Phase-Transfer Catalyzed Esterification of Brominated Poly

Ind. Eng. Chem. Res. 2009, 48, 10759–10764

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Phase-Transfer Catalyzed Esterification of Brominated Poly(Isobutylene-co-Isoprene) J. Keith McLean,† Sergio A. Guille´n-Castellanos,† J. Scott Parent,*,† Ralph A. Whitney,‡ Kevin Kulbaba,§ and Akhtar Osman§ Departments of Chemical Engineering and Chemistry, Queen’s UniVersity, Kingston, Ontario, Canada, and LANXESS Inc., Sarnia, Ontario, Canada

Phase-transfer techniques are used to activate carboxylate nucleophiles for the purpose of preparing ester derivatives of brominated poly(isobutylene-co-isoprene) (BIIR). Studies of the dynamics and yields of stearate ester syntheses reveal the dual role of tetrabutylammonium bromide, as it serves as a phase-transfer catalyst and a catalyst for the isomerization of exomethylene allylic bromide functionality to kinetically more reactive E,Z-BrMe isomers. Knowledge of reaction fundamentals are used to prepare copolymers from BIIR and carboxylate-terminated polybutadiene (cBR) that phase-partition in the manner required for blend compatibilization applications. 1. Introduction

2. Experimental Section

Brominated poly(isobutylene-co-isoprene), or BIIR, is a reactive elastomer that is valued for its air impermeability, oxidative stability, and activity in sulfur-based cure formulations. The latter property is derived from the 1-2 mol % of allylic halide functionality within the polymer, which is susceptible to nucleophilic displacement by elemental sulfur to give thermoset products.1 This chemical reactivity makes BIIR a useful template material for the preparation of isobutylene-rich elastomer derivatives through halide substitution by oxygen-based nucleophiles.2 Applications for these derivatives include interfacial stabilizers, adhesives, composites, and rheology modifiers.3 This report is concerned with BIIR as a reagent in solutionborne, phase-transfer catalyzed (PTC) esterifications (Scheme 1).4,5 These reactions activate alkali metal carboxylates by ion exchange with tetraalkylammonium halides, rendering carboxylate anions soluble in organic solvents and nucleophilic toward σ-bond electrophiles.6,7 Although the direct application of tetraalkylammonium carboxylate salts makes for a simpler process,8,9 it is cost-effective to use catalytic amounts of Bu4NBr in conjunction with KOH. It is this potential to improve reaction economy that has motivated us to study the dynamics and yields of a PTC approach. The chemistry illustrated in Scheme 1 can be used to introduce a wide range of pendant groups to an IIR backbone. Our studies of stearate ester formation are designed to support those interested in preparing IIR derivatives from functional small molecules. Of particular interest is the influence of phase transfer reaction conditions on reaction velocities and product distributions. Having gained an understanding of reaction dynamics, the esterification approach is extended to a polymer coupling process involving BIIR and carboxylate-terminated polybutadiene (cBR). This illustrative example generates a graft copolymer, IIR-g-cBR, that is known to promote phase adhesion in IIR + BR blends,10 but has yet to be prepared through a commercially viable process.

2.1. Materials. Brominated 2,2,4,8,8-pentamethyl-4-nonene (BPMN, 1) was prepared as described previously.11 The following reagents were used as received from Sigma-Aldrich (Oakville, Ontario): Bu4NBr (>98%), 3,5-ditert-butyl-4-hydroxytoluene (BHT, >99%), KOH (>85%), stearic acid (98%). BIIR (BB2030, LANXESS, Sarnia, ON) was purified of calcium stearate by dissolving in xylenes, acidifying with aqueous HCl, precipitating from acetone, and drying under vacuum. 2.2. Synthesis. IIR-g-Stearate. BIIR was dissolved in dried xylenes to produce a 10% w/v solution which was then treated with molecular sieves to remove excess water. The desired amounts of Bu4NBr, KOH, and stearic acid (expressed as molar equivalents relative to the 0.14 mmol/g of allylic bromide functionality in BIIR) were charged and the resulting mixture degassed with nitrogen before heating to 85 °C. Samples were drawn at regular intervals and the polymer recovered by precipitation from acetone. Integration of 1H NMR spectra provided estimates of allylic bromide and ester concentrations using chemical shift assignments generated from model comScheme 1. Phase Transfer Catalyzed Esterification of BIIR

