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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Controlled Catalytic Chain Transfer Polymerization of Isobutylene in the Presence of tert-Butanol as Exo-Enhancer Tota Rajasekhar,† Jack Emert,§ Lawrence M. Wolf,‡ and Rudolf Faust*,† †

Polymer Science Program, Department of Chemistry, and ‡Department of Chemistry, University of Massachusetts Lowell, One University Avenue, Lowell, Massachusetts 01854, United States § Infineum USA, 1900 E. Linden Avenue, Linden, New Jersey 07036, United States S Supporting Information *

ABSTRACT: Catalytic chain transfer polymerization (CCTP) of isobutylene in the presence of alcohol as an exo-enhancer with tert-butyl chloride/ethylaluminum dichloride (EADC)·bis(2chloroethyl) ether (CEE) has been investigated in hexanes at 0 °C. Increasing exo-olefin content was observed with increasing steric bulkiness of the alkyl group of the alcohol, i.e., tert-butyl > isopropyl > methyl. Here, we report that tert-butanol (t-BuOH) is an excellent exo-enhancer compared to other tert-alcohols such as tert-amyl alcohol (AmOH), 2-methyl-2-pentanol (MPOH), and 3-ethyl-3-pentanol (EPOH). The aromatic tert-alcohol cumyl alcohol was not an exo-enhancer but acted as an initiator. In the reaction of EADC.CEE and t-BuOH, t-butoxyaluminum dichloride (t-BuOAlCl2) was formed, which is the real exo-enhancer and is not stable at room temperature. Molecular weights were virtually unchanged in the presence t-BuOAlCl2 with [t-BuOAlCl2]:[EADC.CEE] < 0.5, and exo-olefin content increased ∼15% relative to polymerization in the absence of t-BuOAlCl2. This is presumably due to stabilization of the cation by tBuOAlCl2 which slows isomerization of the PIB+. Stabilization of the cation was confirmed by 1H NMR and UV−vis spectroscopy at 0 °C by adding t-BuOAlCl2 to the diphenylmethyl cation, a representative stable cation. The rate constant of chain transfer (ktr) was determined to be 2 × 108 L mol−1 s−1 at 0 °C, which is not affected by t-BuOAlCl2. Addition of an exoenhancer is especially important for polymerization at CSTR conditions at low steady state monomer concentrations. This is the first report identifying the role of alcohols in CCTP and opens new vistas in the synthesis of highly reactive polyisobutylene.

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INTRODUCTION

of significantly increasing the mixing and temperature control that results in products with lower PDI.10 In preliminary studies of the polymerization under CSTR conditions, the desired molecular weight (∼2200 g/mol) could only be achieved with 70−80% CSTR conversion (i.e., steady state at [IB] = 20−30% of original [IB] in the feed) at 0 °C with [CEE]/[EADC] = 1.5. At higher conversions, molecular weights were too low. Adjusting the [CEE]/[EADC] ratios and temperature might be an effective way to resolve the issue. With decreasing temperature, molecular weights could be increased,1,9 but at the expense of polymerization rate. Decreasing the [CEE]/[EADC] ratios from 1.5 to 1, the molecular weight increases but the exo content drops (from 90 to 70%).1−10 We recently studied the role of various organic nucleophilic impurities such as propionic acid, acetone, methanol, and acetonitrile in CCTP. Interestingly, it was found that exo-olefin content slightly increased in the presence of ppm levels of methanol while molecular weights remained virtually identical to that obtained in the absence of methanol with [CEE]:

Catalytic chain transfer polymerization (CCTP) in nonpolar solvents at moderate temperatures has developed into one of the most adaptable polymerization techniques for the synthesis of highly reactive polyisobutylene (HRPIB) in recent years.1−10 Low molecular weight HRPIB (Mn = 500−5000 g/mol) is a valuable precursor in the preparation of motor oil and fuel auxiliaries.11−13 Initially, metal halide (AlCl3, GaCl3, FeCl3, etc.) ether complexes (MXn·ether) were shown to be better catalysts for the synthesis of HRPIB at moderate temperatures compared to traditional catalysts.14−26 However, their application in nonpolar solvents is limited because of poor solubility, which affects the polymerization rate.27−29 This issue was lately overcome by employing alkylaluminum dichloride ether complexes as hydrocarbon soluble catalysts.1−10 We have shown that the ethylaluminum dichloride (EADC)·bis(2chloroethyl) ether (CEE) complex is an effective catalyst, which not only polymerizes pure IB feed but also polymerizes mixed C4 olefin feeds at 0−10 °C to yield up to 88% exo-olefin content.9 In these studies, polymerization conditions and products have been optimized for a batch reactor. However, polymerization in a continuous stirred tank reactor (CSTR) under steady state conditions is of industrial interest, as a way © XXXX American Chemical Society

