Ether Complexes in Nonpolar

Apr 13, 2012 - News Ed., Am. Chem. .... (1, 2) However, in the presence of DTBP, polymerization was absent, suggesting ..... 21, 0.4, 13, 1200, 1100, ...
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Polymerization of Isobutylene by AlCl3/Ether Complexes in Nonpolar Solvent Philip Dimitrov,† Jack Emert,‡ and Rudolf Faust*,† †

Polymer Science Program, 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: The carbocationic polymerization of isobutylene (IB), co-initiated by AlCl3/ether complexes, has been reexamined and extended to different dialkyl ethers. In the absence of a proton trap, 2,6-di-tert-butylpyridine (DTBP), the polymerization of IB by the cumyl alcohol (CumOH)/AlCl3·nBu2O initiator/co-initiator system in dichloromethane/hexanes (80/20 v/v) at −40 °C gave high conversion to polyisobutylene (PIB) comprising exo-olefins with high selectivity, similar to that reported before by Vasilenko et al.1,2 However, in the presence of DTBP, polymerization was absent, suggesting that CumOH is not an initiator in conjunction with AlCl3·Bu2O, and the true initiator is adventitious water. Similarly, in the presence of DTBP in hexanes at 0 °C, polymerizations were absent not only with CumOH but with CumCl, tert-butanol, and 2-chloro-2,4,4trimethylpentane. The polymerization of IB could be initiated only with adventitious water in the absence of DTBP, but monomer conversions and exo-olefin content (60−70%) were much lower than in a polar solvent and the PIBs exhibited Mn = 700−4200 with high polydispersities (PDI ∼ 3−5). The separate addition of ether followed by AlCl3 to the polymerization mixture resulted in conventional PIB with high trisubstituted olefinic content. The previously proposed mechanism is inadequate, as it cannot explain all the observations. Mechanistic studies suggest that the reaction of water with AlCl3·R2O yields H+AlCl3OH−, which initiates the polymerization, and free ether, which abstracts a β-proton from the growing chain end before it diffuses from the immediate vicinity of the polymer cation. Accordingly, the role of the complex is to deliver the ether to close proximity of the propagating end.

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INTRODUCTION The carbocationic polymerization of isobutylene (IB) is a subject of great scientific and industrial interest.3 Low molecular weight (Mn ∼ 500−5000) olefin end-functional PIB is a precursor to motor oil and fuel additives with worldwide production in excess of 750 000 tons per year. Two major industrial methods are currently employed to produce low molecular weight IB homo- or copolymers with olefinic end groups. The “conventional” process uses a C4 mixture and AlCl3- or EtAlCl2-based catalyst systems and gives polybutenes with high trisubstituted olefinic content.4,5 A mechanism has been proposed recently to explain the complex set of isomerization reactions responsible for tri- and tetrasubstituted olefinic content.6 Because of the low reactivity of the tri- and tetrasubstituted olefinic end groups, polybutenes need to be chlorinated to react with maleic anhydride to give polybutenylsuccinic anhydride, which is reacted with oligoalkylenimines to yield polybutenylsuccinimide ashless dispersants. The © 2012 American Chemical Society

other method employs pure IB and BF3 complexes with either alcohols or ethers and yields highly reactive PIBs (HR PIBs) with 75−85% exo-olefinic end-group content.7 PIB exo-olefins readily react with maleic anhydride in a thermal “ene” reaction to produce PIB succinic anhydride. Because the final product does not contain any chlorine, HR PIB is more desirable than polybutenes. In recent decades, several new methods for the synthesis of HR PIB were reported (adequately reviewed in ref 1), but none of them gained industrial acceptance, likely due to the high cost (low temperature, expensive chemicals). Recently, Vasilenko et al.1 and Liu et al.8 reported that AlCl3·dibutyl ether and AlCl3·diisopropyl ether complexes in dichloromethane (DCM) or in DCM/hexanes 80/20 (v/v) mixtures at moderate Received: February 23, 2012 Revised: April 4, 2012 Published: April 13, 2012 3318

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Table 1. Polymerization of [IB] = 1 M at −40 °C in DCM/Hex = 80/20 v/v for 3 min ([CumOH] = 0.018 M) no. a

1 2 3 [ref 1] 4 5 6 a

[Bu2O·AlCl3] (M)

[DTBP] (M)

conv (%)

initiator effic (%)

Mn (SEC)

PDI (SEC)

Mn (NMR)

[PIB] (M)b

exo (%)

endo (%)

0.020 0.022 0.022 0.022 0.022 0.028 0.032

0 0 0 0 0.006 0.006 0.006

80 78 89 62 0 0 1

74 69 80 75 0 0 3

3700 3300 2300 1560

2.1 2.1 2.1 1.5

2500 2500 3300 n.d.