* To whom correspondence should be addressed. Phone: (613) 5336266. Fax: (613) 533-6637. E-mail: [email protected]. † Department of Chemical Engineering, Queen’s University. ‡ Department of Chemistry, Queen’s University. § LANXESS Inc. 10.1021/ie901284j CCC: $40.75  2009 American Chemical Society Published on Web 11/10/2009

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Figure 1. Downfield 1H NMR spectra illustrating the evolution of allylic bromide and stearate ester (10% BIIR in xylenes, 0.1 equiv Bu4NBr, 1.5 equiv stearic acid, 9 equiv KOH, T ) 85 °C, IIR ) unbrominated isoprene mers).

pound analogues:2 δ 5.02 (Exo-Br, 1H, s), δ 4.00-4.10 (E,ZBrMe, 2H, s), δ 4.60-4.44 (E,Z-endoester, 2H, s), δ 5.11 (exoester. 1H, s). Carboxylated Polybutadiene (cBR). Carboxylated polybutadiene (cBR, 21% cis-CdC, 13% trans-CdC, 66% vinylCdC, Mn ) 5200 g/mol, polydispersity ) 1.1) was used as received from LANXESS Inc. (Sarnia, ON). PMN-g-cBR. c-BR (1.5 g, ∼0.18 mmol), xylene (5 mL), KOH (0.02 g, 0.35 mmol), and Bu4NBr (0.1 g, 0.31 mmol) were stirred for 2 min at 25 °C prior to the addition of BPMN (50 mg, 0.18 mmol). The flask was sealed, backfilled with nitrogen, and heated to 100 °C for 1.5 h. The product was filtered and subjected to Kugelrohr distillation (P ) 80 Pa, T ) 60 °C) to yield a yellow residue. FT-IR: 1736 cm-1 (CdO). 1H NMR (CDCl3); δ 4.59 (s, Z-CH2OC(O)-); δ 4.51 (s, E-CH2OC(O)-). IIR-g-cBR. BIIR was dissolved in dried xylenes to produce a 10% w/v solution which was then treated with molecular sieves to remove excess water. The dried solution was then heated to 85 °C under nitrogen with 0.2 equiv of Bu4NI for 120 min to produce an isomerized starting material containing 0.037 mmol/g of Exo-Br and 0.085 mmol/g of BrMe functionality. The resulting mixture was charged with cBR, KOH, and BHT and heated to 85 °C under nitrogen for 60 min to give crude IIR-g-cBR. The product was diluted with xylenes to give a 2.5% w/v solution prior to isolating by precipitating into acetone. Residual solvent was decanted, and the process was repeated twice to give purified IIR-g-cBR, which was stabilized with BHT and stored under refrigeration. FT-IR: 1736 cm-1 (CdO). 1H NMR spectrum integration provided the amount of the following products: δ 5.02 ppm (Exo-Br, 1H, s), δ ) 4.60-4.44 ppm (endoester, 2H, s), δ 4.10-4.00 ppm (E,ZBrMe, 2H, s). 2.3. Analysis. NMR spectra were acquired with a Bruker AC-500 spectrometer in CDCl3, with chemical shifts referenced to residual protons in chloroform. Gel permeation chromatography (GPC) was conducted with a Waters Associates GPC system equipped with a 515 HPLC pump, a 410 differential refractometer, a Waters 464 differential absorbance detector, and six Styragel columns (100, 500, 10(3), 10(4), 10(5), and

10(6) Å) using a THF flow rate of 0.5 mL/min at 35 °C. Differential scanning calorimetry (DSC) measurements were acquired with a DSCQ100 calorimeter from TA Instruments using a heating rate of 10 °C/min. Reported results were generated from the second of two heating cycles. Transmission electron microscopy (TEM) samples were sectioned at -100 °C using a Leica Ultracut cryoultramicrotome and placed on Formvar-coated copper grids. Images were recorded using a FEI Tecnai 20 transmission electron microscope operated at 200 kV. 3. Results 3.1. Stearic Acid Alkylations. Although polar, aprotic solvents are often preferred for nucleophilic substitutions,12 BIIR is only soluble in nonpolar organics of relatively low dielectric constant. As a result, appreciable esterification rates are only observed at elevated temperature, making alkylated aromatics particularly well-suited for the 85 °C reactions described in this report. Viscosity limitations restricted our experiments to polymer solutions containing 10% w/v of BIIR in xylenes. However, concentrations as high as 20% w/v can be accommodated if magnetic stirring is replaced by direct mechanical agitation. Our starting material was purified to remove calcium stearate and the other additives introduced during its manufacture. The elastomer contained 0.14 mmol of allylic bromide functionality per gram of elastomer, the majority of which was the exomethylene isomer (Exo-Br) illustrated in Scheme 1. This is a kinetically favored bromination product that rearranges at high temperature to give more thermodynamically stable E,Z-BrMe isomers. However, isomerization was not observed when BIIR alone was maintained at 85 °C in a dilute xylenes solution, nor was dehydrobromination to give HBr + conjugated diene.11 Figure 1 shows the downfield region of a 1H NMR spectrum of BIIR at t ) 0 min of reaction time. BIIR did not react when a xylene solution was heated to 85 °C with 1.5 equiv of potassium stearate relative to the amount of allylic bromide functionality in the elastomer. This is expected, given the poor solubility and nucleophilicity of alkali

