first order plots, an equilibrium between oxonium (dormant) and carbenium (active) ions is proposed. The existence of oxonium ions was confirmed by 1H NMR studies and by polymerizations initiated from preformed oxonium ions. While identical rates and olefin distributions were obtained using EADC solutions in hexanes or toluene to form the EADC·CEE complex at short reaction times, at long reaction times, the exoolefin content decreased when EADC solution in toluene was used. This was attributed to a slow tert-butylation of toluene yielding 95% para isomer and the concomitant formation of AlCl3.
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INTRODUCTION Olefin end-functional polybutenes and polyisobutylenes (PIBs) are valuable intermediates for motor oil and fuel additives.1,2 In contrast to tri- and tetra-substituted olefin ends in polybutenes,3−5 PIB containing >80% terminal vinylidene functionality1 is highly reactive toward maleic anhydride to give polyisobutenylsuccinic anhydrides (PIBSA) and subsequently polyisobutenylsuccinimides (PIBSI) as shown in Scheme 1. Many recent efforts have been reported aimed at replacing the BF3 catalyst used in the current industrial process for producing highly reactive PIB (HR PIB). These have been reviewed recently.6−8 Vierle et al. synthesized HR PIB using Mn(II) complexes as initiators.9 Bochmann and co-workers developed a new zinc-based initiator system to produce HR PIB at room temperature.10 Krossing and co-workers employed novel univalent gallium salts [Ga(C6H5F)2]+[Al(ORF)4]− and [Ga(1,3,5-Me3C6H3)2]+[Al(ORF)4]− (where RF = C(CF3)3) to achieve synthesis of HR-PIB in several solvents including CH2Cl2 and toluene.11 Voit and co-workers reported synthesis of HR PIB using M(II) complexes (M = Mn, Cu, Zn, Mo) as catalysts.12−14 Storey’s group reported synthesis of HR PIB by © XXXX American Chemical Society
quenching of living PIB with dialkyl ether/base, dialkyl (or) diaryl sulfide/base or with hindered base.15−17 The most promising methods employed catalytic chain transfer mediated by Lewis acid•Lewis base base complexes.18−22 Wu and coworkers reported that the initiating system for synthesis of HRPIB in the presence of a small amount of H2O contains component A (AlR3−nCln) and component B (phenol, piperidine, 6−16C dialkyl ether), in which the molar ratio of component B to A was in the range of 0.05−2.0.23 The limitations due to poor solubility of the AlCl3•dialkyl ether, FeCl3•dialkyl ether, GaCl3•dialkyl ether complexes,18−21,24,25 and their protonated salts in the preferred nonpolar solvents have been recently overcome by employing a soluble Lewis acid, ethylaluminum dichloride (EADC)26 that is currently used as catalyst to produce polybutenes. We have recently shown that while complexes of isopropyl ether and 2-chloroethyl ethyl ether with EADC are inactive, the EADC•bis(2-chloroethyl) Received: July 1, 2015 Revised: July 31, 2015
A
DOI: 10.1021/acs.macromol.5b01441 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthesis of PIBSA and PIBSI from HR PIB
ization using preformed oxonium ions, the initiator (t-BuCl) and EADC·CEE complex were aged in a separate reactor at 0 °C for the predetermined time. This (t-BuCl + EADC·CEE) mixture was then added to the reactor containing IB and hexanes at 0 °C to start the polymerization. Characterization. Karl Fisher Titration. Water content of hexanes before use in the polymerization of IB was measured using a Mettler Toledo KF coulometer DL39. Size Exclusion Chromatography. Molecular weights and polydispersities were obtained from size exclusion chromatography (SEC) with universal calibration using a Waters 717 Plus autosampler, a 515 HPLC pump, a 2410 differential refractometer, a 2487 UV−vis detector, a MiniDawn multi angle laser light scattering (MALLS) detector (measurement angles are 44.7°, 90.0°, and 135.4°) from Wyatt Technology Inc., a ViscoStar viscosity detector from Wyatt, and five Styragel HR GPC columns connected in the following order: 500, 103, 104, 105, and 100 Å. The RI was the concentration detector. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL min−1 at room temperature. The results were processed using the Astra 5.4 software from Wyatt Technology Inc. Figure S1 depicts a typical SEC RI trace of a representative HR PIB sample prepared in this study. Nuclear Magnetic Resonance. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker 500 MHz spectrometer using CDCl3 or cyclohexane-d12 as solvents (Cambridge Isotope Laboratory, Inc.). Figure 1 depicts a typical 1H NMR spectrum of a representative HR PIB sample prepared in this study. The two protons characteristic of the exo-olefin end group (structure P1, protons a1 and a2) appeared as two well resolved peaks at 4.85 and 4.64 ppm, respectively, while the endo-olefin end group (structure P2, proton d) appeared as a single peak at 5.15 ppm. Small amounts of the E and Z
ether (CEE) complex readily catalyzed the fast polymerization of isobutylene (IB) in hexanes at −20 to +10 °C to yield up to 92% exo-olefin end-group.26 While this development is of great industrial importance, aspects of the polymerization are still unknown. In this publication, we report the results of a detailed study on the kinetics and mechanism of the polymerization.
