Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
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Cationic Copolymerization and Multicomponent Polymerization of Isobutylene with C4 Olefins Tota Rajasekhar,† Ujjal Haldar,† 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, Nadia, Mohanpur 741246, West Bengal, India § Infineum USA, 1900 E. Linden Avenue, Linden, New Jersey 07036, United States S Supporting Information *
ABSTRACT: Cationic polymerization of isobutylene (IB) in the presence of other C4 olefins, 1-butene (B1), cis-2-butene (C2B), and 1,3-butadiene (BD) using the ethylaluminum dichloride (EADC)·bis(2-chloroethyl) ether (CEE) complex in conjunction with tert-butyl chloride (t-BuCl) as initiator in hexanes at 0 °C has been investigated. The reactivity ratio of IB rIB = 1100 was determined for copolymerization of IB and B1 at low conversions, using product compositions obtained from inverse gated 13C NMR analysis. The reactivity ratio of B1 rB1 = 1 was deduced from theoretical considerations. At low B1 incorporation levels, exo-olefin contents remained high in the copolymers, and the molecular weights were virtually unchanged relative to the experiments with IB alone [Banerjee, S.; et al. Macromolecules 2015, 48, 5474]. Close to linear firstorder plots of ln{[M]0/[M]} versus time were obtained ([M]0 and [M] are IB concentrations at time t = 0 and t, respectively) when the copolymerization was carried out with [IB] > 2 M. This is because propagation rate increases with increasing [IB] while termination (ion collapse/sec-alkyloxonium ion formation) is independent of [IB]. The formation of polyisobutylene (PIB) secalkyloxonium ion after B1 incorporation was confirmed by 1H NMR spectroscopy and GC-MS analysis by adding EADC·CEE to a mixture of B1 and 2-chloro-2,4,4-trimethylpentane (TMPCl), a model for the PIB chain end, at 0 °C in cyclohexane-d12. Olefin formation and ion collapse to TMP-B1-Cl from TMP+-capped B1 were observed by quenching the sec-alkyloxonium ion with methanol at 0 °C. These results are important to understand the mechanism of commercially important highly reactive polyisobutylene (HRPIB) synthesis using mixed C4 olefin feeds.
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INTRODUCTION Olefin end-functionalized polyisobutylenes (PIBs) with low molecular weights (Mn = 500−5000 g/mol) and more than 65−75 mol % of exo-olefin terminal ends are commonly known as highly reactive polyisobutylene (HRPIB).1−5 HRPIB is an important intermediate in the preparation of motor oils and fuel additives. Commercial polybutenes contain high percentages of tri- and tetrasubstituted olefinic chain ends, primarily produced from C4 olefin mixtures (isobutylene (IB), 1-butene (B1), cis-2-butene (C2B), trans-2-butene (T2B), and 1,3butadiene (BD)) using aluminum-based Lewis acid catalysts such as AlCl3 and EtAlCl2.6 Because of the high content of triand tetrasubstituted olefin double bond chain ends, it is difficult to further functionalize with maleic anhydride (MA). HRPIB with a high percentage of reactive terminal vinyl chain ends undergoes facile thermal “ene” reaction with MA to produce the HRPIB−MA adduct, which is a vital intermediate for the preparation of lubricant and fuel additives.2,5,6 Currently boron trifluoride (BF3) is used as a Lewis acid catalyst to prepare HRPIB.7 However, BF3 is difficult to handle and is detrimental to equipment. Thus, in the past few years © XXXX American Chemical Society
several new catalyst systems have been developed to synthesize HRPIB.8−25 Among the various approaches, high percentage (up to 90%) of exo-olefin content could be achieved in either dehydrochlorination of tert-chlorine-terminated PIB (PIBCl) or quenching the living PIB cations with dialkyl ether/base, dialkyl (or) diaryl sulfide/base, or hindered bases.9,10,12 Solvent-ligated [M(NCMe)6]2+ (MII = Mn, Cu, Mo, Fe, or Zn) complexes with weakly coordinating borate or aluminate anions were also established as an effective initiating systems to prepare HRPIB with high exo-olefin functionality.26 Very recently, Lewis acid (LA) and Lewis base (LB) complexes were reported for the catalytic chain transfer polymerization of IB, and several research groups have made significant contributions to develop effective LA·LB initiating systems to prepare HRPIB.13,23,27,28 In our earlier attempts, we developed MCl3·ether complexes (M = Al, Ga, and Fe) to obtain high exo-functionality (up to 85%) at −20 to +10 °C.29−32 We also developed an effective Received: September 7, 2017 Revised: October 2, 2017