Received: February 11, 2018 Revised: March 30, 2018

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DOI: 10.1021/acs.macromol.8b00327 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules [EADC] = 1.30 There are few reports about controlled cationic polymerization for HRPIB synthesis in the presence of alcohols,31−34 and the reason for the exo content increase is still unclear. Moreover, relatively strong nucleophiles inactivate the polymerization or decrease the concentration of active species. 30,34−37 In this report, we present a detailed investigation on the mechanistic role of alcohol in EADC· CEE catalyzed chain transfer polymerization of IB as well as discuss the effectiveness of tertiary alcohols compared to secondary and primary alcohols (Scheme 1).

predetermined time, polymerization was terminated by addition of 0.3 mL of prechilled methanol. For polymerization in the presence of preformed t-BuOAlCl2, the t-BuOH and EADC·CEE (required amounts) complex were aged in a separate reactor at 0 °C for the predetermined time. This mixture was then added to the reactor containing monomers, initiator, and hexanes at 0 °C to start the polymerization. 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. Preparation of tert-Butoxyaluminum Dichloride (tBuOAlCl2). In the glovebox, 100 mL solution of EADC in dry hexanes (0.026 mol) was placed in a round-bottomed flask. t-BuOH (1.95 g, 0.026 mol) dissolved in 50 mL of dry hexanes was then added dropwise to the EADC solution at 0 °C under stirring. Then the reaction mixture was allowed to warm to room temperature and stirred for 12 h. The solvent was evaporated to give pale yellow t-BuOAlCl2, which was stored at 0 °C with a tight seal. Model Reaction between Diphenylmethyl Cation and tBuOAlCl2. The model reaction was carried out in 1 mL of CD2Cl2 at 0 °C using the following concentrations: [GaCl3] = 0.1 M, [Ph2CHCl] = 0.05 M, and [t-BuOAlCl2] = 0.05 M. In the glovebox, Ph2CHCl was ionized to the diphenylmethyl cation (Ph2CH+) using GaCl3 in CD2Cl2 at 0 °C. The cation solution was added to an NMR tube containing the t-BuOAlCl2 at 0 °C. Then, the mixture was analyzed by 1 H NMR spectroscopy at 0 °C. Characterization. Size Exclusion Chromatography. Numberaverage molecular weight (Mn) 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, and 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 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. Spectroscopy. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker 500 MHz spectrometer using CDCl3, CD2Cl2, benzene-d6, or cyclohexane-d12 as solvents (Cambridge Isotope Laboratory, Inc.). Figure S1 shows a typical 1H NMR spectrum of a representative HRPIB sample prepared in this study. 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 (Figure S1). UV−vis measurements were done on a PerkinElmer Lambda 750 UV−vis spectrophotometer using CH2Cl2 as solvent.

Scheme 1. Different Alcohols Investigated as Potentional Exo-Enhancers in Catalytic Chain Transfer Polymerization of IB Initiated by t-BuCl/EADC·CEE

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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 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. 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%), sodium hydroxide (NaOH, ≥98%), sodium sulfate (Na2SO4), gallium(III) chloride (GaCl3), chlorodiphenylmethane (Ph2CHCl), and cumyl alcohol (CumOH) were purchased from Sigma-Aldrich and used without any further purification. tert-Amyl alcohol (AmOH), 2methyl-2-pentanol (MPOH, 97%), and 3-ethyl-3-pentanol (EPOH, 98%) were purchased from Alfa Aesar. Hexanes, mixture of isomers (Sigma-Aldrich, ≥98.5%, ACS reagent), were refluxed over 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. Other alcohols such as methanol (MeOH), isopropanol (i-PrOH), and tert-butanol (tBuOH) were dried by standard procedures.38 Preparation of EADC·CEE Complex. The complex was prepared just before the polymerization of IB. In a glovebox, the required amount of CEE (117 μL, 1 mmol) was added to 1 mL of EADC in hexanes (1 M) at room temperature and stirred to form a Lewis acid:Lewis base (LA:LB) = 1 complex, followed by addition of 9 mL of hexanes to make the fully soluble 0.1 M complexes. Polymerization of IB with Other C4 Olefins. 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) and alcohol were 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 tBuCl, alcohol, and hexanes. Then, the temperature was raised to 0 °C, and polymerization was started under stirring by the addition of the desired amount of EADC·CEE complex to the reactors at 0 °C. After a

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RESULTS AND DISCUSSION Polymerization of C4 Olefin Mixture under Designed CSTR Conditions. Recently, our group disclosed that fast copolymerization of a C4 olefin mixture could be observed only at high monomer feed concentrations in a batch reactor. We have now investigated polymerizations under simulated CSTR conditions. In a CSTR reactor there is a continuous feed of C4 olefin mixture, initiator, catalyst, and CCT agent into the reactor and products (the main product being the polymer) out of the reactor. After an initial startup period a steady state is reached, preferably with high polymer and low monomer concentrations determined by the residence time. We attempted to mimick CSTR conditions in a batch reactor by

