Effects of Ether on the Cationic Polymerization of Isobutylene Catalyzed by AlCl3

In this work, we prepared different initiator solutions containing ether and AlCl3 by changing the addition sequence of ingredients, studied the interactions between ether and AlCl3 from the evolution of the Fourier transform infrared (FTIR) spectra by comparison, and investigated the catalytic performances of AlCl3 affected by ether for isobutylene polymerization. We observed that different preparation methods of initiator solutions could lead to two kinds of interactions between ether and H2O/AlCl3 in hexane. The strong interaction could stabilize carbenium ions and seriously decrease the catalytic performance, whereas the weak interaction could promote isomerization and proton elimination. Moreover, we found that the preparation method of initiator solutions was not a critical factor in CH2Cl2. Finally, a universal mechanism based on the AlCl3-involved interactions in different solvents was proposed to understand the effects of ether on the cationic polymerization catalyzed by AlCl3 thoroughly.


■ INTRODUCTION
Polyisobutylenes (PIBs), produced by the cationic polymerization of isobutylene (IB), 1−3 have drawn attention from both industry and academia in recent years because of their distinguished properties with respect to thermal stability, flexibility at ambient temperature, impermeability to gases, and so forth. 4−7 These excellent properties lead to their wide applications in auto tyres, medical bottle plugs, sealants, ashless dispersants, gasoline additives, and so forth. 8−10 Different applications of PIBs need different molecular weights (M n ). According to M n , commercially available PIBs can be classified into three categories: (i) high-molecular-weight PIBs (M n > 100 000 g mol −1 ); (ii) medium molecular-weight PIBs (MPIBs, M n = 10 000−100 000 g mol −1 ), and (iii) low-molecular-weight PIBs (LPIBs, M n < 5000 g mol −1 ). 11−13 Recently, many attentions have been paid on the LPIBs with a high content of exo-olefin end groups and narrow molecular weight distribution, denoted as highly reactive PIBs (HRPIBs), because of their high activity in synthesizing lubricants and gasoline additives. 14−19 The initiator solution of H 2 O/aluminum chloride (AlCl 3 ) is widely adopted for IB polymerization because of the low cost and high efficiency, 20−23 in which H 2 O and AlCl 3 work as the initiator and the coinitiator, respectively. However, the polymerization initiated by H 2 O/AlCl 3 involves complicated chain reactions, such as isomerization, chain termination, and so forth. To control the chain reactions, nucleophilic reagents such as ether or ester are usually added in the initiator solution. The basicity and steric hindrance of the nucleophilic reagent as well as the stability of the corresponding complex have great influence on polymerization. 24,25 Through the regulation of nucleophilic reagents, many functional polymers with different molecular weights were attained, such as the HRPIBs, MPIBs with high content of exoolefin, and so forth. 26−33 The availability of various polymers indicates the complicated effects of nucleophilic reagents on polymerization. Although researchers always gave some explanations on specific experimental results, the interactions of initiators, coinitiator and nucleophilic reagents, are still lack of systematic understanding. 34−40 In this work, we prepared different initiator solutions containing ether and AlCl 3 by changing the addition sequence of ingredients, studied the interactions between ether and AlCl 3 from the evolution of the Fourier transform infrared (FTIR) spectra of ether by comparison, investigated the catalytic performance of AlCl 3 affected by ether on IB polymerization according to conversion determination and product characterization, and proposed a universal mechanism to clarify the effects of ethers on the cationic polymerization catalyzed by AlCl 3 . Herein, both normal ether [diethyl ether (Et 2 O), pK a = −3.59] and branched ether [isopropyl ether (iPr 2 O), pK a = −4.30] were used to study the influence of basicity and steric hindrance; both the nonpolar solvent (n-hexane) and the polar solvent [dichloromethane (CH 2 Cl 2 )] were used to introduce the influence of solvent environment; the microflow reactor was exploited as a suitable platform for investigating instantaneous processes including cationic polymerization. 41−43 Through the research, the effects of ether on the cationic polymerization catalyzed by AlCl 3 in different solvents are understood thoroughly to provide a rational and effective access to customized functional polymers.