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EXPERIMENTAL SECTION
Materials. Technical grade hexanes (Doe & Ingalls) were refluxed over H2SO4 for 48 h, then washed with 10% potassium hydroxide (KOH) aqueous solution and finally washed with distilled water until the aqueous layer was neutral. The hexanes were predried by vigorously mixing with anhydrous sodium sulfate (Na2SO4) for 30 min and then refluxing over CaH2 for 48 h. The hexanes were then distilled onto CaH2, refluxed again for 24 h, and freshly distilled. Isobutylene (IB, Matheson Tri Gas) was dried by passing it through in-line gas-purifier columns packed with BaO/Drierite and then condensed in a receiver flask at −30 °C before use. tert-Butyl chloride (t-BuCl, 98%, TCI America) was used as received. Ethylaluminum dichloride (EADC, 25.7 wt % solution in toluene), EADC (1.0 M solution in hexane), bis(2-chloroethyl) ether (CEE, 99%), diisopropyl ether (i-Pr2O, 99%), toluene (anhydrous 99.8%), KOH (90%), sodium hydroxide (NaOH, ≥98%), Na2SO4, 2-propanol (IPA, ≥99.5%), sodium acetate (NaOAc, 99%), ethylenediaminetetraacetic acid (EDTA, ≥98.5%), and dithizone (85%) were purchased from Aldrich and used without any further purification. Determination of Al Concentration of EADC. An 0.5 mL aliquot of EADC solution in toluene was placed in a 20 mL vial and dissolved in deionized water by adding some dilute HCl and by refluxing. The solution was allowed to cool to room temperature, and 15 mL of the standard 0.05 M EDTA solution was added into the beaker containing the decomposed EADC solution. The solution was diluted with about 50 mL of deionized water. The pH was adjusted to 5 with dilute NaOH, and 10 mL of sodium acetate buffer was added to the solution and refluxed for 3 min. The solution was cooled to room temperature, and 10 mL of ammonium acetate, 75 mL of 2-propanol, and 1 mL of dithizone indicator solution were added. The solution was titrated against a standard 0.02 M ZnSO4 solution to a pink end point to give concentration of Al. Preparation of EADC·CEE Complexes. Complexes were prepared just before the polymerization of IB. In a glovebox, the required amount of ether was added to EADC and stirred to form a 1.0 M Lewis acid/ether complex, followed by the addition of a small amount of hexanes to make the fully soluble 0.5 M (using EADC solution in toluene) and 0.1 M (using EADC solution in hexanes) complexes, respectively. Polymerization of IB. Polymerizations were performed under a dry N2 atmosphere in an MBraun glovebox (MBraun, Inc. Stratham, NH). Typically, required amount of hexanes was placed in the polymerization reactors, screw top culture tubes (75 mL), at −30 °C. Then, the initiator (t-BuCl) was added to the reactors. IB was condensed and distributed to the polymerization reactors containing tBuCl and hexanes. The polymerizations were started by adding EADC·CEE complex to the reactors at 0 °C and terminated with either ammonium hydroxide (NH4OH) or methanol. For polymer-
Figure 1. Typical 1H NMR spectrum of HR PIB obtained in this study. The asterisk denotes the CHCl3 resonance. B
DOI: 10.1021/acs.macromol.5b01441 Macromolecules XXXX, XXX, XXX−XXX
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Table 1. Polymerization of IB ([IB] = 1.0 M) Using [EADC•CEE] = 0.01 M, and [t-BuCl] = 0.01 M in Dry Hexanes at 0 °C at [CEE]/[EADC] = 1.0 no.
time (min)
convna (%)
Mn,NMRb (g/mol)
Mn,SECc (g/mol)
PDIc
exod (%)
tri + endod (%)
tetrad (%)
PIB coupledd (%)
1 2 3 4 5 6 7 8 9 10
2 5 10 20 60 2 5 10 20 60
18 50 74 100 100 22 55 79 100 100
4500 3900 3100 2600 2400 4300 4100 3000 2900 2600
4300 3800 2900 2400 2500 4100 3900 3100 2800 2500
3.1 3.3 3.2 3.4 3.4 3.0 3.2 3.1 3.3 3.2
64.9 64.1 66.2 68.0 47.8 68.9 70.9 70.2 70.0 71.4
20.1 19.2 25.2 22.4 28.7 17.5 17.0 23.2 16.6 19.3
13.0 15.4 7.9 8.2 22.0 13.1 9.2 6.3 13.1 8.6
2.0 1.3 0.7 1.4 1.5 0.5 2.9 0.3 0.3 0.7
a
Determined gravimetrically based on monomer feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study. Entries 1−5: EADC solution in toluene was used. Entries 6−10: EADC solution in hexanes was used.