0.0179 0.0175 0.0151 0.022

100 100 97 91

0 0 3

1000

0.0006

97

3

b

[CumOH] = 0.02 M. Mn(NMR) were used for the calculation of [PIB]; Mn(SEC) was used for the calculation of [PIB] if Mn(NMR) was not available. (measurement angles are 44.7°, 90.0°, and 135.4°) from Wyatt Technology Inc., a ViscoStar viscosity detector from Wyatt, and five Ultrastyragel GPC columns connected in the following order: 500, 103, 104, 105, and 100 Å. RI was the concentration detector. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL/min at room temperature. The results were processed by the Astra 5.4 software from Wyatt Technology Inc. Nuclear Magnetic Resonance. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker 500 MHz spectrometer using CDCl3 or CD2Cl2 as solvents (Cambridge Isotope Lab., Inc.). 27 Al NMR spectra were taken on a Bruker spectrometer operating at 62.9 MHz. Reference was [Al(H2O)6]3+ in H2O as external standard.12

temperatures give HR PIB with exo-olefinic end-groups in excess of 90%. Later, Liu et al. also reported the use of FeCl3·diisopropyl ether complexes for the polymerization of IB to yield HR PIB in DCM.9 Most recently, Vasilenko et al. studied the effect of solvent polarity, temperature, and initiator.2 With the CumOH/AlCl3·Bu2O system in hexane, monomer conversions and initiator efficiencies were low,10 and the PIBs exhibited broad molecular weight distributions. The exo-olefinic end group content was also lower compared to that obtained in polar solvent. This new technique is intriguing from a mechanistic point of view and may replace the BF3 process if it can be applied in nonchlorinated hydrocarbon solvents. In the present contribution, the polymerization of IB co-initiated by AlCl3/ether complexes in hexanes was studied with emphasis on the possible initiators, mechanisms of initiation, chain transfer and termination, and mechanisms of deactivation of the AlCl3/ether co-initiator.

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RESULTS AND DISCUSSION

Polymerization of IB by AlCl3/Ether Complexes in DCM/Hexanes 80:20 (v/v). It was reported previously that the polymerization of IB by CumOH/AlCl3·Bu2O initiator/coinitiator system in DCM/hexane (80/20 v/v) at −40 °C gives HR PIB with relatively high IB conversion in 3 min of polymerization time.1 The proposed mechanism suggested ionization of CumOH with free AlCl3 generated by the dissociation of AlCl3·Bu2O complex, propagation, and proton abstraction by the free ether. Interestingly, in the absence of CumOH, IB conversion and exo-olefin content were similar. We performed polymerizations of IB under the same conditions in the absence and in the presence of the proton trap DTBP in order to evaluate the significance of adventitious water for the overall process. Polymerization results are collected in Table 1. It was possible to reproduce the previously reported resultsthe obtained PIBs had up to 100% exo-olefin content, and the apparent initiator efficiency was around 80%. The only difference was the observed molecular weight, which in our case was up to 2 times higher. However, in the presence of DTBP, polymerization was not observed, even at increased AlCl3·Bu2O concentrations (runs 4, 5, and 6, Table 1). Thus, it can be concluded that CumOH is not an initiator with AlCl3·Bu2O alone (Scheme 1). We suggest that ionization

EXPERIMENTAL SECTION

Materials. Hexanes and dichloromethane (DCM) were purified as described previously.6 Isobutylene (IB, Matheson Tri Gas) gas was dried by passing it through in-line gas-purifier columns packed with BaO and then condensed in a receiver flask at −80 °C before use. AlCl3 (Aldrich 99.99%), cumyl alcohol (CumOH, 97%, Aldrich), tertbutyl alcohol (tBuOH, 99.6%, anhydrous, Aldrich), dibutyl ether (Bu2O, Aldrich anhydrous 99.3%), diisopropyl ether (iPr2O, Aldrich, anhydrous 99%), di-sec-butyl ether (sec-Bu2O, 96%, Aldrich), diisobutyl ether (iBu2O, 99%, TCI America), and di-tert-butylpyridine (DTBP, 97%, Aldrich) were used as received. 2-Chloro-2,4,4trimethylpentane (TMPCl) was synthesized according to ref 11. Polymerization of IB. Polymerizations were performed under a dry N2 atmosphere in a MBraun 150-M glovebox (Innovative Technology Inc., Newburyport, MA). IB was condensed and distributed to the polymerization reactors, screw-top culture tubes (75 mL), at −40 °C. Polymerization were done in DCM/hexanes 80/ 20 (v/v), or in hexanes, co-initiated with AlCl3/ether complex (typically from 0.005 to 0.02 M) at monomer concentrations of [IB] = 0.2−1 M, at temperatures ranging from −40 to 0 °C and terminated with NH4OH. The polymer was recovered from the organic layer, solvents were evaporated, and the final PIB was dried until constant weight. Preparation of AlCl3/Ether Complexes. AlCl3/ether complexes were prepared shortly before the polymerization of IB. In the glovebox, dry DCM was added to AlCl3 powder, which was only partially soluble. Then, an equimolar amount of ether was added slowly while stirring to form a 1 M AlCl3/ether complex solution. Characterization. Size Exclusion Chromatography. Molecular weights and dispersities of the polymers 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

