Carbocationic Polymerization of Isobutylene Using Methylaluminum

Chain extension “incremental monomer addition” (IMA) experiments resulting in bimodal distributions demonstrate that termination is operational wi...
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Macromolecules 2006, 39, 7527-7533

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Carbocationic Polymerization of Isobutylene Using Methylaluminum Bromide Coinitiators: Synthesis of Bromoallyl Functional Polyisobutylene Priyadarsi De and Rudolf Faust* Polymer Science Program, Department of Chemistry, UniVersity of Massachusetts Lowell, One UniVersity AVenue, Lowell, Massachusetts 01854 ReceiVed June 29, 2006; ReVised Manuscript ReceiVed August 24, 2006

ABSTRACT: The carbocationic polymerization of isobutylene (IB) was studied in conjunction with AlBr3, MeAlBr2, Me1.5AlBr1.5, and Me2AlBr coinitiators in hexanes(Hex)/methyl chloride (MeCl) 60/40 (v/v) solvent mixtures at -80 °C in the presence of a proton trap, 2,6-di-tert-butylpyridine. The observed Mns were directly proportional to monomer-to-initiator ratio with 2-chloro-2,4,4-trimethylpentane (TMPCl) as initiator and MeAlBr2, Me1.5AlBr1.5, and Me2AlBr coinitiators; however, with AlBr3 the Mns are much lower than the theoretical values. Chain extension “incremental monomer addition” (IMA) experiments resulting in bimodal distributions demonstrate that termination is operational with Me2AlBr. With MeAlBr2 the chain-extended PIBs exhibited close to theoretical Mns, but the molecular weigh distribution was broad. Using Me1.5AlBr1.5, Mn of the polymers increased in direct proportion and the molecular weight distributions remained narrow. 1H and 13C NMR spectroscopy of the polyisobutylene (PIB) obtained with Me1.5AlBr1.5 suggested virtually quantitative bromo end functionality. With MeAlBr2 and Me2AlBr the bromo functionality was lower (0.8-0), decreasing with the increase of Lewis acid concentration and polymerization time. The capping reaction of living polyisobutylene cation (PIB+) with 1,3butadiene (BD) in Hex/MeCl 60/40 (v/v) solvent mixtures at -80 °C was also studied in conjunction with methylaluminum bromide coinitiators. Quantitative crossover reaction from living PIB chain end to BD followed by instantaneous termination and selective formation of 1,4-addition product bromoallyl functional PIB (PIBAllylBr) was obtained only with Me1.5AlBr1.5 coinitiator.

Introduction Functional polymers are of great interest due to their potential applications as surfactants/dispersants, compatibilizers, macroinitiators, etc. Although there are various methods available to obtain functional polyisobutylenes (PIBs) by modification of chloro-functional PIBs, they usually involve a number of steps1 and are rather cumbersome. Telechelic polymers can be prepared most conveniently by termination of living polymers by nucleophilic functional terminators. The selection of terminators is limited, however, since suitable nucleophiles should not react with the Lewis acid (LA). These terminators are mostly π-nucleophiles, and therefore multiple additions should be avoided. This can be accomplished by employing π-nucleophiles that do not homopolymerize, yielding a stable ionic product or a covalent uncharged product either by rapidly losing a cationic fragment, e.g., Me3Si+ (e.g., allyltrimethylsilane) or H+, or by fast ion collapse. When all dormant chain ends are converted to active ionic species, e.g. by monoaddition of diarylethylenes,2 many other nucleophiles, such as NH3, CH3OH, etc., which also quench the Lewis acid, could be used. Because of the cost of diarylethylenes, however, this method is expensive to produce low-Mn polymers, and the resulting functionalities may exhibit lowered reactivity due to steric effects. We have recently reported the synthesis of chloroallyl functional PIBs (PIB-AllylCl) by quantitative monoaddition of 1,3-butadiene to living PIB, followed by instantaneous termination (absence of multiple addition) selectively yielding the 1,4addition product.3 The chloroallyl group offers a great opportunity to obtain other functionalities, in block copolymer * Corresponding author.

synthesis by the combination of cationic and anionic polymerization (by linking with living polymer anions) or as a macroinitiator in cationic polymerization and ATRP, in reaction with polymer amines, etc. However, the bromoallyl functionality would be more suitable for these applications because the C-Br bond is much weaker than the C-Cl bond. For instance, PIBAllylCl does not react with living poly(methyl methacrylate) (PMMALi) in THF at -78 °C.4 In contrast, coupling of bromoallyl functional PIB (PIB-AllylBr) (obtained by halogen exchange reaction of PIB-AllylCl with excess lithium bromide) and PMMALi was efficient and yielded poly(isobutylene-bmethyl methacrylate).4 Halogen exchange is an additional step, however, and elimination of this step would be desired. This could be accomplished by replacing TiCl4 with a metal bromide coinitiator that would terminate by bromine transfer. Generally, the Lewis acid strength decreases by replacing Cl by Br.5 In line with this statement, stepwise replacement of Cl with Br in TiCl4 resulted in drastically decreased rates in the polymerization of IB.6 This prompted us to evaluate alkylaluminum bromides as potential coinitiators for the living polymerization of IB. Following the first report of Me2AlCl-catalyzed living polymerizations of IB7 in the absence of additives, we reported on the living polymerization of IB using 2-chloro-2,4,4trimethylpentane (TMPCl)/2,6-di-tert-butylpyridine (DTBP)/80 °C in Hex/MeCl solvent mixtures in conjunction with MeAlCl2, methylaluminum sesquichloride (Me1.5AlCl1.5), or Me2AlCl.8 With MeAlCl2 or Me1.5AlCl1.5 as coinitiators, the polymerization proceeds very rapidly in a living manner even at low coinitiator concentration (∼0.001 mol L-1). On the other hand, using Me2AlCl the polymerization of IB is very slow even at [Me2AlCl] ∼ 0.01 mol L-1. In the present study, the cationic polymerization of IB and addition reactions of 1,3-butadiene

