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Jul 29, 2013 - ABSTRACT: The functionalization of polyisobutylene-co- isoprene, commonly referred to as butyl rubber, was investigated with the aim of...
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Rubber Functionalization by Diels−Alder Chemistry: From CrossLinking to Multifunctional Graft Copolymer Synthesis Mahmoud M. Abd Rabo Moustafa† and Elizabeth R. Gillies*,†,§ †

Department of Chemistry, The University of Western Ontario, 1151 Richmond St., London, Canada, N6A 5B7 Department of Chemical and Biochemical Engineering, The University of Western Ontario, 1151 Richmond St., London, Canada, N6A 5B9

§

S Supporting Information *

ABSTRACT: The functionalization of polyisobutylene-coisoprene, commonly referred to as butyl rubber, was investigated with the aim of preparing multifunctional materials. First, the acid catalyzed ring-opening of epoxidized butyl rubber was investigated. Ring-opening followed by elimination resulted in the formation of different dienes depending on the reaction conditions. It was possible to cleanly isolate the exo-diene, which readily undergoes Diels− Alder [4 + 2] cycloaddition reactions and it was demonstrated that this chemistry could be used to prepare multifunctional graft copolymers. For example, poly(ethylene oxide) (PEO) or polystyrene (PS) could be grafted while at the same time introducing carboxylic acid moieties along the polymer backbone, and both polymers could be simultaneously grafted to form mixed graft copolymers with tunable tensile properties. Furthermore, it was demonstrated that reaction with a dimaleimide led to efficient cross-linking of the rubber and this cross-linking could be reversed upon heating due to the thermoreversible nature of the Diels−Alder reaction. Thus, this chemistry allows for clean and facile synthesis of new rubber derivatives, opening possibilities for new rubber applications.

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the membrane.13 Our group recently demonstrated that butyl rubber−PEO graft copolymer films resist the adsorption of proteins and the growth of cells, providing nonfouling properties.14,15 It is noteworthy that all of the above examples of PIB-based polymers for biomedical applications involve PIB in combination with other polymers and/or other chemical functionalities. This demonstrates a need for chemical approaches to modify PIB in order to tune and enhance its properties and functions. In particular, for biomedical applications, the reactions must be clean and reproducible in order to avoid the production of byproducts that may exhibit undesirable biological properties. One way to introduce functional groups to polymers is through the introduction of functional monomers during the polymerization. Although this approach works well for a large variety of polymers,16−18 with the exception of small percentages (≤7 mol %) of isoprene, this approach generally fails for PIB because of the incompatibility of the cationic polymerization approach with monomers containing functional groups. Another approach to modify PIB is to employ a postpolymerization functionalization strategy.19 This has been used extensively for telechelic PIB modification.20,21 It typically

INTRODUCTION Polyisobutylene- (PIB-) based elastomers are of significant interest for a diverse array of commercial applications ranging from automobile tires to chewing gum. Some attractive properties of these polymers include high elasticity, impermeability to gas and water, and outstanding chemical stability. Recently, there has also been increased interest in PIB-based materials for biomedical applications.1−3 For example, PIB− polystyrene triblock copolymers (“SIBS”) are currently used in the drug eluting coating on TAXUS vascular stents.3 These polymers have also been investigated in corneal shunts for the treatment of glaucoma,4 synthetic aortic valves,5 and hydrophobic electrospun fiber mats.6 Multiarm PIB−cyanoacrylate (CA) copolymers have been reported as promising materials for intervertebral disk replacement7,8 and tissue adhesives.9 PIB-based polyurethanes have been demonstrated to exhibit unprecedented combinations of mechanical properties and high oxidative, hydrolytic and biological stability.10 PIB−poly(methyl methacrylate) (PMMA) composites have enhanced properties relative to commercial bone cements due to the incorporation of the elastomeric PIB into the glassy PMMA material.11,12 Furthermore, copolymers of PIB with hydrophilic polymers such as poly(N,N-dimethylacrylamide) or poly(ethylene oxide) (PEO) have been used to form membranes that can encapsulate cells while allowing the exchange of oxygen, nutrients, and secreted proteins such as insulin across © XXXX American Chemical Society

Received: May 24, 2013 Revised: July 10, 2013

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Scheme 1. Isomerization/Dehydration Reaction Sequence of Epoxidized Butyl Rubber 1