Table 2. Polymerization of IB ([IB] = 1.0 M) Using [EADC•CEE] = 0.01 M, and [t-BuCl] = 0.01 M in Dry Hexanes at 0 °C at [CEE]/[EADC] = 1.5 no.
time (min)
convna (%)
Mn,NMRb (g/mol)
Mn,SECc (g/mol)
PDIc
exod (%)
tri + endod (%)
tetrad (%)
PIB coupledd (%)
1 2 3 4 5 6 7 8 9 10
2 5 10 20 60 2 5 10 20 60
14 36 66 90 98 15 31 66 91 96
1800 1400 1200 900 900 2600 1800 1500 1200 1000
1900 1300 1000 800 850 2900 2000 1600 1400 1100
3.3 3.4 3.1 3.2 3.2 3.6 3.7 3.4 3.5 3.5
79.7 81.6 82.3 83.7 65.8 81.0 83.0 82.3 83.3 82.9
8.8 9.0 9.9 9.2 18.4 10.5 9.0 9.9 9.2 8.3
11.1 9.0 6.6 6.7 13.8 8.1 7.5 7.4 6.7 6.6
0.4 0.4 1.2 0.4 2.0 0.4 0.5 0.4 0.8 2.2
a
Determined gravimetrically based on monomer feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study. Entries 1−5: EADC solution in toluene was used. Entries 6−10: EADC solution in hexanes was used. configurations of another tri- substituted olefin end group (structure P3, protons e1 and e2) were also observed in some samples at 5.37 and 5.17 ppm. The signal corresponding to the tetra-substituted olefin end group (structure P4, proton f) appeared as a broad multiplet at 2.85 ppm. Resonances for coupled PIB chains (structure P5, protons g) appeared at 4.82 ppm. The methylene protons in the PIBCl end group (structure P6, protons h) which appear at 1.96 ppm were used to calculate the content of PIBCl in the HR PIB. The methylene and methyl protons of the IB repeat unit (structure P1, protons b and c, respectively) usually appeared at 1.42 and 1.11 ppm, respectively. The number average molecular weight of the HR PIB was calculated from 1 H NMR spectroscopic study (Mn,NMR) by using the following formula:
software. Separation was carried out on a nonpolar DB-5 capillary column (Agilent) with length =30 m, ID = 0.250 mm, film thickness =0.25 μm. Helium (purity 99.999%) was employed as carrier gas at a constant column flow of 1.0 mL min−1. The GC oven temperature was programmed from 60 °C (held for 2 min) to 140 °C at 10°C min−1 (held for 1 min). The injector temperature was kept at 260 °C. The injection volume was 2 μL.
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RESULTS AND DISCUSSION Polymerizations were carried out with both EADC solution in toluene and EADC solution in hexanes with different [CEE]/ [EADC] = 1.0, 1.1, 1.2, and 1.5 ratios in dry hexanes at 0 °C. The results for [CEE]/[EADC] = 1.0 and 1.5 are shown in Table 1-2 and those with 1.1 and 1.2 are in the Supporting Information (SI) as Table S1 and S2. In agreement with previously published results,26 the exo-olefin content increases with increasing [CEE]/[EADC] ratios. Conversions and olefin distributions are similar whether EADC solution in toluene or hexanes is used. However, extending the reaction time to 60 min resulted in a drastic decrease of the exo-olefin content when toluene was present but not when it was absent. The first order plots of the polymerization of IB in dry hexanes ([H2O] = 0.3 mM) with different [CEE] /[EADC] ratios at 0 °C are shown in Figure 2A. The first order plots are linear starting at the origin suggesting a constant steady state concentration of the active centers. The slopes of the linear lines are independent whether EADC solution in hexanes or toluene is used and within the experimental error identical at [CEE]/ [EADC] = 1.0−1.5 ratios. This suggests that the active center concentration is independent of excess CEE. From the slope,
M n,NMR = 56.11 × [(b/2)/{(a1 + a 2)/2 + d + e1 + e 2 + f + (g /2) + (h/2)}] where 56.11 is the molecular weight of IB, and a1, a2, b, c, etc. represent the area corresponding to the respective protons as designated in Figure 1. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). ATR-FTIR spectra were taken using a Mettler Toledo React IR 4000 instrument equipped with a DiComp probe connected to an MCT detector with a K6 conduit in the spectral range from 4000 to 650 cm−1 at a resolution of 2 cm−1. Gas Chromatography−Mass Spectrometry Analysis. The gas chromatography−mass spectrometry (GC−MS) analysis was performed using an Agilent 7890A (GC)−Agilent 5975 C inert MSD with triple axis detector and an Agilent 7693 autosampler from Agilent Technologies. The temperatures of the transfer line, the quadrupole and the ion source were set at 320, 150, and 230 °C, respectively. The system was operated by Agilent MSD ChemStation E.02.00.493 C