■ RESULTS AND DISCUSSION
For the cationic polymerization of IB, solvents have great influence on the chain reactions because the polarity of solvents could affect the reactivity of active centers distinctly. Herein, we investigated the polymerization processes in a nonpolar solvent (n-hexane) and a polar solvent (CH 2 Cl 2 ) separately.
Cationic Polymerization of IB in Hexane. Effects of Et 2 O. The initiator solutions prepared with different preparation methods were determined by attenuated total reflectance (ATR)−FTIR to cognize the interaction between Et 2 O and AlCl 3 , as shown in Figure 1. For the free Et 2 O, the characteristic peak appears at 1126 cm −1 . When AlCl 3 comes in direct contact with Et 2 O, denoted as method 1, the absorbance at 1126 cm −1 is much weakened and many new peaks emerge, which indicates the interaction between Et 2 O and AlCl 3 . The complex could react with H 2 O to form an associated ion pair, which would initiate the polymerization, as shown in entries 1−3 of Table 1. PIBs with a relatively high content of exo-olefin, narrow D̵ , and low conversion were obtained with the initiator solution prepared by method 1. A reasonable explanation is that the interaction between Et 2 O and AlCl 3 could catalyze H 2 O to generate stabilized carbenium ions corresponding to slow chain propagation and isomerization. Besides, it should be noted that (1) extending the residence time from 12 to 600 s could not help to increase the conversion, indicating serious and fast chain termination, and (2) both M n and the content of exoolefin decrease when the temperature increases from −20 to 20°C , indicating that high temperature could accelerate the isomerization and proton elimination simultaneously.
The ATR−FTIR spectrum of the initiator solution prepared by modified method 1 is almost the same as that of the initiator solution prepared by method 1, except the absorbance at 1126 cm −1 which becomes quite less because of the removal of free Et 2 O by vacuuming. The corresponding polymerization performance could be seen in entries 4−5 of Table 1. Compared with entries 1−3, the content of exo-olefin becomes even higher and the conversion increases too, which indicates that the free Et 2 O may result in the decrease of the polymerization rate and the promotion of isomerization. However, the conversions in entries 4−5 are still relatively low because the carbenium ions are still stabilized by the strong interaction between AlCl 3 and ether.
For the initiator solution prepared by method 2, the ATR− FTIR spectrum shows some new peaks, such as 1010, 887, and 770 cm −1 , but the characteristic peak of free Et 2 O at 1126 cm −1 is still obvious. The experimental results of IB polymerization (entries 6−13, Table 1) show that both the conversion and D̵ have great improvement, but the content of exo-olefin decreases seriously. A reasonable explanation on the enhanced conversion is that some AlCl 3 combines with H 2 O in hexane in advance, which provides well-dissociated protons with high reactivity. The narrow D̵ corresponds to enhanced proton elimination or isomerization because of the existence of free (or not strongly interacted) Et 2 O. In detail, entries 6−8 correspond to the same reaction conditions except for the ratio of Et 2 O to AlCl 3 . The increased ratio of Et 2 O to AlCl 3 results in a remarkable decrease of M n and the content of exo-olefin, which indicates that the additive Et 2 O promotes isomerization superior to proton elimination. Entries 9−13 reflect the effects of polymerization temperature. Within the range from −20 to 30°C, a maximum conversion was achieved at around 10°C because of the compromise between chain propagation and chain termination, which are both enhanced with temperature increase. Moreover, the lower M n and narrower D̵ at higher temperature were attributed to the higher proton elimination rate.
In general, the interaction between Et 2 O, AlCl 3, and H 2 O, as well as the effects of Et 2 O on the polymerization of IB catalyzed by AlCl 3 in a nonpolar solvent hexane, was highly dependent on the preparation method of the initiator solution. It may