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Scheme 2. Bu4NBr-Catalyzed Isomerization

Figure 2. Sequential BIIR isomerization and esterification (10% BIIR, T ) 85 °C).

metal carboxylates in nonpolar solvents. However, substantial reactivity was observed when 0.1 equiv Bu4NBr was included in the reaction mixture, with 87% of allylic bromide converted to stearate ester within 420 min (Figure 1). This yield amounts to an average of nine bromide substitution events per molecule of Bu4NBrsclear evidence of the catalytic nature of the phasetransfer process. The NMR spectra shown in Figure 1 reveal a preference for generating Endoesters even though none of the analogous BrMe functionality was observed beyond t ) 0 min. This might suggest that stearate alkylation proceeds by an SN2′ mechanism, where bromide displacement from Exo-Br is accompanied by allylic rearrangement.13 However, the data presented in Figure 2 do not support a concerted isomerization-substitution process. In this experiment, allylic functionality was monitored as a BIIR solution was heated for 120 min with 0.1 equiv Bu4NBr alone and for a further 240 min after adding stearic acid and KOH. The first phase of the experiment illustrated the ability of Bu4NBr to catalyze allylic bromide isomerization, as an initial Exo:BrMe ratio of 13:1 progressed to a 0.7:1 ratio within 120 min. Adding stearate to this mixture resulted in the rapid and complete consumption of BrMe functionality to an equivalent amount of endoester. This 60 min period of high activity was followed by a more sedate reaction with dynamics akin to those observed in the one-step process illustrated in Figure 1. These data reveal the dual role of Bu4NBr in BIIR esterifications. In addition to the conventional phase transfer function in which potassium stearate is activated by ion exchange, Bu4NBr catalyzes rearrangement of the Exo-Br isomer that is abundant within BIIR to its corresponding BrMe isomers. Since the latter react very rapidly with tetraalkylammonium carboxylate nucleophiles, Bu4NBr-catalyzed isomerization leads to enhanced esterification rates and higher proportions of endoester (Scheme 2). We note that 18-crown-6 and polymeric analogues such as polyethylene glycol are also effective phase transfer catalysts for BIIR derivitization.2,14 However, they are single purpose catalysts, in that they do not accelerate Exo-Br rearrangement.

Influence of Bu4NBr Concentration. The complexity of our process stems not only from the multiple catalytic functions of Bu4NBr, but from the presence of multiple salts (derived from Bu4N+, K+, Br-, RCOO- and HO-) that may partition into multiple organic and inorganic phases.15 For example, potassium and hydroxide salts do not react with BIIR under the conditions of interest, but they may affect the concentrations of reactive ammonium bromide and carboxylate salts. Given the importance of onium ion speciation,16 the sensitivity of reaction rates to Bu4NBr concentration is of particular interest. The dynamics data presented in Figure 3 show that catalytic amounts of Bu4NBr support an efficient process and that higher levels have a relatively small effect on reaction velocities. For example, the initial reaction rate generated by 0.1 equiv Bu4NBr was improved by a factor of just 2.4 on increasing the catalyst loading to 0.75 equiv and by a factor of only 2.8 when 1.5 equiv of Bu4NBr was employed. It is apparent, therefore, that as important as Bu4NBr is to PTC substitution reactions of BIIR, its influence has limits that can be approached to a considerable degree using a catalytic amount of 0.1 equiv relative to allylic bromide. We further noted that higher Bu4NBr concentrations raised the proportion of Endoesters in relation to the exoester product

Figure 3. Evolution of allylic bromide and stearate ester concentrations (10% BIIR in xylenes, 1.5 equiv stearic acid, 9 equiv KOH, T ) 85 °C).