DOI: 10.1021/acs.macromol.5b01441 Macromolecules XXXX, XXX, XXX−XXX
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Surprisingly, the polymerization was sometimes sensitive to the water content of the hexanes. While the slope of the first order plots obtained in wet hexanes at [CEE]/[EADC] = 1.5 is identical with that determined for dry hexanes, for the other ratios, the slope increases with decreasing [CEE]/[EADC] (Figure 2). At [CEE]/[EADC] = 1, the slope of the first order plot and thus the active center concentration is ∼5 times higher in wet hexanes than in dry hexanes. While water also initiates the polymerization and yields similar exo-olefin content (Table 3), the polymerization rate (34% conversion in 10 min) is much lower compared to that initiated with t-BuCl. Therefore, water is not simply another initiator, its effect is not additive. Mechanistic Studies. Our previously proposed mechanism26 is inadequate to explain the above findings. The proposed modified scheme is shown in Scheme 2. A key modification is the suggested equilibrium between oxonium (dormant) and carbenium (active) ions. These account for the low concentration of active centers determined from the first order plots. We assume that the concentration of tertbutyloxonium ions is much higher than that of PIB−oxonium ions due to back strain in case of the latter. Scheme 2 suggests that tert-butyloxonium ions are stable under polymerization conditions. This assumption was tested by 1H NMR spectroscopy. Figure 4 shows the 1H NMR spectrum of t-BuCl in the presence of varying amounts of EADC·CEE (1:1) complex. Upon complexation with EADC, both methylene protons of CEE shift downfield as expected. Upon addition of t-BuCl, a further downfield shift of the CEE protons is observed that we propose is due to the formation of tert-butyloxonium ions. A similar downfield shift for methine protons has been reported by Ummadisetty and Storey for 2chloro-2,4,4-trimethylpentane/diisopropyl ether oxonium ion adduct as compared to the i-Pr2O complex with TiCl4.17 Importantly, resonances due to EADC·CEE or CEE are absent at 1:1 t-BuCl/EADC·CEE ratio suggesting that all t-BuCl has been converted to tert-butyloxonium ions. This is further corroborated by the complete absence of the methyl protons of t-BuCl. Upon addition of excess EADC·CEE, resonances due to the methylene protons of complexed CEE appear. Resonances due to t-BuCl are observed at 2:1 t-BuCl/EADC·CEE ratio (Figure S2). This further confirms that 1 equiv of CEE relative to EADC is sufficient to ionize 1 equiv of t-BuCl and convert it to tert-butyloxonium ions with a 1:1 t-BuCl/EADC·CEE stoichiometry. The peak positions of the EtAlCl3− anion was confirmed by comparing the spectra with the spectrum of nBu4N+EtAlCl3− (Figure S2). In the ATR-FTIR spectrum of a 1:1 complex between EADC and CEE, shown in Figure S3, free ether is absent as indicated
Figure 2. ln{[M]0/[M]} vs time plot for polymerization of IB initiated by t-BuCl/EADC·CEE at different [CEE]/[EADC] ratios in hexanes at 0 °C. (A) dry hexanes, [H2O] = 0.3 mM and (B) hexanes saturated with water [H2O] = 3 mM. [EADC·CEE] = 0.01 M; [t-BuCl] = 0.01 M; [IB] = 1.0 M.
using the reported27 absolute rate constant of propagation kp = 108 L mol−1s−1, the steady state concentration of propagating cations [Mn+] = 1.8 × 10−11 M was calculated. This value is ∼9 orders of magnitude lower than the concentration of initiator. Since termination has not been found, it is rational to conclude that there is a large pool of dormant species present in equilibrium with active species. We have also studied the effect of IB concentration on polymerization rate, molecular weight and olefin distribution in detail at [CEE]/[EADC] = 1.5. The results shown in the Supporting Information (Table S3−S6) are consistent with the results presented in Table 2. Thus, olefin distribution is independent of IB concentration and conversion. As expected, at low conversion, Mn is proportional to conversion and the slopes of the first order plots are independent of the starting IB concentration (Figure 3).