Table 1. Reaction Conditions and Resulting Product Distributions for the Transformation of Epoxide-Functionalized Rubber to Allylic Alcohol Derivatives and Dienes approximate distribution of the products (%) a

entry

acid

reaction conditions

ab bc c d e f gd

HCl(aq) TsOH TsOH TsOH TMSOTf TMSOTf TMSOTf

CDCl3, rt, 5 min CDCl3, rt, 5 min CDCl3, rt, 15 min toluene, 80 °C, 10 min DBU, toluene, 0 °C, 10 min DBU, toluene, rt, 3 days Et3N,toluene, rt, 10 min

2a,b

3a,b

4

5

6

100 (2a) 32 (2a) 37 (2a) 0 19 (2a) 0 50 (2b)

0 16 (3a) 0 0 15 (3b) 0 50 (3b)

0 18 63 0 65 0 0

0 0 0 0 0 0 0

0 0 0 100 0 100 0

a

The reactions were done under inert atmosphere/dry conditions unless otherwise noted, and the relative percentages of the products were calculated based on 1H NMR analysis. bThe reaction was done as previously reported,14 but CHCl3 and/or CDCl3 was used as a solvent. c Approximately 34% of epoxide starting material remained following this reaction. When an aqueous solution of TsOH was used, no conversion was observed. dLutidine and Et(iPr)2N were also examined and gave similar results to Et3N.

yields and mixtures of products led the authors to conclude that the functionalization of RB via Diels−Alder chemistry was not a viable approach for the preparation of graft copolymers. Here we show that by clean preparation of the exo-diene from the exo-allylic alcohol and subsequent functionalization by Diels− Alder reactions and/or ring-opening of maleic anhydride adducts, it is possible to obtain graft copolymers with polystyrene (PS), PEO, or both, along with carboxylic acid moieties and also thermoreversibly cross-linked rubber.

involves a living cationic polymerization to form an end functionalized PIB, followed by synthetic modification of that terminus. For example, PIBs modified with bromide,22 exoolefin,23−25 phenol,26,27 aliphatic alcohol,28,29 or silane30 have been investigated. Alternatively, butyl rubber (RB) is a copolymer of isobutylene and small percentages of isoprene (typically 1−2 mol %) and the double bonds of the isoprene units throughout the RB backbone provide sites for chemical functionalization. For example, bromination or chlorination leads to the corresponding halogenated rubber derivatives as a mixture of isomers. These halogenated rubbers have been reacted with a variety of nucleophiles such as alcohols or carboxylic acids.31−34 While the products are suitable for some applications, these reactions have often been limited by incomplete displacement as well as side reactions such as eliminations to the conjugated diene. Our group has recently reported the preparation of butyl rubber functionalized with exo-allylic alcohols along the backbone via a clean and mild reaction sequence involving epoxidation of butyl rubber with m-chloroperoxybenzoic acid, followed by ring-opening with HCl in toluene.14 This allylic alcohol functionality has been used to graft PEO with high efficiency15 and also to install cinnamate moieties for UV-curing of rubber films.35 Herein we report the clean conversion of the exo-allylic alcohol functionality to an exo-conjugated diene and application of this diene in Diels−Alder [4 + 2] cycloaddition chemistry a strategic “click” method for the functionalization of polymers.19 In previous work, dehydrohalogenation of brominated RB has been used to prepare diene functionalized rubber.32,36−38 Although, the reaction was carried out in refluxing xylenes for an extended period of time (up to 25 h), incomplete conversion was observed and the diene was obtained as a mixture of isomers.32 In subsequent steps, low

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RESULTS AND DISCUSSION Conversion of Epoxidized Rubber to Diene Derivatives. The starting material for the present work was the epoxide functionalized butyl rubber derivative 1 (Scheme 1) prepared as previously reported.14 In this case, the starting rubber contained approximately 2 mol % isoprene units, and subsequently there was ∼2 mol % epoxide in 1, as measured by 1 H NMR spectroscopy. A quantitative and clean conversion of 1 to the corresponding exo-allylic alcohol 2a by treatment with aqueous HCl in toluene has been previously reported by our group (Table 1, entry a).14 The current investigation began with the treatment of the epoxide under a variety of other conditions (Scheme 1). First the reaction of epoxide 1 with ptoluenesulfonic acid (TsOH) was monitored by 1H NMR spectroscopy (Supporting Information). After 5 min at room temperature, approximately 66% conversion was observed, affording a mixture of the endo-diene 4 and the allylic alcohols 2a (exo) and 3a (endo) (Table 1, entry b). After an additional 10 min, the endo-allylic alcohol 3a had completely dehydrated to the corresponding diene 4 (Table 1, entry c). As described above, diene functionalized butyl rubber derivatives are of interest for Diels−Alder chemistry. However, none of the exoallylic alcohol 2a had undergone dehydration to the B