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DOI: 10.1021/acs.macromol.7b01941 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
The representative industrial Raff-1 feed used contained C4 components in the following proportions: 43% IB, 28% B1, 4% C2B, 11% T2B, 0.3% BD, and 13.7% saturated C4 isomers. Throughout the study, IB and the other C4 olefins were considered as nonpolar solvents, and their volume was added to the volume of hexanes. Conversions were determined gravimetrically based on [IB] in the monomer feed. To control the temperature jump at high monomer concentrations (4−8 M), the total volume of the reaction mixture was reduced to 3.6 mL. Model Reaction of TMPCl with B1. Model reactions were carried out in cyclohexane-d12 at 0−20 °C using the following concentrations: [TMPCl] = 0.05 M, [B1] = 1.5 M, and [EADC·CEE] = 0.05 M with [CEE]/[EADC] = 1.5. Typically, B1 and TMPCl were mixed in cyclohexane-d12 at 0 °C in the glovebox. Then, the required amount of EADC·CEE complex was added to start the reaction. After 2 h, the reaction mixture was quenched with methanol-d4. Finally, reaction products were analyzed by 1H NMR spectroscopy and gas chromatography−mass spectroscopy (GC-MS) analysis. Size Exclusion Chromatography. Molecular weights and molecular weight distributions (polydispersity index, PDI) 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 multiangle 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 columns connected in the following order: 500, 103, 104, 105, and 100 Å. The refractive index (RI) detector was the concentration detector. THF was used as the eluent at a flow rate of 1.0 mL/min at room temperature. The results were processed using the Astra 5.4 software from Wyatt Technology Inc. NMR Spectroscopy. Proton nuclear magnetic resonance (1H NMR) and 13C NMR spectra were recorded on a Bruker 500 MHz spectrometer using CDCl3 or cyclohexane-d12 (C6D12) as solvents. The number-average molecular weights (Mn,NMR) of the HRPIB samples were calculated by the 1H NMR integrals of end-groups and backbone related peaks of the polymer as reported earlier.37 The 13C NMR spectra were recorded using CDCl3 under inverse gated decoupling with a 3 s delay time with 18 000 scans. 30 mg/mL chromium(III) acetylacetonate (Cr(AcAc)3, Sigma-Aldrich) was added to the sample solution to achieve quantitative spectra. Gas Chromatography−Mass Spectrometry (GC−MS) Analysis. The 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. 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 MSD ChemStation E.02.00.493 software (Agilent Technologies). Separation was carried out on a nonpolar DB-5 capillary column (Agilent Technologies) with length = 30 m, i.d. = 0.250 mm, and film thickness = 0.25 μm. Helium (purity 99.999%) was employed as carrier gas at a constant column flow of 1.0 mL/min. The GC oven temperature was programmed from 60 °C (held for 2 min) to 140 °C at 10 °C/min (held for 1 min). The injector temperature was kept at 260 °C. The injection volume was 2 μL.
co-initiating system by using ethylaluminum dichloride (EADC)·bis(2-chloroethyl) ether (CEE) complex, which works efficiently in hexanes (nonpolar solvent) at −20 to 20 °C to reach more than 90% exo-olefin content.1,33−36 Commercial HRPIB is mostly produced from a pure IB feed. Polymerization of C4 mixed feed to yield polymers with high terminal vinylidene content is challenging. In addition to IB, industrial C4 feed contains other C4 olefins mostly B1, and smaller amounts of C2B, T2B, and BD, which are chain transfer agents and/or terminators (poisons) during the cationic polymerization of IB.37 The presence of these C4 olefins during the polymerization of IB may affect the nature and distribution of olefin types in HRPIB, which in turn influence reaction with MA and eventually the product quality of HRPIB−MA adduct. To determine the feasibility of the EADC· CEE catalyst system to prepare HRPIB from C4 mixed feed, we investigated copolymerization of IB in the presence of C4 olefins typically found in industrial Raffinate-1 (Raff-1) feed compositions. Although several hundred thousand metric tons of polybutenes are commercially produced every year from C4 mixed feed,38 there is no report on the reactivity ratio of C4 olefins in their cationic copolymerization. Therefore, the reactivity ratios rIB and rB1 were determined for the binary copolymerization of IB and B1 at low conversions.