Figure 3. ln{[M]0/[M]} vs time plot for polymerization of IB with different IB concentrations 1.0−4.0 M initiated by t-BuCl/EADC·CEE at [CEE]/[EADC] = 1.5 in dry hexanes at 0 °C. [EADC·CEE] = 0.01 M; [t-BuCl] = 0.01 M.
Table 3. [H2O] = 3 Mm As Initiator in the Polymerization of IB with EADC•CEE Complexes in Wet Hexanes at 0 °C. [IB] = 1.0 M, [EADC] = 0.01 M
a
[CEE]/[EADC]
time (min)
convna (%)
Mn,NMRb (g/mol)
exoc (%)
tri + endoc (%)
tetrac (%)
PIB coupledc (%)
1.0 1.0 1.0 1.0 1.5 1.5 1.5 1.5
10 20 40 60 10 20 40 60
34 56 69 86 27 47 71 78
1800 1540 1400 1460 1800 1400 1000 1000
77.0 76.0 74.0 74.0 80.0 83.0 81.0 83.0
12.0 12.0 13.0 13.0 7.0 8.0 8.0 9.0
11.0 12.0 13.0 13.0 13.0 9.0 11.0 8.0
0 0 0 0 0 0 0 0
Determined gravimetrically based on monomer feed. bDetermined from NMR analysis. cCalculated from NMR spectroscopic study. D
DOI: 10.1021/acs.macromol.5b01441 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 2. Suggested Modified Mechanism for the Polymerization of IB by t-BuCl/EADC·CEE
carbenium ion is lower and polymerization may proceed as has been reported by the AlCl3,28,30 GaCl325 and FeCl320,24,25,31 complexes with i-Pr2O. In addition to temperature, steric and electronic effects influence the stability of oxonium ions. With the less nucleophilic CEE, the oxonium-carbenium ion equilibrium constant is presumably lower than that with iPr2O, which explains the faster rate of polymerization with FeCl3·CEE compared to FeCl3·i-Pr2O.32 Notably, we did not observe any oxonium ion formation by mixing t-BuCl and EADC·i-Pr2O (Figure S4) which also confirms the high nucleophilicity of i-Pr2O compared to CEE. Since 1H NMR studies confirmed the formation of stable oxonium ions by mixing t-BuCl and EADC·CEE, polymerization of IB was carried out with preformed oxonium ions in dry hexanes at 0 °C. Here, a mixture of t-BuCl and EADC·CEE was aged at 0 °C for 10 min and then added to the polymerization reactor containing hexanes and IB. The results, summarized in Table S7, are identical to that obtained when the EADC·CEE complex was added last (entries 6−9 in Table 1). When the formation of oxonium ions was studied using EADC solution in toluene, a side reaction, namely alkylation of toluene was observed. Figure 5 shows the 1H NMR spectra at
Figure 4. 1H NMR spectra of [CEE], [EADC·CEE], [EADC·CEE + tBuCl] (1:1), [EADC·CEE + t-BuCl] (1.5:1) and [EADC·CEE + tBuCl] (2:1) at 0 οC in cyclohexane-d12. [t-BuCl] = 0.05 M, [EADC· CEE] = 0.05, 0.075, and 0.1 M, [CEE]/[EADC] = 1. The asterisk denotes the cyclohexane-d11H resonance.
by the absence of a peak at 1125 cm−1. ATR-FTIR spectra of the t-BuCl/EADC·CEE at 1:1, 1:1.5 and 1:2 revealed the appearance of a new peak around 1100 cm−1 that is attributed to the C−O−C stretching bands of the tert-butyloxonium ion. We have proposed previously that the ether molecules are brought into the vicinity of the propagating ends by the complex and remain there for the duration of the propagation,28 so their local concentration is higher than in the bulk. This was necessary to explain why the Mns were lower than predicted based on diffusion limited proton abstraction by free ether, and why separately added ether followed by the addition of the Lewis acid did not produce HR PIB using AlCl3 or GaCl3 in nonpolar solvent. However, under the conditions reported, alkylaluminum dichlorides (EtAlCl2 or i-BuAlCl2)/ ether complexes afforded HR PIB with high exo-olefin end groups content even if Lewis acid and ether were added into to the system separately.22,29 The rapid equilibrium between oxonium and carbenium ions could also explain why the ether stays in close proximity to the carbenium ion, and why excess CEE does not affect the steady state concentration of active centers. Since all t-BuCl is converted to tert-butyloxonium ions and the concentration of carbenium ions is very low, it is possible to estimate that approximately 2 carbenium ions are in equilibrium with 1 billion oxonium ions. Ummadisetty and Storey17 also reported that at −60 °C the oxonium-carbenium ion equilibrium is fully shifted to the stable oxonium ions as the addition of i-Pr2O to a living IB polymerization system stopped the polymerization completely. At higher temperature, the equilibrium constant of oxonium-
Figure 5. 1H NMR spectra of [EADC·CEE + t-BuCl] in the presence of toluene at different times at 0 οC in cyclohexane-d12. [t-BuCl] = 0.05 M, [EADC·CEE] = 0.05 M, [CEE]/[EADC] = 1. The asterisk denotes the cyclohexane-d11 H resonance.