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Figure 4. Evolution of total allylic ester concentration (T ) 85 °C, 10% BIIR in xylenes, 9 equiv KOH, 0.1 equiv Bu4NBr).

(Figure 3). This is the expected result, given the demonstrated ability of Bu4NBr to catalyze allyic bromide rearrangement. Higher Bu4NBr loadings accelerate Exo-Br isomerization to BrMe, whose subsequent SN2 displacement of bromide by carboxylate quickly generates Endoesters. Influence of Stearic Acid Concentration. The rate of a bimolecular substitution process is expected to respond strongly to the concentration of nucleophile in the reaction solution. In the context of a PTC substitution, the concentration of active carboxylate nucleophile is dictated by the positions of various ion-exchange and phase-partitioning equilibria. An upper limit of stearate nucleophile concentration is established by the 0.1 equiv of Bu4NBr used in our process, but precise knowledge of the concentration of tetrabutylammonium stearate, bromide, and hydroxide salts in our reaction mixtures is lacking. The experimental data illustrated in Figure 5 show that, at a constant Bu4NBr catalyst loading, the concentration of stearic acid has a relatively small effect on reaction dynamics (Figure 4). This insensitivity has practical consequences, since final allylic bromide conversions can be adjusted by varying carboxylic acid concentrations without incurring gross changes in reaction velocity. 3.2. IIR-g-cBR Copolymer Synthesis. Solution-borne esterifications can be used to prepare IIR-based graft copolymers from BIIR and a monocarboxylated polymer. A variety of these monochelic materials are commercially available, and others are accessible through ring-opening of cyclic anhydrides by hydroxyl-functionalized precursors.17 We prepared monocarboxy-terminated polybutadiene (cBR) and studied its PTC reactionswithBIIRtoimproveourunderstandingofpolymer-polymer couplings while preparing a graft copolymer, IIR-g-cBR, that holds promise as an interfacial stabilization agent for blends of these two materials.10 Polymeric nucleophiles such as cBR present complications not encountered in the small molecule reactions described above.18 In the first place, the oxidative instability of polybutadiene can lead to undesirable cross-linking, necessitating the use of a N2 atmosphere and a hindered phenolic antioxidant such as BHT, which is unreactive to BIIR under our reaction conditions.14 Copolymer purification can also be difficult when components have similar solubility parameters. Fortunately, IIRg-cBR could be isolated from residual cBR by exploiting differences in molecular weight. Whereas IIR-g-cBR will not dissolve in xylene-acetone mixtures, the low solubility of cBR facilitated its extraction by repeated dissolution/precipitation with this solvent combination.

Figure 5. Downfield 1H NMR spectra of (a) model ester PMN-g-cBr; (b) purified IIR-g-cBR.

Characterization of the structure and yields of IIR-g-cBR is also more complicated than for IIR-g-stearate, in that cBR contains cis-, trans-, and vinyl-CdC groups whose 1H NMR resonances lie in the downfield region used to quantify allylic ester products. Figure 5 provides 1H NMR spectra for a model compound prepared from cBR and brominated 2,2,4,8,8pentamethylnonene (PMN) and the desired copolymer IIR-gcBR. The allylic endoester protons of both materials are clearly evident, but resonances derived from the corresponding exoester structures are obscured by those generated by cBR functionality. Nevertheless, this 1H NMR evidence, when combined with unambiguous absorbances in FT-IR spectra, provides confidence that the intended BIIR esterification has been accomplished. Accurate estimates of overall copolymer composition were derived from gravimetric analysis to give the weight percentages of each component. Our efforts to develop an IIR-g-cBR synthesis were concerned with minimizing the opportunity for cBR cross-linking and reducing the proportion of unidentifiable exoester in the product. Both objectives were achieved by isomerizing BIIR prior to esterification. The rearranged material contained an Exo-Br: BrMe ratio of 0.4:1, compared to the 13.0:1 ratio found in unmodified BIIR. As expected, this material esterified relatively quickly, reaching a cBR-limited conversion plateau within 30 min (Figure 6). Endoesters accounted for the amount of allylic bromide consumed by the reaction, providing confidence that exoester product was not generated in significant quantity. The final material contained approximately 9 wt % of grafted cBR. Fractionation of IIR-g-cBR by gel permeation chromatography showed that the copolymer eluted earlier than the parent material, in keeping with expectations of increased molecular weight (Figure 7). Differences between the refractive index detector response, which is sensitive to mass concentration, and UV detector response, which is sensitive to unsaturation content, were quite revealing. While the refractive index detector signal generated by IIR-g-cBR was less than that recorded for BIIR, its UV signal was significantly greater. This not only confirms that IIR-g-cBR contains a substantial quantity of cBR, it also indicates that cBR grafts are uniformly distributed throughout the copolymer’s molecular weight distribution. We concluded our studies with a brief examination of the solid state morphology of a copolymer containing 21 wt % of grafted cBR (IIR-g-cBR-21%). The degree to which this copolymer can stabilize the interface of an IIR + BR blend

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Figure 9. TEM micrograph of purified IIR-g-cBR-21% stained with RuO4.