Scheme 1

3319

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Table 2. Polymerization of [IB] = 0.91 M Initiated by AlCl3·Bu2O in Hexanes at −20 °C ([AlCl3·Bu2O] = 0.022 M)

a

no.

time (min)

yield (%)

Mn (NMR)

Mn

PDI

tri (%)

exo (%)

endo (%)

tetra (%)

7 8 9 10a

20 40 60 60

14 16 21 21

1800 1782 1970 2000

2600 2100 2300 1800

4.4 12.0 8.7 2.3

0 0 0 0

85 81 82 79

8 9 8 9

7 10 10 11

[TMPCl] = 0.015 M.

Table 3. Polymerization of IB (1 M) by AlCl3/Ether Systems in Hexanes at 0 °C for 20 min PIB end groups distribution f

no.

initiator

[initiator] (M)

conv (%)

[DTBP] (M)

Mn (NMR)

Mn (SEC)

PDI (SEC)

[PIB] (M)

11a 12b 13c 14d 15e

adventitious H2O TMPCl CumCl CumOH tBuOH

∼4 × 10−4 0.015 0.015 0.02 0.02

35 0 0 0 0

0 0.006 0.006 0.006 0.006

4100

4 000

4.1

0.0048

exo (%)

tri + endo (%)

tetra (%)

60

14

26

[AlCl3·iPr2O] = 0.010 M. b[AlCl3·Bu2O] = 0.022 M. c[AlCl3·iPr2O] = 0.022 M. d[AlCl3·Bu2O] = 0.020 M. e[AlCl3·iPr2O] = 0.020 M. fMn(NMR) were used for the calculation of [PIB]. a

Table 4. Polymerization of IB in Hexanes at 0 °C for 20 min Co-initiated by [R2O·AlCl3] = 0.005 M (R = Bu, iPr, sec-Bu, or iBu) PIB end groups distribution

a

no.

[IB] (M)

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8

LA·LB AlCl3·Bu2O

AlCl3·iPr2O

AlCl3·sec-Bu2O

AlCl3·iBu2O

conv (%)

Mn (NMR)

1 2 4 9 10 13 18 30 26 35 43 56 20 28 33 52

1000 1800 2400 2800 700 1200 1700 2000 860 1500 1800 2400 1400 2200 3100 3700

Mn (SEC)

2400 2600 620 1100 1800 2000 1000 1800 2000 2800 1400 2400 3600 4200

PDI

a

[PIB] (M)

exo (%)

endo + tri (%)

tetra (%)

5.8 6.8 5.2 4.3 4.6 4.9 4.5 4.6 5 4.5 5.8 5.1 4.9 4.4

0.0001 0.0002 0.0006 0.0014 0.0016 0.0024 0.0036 0.0067 0.0034 0.0052 0.0080 0.0105 0.0032 0.0076 0.0036 0.0063

72 66 63 63 74 70 65 65 60 53 54 47 54 54 54 52

13 16 15 15 14 14 15 14 24 25 22 24 25 21 19 18

15 18 21 22 12 16 20 21 16 22 24 29 21 25 26 30

Mn(NMR) were used for the calculation of [PIB].