10.1021/ma061463d CCC: $33.50 © 2006 American Chemical Society Published on Web 09/27/2006

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to living PIB+ were studied using AlBr3, MeAlBr2, Me1.5AlBr1.5, and Me2AlBr as coinitiators to obtain PIB-AllylBr in one pot. Experimental Section Materials. Methyl chloride (MeCl) and isobutylene (IB) were dried in the gaseous state by passing them through in-line gaspurifier columns packed with BaO/Drierite. They were condensed in the cold bath of a glovebox prior to polymerization. The 2,6di-tert-butylpyridine (DTBP, Aldrich, 97+%), 1,3-butadiene (BD, Aldrich, 99+%), aluminum bromide (AlBr3, 1.0 M solution in dibromomethane, Aldrich), and trimethylaluminum (Me3Al, 2.0 M solution in hexanes, Aldrich) were used as received. Methylaluminum dibromide (MeAlBr2), methylaluminum sesquibromide (Me1.5AlBr1.5), and dimethylaluminum bromide (Me2AlBr) was obtained by mixing AlBr3 and Me3Al solutions respectively in 2:1, 1:1, and 1:2 ratio at room temperatures. The 2-chloro-2,4,4trimethylpentane (TMPCl)9 and 1,1-di-p-tolylethylene (DTE)10 were synthesized according to the literature. The 2-bromo-2,4,4-trimethylpentane (TMPBr) was synthesized from 2,4,4-trimethyl-1pentene (TMP, Aldrich, 99%) by hydrobromination with dry HBr gas (obtained from the reaction of NaBr and concentrated H2SO4 and by passing the HBr gas through a drying column packed with Drierite) in olefin-free hexanes: Hex/TMP 9:1 (v/v) at -78 °C. After removing hexanes under vacuum it was stored in a refrigerator over CaH2. Finally, TMPBr was distilled under reduced pressure just before use. 1H NMR spectrum: 1.11 (s), 1.94 (s), and 2.12 ppm (s). The 1-tert-butyl-3,5-bis(1-methoxy-1-methylethyl)benzene (t-BudiCumOMe) was obtained from Prof. M. Zsuga (Department of Applied Chemistry, University of Debrecen, Hungary). The synthesis of t-BudiCumOMe has been described in the literature.11 The PIB capped with BD (PIB-BD-Cl) was synthesized as reported in ref 3. All other chemicals and solvents were purified as described previously or used as received.6 Polymerization. Polymerizations were carried out under a dry nitrogen atmosphere ([H2O] < 0.5 ppm) in an MBraun 150-M glovebox (Innovative Technology Inc., Newburyport, MA). Large (75 mL) culture tubes equipped with Teflon-lined caps were used as polymerization reactors. Throughout the study IB was considered as an apolar solvent, and its volume was added to the volume of hexanes (Hex). The total volume of the reaction mixture was 20 or 25 mL. The AlBr3 and Me3Al were mixed in different ratios in hexanes at room temperature and kept for 10 min. Required amounts of this stock solution were added slowly to the culture tubes at -80 °C containing hexanes, DTBP, and MeCl. It was stirred thoroughly and kept at -80 °C for 30 min. The polymerization of IB was initiated by adding the mixture of IB and initiator stock solution. After a predetermined time, the polymerization was terminated by the addition of 1.0 mL of prechilled methanol. The polymer was recovered and purified two times by reprecipitation from hexanes/ methanol. Monomer conversions were determined by gravimetric analysis. In a typical experiment, the capping reaction of PIB+ cation with BD was carried out in Hex/MeCl 60/40 (v/v) at -80 °C using the following concentrations: [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [IB] ) 0.25 mol L-1, [Me1.5AlBr1.5] ) 0.004 mol L-1, and [BD] ) 0.04 mol L-1. Into a 75 mL culture tube at -80 °C 10.9 mL of Hex at room temperature, 8.0 mL of MeCl at -80 °C, and 0.4 mL of DTBP stock solution in Hex (0.2 mol L-1) at -80 °C were added, mixed thoroughly, and kept at -80 °C. 1.0 mL of Me1.5AlBr1.5 (1:1 ) AlBr3:Me3Al mixture in Hex at room temperature, 0.09 mol L-1 stock solution) was added to the culture tube and mixed thoroughly. The polymerization of IB was initiated under stirring at -80 °C by adding a 1.0 mL mixture of IB and TMPCl stock solution ([TMPCl] ) 0.04 mol L-1, [IB] ) 5.0 mol L-1 in Hex at -80 °C). After 3 min of IB polymerization, one of the tubes was quenched with 1.0 mL of prechilled methanol for the characterization of original PIB, and to the rest 0.5 mL of BD (1.6 mol L-1, in Hex/MeCl 60/40 (v/v) at -80 °C) was added under stirring. After predetermined times, parallel runs were terminated by the addition of 1.0 mL of prechilled methanol at -80 °C.