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Table 1. Polymerization of C4 Olefin Mixture ([IB + B1 + C2B + BD] = 0.25 to 1.5 + 3 + 0.4 + 0.02 M) Using [EADC·CEE] = 0.01 M and [t-BuCl] = 0.01 M in Dry Hexanes at 0 °C with [CEE]/[EADC] = 1.5 and 1 and Polymerization Time = 2−4 min CSTR conv (%)

conva (%)

Mn,NMRb (g/mol)

Mn,SECc (g/mol)

PDIc

exod (%)

tri + endod (%)

tetrad (%)

−CHCl−d (%)

BD endsd (%)

70 70e 80 80e 90 90e 95 95e 90f 95f

8 10 9 8 8 9 10 8 6 4

2300 2200 1900 2000 1000 900 800 800 2400 2100

2500 2500 2100 2200 1200 1000 800 900 2600 2200

3.2 2.4 3.1 2.3 3.3 2.4 3.2 2.5 2.5 2.3

78 82 76 81 79 83 78 81 72 70

12 10 14 9 13 9 13 10 15 16

9 7 9 9 7 7 7 8 13 14

0.4 0.3 0.6 0.4 0.5 0.6 1.0 0.7 0.2 0.3

0.4 0.4 0.5 0.3 0.2 0.4 0.5 0.5 0.2 0.4

a

Determined gravimetrically based on [IB] in feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study. eUnder continuous stirring with rpm ∼500. fUnder continuous stirring with [CEE]/[EADC] = 1.

studying polymerizations at monomer concentrations equivalent to 70%, 80%, 90%, and 95% steady state IB conversions with continuous stirring (∼500 rpm). In these experiments the monomer conversion was kept low to be able to neglect change in the monomer concentrations that affects the Mn. The simulated steady state [IB] concentrations were calculated assuming that the other C4 olefins did not react due to the much higher reactivity of IB compared with 1B, C2B, and BD. In actuality, the HRPIB discussed here may be a pure homopolymer of IB or a copolymer of IB with low levels of 1B, C2B, and BD incorporated (Figure S1).9 Results for the polymerization of a C4 olefin mixture at simulated CSTR monomer concentrations corresponding to 70−95% CSTR conversion in hexanes using EADC·CEE ([CEE]:[EADC] = 1.5) are tabulated in Tables S1−S4, and their comparative first-order plots are shown in Figure S2a. The first-order plots move downward with decreasing [IB] similar to the trend observed for polymerization in a batch reactor.9 Moreover, each first-order plot curves downward with increasing polymerization time due to formation of stable PIB sec-alkoxonium ions via end-capping.9 Moreover, exo contents are ∼80% and not affected by polymerization time/conversion (Figure S2b). In Table 1, the data are tabulated for low conversions for two sets of conditions, stirring at the beginning of the polymerization only (no remark) or with continuous stirring during the polymerization (experiments with remark e and f), the latter which consistently gave 3−5% higher exo content. Independent of the mode of stirring, Table 1 reveals that the desired molecular weight can be achieved only at 70−80% CSTR conversions at 0 °C with [CEE]/[EADC] = 1.5. Molecular weights decrease with increasing IB conversion (decreasing IB concentration) to below 1000 g/mol at 90−95% CSTR conversions due to a decrease in the [IB]/[CEE] ratio that determines the molecular weight at constant temperature. To increase molecular weights at low [IB], [CEE] can be decreased. Indeed at [CEE]/[EADC] = 1, the desired Mn = 2200 can be obtained at [IB] equivalent to 90−95% conversion in the CSTR reactor (last two entries in Table 1). However, exo-olefin content decreases to ∼70% (Table 1) due to either an increase in the rate of isomerization or a decrease in βproton abstraction by CEE. Thus, high [CEE] is necessary to maintain high exo content. Determination of Rate Constant of Chain Transfer (ktr). Based on the general mechanism for cationic polymerization of IB (Scheme 2), a propagating PIB+ cation, once

Scheme 2. Schematic Depiction of the Possible Choices of PIB+ in Chain Transfer Polymerization

generated, has four choices: it can propagate (addition of IB), it can undergo termination by ion collapse, or it can isomerize to hindered olefin, or it can transfer a proton to a chain transfer agent (CEE). The degree of polymerization (DPn) at any given instant of the polymerization, can be determined from the rates of propagation and termination.39 DPn =

rate of propagation (R p) sum of the rates of all termination reactions (R ic + R iso + R tr)

(1)

At elevated temperatures, rate of ion collapse can be neglected,9 and eq 1 is rearranged as R R 1 = iso + tr DPn Rp Rp (2) where the degree of polymerization in the absence of chain transfer agent DPn0 = Rp/Riso, rate of chain transfer Rtr = ktr[PIB+][CEE], and rate of propagation, Rp = kp [PIB+][IB]. Hence, substitution of these in eq 2 gives the Mayo equation k [CEE] 1 1 = + tr DPn DPn0 k p[IB]