of CumOH is an indirect process and requires an initial reaction of the AlCl3 with the adventitious H2O to give H+ AlCl3OH−, which ionizes CumOH (Scheme 1). As expected, when tBuOH was used instead of CumOH at the same conditions (Table S2), polymerization was also absent in the presence of 0.006 M DTBP. Polymerization of IB by AlCl3/Ether Complexes in Hexanes. In hexanes, IB conversions (Table 2) were generally much lower than in DCM/hexanes (80/20 v/v) and also lower than that (65% in 30 min) reported by Vasilenko and coworkers at −20 °C, possibly due to the lower concentration of adventitious water in our case.2 As expected in the presence of TMPCl conversions and Mns remained unchanged, indicating that TMPCl does not initiate the polymerization in conjunction with AlCl3·Bu2O. Polymerizations were also performed in hexanes at 0 °C typically for 20 min. Similarly to the DCM/hexanes (80/20 v/ v) system, CumOH and tBuOH did not initiate the

polymerization of IB (Table 3). Also, even the most efficient cationic initiators for IB polymerization such as TMPCl and CumCl could not initiate polymerization in the presence of a proton trap (Table 3). Apparently, the reactivity of AlCl3/ether complex is too low, and active cationic species could be generated only from adventitious moisture. The results of polymerizations utilizing different dialkyl ethers as a function of IB concentration are summarized in Table 4. The polydispersity indices of the polymers were high (PDI = 4.3−6.8), which may be attributed to the heterogeneous nature of the polymerization. Indeed, upon the addition of AlCl3/ether complex to the polymerization mixture, a fine white precipitate was observed. Other PIB end groups as well as tri- and tetrasubstituted olefins were also observed as a result of carbocationic rearrangements and chain scission at the end of the PIB chain due to delayed proton abstraction from the growing PIB+ by the ether molecule (Figure 1).13 The yields and the exo-olefinic content of the PIBs were directly related to 3320

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Polymerization of IB in Hexanes Co-initiated by AlCl3 and Separately Added Ether. Polymerizations with separately added iPr2O and AlCl3 were performed in hexanes at 0 °C in the presence of TMPCl. If free iPr2O is responsible for proton abstraction, adding iPr2O separately would be a convenient way to obtain HR PIB of high yields due to the introduction of the efficient initiator TMPCl. Unfortunately, as seen in Table 8, only conventional PIB with mainly trisubstituted olefin ends was obtained even if the concentration of iPr2O was orders of magnitude higher than the apparent concentration of AlCl3, which is largely insoluble in hexanes. Mechanistic Studies. Formation of AlCl3/Ether Complexes. The formation of AlCl3/ether complexes is an exothermic process, which is possible only in polar solvents, such as DCM. Complexation of AlCl3 with ethers does not proceed in nonpolar solvents such as hexanes. Most AlCl3 also remained undissolved in pure iPr2O, but AlCl3 formed a 1:1 complex readily with iPr2O in DCM at 25 °C though AlCl3 has low solubility in DCM (∼1.2 × 10−3 M at room temperature).15 There is no conclusive data whether AlCl3 is in monomeric or dimeric form in DCM. In order to investigate the complexation of AlCl3 with ethers in detail, 1H NMR and FTIR spectroscopies were used. The FTIR spectrum of iPr2O is presented in Figure 2, where the characteristic peaks for ether C−O bond stretching are situated at 1010, 1108, and 1130 cm−1. Upon complexation of iPr2O with AlCl3, the peak at 1010 cm−1 disappeared completely and two new peaks appeared at 902 and 1097 cm−1. For AlCl3·iPr2O at a 1:1 ratio, there was no detectable peak at 1010 cm−1 (Figure 2), which indicates a very low concentration of free ether and free AlCl3. Provided that saturated solutions of AlCl3 in DCM are around ∼1.2 × 10−3 M, this concentration should be also the upper limit for the uncomplexed species. An additional half equivalent of iPr2O (AlCl3:iPr2O = 1.5) did not lead to the appearance of the peak at 1010 cm−1. However, a broad peak appeared between 902 and 1010 cm−1, suggesting the existence of another complex with a higher ratio of ether to AlCl3 (e.g., AlCl3·2R2O). Free ether was only observed at stoichiometries of 1:2 or higher (Figure 2). This behavior is different than in the case of FeCl3/iPr2O complexes, where only 1:1 complexes were reportedly formed.9 The 1 H NMR spectra in DCM-d 2 of AlCl 3 ·iPr 2 O, AlCl3·Bu2O, and AlCl3·iBu2O are presented in Figure 3, Figure S1, and Figure S2, respectively. All of the characteristic ether peaks move downfield as a result of complexation with AlCl3, especially the protons on the carbons next to oxygen. For AlCl3·Bu2O and AlCl3·iBu2O, the peaks, most notably the CH2−O peak at 4.2 ppm, were also broadened by complexation to AlCl3.