Macromolecules, Vol. 39, No. 22, 2006 Table 1. Experimental Results for the Polymerization of IB at Different Methylaluminum Bromide Coinitiators Concentration Using t-BudiCumOMe as Initiator in Hex/MeCl 60/40 (v/v) Solvent Mixture at -80 °Ca expt no.

[LA] (mol L-1)

time (min)

conv (%)

1 2 3 4 5 6 7 8 9

[MeAlBr2] ) 0.004 [MeAlBr2] ) 0.004 [MeAlBr2] ) 0.008 [MeAlBr2] ) 0.024 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.008 [Me1.5AlBr1.5] ) 0.024 [Me2AlBr] ) 0.036

4 20 1 1 4 20 10 2 300

13.6 14.9 100 100 9.8 10.2 4 100 0

Mn (GPC)

PDI

530000 610000

1.47 1.67

580000

1.55

a [DTBP] ) 0.004 mol L-1. [t-BudiCumOMe] ) 0.001 mol L-1, [IB] ) 0.25 mol L-1. Mn,theoretical ) 14 300.

Table 2. Experimental Results for the Polymerization of IB at Different Methylaluminum Bromide Coinitiators Concentration Using TMPCl as Initiator in Hex/MeCl 60/40 (v/v) Solvent Mixture at -80 °Ca expt no.

[LA] (mol L-1)

time (min)

conv (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

[MeAlBr2] ) 0.001 [MeAlBr2] ) 0.002 [MeAlBr2] ) 0.003 [MeAlBr2] ) 0.004 [MeAlBr2] ) 0.005 [MeAlBr2] ) 0.01 [Me1.5AlBr1.5] ) 0.001 [Me1.5AlBr1.5] ) 0.002 [Me1.5AlBr1.5] ) 0.003 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.005 [Me1.5AlBr1.5] ) 0.01 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08

4 4 1 1 2 2 4 4 4 0.25 1 3 2 2 10 15 40 120

0 100 100 100 100 100 0 0 17.7 20.0 86.0 100 100 100 27.6 48.0 93.7 100

Mn (GPC)

PDI

Mn,theo

8100 8400 7800 10400 8800

1.06 1.37 1.47 1.43 1.57

7200 7200 7200 7200 7200

2160 6600 7600 8200 8400 4000 5700 11000 11900

1.25 1.05 1.06 1.04 1.30 1.14 1.12 1.05 1.07

1440 6100 7200 7200 7200 3200 5500 10700 11400

a [DTBP] ) 0.004 mol L-1. For experiments 1-14: [TMPCl] ) 0.002 mol L-1 and [IB] ) 0.25 mol L-1. For experiments 15-18: [TMPCl] ) 0.01 mol L-1 and [IB] ) 2.0 mol L-1.

Characterization. Molecular weights were measured with a Waters HPLC system equipped with a model 510 HPLC pump, a model 410 differential refractometer, a model 441 absorbance detector, an on-line multiangle laser light scattering (MALLS) detector (MiniDawn, Wyatt Technology Inc.), a model 712 sample processor, and five Ultrastyragel GPC columns connected in the following series: 500, 103, 104, 105, and 100 Å. Tetrahydrofuran was used as eluent at a flow rate of 1.0 mL/min at room temperature. The measurements were carried out at room temperature. NMR spectroscopy was carried out on a Bruker 500 MHz spectrometer using CDCl3 as a solvent (Cambridge Isotope Lab., Inc.). 1H and 13C NMR spectra of solutions in CDCl were calibrated to 3 tetramethylsilane as internal standard (δH 0.00) or to the solvent signal (δC 77.0), respectively.

Results and Discussion Cationic Polymerization of IB in Hex/MeCl 60/40 (v/v) at -80 °C Using Methyaluminum Bromides. Preliminary experiments were carried out with MeAlBr2, Me1.5AlBr1.5, or Me2AlBr as Lewis acid coinitiators at different concentrations using two different initiators: the difunctional t-BudiCumOMe (Table 1) and the monofunctional TMPCl (Table 2). The hindered pyridine DTBP was used throughout the study to prevent protic initiation. With t-BudiCumOMe as initiator, the polymerization was absent with Me2AlBr, and only low conversions were obtained

Macromolecules, Vol. 39, No. 22, 2006

Figure 1. GPC RI traces of the PIB obtained after different polymerization time in Hex/MeCl 60/40 (v/v) at -80 °C using [TMPCl] ) 0.01 mol L-1, [IB] ) 2.0 mol L-1, [DTBP] ) 0.004 mol L-1, and [Me2AlBr] ) 0.08 mol L-1.