(3)

where [CEE] is the free CEE concentration, [IB] is the concentration of the isobutylene, and kp = 108 L mol−1 s−1 is the rate constant of propagation.5 From eq 3, the ktr value could be obtained from the slope of the 1/DPn vs free [CEE] plot. Polymerizations were conducted at different free [CEE] = 0−20 mM with a constant [IB] = 1 M and initiated by t-BuCl/EADC·CEE (Tables S5 and S6). The data points with free [CEE] = 0, 1, 2, and 5 mM were directly collected from our previous publication.5 Figure 1A shows Mn C

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presented in this early disclosure.30 Herein, the study was extended to various alcohols, such as MeOH (primary alcohol), i-PrOH (secondary alcohol), and t-BuOH (tertiary alcohol), to understand the steric effects. In the presence of these alcohols, the exo-olefin content clearly increased with increasing [alcohol] (Table 3 and Tables S7, S8). The comparative exo content vs [alcohol] plots are shown in Figure 2, which showed that tert-butanol is the best exo-enhancer. Increasing exo-olefin content was observed in the order of increasing steric bulkiness of the alkyl group of the alcohol, i.e., tertiary alcohol > secondary alcohol > primary alcohol. To further study other potential exo-enhancers, four branched tertiary alcohols were chosen, such as AmOH, MPOH, EPOH, and CumOH, and their exo-enhancing behavior was tested in the polymerization of IB. The results are summarized in Table 4 and Table S9. The results clearly indicate that AmOH, MPOH, and EPOH act as exo-enhancers, but their efficiency is slightly lower than t-BuOH. As with tBuOH, exo content increased and polymerization conversion decreased with increasing concentration of branched alcohols. The reason for this will be discussed in the following sections. Remarkably, CumOH failed to increase the exo content (Table S10). To understand this behavior, an NMR experiment was conducted using [CumOH] = 0.05 M and [EADC·CEE] = 0.05 M with [CEE]/[EADC] = 1 at 0 °C in cyclohexane-d12. Figure S3 shows that CumOH is ionized to Cum+ by EADC· CEE, which converts to α-methylstyrene (αMeSt) by proton elimination. Furthermore, the αMeSt dimerizes. To examine the ionization under polymerization conditions, IB polymerization was carried out using EADC·CEE complex and CumOH as an initiator. The polymerization results are summarized in Table 5. The resulting HRPIB exhibited sharp peaks at 7.3−6.9 ppm in the 1H NMR spectrum (Figure S4) that are associated with a cumyl moiety. Moreover, the SEC trace obtained with an RI detector follows the SEC trace obtained with a UV detector at 254 nm, indicating that the cumyl moiety is incorporated into the polymer (Figure S5). The slow polymerization (19% monomer conversion in 30 min) could be due to the low initiator efficiency of CumOH in dry hexanes.19 Formation and Stability of tert-Butoxyaluminum Dichloride (t-BuOAlCl2). Szumacher et al.40 studied the formation of t-BuOAlCl2 by reaction of methylaluminum dichloride with t-BuOH, and a single peak was observed at 1.25 ppm in the 1H NMR spectrum in benzene-d6. In the reaction of EADC·CEE with t-BuOH, t-BuOAlCl2 was also formed as indicated by the formation of ethane and a t-BuO

Figure 1. (A) Correlation between conversion and Mn at different free [CEE]: (1) 0 mM, [LB]:[LA] = 1; (2) 1 mM, [LB]:[LA] = 1.1; (3) 2 mM, [LB]:[LA] = 1.2; (4) 5 mM, [LB]:[LA] = 1.5; (5) 10 mM, [LB]: [LA] = 2; and (6) 20 mM, [LB]:[LA] = 3. (B) Plot of 1/DPn vs free [CEE] for the polymerization of IB with t-BuCl/EADC.CEE.

vs conversion plots extrapolated to zero conversion to determine Mn and DPn at low conversions to be able to set initial concentrations to simulate CSTR conditions. The representative 1/(DPn) vs free [CEE] plot shown in Figure 1B yields a straight line with slope ktr/kp[IB] and intercept 1/ DPn0 (see eq 3), from which ktr = 2 × 108 L mol−1 s−1 and DPn0 = 82 were obtained. The value DPn0 = 82 indicates that molecular weight should be more than 4600 g/mol for the polymerization in the absence of free chain transfer agent, CEE. Surprisingly, at low conversion, Mn is ∼1800 g/mol for the polymerization of IB in the absence of CEE (Table 2). Thus, we must conclude that Table 2. Polymerization of IB ([IB] = 2 M) Using [EADC] = [t-BuCl] in the Absence of Chain Transfer Agent (CEE) and Polymerization Time = 2 min in Dry Hexanes at 0 °C [EADC] (mM)

conva (%)

Mn,NMRb (g/mol)

Mn,SECc (g/mol)