different times from 8 to 120 min. There is a decrease of the intensity of the methyl resonance of t-BuCl at 1.6 ppm and appearance of a new resonance at 1.3 ppm with time. After 2 h, the reaction is virtually complete. Figure 6 shows the magnified part (0.0−1.2 ppm) of the NMR spectra shown in Figure 5. The signals corresponding to E
DOI: 10.1021/acs.macromol.5b01441 Macromolecules XXXX, XXX, XXX−XXX
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the polymerization rates remain unaffected. At lower excess, the carbenium to oxonium ion rate might be affected. Effect of t-BuCl and EADC·CEE Complex Concentration on the Polymerization. Polymerizations of IB were carried out at different [t-BuCl] while [EADC·CEE] was kept constant at 0.01 M, and also at different [EADC·CEE] while [tBuCl] was kept constant at 0.01 M. Results are summarized in Table S8−S12 and the first order plots are shown in Figure 7A.
Figure 6. 1H NMR spectra at different times at 0 °C in cyclohexaned12 showing loss of ethyl group of EADC and formation of ethane. [tBuCl] = 0.05 M, [EADC·CEE] = 0.05 M and [CEE]/[EADC] = 1.
the −CH3 protons (at ∼1.07 ppm) and the −CH2− protons (at ∼0.15 ppm) of EADC decrease with time. This decrease is accompanied by appearance and subsequent increase of a resonance signal at ∼0.87 ppm. This suggests loss of ethyl group from EADC leading to the formation of ethane and consequently AlCl3. Notably, EADC·CEE complex solution in cyclohexane-d12 is stable at 0 °C up to 24 h and there is no precipitation. In these experiments, both the concentration of toluene and EADC·CEE complex were 5 times higher than in the polymerization experiments, so alkylation is 25 times faster. Thus, we propose that the decrease of the exo-olefin content at long reaction times when EADC solution in toluene solution was used is due to the formation of AlCl3 (see Scheme S1 for detailed mechanism of alkylation of toluene). AlCl3 catalyzed conventional polymerization of IB (when complete conversion of monomer was not achieved) and the isomerization/chain cleavage induced by AlCl 3 (after complete monomer conversion) lead to reduction of exo olefin content at longer reaction time when EADC in toluene solution was used. While tert-butylation of toluene readily proceeds to completion in 2 h based on 1H NMR spectra in Figure 5, alkylation of toluene by PIB+ has not been observed, as no aromatic moiety was found in the purified PIB product. This can be attributed to the more than 100 fold higher reactivity of the t-Bu+ compared to PIB+ due to the reverse of back strain.33 To identify the tert-butylated product of toluene discussed above, the product was analyzed by GC-MS. GC chromatogram of the product (Figure S5) reveals the existence of two products eluting at 5.90 min (∼5%) and 5.98 min (∼95%), which were determined by MS to be the meta- and paraisomers, respectively. Friedel−Crafts reactions generally do not readily proceed in nonpolar solvents due to the limited solubility of the Friedel−Crafts catalysts. Therefore, Friedel− Crafts reactions with Friedel−Crafts acid•Lewis base complexes in nonpolar solvents may be of importance. We are currently undertaking a separate study on this subject and results will be reported in due time. On the basis of Scheme 2, the higher rate of polymerization in wet hexanes compared to dry hexanes when [CEE]/[EADC] = 1 suggests higher steady state concentration of carbenium ions in wet hexanes as compared to dry hexanes. At large excess of CEE, relative to EADC (such as [CEE]/[EADC] = 1.5), water presumably forms hydrogen bonds with the excess free CEE and the oxonium−carbenium ion equilibrium and thereby
Figure 7. ln{[M]0/[M]} vs time plot for polymerization of IB (1.0 M) initiated by t-BuCl/EADC·CEE at varying t-BuCl and EADC·CEE concentrations in dry hexanes at 0 °C. (A) [EADC·CEE] added last; (B) (t-BuCl + EADC·CEE) was aged for 10 min at 0 °C to allow for tert-butyloxonium ion formation. 1: [t-BuCl] = 0.005 M, [EADC· CEE] = 0.01 M. 2: [t-BuCl] = 0.01 M, [EADC·CEE] = 0.01 M. 3: [tBuCl] = 0.02 M, [EADC·CEE] = 0.01 M. 4: [t-BuCl] = 0.01 M, [EADC·CEE] = 0.02 M. 5: [t-BuCl] = 0.01 M, [EADC·CEE] = 0.03 M.