Figure 1. 1H NMR spectrum of PIBs obtained by polymerization of IB in hexanes at 0 °C for 20 min co-initiated by [AlCl3·Bu2O] = 0.005 M and adventitious H2O.

the type of ether used. The ethers employed have similar proton affinities (Table S1) so steric factors must have influenced the proton abstraction process. AlCl3·Bu2O gave PIB with high exo-olefinic end-group content, but yields were below 10%. AlCl3·iPr2O gave similar end-group distribution and somewhat higher IB conversions. More sterically hindered ethers, such as sec-Bu2O and iBu2O, gave higher IB conversions up to 56%, but the exo-olefinic end-group content was lower. With increasing solvent polarity conversions and exo-olefinic content increased (Table S3) up to 100% and 89%, respectively, with AlCl3·iPr2O in DCM/hexanes (80/20 v/v) at 0 °C. An attempt to increase monomer conversion was made by saturating hexanes with water ([H2O]sat = 4 × 10−4 M at 0 °C)14 by washing with deionized water. As seen in Table 4, the IB conversion increased to 90% while the exo-olefin content remained similar. Attempts were also made to increase the exoolefin content by increasing the concentration of free ether. The exo-olefin content did increase (Table 5), suggesting faster proton abstraction; however, monomer conversion decreased. Time studies in Table 6 revealed that the polymerization was very rapid, and 60% conversion was obtained within the first minute of polymerization. Additional AlCl3·iPr2O introduced after 20 min increased the yield only marginally (sample 40, Table 6), indicating that water was already consumed. At the same time, if the co-initiator was aged (Table 7), the yield dropped significantly. Most likely, in that case, the co-initiator was deactivated by hydrolysis of AlCl3 that led to the release of HCl, which is not an initiator in conjunction with AlCl3·ether complexes.

Table 5. Polymerization of [IB] = 1 M by [AlCl3·iPr2O] = 0.01 M in Hexanes Saturated with H2O at 0 °C for 20 min PIB end group distribution

a

no.

added [iPr2O] (M)

conv (%)

Mn (NMR)

Mn (SEC)

PDI (SEC)

[PIB]a (M)

exo (%)

tri + endo (%)

tetra (%)

32 33 34 35 36

0 0.005 0.010 0.015 0.020

90 27 16 7 4

2100 1600 1300 1100 984

1 900 1 000 800 600

4.5 7.3 8.7 8.9

0.0240 0.0095 0.0069 0.0036 0.0023

62 78 80 84 88

15 8 8 7 4

23 14 12 9 9

Mn(NMR) were used for the calculation of [PIB]. 3321

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Table 6. Polymerization of [IB] = 1 M by [AlCl3·iPr2O] in Hexanes Saturated with Water at 0 °C

a

no.

time (min)

conv (%)

Mn (SEC)

PDI (SEC)

Mn (NMR)

[PIB]b (M)

exo (%)

tri + endo (%)

tetra (%)

37 38 39 40a

1 20 40 20 + 20

60 75 90 83

4000 3100 2300 2700

2.8 3.5 4.4 5.2

3700 2900 2400 3100

0.0091 0.0145 0.0210 0.0150

63 62 63 61

15 16 18 13

21 22 20 25

[AlCl3·iPr2O] = 0.01 M. After 20 min another [AlCl3·iPr2O] = 0.01 M was added. bMn(NMR) were used for the calculation of [PIB].

Table 7. Polymerization of [IB] = 1 M by [iPr2O·AlCl3] = 0.02 M at 0 °C for 20 min

a b

no.

conv (%)

Mn (SEC)

41 42a

86 25

2500 2000

PDI Mn (SEC) (NMR) 3.9 5.1

1900 1900

[PIB]b (M)

exo (%)

tri + endo (%)

tetra (%)

0.0253 0.0074

60 56

17 18

23 26

iPr2O·AlCl3 was initially aged in hexanes for 20 min at 0 °C. Mn(NMR) were used for the calculation of [PIB].