PDI

2 3

11 44

1700 1200

1800 1100

3.1 2.8

c

exod (%)

tri + endod (%)

tetrad (%)

3 2

70 76

27 22

a

Determined gravimetrically based on monomer feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study.

in addition to being a chain transfer agent, CEE also stabilizes the active PIB+ cation to isomerization (Scheme 3). This finding will help elucidate the role of alcohol in CCTP and will be discussed in the interpretation of further results. Polymerization of IB in the Presence of Different Alcohols. In our previous study, exo-olefin content increased in the presence of methanol, but no reaction mechanism was

Scheme 3. Proposed Intermediate Stage for PIB Cation−Ether Pair in the Chain Transfer Polymerization

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Table 3. Polymerization of IB ([IB] = 1.0 M) Using [EADC·CEE] = 0.01 M, [t-BuCl] = 0.01 M, and [t-BuOH] = 0−5 mM in Dry Hexanes at 0 °C with [CEE]/[EADC] = 1.0 time (min)

[t-BuOH] mM

conva (%)

Mn,NMRb (g/mol)

Mn,SECc (g/mol)

PDIc

exod (%)

tri + endod (%)

tetrad (%)

20 20 20 60 20 60 60 60

0 0.01 0.3 0.3 1.0 1.0 2.0 5.0

100 100 96 100 76 100 75 16

2400 2300 2300 1900 1900 1400 2000 1800

2500 2500 2400 2000 1800 1300 1900 2000

3.3 3.4 3.2 3.3 3.4 3.2 3.3 3.1

68 77 83 82 84 86 90 96

20 15 8 11 8 8 5 3

12 8 9 7 8 6 5 1

a Determined gravimetrically based on monomer feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study.

Table 5. Polymerization of IB ([IB] = 1.0 M) Using [EADC· CEE] = 0.01 M and [CumOH] = 0.01 M in Dry Hexanes at 0 °C with [CEE]/[EADC] = 1.0 time (min)

conva (%)

Mn,NMRb (g/mol)

2 5 10 20 30

4 6 10 15 19

1300 1200 1100 900 800

Mn,SECc (g/mol)

1100 1000 900

PDI

c

exod (%)

tri + endod (%)

tetrad (%)

65 68 69 68 67

22 19 19 18 20

13 13 12 14 13

2.6 2.8 2.7

a

Determined gravimetrically based on monomer feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study.

suppressed rate of isomerization of PIB+ cation is proposed (Scheme 4) and will be discussed in detail in the Mechanistic Studies section. In order to determine the stability of t-BuOAlCl2, EADC· CEE was aged with the required amount of t-BuOH for 2−60 min, prior to polymerization (Table S11). The results are identical to those obtained when the EADC·CEE + t-BuOH mixture was added to the reaction mixture without aging (Table 6). Thus, the t-BuOAlCl2 is stable at 0 °C for at least 1 h. This is further corroborated by 1H NMR spectroscopy. Figure S6 shows the 1H NMR spectra of the EADC·CEE + tBuOH mixture in cyclohexane-d12 at 0 °C at different time intervals, where t-BuOAlCl2 does not show any decomposition/ precipitation as expected. When EADC·CEE was added to tBuOH at room temperature (Table S12), the increase of exoolefin content is small. This is because t-BuOAlCl2 decomposes slowly at room temperature with gas evolution forming an insoluble solid (AlCl3) (see Scheme S1 for detailed mechanism of the decomposition).40,41 Faust and co-workers also reported5

Figure 2. Plot of exo content vs [alcohol] for polymerization of IB initiated by t-BuCl/EADC·CEE with [CEE]/[EADC] = 1 at 0 °C in the presence of different alcohols.

group signal at 1.68 ppm in the 1H NMR spectrum in cyclohexane-d12 and at 1.25 ppm in benzene-d6 at 0° (Figure 3). This suggests that the real exo-enhancer is likely t-BuOAlCl2. This was further confirmed by conducting the polymerization with preformed t-BuOAlCl2 (Table 6), where the exo content increased with increasing [t-BuOAlCl2]. However, conversions are higher compared to the polymerization in the presence of tBuOH (Table 3). This is because free t-BuOH reacts with EADC·CEE, leading to the decrease of active species concentration, so the polymerization is slower. However, the molecular weights of HRPIB were virtually unaffected in the presence of [t-BuOAlCl2] < 5 mM compared with those in the absence of the exo-enhancer. This clearly indicates that ktr is not affected by the presence of t-BuOAlCl2. To justify the increase of exo content in the presence of exo-enhancer, a

Table 4. Polymerization of IB ([IB] = 1.0 M) Using [EADC·CEE] = 0.01 M, [t-BuCl] = 0.01 M, and [Alcohol] = 1 mM in Dry Hexanes at 0 °C with [CEE]/[EADC] = 1.0 time (min)

alcohol

conva (%)

Mn,NMRb (g/mol)

Mn,SECc (g/mol)

PDIc

exod (%)

tri + endod (%)

tetrad (%)

20 60 20 60 60 60

AmOH AmOH MPOH MPOH EPOH EPOH

63 88 61 85 58 87

2200 2000 2100 1900 2100 1900

2400 2100 2300 2100 2200 2000

2.9 2.8 3.1 3.0 2.9 3.2

80 81 79 80 81 80

10 10 11 10 9 9

10 9 10 10 10 11

a Determined gravimetrically based on monomer feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study.