Figure 6. Dynamics of IIR-g-cBR formation from isomerized BIIR (5% BIIR, 0.2 equiv Bu4NI, 0.81 equiv BR-COOH, 9 equiv KOH, T ) 85 °C).

electron microscopy (TEM) images showed a continuous IIR matrix and a dispersed cBR phase that was preferentially stained with RuO4 according to differences in CdC unsaturation (Figure 9). 4. Conclusions Bu4NBr plays a dual catalytic role in PTC esterifications of BIIR by rendering carboxylate anions nucleophilic and by isomerising exoallylic bromide into more reactive BrMe electrophiles. The solution-borne reaction is robust, showing little sensitivity to Bu4NBr and potassium stearate concentrations, and can be adapted to produce IIR-g-cBR copolymers in high yield for use in blend stabilization applications. Acknowledgment Funding from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

Figure 7. GPC detector signals for BIIR and its IIR-g-cBR derivative.

Figure 8. DSC of BIIR, cBR, and their IIR-g-cBR-21% derivative.

will depend on the extent to which its components phaseseparate.19 Despite the relatively low molecular weight of individual cBR side-chains added to the IIR backbone, the total amount of grafted cBR was sufficient to support a phasepartitioned morphology. Differential scanning calorimetry (DSC) analysis revealed two distinct glass transitions at temperatures near those of the parent materials (Figure 8), and transmission

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(10) Ho, C. H.; Hopkins, W. Halogenated Butyl Rubber Graft Copolymers. United States Patent 5,264,494, 1993. (11) Parent, J. S.; Thom, D. J.; White, G.; Whitney, R. A.; Hopkins, W. Thermal Stability of Brominated Poly(isobutylene-co-isoprene). J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2019–2026. (12) Pfeffer, P. E.; Silbert, L. S. Esterification by alkylation of carboxylate salts. Influence of steric factors and other parameters on reaction rates. J. Org. Chem. 1976, 41 (8), 1373–1379. (13) Bordwell, F. G.; Clemens, A. H.; Cheng, J. P. Reactions of 9-Subsituted Fluorenide Carbanions with Allyl Chlorides by SN2 and SN2′ Mechanisms. J. Am. Chem. Soc. 1987, 109, 1773–1782. (14) Guille´n-Castellanos, S. A.; Parent, J. S.; Whitney, R. A. Synthesis and Characterization of Ether Derivatives of Brominated Poly(isobutylene-co-isoprene). J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 983– 992. (15) Asai, S.; Nakamura, H.; Tanabe, M.; Sakamoto, K. Distribution and Dissociation Equilibria of Phase-Transfer Catalysts, Tetrabutylammonium Salts. Ind. Eng. Chem. Res. 1993, 32, 1438–1441.

(16) Yufit, S. S.; Zinovyev, S. S. A Comparative Study of Nucleophilic Subsitution under PTC Conditions in Liquid-Liquid and Solid-Liquid Systems. Tetrahedron 1999, 55, 6319–6328. (17) McLean, J. K.; Guillen-Castellanos, S. A.; Parent, J. S.; Whitney, R. A.; Resendes, R. Synthesis of Graft Copolymer Derivatives of Brominated Poly(isobutylene-co-isoprene. Eur. Polym. J. 2007, 43, 4619–4627. (18) Yamashita, S.; Kodama, K.; Ikeda, Y.; Kohjiya, S. Chemical Modification of Butyl Rubber. I. Synthesis and Properties of Poly(ethylene oxide)-Grafted Butyl Rubber. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2437–2444. (19) Ajji, A.; Utracki, L. A. Interphase and Compatibilization of Polymer Blends. Polym. Eng. Sci. 1996, 36, 1574–1585.

ReceiVed for reView August 14, 2009 ReVised manuscript receiVed September 18, 2009 Accepted October 16, 2009 IE901284J