In the case of AlCl3·iPr2O, two types of complexes were observed (Figure 3-2) as also suggested by the FTIR spectra. One complex exhibits peaks at 5.15 ppm (OCH, m) and at 1.65 ppm (CH3)2CH, d) that are well resolved and appear more downfield with the chemical shift independent of the amount of ether. Another set of resonance peaks at 4.2 ppm (OCH, m) and 1.4 ppm ((CH3)2CH, d) are further upfield, consistent with a lower electron-withdrawing environment. The second set of peaks also have broader line widths, indicating ongoing exchange reactions.16 By lowering the temperature to 0 °C both peaks move downfield to 4.6 and 1.5 ppm. At iPr2O-starved conditions (AlCl3:iPr2O = 2:1, Figure 3-1), the peak (Figure 3-2) at 4.2 ppm was absent. At this ratio, about half of the added AlCl3 remained undissolved, so the real ratio of the formed complex is closer to 1:1 with a small amount of free AlCl3 at a concentration determined by its solubility. This resulted in formation of a stable complex, where the peak at 5.2 ppm is well resolved due to decreased exchange reactions. As more ether is added, a small peak appears at 4.2 ppm (CHO) which grows with increasing amounts of iPr2O moving upfield due to exchange with free ether (Figure 3-3). This suggests the presence of an additional, less stable complex where the iPr2O is in an environment of lower acidity (Scheme 2). Initiation Step. In order to investigate the initiation process, a 1:1 AlCl3·iPr2O complex was reacted with tBuOH and H2O, and the products of these reactions were monitored in situ by 1 H NMR spectroscopy. As a result of the addition of tBuOH (0.015 M) to a 0.02 M solution of AlCl3·iPr2O in DCM-d2, the CHO peak characteristic of the complex at 5.15 ppm (Figure 4-1) decreased in area, and the CHO peak characteristic of the second complex at 4.2

Figure 2. ATR-FTIR spectra overlay of [iPr2O] = 0.1 M (solid line) and AlCl3·iPr2O complexes at different AlCl3/iPr2O ratios at 25 °C where [AlCl3] = 0.1 M.

Figure 3. 1H NMR spectra overlay of AlCl3·iPr2O complexes in DCMd2 at 25 °C [AlCl3] = 0.02 M: (1) AlCl3:iPr2O = 2:1 (soluble portion); (2) AlCl3:iPr2O = 1:1; (3) AlCl3:iPr2O = 1:2; (4) spectrum of iPr2O.

ppm shifted upfield and increased in intensity (Figure 4-2). At the same time, the added tBuOH formed a complex with AlCl3

Table 8. Polymerization of [IB] = 1 M by [AlCl3] = 0.035 M and [TMPCl] = 0.02 M in Hexanes at 0 °C for 20 mina PIB end groups distribution b

no.

[iPr2O] (M)

conv (%)

Mn (SEC)

PDI (SEC)

Mn (NMR)

[PIB] (M)

exo (%)

tri + endo (%)

tetra (%)

43 44 45 46

0.005 0.010 0.015 0.020

98 85 84 79

350 400 360 370

3.6 3.9 3.6 3.5

599 830 705 768

0.0917 0.0574 0.0668 0.0577

4 7 8 11

75 73 71 70

21 20 20 19

a

The hexane solution of IB, iPr2O, and TMPCl was added to a glass culture tube containing AlCl3. bMn(NMR) were used for the calculation of [PIB]. 3322

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Scheme 2

Figure 5. 1H NMR spectrum of the soluble fraction of [AlCl3·iPr2O] = 0.02 M in DCM-d2 after treatment with [H2O] = 0.02 M.

characterized by 1H NMR and 27Al NMR spectroscopies. 1H NMR spectroscopy revealed that iPr2O was completely released after the formation of AlCl3OH−, as indicated by the almost

Figure 4. 1H NMR spectra overlay of AlCl3·iP2O complexes in DCMd2 at 25 °C: (1) AlCl3·iPr2O; (2) AlCl3·iPr2O and AlCl3·tBuOH; (3) spectrum of free iPr2O; (4) spectrum of free tBuOH.

(a, Figure 4-2), as indicated by the shift of the characteristic peak for the tert-butyl group downfield from 1.5 to 1.7 ppm. The AlCl3·tBuOH complex was formed at the expense of the AlCl3·iPr2O complex due to the displacement of iPr2O by tBuOH or the additional tBuOH added to an existing AlCl 3 ·iPr 2 O complex forming the more exchangeable tBuOH·AlCl3·iPr2O. In both of these cases the released free iPr2O would interact with other AlCl3·iPr2O complexes, leading to complexes with higher stoichiometry of iPr2O. When an equimolar amount of water was added to a 1:1 AlCl3·iPr2O complex, similar results were observed. The characteristic peaks for the complex at 5.15 and 1.65 ppm were still present, but the amount of free ether also increased due to its displacement by water (Figure 5). The released free ether was in equilibrium with the complex species from which the minor CHO peak (from 4.2 to 3.8 ppm) originates. With a decrease of the temperature, the quantity of the major species (with CHO peak at 5.2 ppm) increases (Figure 5). This as an indication that the two complex species are indeed in dynamic equilibrium. Nature of Counteranion. The original publications by the Vasilenko1 and Liu8 and co-workers suggest that the ether molecules remain associated with the MXnOH− counteranions. This is difficult to rationalize since both are Lewis bases. In order to clarify this issue AlCl3/ether complex was mixed with TBAOH in DCM-d2 and the products of the reaction were