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interaction of carbenium ion and t-BuOAlCl2 was studied by low-temperature NMR spectroscopy using a stable cation, Ph2CH+. Figure 4 shows the 1H NMR of Ph2CHCl, Ph2CH+, tBuOAlCl2, and [Ph2CH+ + t-BuOAlCl2] in CD2Cl2 at 0 °C. The Ph2CH+ was obtained by ionization of Ph2CHCl in CD2Cl2 at 0 °C, where 2 equiv of GaCl3 is essential for complete ionization. Upon complete ionization, methine and aromatic protons of Ph2CHCl shift downfield as anticipated. A similar downfield shift for methine and aromatic protons has been reported by De et al. for the cation generated from ionization of 1-chloro-1-(2,4,6-trimethylphenyl)ethane with GaCl3 at −78 °C.42 Upon addition of t-BuOAlCl2 to Ph2CH+, an upfield shift of the Ph2CH+ protons and a downfield shift of the t-BuOAlCl2 protons are observed which we propose is due to the formation of Ph2CH·t-BuOAlCl2+ based on computational results detailed below. The structural identity of the proposed interaction complex was supported computationally using density functional theory (DFT).43 The optimized geometry of the [Ph2CH+ + tBuOAlCl2] complex included an interaction between the Al center and a π system of one of the aromatic rings to produce an Al···π type interaction (Figure 5). This interaction was detected with and without the inclusion of solvent in the optimization. The separation between the Al and the ortho carbon is only 2.28 Å, suggesting some favorable contact. Both the ortho carbon and the Al center exhibit some degree of pyramidalization, confirming the presence of a bonding-type interaction between these two centers. To determine how closely this calculated complex agreed with experiment, the 1H NMR spectrum was calculated. The chemical shifts for Ph2CHCl, Ph2CH+, and [Ph2CH+ + tBuOAlCl2] were calculated at the mPW1PW91(DCM)/311+G(2d,p)//B3LYP-D3/6-31+G(d,p) level of DFT applying suitable scaling factors.43 The predicted chemical shifts for Ph2CHCl and Ph2CH+ agree well with the observed shifts, serving to provide partial validation of the method. The predicted shifts for the [Ph2CH+ + t-BuOAlCl2] complex accurately portray the asymmetric nature of the aromatic region as the protons of the aromatic ring complexed to Al are more downshifted than those protons belonging to the second aromatic ring. Al complexation thus leads to a more electrondeficient ring. The attenuation of the methine chemical shift with complexation relative to Ph2CH+ is qualitatively predicted here. While the predicted methine shift is approximately 0.8 ppm greater than that observed, it is still within a reasonable range for the prediction of cations.

Figure 3. 1H NMR spectra of (A) [t-BuOH] and (B) [EADC·CEE + t-BuOH] (1:1) in cyclohexane-d12 and (C) [EADC·CEE + t-BuOH] (1:1) and (D) [t-BuOH] in benzene-d6 at 0 οC. [t-BuOH] = 0.05 M, [EADC·CEE] = 0.05 M, and [CEE]/[EADC] = 1. The asterisk denotes solvent resonance.

that decrease of exo content has been observed upon in situ formation of AlCl3 in the CCTP of isobutylene in the presence of toluene initiated by t-BuCl/EADC·CEE. Mechanistic Studies. The results discussed above suggest that t-BuOAlCl2 suppresses the isomerization of PIB+ but does not act as a chain transfer agent (Scheme 4). The exo content and molecular weight increased with increasing [t-BuOAlCl2] in the absence of chain transfer agent (Table S13). This supports the premise that t-BuOAlCl2 stabilizes the cation similar to that observed above with CEE, resulting in slow isomerization of the PIB+. Conversions decreased slightly with increasing [t-BuOAlCl2] (Table 6 and Table S13), which might suggest a competition between the monomer and t-BuOAlCl2 for the proton. The protonated t-BuOAlCl2 is not expected to be an efficient initiator due to the lack of steric strain that exists in the stabilized PIB chain end. It is noteworthy that the increase in exo content is not observed for the polymerization with [CEE]/[EADC] = 1.5. In these conditions, free CEE competes with the exo-enhancer to the interaction with PIB+ cation, which could be due to the relatively low nucleophilicity of t-BuOAlCl2. Model Studies with Stable Cation (Ph2CH+). We proposed that the slow isomerization of PIB+ in the presence of t-BuOAlCl2 is due to an interaction between them. The