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Article bring complexities as well as challenges for this system in regulating the process and product. Effects of iPr 2 O. Compared with Et 2 O, iPr 2 O as the nucleophilic reagent has lower basicity and certain steric hindrance. Similar methods were exploited to investigate the interaction between iPr 2 O and AlCl 3 and its influences on the polymerization catalyzed by AlCl 3 . Figure 2 shows the ATR− FTIR spectra of initiator solutions prepared by different methods. When AlCl 3 comes in direct contact with iPr 2 O (method 1), a new peak at 1100 cm −1 emerges; one of the characteristic peaks of iPr 2 O at 1170 cm −1 disappears; the peaks at 1012 and 904 cm −1 assigned to C−O−C stretch in iPr 2 O just show changes in their intensity. In contrast to the difference between free Et 2 O and the initiator solution prepared by method 1 (Figure 3), the interaction between iPr 2 O and AlCl 3 seems to be weaker than that between Et 2 O and AlCl 3 because iPr 2 O has relatively low basicity and high steric hindrance. Table 2 correspond to the polymerization process using initiator solutions prepared by method 1. Compared with entries 1−3 in Table 1, the conversion was improved by substituting Et 2 O with iPr 2 O. Meanwhile, the content of exo-olefin decreases remarkably. These phenomena were related to the weakened interaction between ether and AlCl 3 . It allows AlCl 3 /iPr 2 O complexes to catalyze the proton dissociation more effectively and weakens the effect of iPr 2 O on the stability of carbenium ions. We could also observe from entries 14−17 that (1) the conversion has a remarkable increase with the residence time increasing from 60 to 600 s, indicating the delay of chain termination, and (2) the content of exo-olefin increases and the M n decreases with the increase of temperature, indicating the enhancement of proton elimination. Compared with Et 2 O, iPr 2 O did not show the clear effect of isomerization because the steric hindrance may cause it to have a substantial effect on the end of propagating chains only.

Entries 14−17 in
For the initiator solution prepared by method 2, the ATR− FTIR spectrum shows that most of characteristic peaks are accordant with those of free iPr 2 O. The corresponding polymerization experiments were listed as entries 18−19 in Table 2. The results are similar with those in entries 6−13 of Table 1. High conversion and broad D̵ were obtained. H 2 O catalyzed by AlCl 3 could provide a well-dissociated proton to start fast initiation, which is affected by the little amount of additive iPr 2 O as iPr 2 O does not participate in the interaction between H 2 O and AlCl 3 . Meanwhile, iPr 2 O could promote chain transfer as well as inhibit chain termination to achieve high conversion.
Cationic Polymerization of IB in CH 2 Cl 2 . According to the above-mentioned experiments, the effects of the addition sequence of solvents on the interaction between AlCl 3 , ether, and H 2 O as well as polymerization performance were quite distinct in hexane (the nonpolar solvent). Herein, to recognize whether the addition sequence always needs serious consideration, CH 2 Cl 2 as a typical polar solvent was adopted to carry out similar investigations.
From the ATR−FTIR spectra shown in Figure 3, for the initiator solution containing AlCl 3 and Et 2 O, the addition sequence of CH 2 Cl 2 has a little influence on the composition of the initiator solution. In detail, the characteristic peak of Et 2 O at 1126 cm −1 disappears and some new peaks emerge, such as the peaks at 990 and 975 cm −1 . The spectrum indicates that there exists intensive interaction between Et 2 O and AlCl 3 in CH 2 Cl 2 . Table 3 shows the polymerization performances of various initiator solutions. Evidently, the influence of the preparation method on the polymerization could be neglected. The high content of exo-olefin is resulted from the intensive interaction between Et 2 O and AlCl 3 , which stabilizes the carbenium ions effectively. On the other hand, the low

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Article conversion is resulted from the poor dissociation of protons from H 2 O because the catalytic effect of AlCl 3 is inhibited by Et 2 O seriously. Besides, the higher ratio of Et 2 O to AlCl 3 corresponds to lower M n and higher conversion, indicating that excessive Et 2 O could promote proton elimination. The initiator solutions containing AlCl 3 and iPr 2 O prepared by method 1 and method 2 also presented similar ATR−FTIR spectra (Figure 4), and the polymerization performances shown in Table 4 were quite similar too. Compared with the interaction between Et 2 O and AlCl 3 , the interaction between iPr 2 O and AlCl 3 is weaker. However, it could effectively accelerate proton elimination and chain transfer to obtain the high content of exo-olefin and high conversion.
In general, the effects of ether on the IB polymerization catalyzed by AlCl 3 in CH 2 Cl 2 are independent on the preparation method of initiator solutions. It is a totally different scenario with that in hexane. The inspiration of these phenomena is that the interaction between ether, AlCl, 3 and H 2 O is easy to control and reproduce in CH 2 Cl 2 , a typical polar solvent. On the contrary, the interaction has the potential to be regulated facilely toward various requirements in hexane, a typical nonpolar solvent. If we regulate the cationic polymerization catalyzed by AlCl 3 or other Lewis acid in hexane by adding nucleophilic reagents, the addition sequence of reagents should be considered first.
Investigation of Polymerization Mechanism. To understand the various effects of ether on IB polymerization catalyzed by AlCl 3 , we proposed the evolution of AlCl 3 -involved interactions during the preparation of initiator solutions containing AlCl 3 and ether, as shown in Scheme 1. For the same pair of interacted groups, AlCl 3 /ether or AlCl 3 /H 2 O, the solid line and the dashed line correspond to strong and weak interactions, respectively. All the interactions exist between Lewis acid (AlCl 3 ) and Lewis base (H 2 O or ether), of which the intensity is determined by the basicity of the Lewis base and the polarity of the solvent together. Commonly, the stronger the basicity is, the stronger the interaction is, and the lower the polarity of solvent is, the stronger the interaction is. AlCl 3 that already strongly interacted with one Lewis base can only have weak interaction with the other added Lewis base.
The ether that strongly interacted with H 2 O/AlCl 3 can stabilize carbenium ions to increase the content of exo-olefin, and Et 2 O is found to be more effective than iPr 2 O. The ether that weakly interacted with H 2 O/AlCl 3 or free ether can promote proton elimination, isomerization, and delay chain termination, achieving low M n , low content of exo-olefin, or high conversion. Different preparation methods correspond to different evolution routes of AlCl 3 -involved interactions. When the initiator solution was prepared by method 2, H 2 O comes in contact with AlCl 3 first, denoted as sequence 1. When the initiator solution was prepared by method 1, ether comes in