Figure 6. 1H NMR spectrum of products of reaction between AlCl3·iPr2O and TBAOH in DCM-d2.

quantitative integral values of TBA+ and iPr2O (Figure 6). The 27 Al NMR spectrum of the AlCl3·iPr2O complex shows a broad singlet at around 100 ppm and a much smaller narrow singlet at 103 ppm, corresponding to AlCl3OH− formed by adventitious water (Figure 7-1). After the reaction of AlCl3·iPr2O with TBAOH, only the sharp singlet at 103 ppm remained (Figure 7-2), which confirmed that AlCl3 was no longer complexed with iPr2O. Termination Reactions. PIBs obtained by AlCl3/ether systems in hexanes at 0 °C possessed a detectable amount of PIB-Cl end-groups (Figure S3), formed by ion collapse (Scheme 3), which cannot be reionized by the AlCl3·ether 3323

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methanol. According to 1H NMR spectroscopy, which is less sensitive than UV−vis, ∼ 2% from PIB was capped with DTE, which corresponds to 2% PIB-Cl end-groups (Figure S4). Proposed Mechanism. Since in the presence of DTBP polymerization of IB was absent when CumOH, tBuOH, tBuCl, or TMPCl initiators were used, the true initiator for IB polymerization is adventitious water. It is also unlikely that free uncomplexed AlCl3 is the true co-initiator, as proposed by Vasilenko and co-workers.1 Free uncomplexed AlCl3 should be able to ionize tBuCl or TMPCl that would lead to a rapid polymerization of IB, which was not observed. The reaction of water with AlCl3·ether complex yields H+AlCl3OH− with the simultaneous release of the ether. Protonation of the monomer and propagation takes place until ether assisted chain transfer to monomer yields PIB exo-olefin and a new protonated monomer. Termination by ion collapse yields PIB-Cl, which cannot be reionized. This mechanism adequately explains all but one observation, i.e., that adding AlCl3 and an ether separately does not produce PIB exo-olefin. In order to explain this finding, we reconsider the data in Table 3. Even if all ether molecules are simultaneously displaced from the complex by water, its maximum concentration would be 0.005 M. Considering that the propagation rate constant of IB is close to diffusion limited and the proton abstraction from PIB+ by the ether molecule at maximum could also be diffusion limited, the DPns controlled by the [M]/[R2O] ratio should be >40− 160 at low conversion. Clearly the observed DPns are much lower. To explain these findings, we propose that the ether molecules are brought into the vicinity of propagating ends by the complex and remain there for the duration of the propagation (perhaps solvating the cation), so their local concentration is higher than in the bulk. The proposed mechanism is shown in Scheme 4. This mechanism also

Figure 7. 27Al NMR spectra overlay in DCM/DCM-d2 = 66/33 (v/v) of (1) [AlCl3·iPr2O] = 0.1 M and (2) AlCl3·iPr2O + TBAOH = 0.1 M.

Scheme 3

complex. This reaction also contributed to the overall deactivation of the co-initiator by releasing AlCl2OH (Scheme 3). Because of the overlapping peaks in the 1H NMR spectra, it was not possible to accurately determine the amount of PIB-Cl. In order to determine the content of PIB-Cl more accurately, end-capping of the PIB-Cl groups with ditolylethylene (DTE) was performed at the following conditions: [PIB] = 0.01 M, [TiCl4] = 0.036 M, [DTBP] = 0.005 M, [DTE] = 0.01 M, hexanes/MeCl (60/40, v/v), −80 °C for 30 min. Under these conditions PIB-Cl group can be ionized by TiCl4, and the resulting PIB+ is end-capped with DTE to form PIB-DTE+. The [PIB-DTE+] = 2.7 × 10−4 M could be determined from the absorbance at 460 nm characteristic for DTE+ (Figure 8),

Scheme 4

Figure 8. UV−vis spectrum of PIB-DTE+ obtained at [PIB] = 0.01 M, [TiCl4] = 0.036 M, [DTBP] = 0.005 M, [DTE] = 0.01 M, hexanes/ MeCl = 60/40, −80 °C, 10 min.

explains why alkyl halides, which are generally efficient initiators with AlCl3, are completely inactive with the ether complexes.