At all initiator concentrations keeping [EADC·CEE] constant at 0.01 M, a fast polymerization was observed and the slope of the first order plots were approximately proportional to [tBuCl]. When the [t-BuCl] concentration is increased from 0.005 to 0.01 M, the rate increases approximately 2-fold as expected from the mechanism of the polymerization (Scheme 2). However, the further increase of the rate upon increasing [tBuCl] from 0.01 to 0.02 M is unexpected. We expected the polymerization rate to be unaffected by increase of [EADC· CEE] or [t-BuCl] above a 1:1 stoichiometry since according to our findings, oxonium ion formation is complete at a 1:1 ratio of [t-BuCl] to [EADC·CEE] (0.01 M). Thus, further increase of [EADC·CEE] to 0.02 and 0.03 M should not affect the polymerization rate. Instead we observed that the polymerization rate increases, albeit slightly with increase in [EADC· CEE] concentration keeping [t-BuCl] constant at 0.01 M. The results suggest that tert-butyloxonium ion formation is not instantaneous. To prove this we aged (EADC·CEE + tBuCl) for 10 min at 0 °C and carried out the polymerization with this mixture added last to the polymerization reactor containing IB and hexanes at 0 °C (Table S13−S17). The first order plots (Figure 7B) revealed that when EADC·CEE was aged with t-BuCl, there is no further increase of polymerization rate upon increasing [EADC·CEE] to 0.02 and 0.03 M keeping [t-BuCl] constant at 0.01 M or increasing [t-BuCl] to 0.02 M keeping [EADC·CEE] constant at 0.01 M. In order to determine the time required for tertbutyloxonium ion formation, we aged EADC·CEE with tBuCl for 1−5 min, and carried out the polymerization (Table S18−S32, Figure 8 and Figures S6−S7). The first order plots after 4 and/or 5 min of aging are similar to that obtained with [t-BuCl] = 0.01 M, [EADC·CEE] = 0.01 M, whereas after 1 and/or 2 min of aging, polymerization was faster as obtained using a higher concentration of [EADC·CEE] or [t-BuCl] when EADC·CEE was added last. Thus, tert-butyloxonium ion F
DOI: 10.1021/acs.macromol.5b01441 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
MS data. Financial support from Infineum USA is greatly appreciated.
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Figure 8. Ln ([M]0/[M]) vs time plot for polymerization of IB (1.0 M) initiated by t-BuCl/EADC·CEE at varying aging time of (t-BuCl+ EADC·CEE) in dry hexanes at 0 °C using [t-BuCl] = 0.01 M and [EADC·CEE] = 0.02 M. [t-BuCl]/[EADC·CEE] = 0.01/0.02 and [tBuCl]/[EADC·CEE] = 0.01/0.01 represent reactions where the EADC·CEE complex was added last to the solution containing hexanes, IB, and t-BuCl.
formation takes around 4 min to completion. When tertbutyloxonium ion formation is complete, polymerization goes exclusively via the oxonium ion pathway described in Scheme 2 and the reaction rate is unaffected by increasing the [t-BuCl] and [EADC·CEE] concentration from 0.01 M.