with Me1.5AlBr1.5 and MeAlBr2 at 0.004 mol L-1 Lewis acid concentration. This may be due to complex formation between the initiator and coinitiator. Since organoaluminum compounds form dimers in Hex/MeCl 60/40 (v/v) solvent mixture at -80 °C,7,12,13 free Lewis acid may not be present. At higher Lewis acid concentrations the polymerization was rapid but uncontrolled since it yielded polymers with Mns much higher than the theoretical value and broad molecular weight distribution. With TMPCl using MeAlBr2, theoretical molecular weights were obtained at [MeAlBr2] ) 0.002-0.01 mol L-1, but because of the extremely rapid nature of the polymerization, the molecular weight distribution is broad (PDI ) 1.1-1.6). With Me1.5AlBr1.5, theoretical molecular weights and narrow polydispersities were obtained at [Me1.5AlBr1.5] ) 0.004-0.01 mol L-1. The polymerization induced by Me2AlBr proceeded with a moderate rate, reaching complete conversion in 2 h. The polymers exhibit close to theoretical Mns increasing in direct proportion with conversion, and the molecular weight distribution is narrow (PDI < 1.15) (Figure 1). Based on the above study all further polymerizations were carried out with [MeAlBr2] ) 0.003-0.004 mol L-1, [Me1.5AlBr1.5] ) 0.004-0.007 mol L-1, and [Me2AlBr] ) 0.08 mol L-1. Cationic Polymerization of IB in Hex/MeCl 60/40 (v/v) at -80 °C Using AlBr3, MeAlBr2, Me1.5AlBr1.5, and Me2AlBr: All Monomer In (AMI) Experiments. Since the polymerization was too fast for sampling with MeAlBr2 and Me1.5AlBr1.5 coinitiators, the diagnostic first-order and Mn vs conversion plots could not be constructed. To confirm the absence of chain transfer and termination, a series of experiments were carried out by varying the initial monomer to initiator molar ratio. In addition to MeAlBr2, Me1.5AlBr1.5, and Me2AlBr, experimentation was also carried out with AlBr3. As shown in Table 3, the cationic polymerization of IB using AlBr3 as coinitiator is uncontrolled and yields polymers with Mns lower than the theoretical value and broad molecular weight distribution (PDI ) 1.6-2.4). With the other three methylaluminum bromides the Mns are proportional to the [IB]/[TMPCl] ratio, and the Mn values are in acceptable agreement with the theoretical Mns calculated with the assumption that chain transfer is absent and one molecule of TMPCl initiates one polymer chain. Thus, the TMPCl/MeAlBr2, TMPCl/Me1.5AlBr1.5, and TMPCl/Me2AlBr initiating systems efficiently generate high molecular weight PIB with narrow PDI. Cationic Polymerization of IB in Hex/MeCl 60/40 (v/v) at -80 °C Using MeAlBr2, Me1.5AlBr1.5, and Me2AlBr: Incremental Monomer Addition (IMA) Experiments. The living nature of the IB polymerization with the TMPCl initiator was further studied by chain extension, also known as incremental monomer addition (IMA). In this technique, IB was

Bromoallyl Functional Polyisobutylene 7529

polymerized for 2, 3, and 60 min using MeAlBr2, Me1.5AlBr1.5, and Me2AlBr, respectively. Then a second and subsequently a third feed of IB were added to a polymerization system under stirring. The results are summarized in Table 4. The additional IB smoothly polymerized without a noticeable decrease in the polymerization rate, the Mn increased, and the molecular weight distribution stayed narrow. The GPC RI traces of the original PIB and PIB obtained after the incremental monomer addition are shown in Figures 2, 3, and 4 respectively with MeAlBr2, Me1.5AlBr1.5, and Me2AlBr coinitiators. Using MeAlBr2, the experimental molecular weight is somewhat lower than the theoretical molecular weight after the second monomer increment, and there is tailing toward the low molecular weight region in the GPC RI traces (Figure 2). This may indicate that a small fraction of the PIB chains are terminated under monomer starved conditions. With Me1.5AlBr1.5 the Mns are close to the theoretical values, and the GPC RI traces (Figure 3) are monomodal and symmetrical. Although theoretical molecular weight was obtained after the first monomer addition with Me2AlBr, the GPC RI trace of the product of chain extension experiment is bimodal (Figure 4) and indicates that ∼22% of chain ends are dead after 60 min. Analysis of the End Group. The 1H NMR spectrum of PIB obtained with the TMPCl/MeAlBr2 initiating system at [MeAlBr2] ) 0.004 mol L-1 is shown in Figure 5 together with the 1H NMR spectrum of PIB obtained with the TMPCl/TiCl 4 initiating system (see ref 3). The peaks at 0.89 and 1.27 ppm are due to the residual hexanes in the polymer matrix, which are often hard to remove. Quenching living PIB, produced with the TMPCl/TiCl4 initiating system, with methanol yields PIB with a terminal chlorine group (PIB-Cl, i.e., PIB-CH2-C(CH3)2-Cl), and the 1H NMR spectrum of PIB-Cl exhibits characteristic resonance signals at δ ) 1.94 and 1.67 ppm, corresponding respectively to -CH2- and -CH3 protons next to the terminal chloro group.14 These signals are absent for the PIB samples obtained with the TMPCl/MeAlBr2 initiating system. The signals at 1.95 and 2.2 ppm are due to the -CH2- and -CH3 protons next to the terminal bromo atom, respectively (PIB-Br, i.e., PIB-CH2C(CH3)2-Br). The resonance signals at 1.1-1.2 and 1.4-1.5 ppm are due to the -CH3 and -CH2 protons in the main chain of the polymer, respectively. The peak at 1.02 ppm corresponds to the terminal -CH3 protons from the initiator fragment (TMPCl). At [MeAlBr2] ) 0.004 mol L-1 repeated experiments showed consistently 25-40% tert-bromide chain end in the PIB obtained after 1 min; however, after 2 min all tert-bromide chain ends were absent. This indicates that the tert-bromide chain ends were transformed into something else in the presence of MeAlBr2 coinitiator. The absence of peaks in the 4-6 ppm region of the 1H NMR spectra indicates that the PIBs are free of unsaturation and proton elimination did not occur. The absence of tetrasubstituted olefinic end groups was confirmed by 13C NMR spectroscopy. While the NMR analysis shows the absence of tert-bromide chain end after 2 min, the results of IMA experiments indicate that most of the chains are still active and can reinitiate the polymerization of IB. These results are similar to the observation reported by Storey et al. for the decomposition of PIB+ chain ends at high TiCl4 concentrations.15 Storey et al. suggested that the ionized PIB chain ends undergo slow carbenium ion rearrangements and produce various isomeric structures, which, however, are able to reinitiate the polymerization of IB. Since the polymerization is very rapid and possibly over in seconds, lowering the MeAlBr2 concentration would lower the rate of chain end decomposition. The 1H