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EXPERIMENTAL SECTION
Materials. Isobutylene (IB, Matheson Tri Gas) was dried by passing it through in-line gas-purifier columns packed with barium oxide (BaO)/drierite and then condensed in a glass tube at −30 °C. tert-Butyl chloride (t-BuCl, 98%, TCI America) was used without further purification. 1-Butene (B1, ≥97%), cis-2-butene (C2B, ≥99%), 1,3-butadiene (BD, ≥99%), ethylaluminum dichloride (EADC, 1.0 M solution in hexane), bis(2-chloroethyl) ether (CEE, 99%), potassium hydroxide (KOH) (90%), and anhydrous sodium sulfate (Na2SO4) were purchased from Sigma-Aldrich and used without any further purification. CDCl3 (99.8% D), methanol-d4, and cyclohexane-d12 (99.6% D) were obtained from Cambridge Isotope Laboratories, Inc., USA, for NMR study. 2-Chloro-2,4,4-trimethylpentane (TMPCl) was synthesized according to the earlier reported literature procedure.39 Hexanes (mixture of isomers, Sigma-Aldrich, ≥98.5%, ACS reagent) were refluxed over concentrated H2SO4 for 48 h, then washed with 10% KOH aqueous solution, and finally washed with distilled water until the aqueous layer was neutral. The hexanes were predried by vigorously mixing with anhydrous Na2SO4 for 30 min and then refluxed over CaH2 for 48 h. The hexanes were then distilled onto CaH2, refluxed again for 24 h, and freshly distilled just before the polymerization. Other solvents such as methanol (MeOH), tetrahydrofuran (THF), etc., were purified by standard procedures. Preparation of EADC·CEE Complexes. Complexes were prepared just before the polymerization of IB. In a glovebox, the required amount of CEE (117 μL, 1.0 mmol) was added to 1.0 mL of EADC in hexanes (1.0 M) at room temperature and stirred to form a Lewis acid:Lewis base (LA:LB) = 1 complex. Then, 9.0 mL of hexanes was added to make fully soluble 0.1 M complex. Copolymerization of IB with Other C4 Monomers. Polymerizations were performed under a dry N2 atmosphere in an MBraun glovebox (MBraun, Inc., Stratham, NH). Typically, the required amount of dry hexanes was placed in the screw top glass culture tube (75 mL) at −30 °C. Then, the initiator (t-BuCl) was added to the culture tube. IB and other C4 olefins (B1, C2B, and BD) were condensed at −30 °C and distributed to the polymerization reactors containing t-BuCl and hexanes. Then, the temperature was raised to 0 °C, and the polymerization was started under stirring by the addition of desired amount of EADC·CEE complex to the reactors at 0 °C. After a predetermined time, polymerization was terminated by addition of 0.3 mL of prechilled methanol.