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CONCLUSION The kinetics and mechanism of the polymerization of IB initiated by t-BuCl and catalyzed by EADC·CEE complex was studied in hexanes at 0 °C. The polymerization kinetics, first order in [IB] and independent of the [CEE]/[EADC] ratio, could only be explained by postulating an equilibrium between dormant oxonium and active carbenium ions. The existence of oxonium ions could be corroborated by 1H NMR studies and by polymerizations initiated from preformed oxonium ions. This oxonium/carbenium ion equilibrium is affected by moisture at [CEE]/[EADC] = 1 but remains unaffected at [CEE]/[EADC] = 1.5. The EADC·CEE complex is an efficient catalyst for the Friedel−Crafts tert-butylation of toluene in a nonpolar medium yielding 95% para isomer close to quantitative yield at 0 °C.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01441. Tables of polymerization data, a scheme showing the suggested mechanism for alkylation, and figures showing additional analytical analysis including spectral data (PDF)
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REFERENCES
(1) Mach, H.; Rath, P. Lubr. Sci. 1999, 11, 175−185. (2) Balzano, F.; Pucci, A.; Rausa, R.; Uccello-Barretta, G. Polym. Int. 2012, 61, 1256−1262. (3) Puskas, I.; Banas, E. M.; Nerheim, A. G. J. Polym. Sci., Polym. Symp. 1976, 56, 191−202. (4) Puskas, I.; Meyerson, S. J. Org. Chem. 1984, 49, 258−262. (5) Harrison, J. J.; Mijares, C. M.; Cheng, M. T.; Hudson, J. Macromolecules 2002, 35, 2494−2500. (6) Li, Y.; Cokoja, M.; Kuhn, F. E. Coord. Chem. Rev. 2011, 255, 1541−1557. (7) Kostjuk, S. V.; Yeong, H. Y.; Voit, B. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 471−486. (8) Kostjuk, S. V. RSC Adv. 2015, 5, 13125−13144. (9) Vierle, M.; Zhang, Y.; Herdtweck, E.; Bohnenpoll, M.; Nuyken, O.; Kühn, F. E. Angew. Chem., Int. Ed. 2003, 42, 1307−1310. (10) Guerrero, A.; Kulbaba, K.; Bochmann, M. Macromolecules 2007, 40, 4124−4126. (11) Lichtenthaler, M. R.; Higelin, A.; Kraft, A.; Hughes, S.; Steffani, A.; Plattner, D. A.; Slattery, J. M.; Krossing, I. Organometallics 2013, 32, 6725−6735. (12) Hijazi, A. K.; Radhakrishnan, N.; Jain, K. R.; Herdtweck, E.; Nuyken, O.; Walter, H.-M.; Hanefeld, P.; Voit, B.; Kühn, F. E. Angew. Chem., Int. Ed. 2007, 46, 7290−7292. (13) Radhakrishnan, N.; Hijazi, A. K.; Komber, H.; Voit, B.; Zschoche, S.; Kühn, F. E.; Nuyken, O.; Walter, M.; Hanefeld, P. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5636−5648. (14) Yeong, H. Y.; Li, Y.; Kühn, F. E.; Voit, B. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 158−167. (15) Ummadisetty, S.; Morgan, D. L.; Stokes, C. D.; Storey, R. F. Macromolecules 2011, 44, 7901−7910. (16) Morgan, D. L.; Harrison, J. J.; Stokes, C. D.; Storey, R. F. Macromolecules 2011, 44, 2438−2443. (17) Ummadisetty, S.; Storey, R. F. Macromolecules 2013, 46, 2049− 2059. (18) Vasilenko, I. V.; Frolov, A. N.; Kostjuk, S. V. Macromolecules 2010, 43, 5503−5507. (19) Vasilenko, I. V.; Shiman, D. I.; Kostjuk, S. V. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 750−758. (20) Liu, Q.; Wu, Y.; Yan, P.; Zhang, Y.; Xu, R. Macromolecules 2011, 44, 1866−1875. (21) Liu, Q.; Wu, Y.-X.; Zhang, Y.; Yan, P.-F.; Xu, R.-W. Polymer 2010, 51, 5960−5969. (22) Vasilenko, I. V.; Shiman, D. I.; Kostjuk, S. V. Polym. Chem. 2014, 5, 3855−3866. (23) Wu, Y. X.; Zhang, L. B.; Liu, Q.; Zhou, P. CN 101613423 B, May 18, 2011. (24) Bartelson, K. J.; De, P.; Kumar, R.; Emert, J.; Faust, R. Polymer 2013, 54, 4858−4863. (25) Kumar, R.; Dimitrov, P.; Bartelson, K. J.; Emert, J.; Faust, R. Macromolecules 2012, 45, 8598−8603. (26) Kumar, R.; Zheng, B.; Huang, K.-W.; Emert, J.; Faust, R. Macromolecules 2014, 47, 1959−1965. (27) Sipos, L.; De, P.; Faust, R. Macromolecules 2003, 36, 8282− 8290. (28) Dimitrov, P.; Emert, J.; Faust, R. Macromolecules 2012, 45, 3318−3325. (29) Shiman, D. I.; Vasilenko, I. V.; Kostjuk, S. V. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2386−2393. (30) Shiman, D. I.; Vasilenko, I. V.; Kostjuk, S. V. Polymer 2013, 54, 2235−2242. (31) Kumar, R.; Emert, J.; Faust, R. Polym. Bull. 2015, 72, 49−60. (32) Kumar, R.; De, P.; Zheng, B.; Huang, K.-W.; Emert, J.; Faust, R. Polym. Chem. 2015, 6, 322−329. (33) Roth, M.; Mayr, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2250− 2252.
AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected] (R.F.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Mrs. Wendy Gavin, University of Massachusetts Lowell, for assistance in acquiring the 1H NMR spectra at 0 °C and Mr. Mahesh Jayamanna, University of Massachusetts Lowell, for assistance in acquiring the GC− G
DOI: 10.1021/acs.macromol.5b01441 Macromolecules XXXX, XXX, XXX−XXX