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Table 3. Experimental Results for the Polymerization of IB Using Methylaluminum Bromide Coinitiators at Different Initial Monomer to Initiator Molar Ratio in Hex/MeCl 60/40 (v/v) Solvent Mixture at -80 °Ca expt no.

[IB]/[TMPCl]

[LA] (mol L-1)

time (min)

conv (%)

Mn (GPC)

PDI

Mn,theo

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

125 250 700 1100 125 250 450 700 40 125 250 450 700 1100 50 200 400

[AlBr3] ) 0.004 [AlBr3] ) 0.004 [AlBr3] ) 0.004 [AlBr3] ) 0.004 [MeAlBr2] ) 0.003 [MeAlBr2] ) 0.003 [MeAlBr2] ) 0.003 [MeAlBr2] ) 0.003 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08

2 2 2 2 2 2 2 2 3 3 3 3 3 3 60 60 60

86 96 100 99 100 100 100 100 100 100 100 100 100 100 100 100 100

7200 6200 10200 10300 7600 15500 28400 38500 3100 7900 15700 27800 41600 61000 3800 12400 24500

1.65 2.00 2.25 2.40 1.53 1.44 1.46 1.53 1.08 1.04 1.11 1.12 1.13 1.17 1.07 1.05 1.09

6200 13600 39400 61200 7200 14200 25400 39400 2400 7200 14200 25400 39400 61900 3000 11400 22600

a

[DTBP] ) 0.004 mol L-1. For experiments 1-14: [TMPCl] ) 0.002 mol L-1. For experiments 15-17: [TMPCl] ) 0.01 mol L-1.

Table 4. Experimental Results from the Chain Extension Experiments Using Methylaluminum Bromide Coinitiators for the Polymerization of IB in Hex/MeCl 60/40 (v/v) Solvent Mixture at -80 °Ca expt no.

[LA] (mol L-1)

time (min)

conv (%)

Mn (GPC)

PDI

Mn,theo

1 2 3 4 5 6 7 8

[MeAlBr2] ) 0.003 [MeAlBr2] ) 0.003 [MeAlBr2] ) 0.003 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08

2 2+2 2+2+2 3 3+3 3+3+3 60 60 + 60

100 200 300 100 200 300 100 200

7600 19400 30800 8400 23500 34600 12400 23700

1.53 1.56 1.52 1.05 1.07 1.12 1.05 1.30

7200 21200 35200 7200 21200 35200 11400 22600

a [DTBP] ) 0.004 mol L-1. For experiments 1-6: [TMPCl] ) 0.002 mol L-1, [IB] ) 0.25 mol L-1, and [IB] ) [IB] ) 0.5 mol L-1. For experiments 1 2 3 7 and 8: [TMPCl] ) 0.01 mol L-1 and [IB]1 ) [IB]2 ) 2.0 mol L-1.

Figure 2. GPC RI traces of the original PIB and PIB obtained after the incremental monomer addition (IMA) experiments in Hex/MeCl 60/40 (v/v) at -80 °C using [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [MeAlBr2] ) 0.003 mol L-1, [IB]1 ) 0.25 mol L-1, and [IB]2 ) [IB]3 ) 0.5 mol L-1.

Figure 3. GPC RI traces of the original PIB and PIB obtained after the incremental monomer addition (IMA) experiments in Hex/MeCl 60/40 (v/v) at -80 °C using [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [Me1.5AlBr1.5] ) 0.004 mol L-1, [IB]1 ) 0.25 mol L-1, and [IB]2 ) [IB]3 ) 0.5 mol L-1.

NMR spectrum of the product obtained at [MeAlBr2] ) 0.002 mol L-1 in 4 min indeed gave ∼95% tert-bromide chain end. However, when the same experiment was repeated under identical conditions, only ∼45% tert-bromide chain end was observed, indicating the irreproducibility of the experiments. Similar irreproducible results were obtained at [MeAlBr2] ) 0.003 mol L-1. This can be attributed to the low but irreproducible effective MeAlBr2 concentration after reaction with traces of moisture estimated to be 0.001-0.002 mol L-1. Figure 6 depicts the 1H NMR spectra of PIB samples obtained with TMPCl/Me1.5AlBr1.5 initiating system ([Me1.5AlBr1.5] ) 0.004 mol L-1) at different conversions together with the 1H NMR spectrum of PIB obtained with the TMPCl/TiCl4 initiating system. The signals due to terminal chloro group are absent, and the PIB has tert-bromide chain ends. The 13C NMR spectra of the PIB obtained with the TMPCl/Me1.5AlBr1.5 initiating