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Article contact with AlCl 3 first, denoted as sequence 2. In nonpolar hexane, sequence 1 leads to a strong interaction between AlCl 3 and H 2 O, and the interaction between AlCl 3 and ether will be weak. With the electrophilicity of AlCl 3 , the proton would be ionized easily to show high reactivity. The additive ether could promote the proton elimination or isomerization (for Et 2 O only in this work) greatly and delay chain termination to achieve high conversion or low content of exo-olefin. Sequence 2 leads to a strong interaction between AlCl 3 and ether, which is helpful to stabilize carbenium ions and obtain a high content of exo-olefin. The interaction between AlCl 3 and H 2 O will be weak, inhibiting the dissociation of protons to decrease conversion seriously. Compared with Et 2 O, sequence 1 allows iPr 2 O to have a weaker effect in promoting chain transfer and stronger effect in delaying chain termination, and sequence 2 allows iPr 2 O to have a weaker effect in stabilizing carbenium ions.
In CH 2 Cl 2 with high polarity, the interaction between AlCl 3 and H 2 O and the interaction between AlCl 3 and ether are always of weak interaction. One weak interaction may allow AlCl 3 to have another weak interaction with the other Lewis base. Thus, there will be no difference between sequence 1 and sequence 2. Moreover, the proton is prone to dissociating from H 2 O in polar solvents and is loosely bounded by the counterion, presenting a relatively high reactivity.
Preparation of Initiator Solutions. The initiator solution, consisted of AlCl 3 , iPr 2 O or Et 2 O, and n-hexane or CH 2 Cl 2 , was prepared in a glovebox, shortly before experiments. Two methods were taken to prepare the initiator solution. Method 1: ether was added onto the AlCl 3 solid first to form an AlCl 3 − ether complex, and then the solvent, n-hexane or CH 2 Cl 2 , was added and stirred by using a magnetic stirrer (IKA, Germany) to obtain the initiator solution. To remove the ether not combining with AlCl 3 further, modified method 1 was also proposed. In modified method 1, vacuum treatment on the AlCl 3 −ether complex was carried out before adding solvents. Method 2: hexane or CH 2 Cl 2 was added onto the AlCl 3 solid first, and then ether was added and stirred to obtain the initiator solution. The content of AlCl 3 in the initiator solution was determined by a UV−vis spectrophotometer (UV-2450, Shimadzu).
Polymerization of IB. The polymerization of IB was carried out in a microflow system composed of three T-shaped micromixers (M1 for the mixing of IB and diluent (n-hexane or CH 2 Cl 2 ), M2 for the mixing of the IB and initiator solutions, and M3 for the injection of the terminator agent, ethanol), two precooling (or preheating) coiled tubes (C1 and C2, inner diameter 900 μm), and a microtube reactor (R, inner diameter 900 μm), as shown in Figure 5. The polymerization of IB proceeded throughout R1, and the reaction time could be changed by the flow rate and the length of R1. Four syringe pumps were used to deliver IB, n-hexane, initiator solution, and terminator, respectively. IB was transferred as the liquid from the bottom of the IB cylinder into the syringe and then mixed with n-hexane in the tube as the liquid under the pressure of 3 bar.
Characterization. Size Exclusion Chromatography. Molecular weight and dispersity value (D̵ ) of the polymers were measured by a Waters gel permeation chromatography (GPC) instrument composed of a Waters 2707 autosampler, a 1515 Isocratic HPLC pump, a 2414 refractive index detector, and three Styragel GPC columns, Styragel HT3, HT4, and HT5 (o.d., 7.8 mm; length, 300 mm; particle size, 10 μm; the detection range of molecular weight, 500 to 4 × 10 6 ), thermostated at 38°C. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL/min. The instrument was calibrated with polystyrene standards. The results were processed by the Breeze 2 software from Waters.
Nuclear Magnetic Resonance (NMR) Spectroscopy. Proton NMR ( 1 H NMR) spectra were recorded on a JNM-ECA 600 MHz spectrometer using CDCl 3 as the solvent. The PIB endgroup content was calculated from 1 H NMR spectroscopy. Figure 6 shows a typical 1 H NMR spectrum. As seen, the main Figure 5. Schematic diagram of flow synthesis setup. M1, M2, and M3 (tees in shape) are used as micromixers; C1 and C2 are curved tubes that are used to achieve the preset temperature; and R1 is a microtube that is used as the reactor.