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CONCLUSIONS A reexamination of the polymerization of IB with the CumOH/ AlCl3·Bu2O initiator/co-initiator system in dichloromethane/ hexanes (80/20 v/v) or in hexanes reported by Vasilenko and co-workers before1,2 revealed that CumOH is not an initiator in conjunction with AlCl3·Bu2O, and the true initiator is

which corresponds to ∼5.4% of PIB-Cl chains from PIB of Mn = 2000 obtained by polymerization of IB co-initiated by AlCl3·iPr2O in hexanes for 20 min. PIB-DTE+ was also terminated with the hydride donor [Bu3SiH] = 0.01 M for 30 min followed by rigorous reprecipitation of the polymer five times from hexanes/ 3324

dx.doi.org/10.1021/ma3003856 | Macromolecules 2012, 45, 3318−3325

Macromolecules

Article

adventitious water. Similarly, in hexanes at 0 °C none of the conventional cationic initiators such as CumCl or TMPCl could initiate the polymerization of IB. The polymerization of IB can only be initiated with water in the absence of DTBP, but monomer conversions and exo-olefin content (60−70%) are much lower than in polar solvent and the polymers exhibit broad molecular weight distributions. Among the AlCl3 complexes with Bu2O, iPr2O, sec-Bu2O, and iBu2O, the highest IB conversions and exo-olefin content could be reached with AlCl3·iPr2O. Mechanistic studies suggest that the reaction of water with AlCl3·R2O yields H+AlCl3OH− that initiates the polymerization and free ether, which abstracts a β-proton from the growing chain end before it diffuses from the immediate vicinity of the polymer cation. A separate addition of AlCl3 and ether to the polymerization mixture results in conventional PIB with high trisubstituted olefinic end groups due to slow proton abstraction. Accordingly, the role of the complex is to deliver the ether to the close proximity of the propagating end. Since the AlCl3·R2O complex yields PIB exo olefins with high selectivity only in polar solvent and only water initiates the polymerization, the industrial relevance of this process is questionable.

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(11) Fodor, Zs.; Faust, R. J. Macromol. Sci., Pure Appl. Chem. 1996, A33, 305. (12) Cerný, Z.; Machácek, J.; Fusek, J.; Cásenský, B.; Kriz, O.; Tuck, D. G. J. Chem. Soc., Dalton Trans. 1998, 1439−1446. (13) Dimitrov, P.; Emert, J.; Hua, J.; Keki, S.; Faust, R. Macromolecules 2011, 44, 1831−1840. (14) Englin, B. A.; Plate, A. F.; Tugolukov, V. M.; Pryanishnikova, M. A. Khim. Technol. Topl. Masel 1965, 9, 42−46. (15) Masure, M.; Sauvet, G.; Sigwalt, P. J. Polym. Sci. 1978, 16, 3065−3076. (16) Saunders, M.; Kate, M. R. J. Am. Chem. Soc. 1978, 100, 7082− 7083.

ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra of AlCl3/ether complexes, PIB-Cl, and PIB end-capped with DTE; tables with additional polymerization data and DFT calculations for the proton affinities of the ethers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: e-mail:[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Infineum is greatly appreciated. REFERENCES

(1) Vasilenko, I. V.; Frolov, A. N.; Kostjuk, S. V. Macromolecules 2010, 43 (13), 5503−5507. (2) Vasilenko, I. V.; Shiman, D. I.; Kostjuk, S. V. J. Polym. Sci. 2012, 50 (4), 750−758. (3) De, P.; Faust, R. Carbocationic Polymerization. In Macromolecular Engineering. Precise Synthesis, Materials Properties, Applications; Matyjaszewski, K., Gnanou, Y., Leibler, L., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2007; pp 1−45. (4) Puskas, I.; Banas, E. M.; Nerheim, A. G. J. Polym. Sci., Polym. Symp. 1976, 56, 191−202. (5) Puskas, I.; Meyerson, S. J. Org. Chem. 1984, 49, 258−262. (6) Dimitrov, P.; Faust, R. Macromolecules 2010, 43 (4), 1724−1729. (7) Mach, H.; Rath, P. Lubr. Sci. 1999, 11−2, 175−185. (8) Liu, Q.; Wu, Y.-X.; Zhang, Y.; Yan, P.-F.; Xu, R.-W. Polymer 2010, 51, 5960−5969. (9) Liu, Q.; Wu, Y.; Yan, P.; Zhang, Y.; Xu, R. Macromolecules 2011, 44, 1866−1875. (10) In ref 8, the initiator efficiencies for CumOH were calculated by the formula Ieff = Mn,theor/Mn,exp × 100. This is correct only when initiation from adventitious water and chain transfer are absent. Based on the low Fn values especially for hexane where less than half of the chains contain the Cum moiety, this is not the case. The corrected Ieffs ∼ 23%. 3325

dx.doi.org/10.1021/ma3003856 | Macromolecules 2012, 45, 3318−3325