system confirmed the absence of the terminal chloro group (peak due to PIB-CH2-C(CH3)2-Cl at 72.2 ppm) and the exclusive formation of terminal bromo group (peak due to PIB-CH2C(CH3)2-Br at 69.9 ppm). The number-average degree of polymerization Mn,NMR ) 8500 for PIB obtained at complete conversion determined from the 1H NMR spectrum (from the peak intensity ratio of the -CH2 protons in the main chain of the polymer at 1.4-1.5 ppm to the -CH2 terminal group at 2.2 ppm) is in excellent agreement with the Mn value obtained by GPC (Mn,GPC ) 8400). These results indicate that the preferential termination is by bromine rather than chlorine transfer similar to the observation reported by Santos et al. for PIB+BCl3Br-.16 The 1H NMR spectrum of PIB obtained with the TMPCl/ Me2AlBr initiating system ([Me2AlBr] ) 0.08 mol L-1) shows the absence of a terminal chloro group and the presence of a terminal bromo group (Figure 7). However, the bromo end

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Figure 4. GPC RI traces of the original PIB and PIB obtained after the incremental monomer addition (IMA) experiments in Hex/MeCl 60/40 (v/v) at -80 °C using [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [Me2AlBr] ) 0.08 mol L-1, and [IB]1 ) [IB]2 ) 2.0 mol L-1. Figure 7. 1H NMR spectra of the PIB (and PIB-Cl for comparison) obtained at different times in Hex/MeCl 60/40 (v/v) solvent mixture at -80 °C using [TMPCl] ) 0.01 mol L-1, [IB] ) 2.0 mol L-1, [DTBP] ) 0.004 mol L-1, and [Me2AlBr] ) 0.08 mol L-1.

Figure 5. 1H NMR spectra of the PIB (and PIB-Cl for comparison) obtained in Hex/MeCl 60/40 (v/v) solvent mixture at -80 °C using [IB] ) 0.25 mol L-1, [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, and [MeAlBr2] ) 0.004 mol L-1.

Figure 6. 1H NMR spectra of the PIB-Br (and PIB-Cl for comparison) obtained at different conversions in Hex/MeCl 60/40 (v/v) at -80 °C using [IB] ) 0.25 mol L-1, [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, and [Me1.5AlBr1.5] ) 0.004 mol L-1.

functionality decreases as the polymerization proceeds. At 10, 40, 60, and 120 min, there was 98%, 84%, 62%, and ∼2% -Br chain ends, respectively. The absence of peaks in the 4-6 ppm region of the 1H NMR spectra indicates that the PIBs are free of unsaturation, and 13C NMR spectroscopy confirmed the absence of tetrasubstituted olefinic end groups. The GPC RI trace of the chain-extended PIB shown in Figure 4 suggests

that this is due to termination most likely by methide transfer. Kennedy et al. reported alkylation and hydridation as the main termination pathways with Et2AlCl in conventional systems.17 Capping of PIB+ with 1,3-Butadiene in Hex/MeCl 60/40 (v/v) at -80 °C Using Methylaluminum Bromides. To study the capping reaction of PIB+ cation with BD, IB was polymerized for 2, 3, and 60 min using MeAlBr2, Me1.5AlBr1.5, and Me2AlBr, respectively, at [DTBP] ) 0.004 mol L-1. Then the capping agent [BD] ) 0.04 mol L-1 was added, and after different polymerization time, the reaction was quenched with methanol. The results are summarized in Table 5. The identical GPC RI traces of the original PIB and PIB capped with BD at different times confirmed that the number-average molecular weight and molecular weight distributions remain unchanged for all three Lewis acids (Figure 1 in the Supporting Information shows the results with Me1.5AlBr1.5). The 1H NMR spectra of the original PIB and PIB obtained after the capping reaction with BD at different times for MeAlBr2 shows inefficient capping reaction (Figure 2 in the Supporting Information). This result is not unexpected because even if BD adds to the isomerized PIB+, our recent results indicate that MeAlCl2 can ionize the PIB-AllylCl end, which undergoes decomposition.18 The 1H NMR spectra of the original PIB and PIB capped with BD at different times are shown in Figure 8 for the capping reaction using Me1.5AlBr1.5 coinitiator at [Me1.5AlBr1.5] ) 0.004 mol L-1, [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [IB] ) 0.25 mol L-1, and [BD] ) 0.04 mol L-1. The olefinic protons of the end group shows two characteristic multiplets of the ABX2 spin system at 5.72 and 5.84 ppm. The PIB-CH2CHdCH-CH2-Br group appears as doublet at 4.01 ppm, while the allylic CH2 on the PIB side gives a doublet at 2.06 ppm. The 1H NMR spectrum shows the exclusive formation of 1,4addition product PIB-AllylBr. A separate experiment with decreased IB concentration (0.08 mol L-1) to prepare low molecular weight PIBs at otherwise identical conditions also showed quantitative monoaddition of BD at [BD] ) 0.04 mol L-1. According to the NMR analysis, quantitative bromoallyl functionality is obtained in 20 min (see Figure 3 in the Supporting Information for the 1H NMR spectra of the original PIB and PIB obtained after the capping reaction with BD at different times). The Mn,GPC ) 3100 is in good agreement with the molecular weight obtained from NMR (Mn ) 3000). Similar

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Macromolecules, Vol. 39, No. 22, 2006

Table 5. Experimental Results for the Capping Reactions of Living PIB+ Cation with BD in Conjunction with Methylaluminum Bromide Coinitiators in Hex/MeCl 60/40 (v/v) Solvent Mixture at -80 °Ca expt no.