Table 6. 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 with [CEE]/[EADC] = 1.0 time (min)

[t-BuOAlCl2] (mM)

conva (%)

Mn,NMRb (g/mol)

Mn,SECc (g/mol)

PDIc

exod (%)

tri + endod (%)

tetrad (%)

20 60 20 60 20 60 20 60

0.3 0.3 1.0 1.0 2.0 2.0 5.0 5.0

100 100 90 100 80 96 71 94

2900 2800 3100 2800 2900 2600 2700 2200

3100 3000 3300 3000 3200 2900 2800 2300

2.9 3.1 3.2 3.0 3.3 3.2 3.0 2.9

82 84 84 85 85 86 87 89

10 10 8 8 9 7 9 8

8 6 8 7 6 7 4 3

a Determined gravimetrically based on monomer feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study.

F

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Macromolecules Scheme 4. Possible Mechanism for Polymerization of IB with t-BuCl/EADC.CEE in the Presence of t-BuOH

Figure 6. DFT predicted 1H NMR chemical shifts in dichloromethane for [Ph2CH+ + t-BuOAlCl2], Ph2CH+, and Ph2CHCl. The values in parentheses were obtained from experiment.

species, UV−vis spectroscopy could be used to monitor the interaction. The absorption maxima (λmax) of Ph2CH+ and (Ph2CH+ + t-BuOAlCl2) appear at 422 and 303 nm, respectively (Figure 5). The λmax and extinction coefficients of the peaks for (Ph2CH+ + t-BuOAlCl2) were shifted significantly to lower wavelength relative to Ph 2 CH + , supporting a substantial interaction. Controlled Preparation of HRPIB in CSTR Conditions in the Presence of Exo-Enhancer. We next sought to optimize the polymerization in the presence of exo-enhancer

Figure 4. 1H NMR spectra of (A) [Ph2CHCl], (B) [Ph2CH+], (C) [tBuOAlCl2], and (D) [Ph2CH+ + t-BuOAlCl2] in CD2Cl2 at 0 °C. [GaCl3] = 0.1 M; [Ph2CHCl] = [t-BuOAlCl2] = 0.05 M. The asterisk denotes the CDHCl2 resonance.

Figure 5. Calculated geometry for the [Ph2CH+ + t-BuOAlCl2] complex obtained at the B3LYP-D3/6-31+G(d,p) level of DFT.

A UV−vis spectroscopic study was also carried out at 0 °C to study the interaction of the stable cation Ph2CH+ with tBuOAlCl2 in CH2Cl2 solvent. Since ionization of diphenylmethyl chloride with GaCl3 generates a colored Ph2CH+ cation

Figure 7. UV−vis spectra of [Ph2CH+] and [Ph2CH+ + t-BuOAlCl2] in CH2Cl2 solvent at 0 °C. G

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Macromolecules

Table 7. Polymerization of C4 Olefin Mixture ([IB + B1 + C2B + BD] = 0.25 to 1.5 + 3 + 0.4 + 0.02 M) Using [EADC·CEE] = 0.01 M, [t-BuCl] = 0.01 M, and [t-BuOH] = 0.4 mM in Dry Hexanes at 0 °C with [CEE]/[EADC] = 1 under Continuous Stirring with rpm ∼ 500 and Polymerization Time = 2 min CSTR conv (%)

conva (%)

Mn,NMRb (g/mol)

Mn,SECc (g/mol)

PDIc

exod (%)

tri + endod (%)

tetrad (%)

−CHCl−d (%)

BD endsd (%)

70 80 90 95

11 8 5 4

3300 3000 2300 2100

3600 3100 2500 2300

2.5 2.5 2.5 2.3

84 85 83 84

10 9 10 10

6 6 7 6

0.3 0.4 0.3 0.1

0.1 0.2 0.1 0.1

a Determined gravimetrically based on [IB] in feed. bDetermined from NMR analysis. cObtained from SEC measurements. dCalculated from 1H NMR spectroscopic study.

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ACKNOWLEDGMENTS Financial support from Infineum USA is greatly respected. The authors also acknowledge the Infineum HRPIB team for supportive discussions.

under simulated CSTR conditions (70−95% CSTR conversions). To obtain the desired molecular weights, the polymerization was examined at [CEE]/[EADC] = 1 at 0 °C in the presence of [t-BuOH] = 0.4 mM (Tables S14−S17). The results tabulated in Table 7 clearly show that HRPIB with target Mn and high exo content ∼85% can be prepared at 0 °C at [CEE]:[EADC] = 1 using t-BuOH as exo-enhancer under CSTR conditions at monomer concentrations equivalent to 90−95% conversion.