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RESULTS AND DISCUSSION Determination of the Reactivity Ratio for Copolymerization of IB with B1. Since a representative commercial Raff-1 feed contains 43% IB and 28% B1, we determined the reactivity ratios for the cationic copolymerization of IB and B1. The higher reactivity of IB leads to low incorporation of B1 so copolymerization reactions were carried out at [IB]/[B1] = 4:96, 8:92, 12:88, and 16:84 feed ratios in the presence of [tBuCl] = 0.01 M, [EADC·CEE] = 0.01 M, and [IB+B1] = 2.0 M in dry hexanes at 0 °C for 75 min. We obtained ∼7−10% total monomer conversion and applied the following copolymerization equation to determine reactivity ratios:40 B
DOI: 10.1021/acs.macromol.7b01941 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules f (r1f + f2 ) F1 = 1 1 F2 f2 (r2f2 + f1 )
f⎞ f F1 ⎛ ⎜⎜1 + 2 ⎟⎟ = r1 1 + 1 F2 ⎝ f1 ⎠ f2
(1)
According to eq 3, the F1/F2(1 + f 2/f1) vs f1/f 2 plot should yield a straight line, with the slope and intercept of r1 and 1, respectively (Figure 1). In Figure 1, r1 = 1100 was calculated
F1 and F2 are mole fractions of IB and B1 in the copolymer, respectively. Monomer mole fractions in the feed are represented as f1 and f 2, respectively, for IB and B1. r1 = kIB−IB/kIB−B1 and r2 = kB1−B1/kB1−IB are monomer reactivity ratios, where kIB−IB, kIB−B1, kB1−B1, and kB1‑IB are the rate constants for addition of IB to ···IB+, B1 to ···IB+, B1 to ···B1+, and IB to ···IB+, respectively. The F1 and F2 values were determined from analysis of inverse gated 13C NMR spectra (Figure S1) by comparing signals from IB and B1 units. All results are summarized in Table S1. The 13C NMR spectrum of the copolymer shows signals corresponding to the IB carbons present in the copolymer at 59.1, 37.8, and 30.9 ppm for −CH2−, −C(CH3)2−, and −CH3, respectively. Interestingly, the resonance due to the ethyl branch of the regular 1,2enchained B1 repeat unit is very small. Corno et al.41 already reported that extensive isomerization takes place during cationic polymerization of B1, and they identified nine structural units by 13C NMR spectroscopy. The most intense signals at 14.1 and 14.6 ppm were assigned to methyl carbons of n-butyl and n-propyl groups formed by intramolecular hydride shifts. These are also the most intense peaks in the methyl region of the 13C NMR spectrum in Figure S1. To determine the copolymer composition, the ratios of peak areas of the main chain −C(CH3)2− carbons of IB units (37.8 ppm) to the CH3− carbons of B1 units (8.5−16.6 ppm region) were compared. The absolute rate constant of propagation (kIB−IB) is reported for IB polymerization as 3.6 × 108 to 1.7 × 109 L mol−1 s−1, which was independent of temperature and nature of Lewis acid.42 The structure−reactivity relationship of monomers and their cations showed that by decreasing monomer reactivity, reactivity of the corresponding cation increases.43 However, the effect of substituents on the reactivity of cations is much larger than their effect on the reactivity of monomers. For instance, the propagation rate constant of substituted styrenes increases with decreasing monomer reactivity and becomes diffusion limited for styrene. It is obvious, therefore, that the propagation rate constant of any monomer that is less reactive than styrene will also be diffusion limited (e.g., p-chlorostyrene). The linear free energy relationship (LFER)44−47 is usually used to predict rates of reactions of carbocations with alkenes and other types of nucleophiles: log k 20 °C = s(N + E)
(3)
Figure 1. Plot of F1/F2(1 + f 2/f1) vs f1/f 2 to determine r1 for the cationic copolymerization of IB and B1.
from the slope by fixing the intercept = 1. This r1 value is almost double the value (543) reported by us previously from capping of living PIB+ cation with B1 at −40 to −80 °C in hexanes/methyl chloride 80/20 to 50/50 (v/v) solvent mixtures and extrapolating the values to 0 °C in pure hexanes.37 Double extrapolation (solvent and temperature) might have given a lower number of r1. Also, r1 = 1100 is somewhat lower than the theoretical prediction from LFER, where r1 = 3162 was calculated by using s = 1 and N = −2.4 and +1.11 for B1 and IB, respectively. Because of the very large reactivity difference between IB and B1, the initiating t-Bu+ and propagating PIB+ cations tend to sequence IB even when [B1] is 5−20 times higher than [IB]. Considering the low Mn values, not all polymer chains will contain B1. However, after the crossover reaction the PIB-B1+ cation will tend to sequence B1 due to higher concentration of B1 considering that the addition rates of B1 and IB are identical (both diffusion limited). This could yield a copolymer with