[LA] (mol L-1)

time (min)

conv (%)

Mn (GPC)

PDI

Mn,NMR

1 2 3 4 5 6 7 8 9 10 11 12

[MeAlBr2] ) 0.003 [MeAlBr2] ) 0.003 [MeAlBr2] ) 0.003 [MeAlBr2] ) 0.003 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me1.5AlBr1.5] ) 0.004 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08 [Me2AlBr] ) 0.08

2 2+2 2 + 15 2 + 30 3 3+2 3 + 10 3 + 30 60 60 + 5 60 + 16 60 + 30

100 100 100 100 100 100 100 100 100 100 100 100

7600 7900 8500 8600 8400 8300 8300 8600 11900 13200 12900 12500

1.53 1.51 1.41 1.40 1.05 1.04 1.07 1.04 1.05 1.12 1.12 1.05

8500 9500 9200 8500 14400 14600 16500 17400

a [DTBP] ) 0.004 mol L-1. For experiments 1-8: [TMPCl] ) 0.002 mol L-1, [IB] ) 0.25 mol L-1, and M n,theo ) 7200. For experiments 9-12: [TMPCl] ) 0.01 mol L-1, [IB] ) 2.0 mol L-1, and Mn,theo ) 11 400.

Figure 8. 1H NMR spectra of the PIB-Br (and PIB-Cl for comparison) and PIB obtained after the capping reaction with BD at different times in Hex/MeCl 60/40 (v/v) at -80 °C using [IB] ) 0.25 mol L-1, [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [Me1.5AlBr1.5] ) 0.004 mol L-1, and [BD] ) 0.04 mol L-1. Scheme 1. Capping Reaction of PIB+ Cation with BD in Conjunction with Me1.5AlBr1.5

results were also obtained using TMPBr as initiator at otherwise identical conditions. The NMR analysis confirmed the formation of bromoallyl functional PIB (Scheme 1) in 5 min. With TMPCl/Me2AlBr, the bromoallyl end functionality was 0.5-0.8 (Figure 4 in the Supporting Information). This can be attributed to termination discussed earlier. While we succeeded in the one-pot synthesis of PIB-AllylBr by utilizing the TMPCl/Me1.5AlBr1.5 initiating system, in this system Me1.5AlBr1.5 cannot be considered strictly as catalyst only since it is also the source of bromine. In other words, PIBAllylBr could be prepared in the previous experiments since 1.5[Me1.5AlBr1.5] > [TMPCl]. To determine whether TMPAllylBr could be synthesized at 1.5[Me1.5AlBr1.5] ) [TMPCl], the polymerization of IB was carried out at [TMPCl] ) 0.01 mol L-1, [IB] ) 0.4 mol L-1, and [Me1.5AlBr1.5] ) 0.007 mol L-1. After 3 min the capping agent BD at 0.025, 0.035, and

Figure 9. 1H NMR spectra of the PIB-Br and PIB obtained after the capping reaction with BD at different times in Hex/MeCl 60/40 (v/v) at -80 °C using [IB] ) 0.4 mol L-1, [TMPCl] ) 0.01 mol L-1, [DTBP] ) 0.004 mol L-1, [Me1.5AlBr1.5] ) 0.007 mol L-1, and [BD] ) 0.035 mol L-1.

0.045 mol L-1 concentrations was added to the reaction mixture under stirring, and after different times the polymerization was quenched with methanol. 100% conversion based on IB was obtained for all the reactions. The 1H NMR spectra are shown in Figure 9 for the capping reaction of PIB+ cation with BD at [BD] ) 0.035 mol L-1 (see Figure 5 in the Supporting Information for the 1H NMR spectra of the original PIB and PIB obtained after the capping reaction with BD at different times at [Me1.5AlBr1.5] ) 0.007 mol L-1 and [BD] ) 0.045 mol L-1). Figure 9 indicates that the PIB obtained after 2 min capping reaction time contains PIB-Br, PIB-AllylBr, and PIB-AllylCl chain ends. However, the characteristic resonance signals for PIB-Br and PIB-AllylCl diminish with increasing time to 20 min. The ionization study of PIB-AllylCl with Me1.5AlBr1.5 in the presence and absence of DTBP shows complete and virtually quantitative transformation from PIB-AllylCl to PIB-AllylBr in Hex/MeCl 60/40 (v/v) at -80 °C (see Figure 6 in the Supporting Information for the 1H NMR spectra). However, addition of 1,1-ditolylethylene to PIB-AllylCl was absent in conjunction with Me1.5AlBr1.5 in Hex/MeCl 60/40 (v/v) at -80 °C, which suggests that Me1.5AlBr1.5 is unable to ionize PIB-AllylCl. Therefore, the transformation of PIB-AllylCl to PIB-AllylBr chain ends could possibly due to the halogen exchange reaction similar to that of the methyl chloride-aluminum bromide halogen exchange, which proceeds to completion without ionization.19