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Article resonance signals observed are located at δ = 1.1 (z), 1.41 (y), 0.99 (x), 4.85 (a1), 4.64 (a2), 5.17 (c1), 5.37 (c2), 5.15 (d), and 2.83 (e). The two characteristic protons of the exo-olefin end group (structure A, protons a1 and a2) appear as two wellresolved peaks at 4.85 and 4.64 ppm, respectively, while small amounts of the E and Z configurations of the trisubstituted olefin end group (structure C, protons c1 and c2) appear at 5.37 and 5.17 ppm. The one characteristic proton of the endoolefin end group (structure D, proton d) appears at 5.15 ppm. The signal corresponding to the tetrasubstituted olefin end group (structure E, proton e) is a broad multiplet at 2.85 ppm. The methylene, methyl, and end methyl protons of the PIB chains (structure A, protons y, z, and x, respectively) usually appear at 1.41, 1.11, and 0.99 ppm, respectively.
ATR−FTIR Spectroscopy. The ATR−FTIR spectra were recorded in situ by using a Mettler Toledo ReactIR 15 instrument with a DiComp probe coupled to an MCT detector via AgX fiber. Each spectrum was collected every 256 s by accumulating 256 scans with a wavenumber resolution of 4 cm −1 over the spectral range of 650−3000 cm −1 . The ATR− FTIR spectrum of the diluent (n-hexane or CH 2 Cl 2 ) was chosen as the background. The measurement temperature was 25°C.

■ CONCLUSIONS
For the cationic polymerization of IB in hexane, we exploited different preparation methods to obtain different initiator solutions containing AlCl 3 and ether. Through FTIR measurement and reaction evaluation, two kinds of interactions between ether and H 2 O/AlCl 3 and their different influences on chain reactions were revealed, which areas follows: (1) when AlCl 3 comes in contact with ether first, a strong interaction can take place to stabilize H 2 O/AlCl 3 and decrease the reactivity of polymerization of IB catalyzed by AlCl 3 in hexane and (2) when AlCl 3 comes in contact with hexane first to interact with H 2 O in hexane, only weak interaction between ether and H 2 O/ AlCl 3 can exist to promote proton elimination and isomerization. As for the AlCl 3 -catalyzed polymerization of IB in CH 2 Cl 2 , the effects of ether are similar because of the weak interactions of H 2 O, AlCl 3, and ether in polar solvents. Furthermore, the evolution routes of the AlCl 3 -involved interactions were proposed to analyze the effect of ether on the polymerization when using different preparation methods of initiator solutions. In general, as ether was adopted to regulate the AlCl 3 -catalyzed polymerization of IB, the reproducible process could be facilely achieved in polar solvents, while various processes with more potential are worth expecting in nonpolar solvents. This work deepens the understanding on the effects of ether on AlCl 3 -catalyzed cationic polymerization and may provide support to achieve the rational process design for customized polymers.