blocky structure if enough B1 is incorporated and polymerization is not terminated by isomerization or ion collapse. Since a very small amount of C2B and BD (4% C2B and 0.3% BD) is present in Raff-1 feed and T2B (11% in Raff-1 feed51) does not react at all with PIB+,37 we have not determined reactivity ratios for these monomers during their copolymerization with IB. In the next stage the copolymerization of IB with each of the C4 olefins was examined individually to understand the effect of each component on the polymerization rate and chain-end olefin distribution. Copolymerization of Isobutylene with 1-Butene. Polymerization of IB in the presence of B1 with t-BuCl and EADC·CEE was conducted in dry hexanes at 0 °C with different [IB]/[B1] = 95/05, 90/10, 80/20, 70/30, 60/40, and 10/90 ratios, at [IB+B1] = 1 M. The results are tabulated in Tables S2−S7, and their comparative first-order plots of ln{[M]0/[M]} versus time are shown in Figure 2A (where [M]0 and [M] are IB concentrations at time, t = 0 and t, respectively). The first-order plots suggesting that a decrease of active species concentration with time. The 1H NMR spectra of HRPIB exhibit a characteristic resonance signal at 3.95 ppm
(2)
where E is an electrophilicity parameter, N is a nucleophilicity parameter, and s is a nucleophile-specific slope parameter, which is usually close to 1 and can be neglected in semiquantitative treatments of reactivity. Therefore, monomers with N values less than styrene (0.78)48 should show diffusionlimited propagation. B1 has N = −2.4, and the isomerized structures formed during IB/B1 copolymerization should also have N values less than 0.78; thus, eq 3 is valid to determine r1. Both T2B (N = −2.45)49 and C2B (N = −2.44)50 are much less reactive; therefore, their homopropagation is also diffusion limited. Since IB is much more reactive than B1 (N parameter for IB is 1.1150), cross-propagation from B1 to IB should also be limited by diffusion; thus r2 = 1. Using r2 = 1, eq 1 can be rearranged as C
DOI: 10.1021/acs.macromol.7b01941 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
decrease with decreasing [IB]/[B1] ratio. However, molecular weights were not affected by [IB]/[B1] ratio, since Mn is highly dependent on [IB]. To reduce termination, copolymerization was conducted with [IB+B1] ≥ 2 M up to 8 M, with [IB]/[B1] = 60/40 (ratio in Raff-1 feed). The results are presented in Table 1 and Tables S13−S15. Remarkably, first-order plots move upward with increasing [IB] (Figure 2B). This clearly indicates that termination was suppressed at [IB+B1] ≥ 2 M relative to propagation (Scheme 1). Moreover, Mn values increased with increasing [IB] with high exo content up to 89%. Figure S4 depicts quantitative 13C NMR spectra of HRPIB prepared at different polymerization times with [IB+B1] = 2 M. As anticipated, the B1/IB ratio in the copolymer increased with increasing polymerization conversion as shown in Table 2. The IB/B1 ratio in the copolymer at IB:B1 = 60:40 extrapolated to zero conversion is 825, in line with our r1 = 1100. Copolymerization of Isobutylene with 2-Butene. Polymerization of IB in the presence of C2B with [IB]/ [C2B] = 92/08 (ratio in Raff-1 feed) was carried out using tBuCl and EADC·CEE in dry hexanes at 0 °C at different [IB +C2B] = 1, 2, 4, and 6 M. The results are tabulated in Table 3 and Tables S16−S18, and their comparative first-order plots are shown in Figure 4. The polymerization rate is quite low at [IB +C2B] = 1 and 2 M, even though reactivity of C2B is ∼294 times less than IB in hexanes at 0 °C.37 This is probably due to formation of sec-alkyloxoniumions during the polymerization. However, close to linear first-order plots were obtained with [IB+C2B] ≥ 2 M due to the increase in propagation rate relative to termination. Moreover, Mn increased with increasing [IB], and the exo content remained high, up to 92%. The 1H NMR spectra of the HRPIB obtained in the copolymerization of IB with C2B were similar to the homopolymerization of IB due to low C2B incorporation in the copolymer. Since T2B was unreactive toward the PIB+ cation,37 we did not study copolymerization of IB in the presence of T2B.
Figure 2. Plot of ln{[M]0/[M]} vs time for the polymerization of IB +B1 with different (A) IB:B1 ratios at [IB+B1] = 1 M and (B) [IB +B1] at IB:B1 = 60:40, initiated by [t-BuCl] = 0.01 M and [EADC· CEE] = 0.01 M with [CEE]/[EADC] = 1.5 at 0 °C.
corresponding to −CHCl− as shown in Figure 3. This indicates that capping of PIB+ cation with B1 gives PIB−B1−Cl due to termination by ion collapse.37 Furthermore, the ion collapse rate increased moderately with decrease of [IB]/[B1] ratio (Figure S2 and Figure 3). However, the [PIB−B1−Cl] is still