Macromolecules, Vol. 39, No. 22, 2006

By employing the bromide initiator TMPBr, one can utilize Me1.5AlBr1.5 in catalytic amounts since the Lewis acid is no longer the source of bromine. Conclusions. PIB-AllylBr can be synthesized in one pot by the cationic polymerization of IB followed by capping with BD in Hex/MeCl 60/40 (v/v) at -80 °C using TMPCl or TMPBr as initiator in conjunction with Me1.5AlBr1.5. With MeAlBr2 the polymerization is extremely fast, and under monomer starved conditions the chain ends undergo rapid decomposition, suggested to be similar in nature to that reported by Storey et al. for high concentrations of TiCl4.15 The polymerization is too fast even at very low MeAlBr2 concentrations, and therefore the polymerization and decomposition of the chain end cannot be separated into two distinct kinetic events. The polymerization is much slower with Me2AlBr, but termination most likely by methide transfer is operational. Acknowledgment. Support by the National Science Foundation (CHE-0548466) is gratefully acknowledged. Supporting Information Available: GPC RI traces of the original PIB and PIB obtained after the capping reaction with BD at different times in Hex/MeCl 60/40 (v/v)at -80 °C using [IB] ) 0.25 mol L-1, [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [Me1.5AlBr1.5] ) 0.004 mol L-1, and [BD] ) 0.04 mol L-1 (Figure 1); 1H NMR spectra of the PIB-Br (and PIB-Cl for comparison) and PIB obtained after the capping reaction with BD at different times in Hex/MeCl 60/40 (v/v) at -80 °C using [IB] ) 0.25 mol L-1, [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [MeAlBr2] ) 0.003 mol L-1, and [BD] ) 0.04 mol L-1 (Figure 2); 1H NMR spectra of the PIB-Br (and PIB-Cl for comparison) and PIB obtained after the capping reaction with BD at different times in Hex/MeCl 60/40 (v/v) at -80 °C using [IB] ) 0.08 mol L-1, [TMPCl] ) 0.002 mol L-1, [DTBP] ) 0.004 mol L-1, [Me1.5AlBr1.5] ) 0.004 mol L-1, and [BD] ) 0.04 mol L-1 (Figure 3); 1H NMR spectra of the PIB-Br (and PIB-Cl for comparison) and PIB obtained after the capping reaction with BD at different times in Hex/MeCl 60/40 (v/v) at -80 °C using [IB] ) 2.0 mol L-1, [TMPCl] ) 0.01 mol L-1, [DTBP] ) 0.004 mol L-1, [Me2AlBr] ) 0.08 mol L-1, and [BD] ) 0.04 mol L-1 (Figure 4); 1H NMR

Bromoallyl Functional Polyisobutylene 7533 spectra of the PIB-Br and PIB obtained after the capping reaction with BD at different times in Hex/MeCl 60/40 (v/v) at -80 °C using [IB] ) 0.4 mol L-1, [TMPCl] ) 0.01 mol L-1, [DTBP] ) 0.004 mol L-1, [Me1.5AlBr1.5] ) 0.007 mol L-1, and [BD] ) 0.045 mol L-1 (Figure 5); 1H NMR spectra of the PIB before (PIB-BDCl) and after (PIB-BD-Br) the ionization study in Hex/MeCl 60/ 40 (v/v) at -80 °C using [PIB-BD-Cl] ) 0.004 mol L-1 and [Me1.5AlBr1.5] ) 0.004 mol L-1 (Figure 6). This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes (1) Kennedy, J. P.; Marechal, E. In Carbocationic Polymerization; John Wiley & Sons: New York, 1982; p 486. (2) Schlaad, H.; Erentova, K.; Faust, R.; Charleux, B.; Moreau, M.; Vairon, J.-P.; Mayr, H. Macromolecules 1998, 31, 8058-8062. (3) De, P.; Faust, R. Polym. Prepr. 2005, 46, 847-848. (4) Higashihara, T.; Feng, D.; Faust, R. Macromolecules 2006, 39, 52755279. (5) In Cationic Polymerizations: Mechanisms, Synthesis, and Applications; Matyjaszewski, K., Ed.; Marcel Dekker: New York, 1996; p 173. (6) Tawada, M.; Faust, R. Macromolecules 2005, 38, 4989-4995. (7) Bahadur, M.; Shaffer, T. D.; Ashbaugh, J. R. Macromolecules 2000, 33, 9548-9552. (8) Hadjikyriacou, S.; Acar, M.; Faust, R. Macromolecules 2004, 37, 7543-7547. (9) Fodor, Zs.; Faust, R. J. Macromol. Sci., Pure Appl. Chem. 1996, A33, 305-324. (10) Drabowicz, J.; Luczak, J.; Mikolajczyk, M. J. Org. Chem. 1998, 63, 9565-9568. (11) (a) Gyor, M.; Wang, H.-C.; Faust, R. J. Macromol. Sci., Pure Appl. Chem. 1992, A29, 639-653. (b) Wang, B.; Mishra, M. K.; Kennedy, J. P. Polym. Bull. (Berlin) 1987, 17, 205-211. (12) Mole, T.; Jeffery, E. A. Organoaluminum Compounds; Elsevier: Amsterdam, 1972. (13) Dimitrov, I.; Faust, R. Macromolecules 2004, 37, 9753-9760. (14) Si, J.; Kennedy, J. P. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 2011-2021. (15) Storey, R. F.; Curry, C. L.; Brister, L. B. Macromolecules 1998, 31, 1058-1063. (16) Santos, R.; Kennedy, J. P.; Toman, L. Polym. Bull. (Berlin) 1984, 11, 341-348. (17) Kennedy, J. P.; Johnston, J. E. AdV. Polym. Sci. 1975, 19, 57-95. (18) De, P.; Faust, R. Unpublished results. (19) Brown, H. C.; Wallace, W. J. J. Am. Chem. Soc. 1953, 75, 6279-6285.

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