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(1) Kumar, R.; Zheng, B.; Huang, K.-W.; Emert, J.; Faust, R. Synthesis of Highly Reactive Polyisobutylene Catalyzed by EtAlCl2/ Bis(2-chloroethyl) Ether Soluble Complex in Hexanes. Macromolecules 2014, 47, 1959−1965. (2) Vasilenko, I. V.; Shiman, D. I.; Kostjuk, S. V. Alkylaluminum Dichloride−ether Complexes Which are Fully Soluble in Hydrocarbons as Catalysts for the Synthesis of exo-Olefin Terminated Polyisobutylene at Room Temperature. Polym. Chem. 2014, 5, 3855− 3866. (3) Shiman, D. I.; Vasilenko, I. V.; Kostjuk, S. V. Cationic polymerization of isobutylene by complexes of alkylaluminum dichlorides with diisopropyl ether: An activating effect of water. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2386−2393. (4) Banerjee, S.; Emert, J.; Wright, P.; Skourlis, T.; Severt, R.; Faust, R. Polymerization of Isobutylene Catalyzed by EtAlCl2/Bis(2chloroethyl) Ether Complex in Steel Vessels. Polym. Chem. 2015, 6, 4902−4910. (5) Banerjee, S.; Jha, B. N.; De, P.; Emert, J.; Faust, R. Kinetic and Mechanistic Studies of the Polymerization of Isobutylene Catalyzed by EtAlCl2/Bis(2-chloroethyl) Ether Complex in Hexanes. Macromolecules 2015, 48, 5474−5480. (6) Shiman, D. I.; Vasilenko, I. V.; Kostjuk, S. V. Cationic Polymerization of Isobutylene Catalyzed by iBuAlCl2 in the Presence of Ethers: Effect of Catalyst Pre-activation and Mixture of Two Ethers. Polymer 2016, 99, 633−641. (7) Vasilenko, I. V.; Nikishev, P. A.; Shiman, D. I.; Kostjuk, S. V. Cationic Polymerization of Isobutylene in Toluene: Toward Welldefined exo-olefin Terminated Medium Molecular Weight Polyisobutylenes Under Mild Conditions. Polym. Chem. 2017, 8, 1417− 1425. (8) Rajasekhar, T.; Emert, J.; Faust, R. Synthesis of Highly Reactive Polyisobutylene by Catalytic Chain Transfer in Hexanes at Elevated Temperatures; Determination of the Kinetic Parameters. Polym. Chem. 2017, 8, 2852−2859. (9) Rajasekhar, T.; Haldar, U.; De, P.; Emert, J.; Faust, R. Cationic Copolymerization and Multicomponent Polymerization of Isobutylene with C4 Olefins. Macromolecules 2017, 50, 8325−8333. (10) Zhu, S.; Lu, Y.; Faust, R. Micromixing Enhanced Synthesis of HRPIBs Catalyzed by EADC/Bis(2-chloroethyl)Ether Complex. RSC Adv. 2017, 7, 27629−27636. (11) Harrison, J. J.; Mijares, C. M.; Cheng, M. T.; Hudson, J. Negative Ion Electrospray Ionization Mass Spectrum of Polyisobutenylsuccinic Anhydride: Implications for Isobutylene Polymerization Mechanism. Macromolecules 2002, 35, 2494−2500. (12) Balzano, F.; Pucci, A.; Rausa, R.; Uccello-Barretta, G. Alder-ene Addition of Maleic Anhydride to Polyisobutene: Nuclear Magnetic Resonanceevidence for an Unconventional Mechanism. Polym. Int. 2012, 61, 1256−1262.

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CONCLUSION HRPIB with the desired Mn = 2200 g/mol and high exo content cannot be prepared by the EADC·CEE catalyst system under simulated steady state conditions at 90−95% IB conversion in a CSTR reactor. At [CEE]/[EADC] = 1.5 the exo content is satisfactory but the Mns are too low, while at [CEE]/[EADC] = 1 the Mns are satisfactory but the exo content is only ∼70%. Alcohols can be used as exo-enhancers in the chain transfer polymerization of IB. Increasing steric bulkiness of the alkyl group of the alcohol increased the exoolefin content, t-BuOH being the most effective. The increase of the exo content is attributed to the stabilization of PIB+ cation by t-BuAlCl2 that arises via the reaction of EADC·CEE with t-BuOH. Spectroscopic studies confirmed the interaction between t-BuOAlCl2 and carbocation. CEE not only acts as a chain transfer agent, but it is also involved in stabilization of the PIB+. In addition to the obvious industrial importance this is the first time that stabilization of carbocations from hydrocarbon monomers has been proven.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00327.

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REFERENCES

Tables S1−S18, Figures S1−S7, Scheme S1, and references (PDF)

AUTHOR INFORMATION

Corresponding Author

*(R.F.) E-mail [email protected]; Ph 9789343675; Fax 9789343013. ORCID

Tota Rajasekhar: 0000-0002-2266-3898 Notes

The authors declare no competing financial interest. H

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I

DOI: 10.1021/acs.macromol.8b00327 Macromolecules XXXX, XXX, XXX−XXX