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Scheme 2. Proposed Mechanisms for the Formation of the exo- versus endo-Allylic Alcohols

deprotonation unselectively to provide a mixture of exo (Hofmann) and endo (Zaitsev) products. Second, when 3a is prepared, it is unstable under the reaction conditions and undergoes rapid elimination to the diene 4. In contrast, exoallylic alcohol 2a undergoes elimination less readily. Lastly, the endo-diene 4 and the exo-diene 5 cannot be cleanly isolated, yet they can be observed under the reaction conditions by 1H NMR spectroscopy (Supporting Information). They undergo gradual conversion to the diene 6 under the reaction conditions. Though the isolation of diene 6 was interesting from a mechanistic point of view, it was not anticipated to be of great synthetic utility in the Diels−Alder reaction due to the steric repulsions that would be expected in the single bond-cis (s-cis) conformation that is required for this reaction. Therefore, an efficient and highly selective approach for the synthesis of exodiene 5 from the exo-allylic alcohol 2a was developed. The mesylate derivative 9 was prepared by reaction of 2a with methanesulfonyl chloride in the presence of NEt3 and K2CO3 (Scheme 3, Figure 1a). Refluxing of the freshly prepared 9 in

corresponding exo-diene 5, a diene that was previously shown to undergo Diels−Alder reactions with maleic anhydride.32 An attempt to force this dehydration by heating (Table 1, entry d) afforded another product, which we propose is the endo-diene 6 as a mixture of (E) and (Z) isomers, presumably through acid catalyzed isomerization of 5 and any transiently formed 4. Interestingly, this diene has not previously been identified by Parent and co-workers in their studies of the elimination of brominated butyl rubber derivatives, though they have performed their elimination reactions in the presence of acid scavengers.32,39 Unlike the brominated derivatives, the allylic alcohol derivatives seem to strongly favor elimination processes over rearrangements, even at modest temperatures. In order to further explore the reactivity of the epoxide, allylic alcohol, and alkene products with the aim of generating specific reactive rubber derivatives, catalysis with trimethylsilyl trifluoromethansulfonate (TMSOTf) in presence of a variety of bases (Table 1, entries e−g) was also explored as these are commonly used conditions for the opening of epoxides in synthetic organic chemistry.40 When the reaction was performed in the presence of 1,8-diazabicyclo[5.4.0]undec-7ene (DBU), the expected silyl protected allylic alcohols 2b (exo) and 3b (endo) were observed in low yields and the major products were the unprotected allylic alcohols 2a (exo) and/or the endo-diene 4 (Table 1, entries e). When the reaction was left at room temperature for longer time, complete isomerization to the diene 6 was again observed (Table 1, entry f). Presumably, the silyl group is labile under these conditions. In contrast to DBU, when the reaction was performed using Et3N, the silyl protected allylic alcohols 2b (exo) and 3b (endo) were observed in 1:1 ratio without any sign of deprotection and/or dehydration (Table 1, entry g). Therefore, the desired exodiene 5 was not observed. These results reveal several findings. First, while it was possible to isolate the exo-allylic alcohol 2a with high selectivity using aqueous HCl in CHCl3, it was not possible to obtain the endo-allylic alcohol 3a with high selectivity under any of the investigated conditions. Although a detailed mechanistic study is outside the scope of this work, we propose that this can be explained on the mechanistic basis previously studied for the related brominated butyl rubber derivatives, as shown in Scheme 2.39,41 In the case of HCl, it is proposed that an oxonium ion intermediate 7 is selectively deprotonated at the less sterically hindered exo site, leading to the exo (Hofmann) product. On the other hand, in the case of TsOH or TMSOTf a carbocation intermediate 8 is formed which undergoes

Scheme 3. Synthesis of the exo-Diene Butyl Rubber 5

xylenes for 3 h in the presence of Et(iPr)2N led to clean conversion to the exo-diene 5.42 Although the reaction sequence involved multiple steps, the target diene was obtained in high yield as a very clean (E)-isomer (Figure 1b). In addition to 1H NMR characterization, the polymers were characterized by infrared (IR) spectroscopy and size exclusion chromatography (SEC), which supported the proposed structures and confirmed that degradation of the polymer backbone did not occur throughout the synthetic process, with the exception of diene 6 which exhibited a modest reduction in molecular weight, likely attributable to acid induced degradation (Supporting Information).32 Investigation of Diels−Alder Chemistry. Having the strategic exo-diene 5 in hand, the Diels−Alder [4 + 2] cycloaddition was investigated. It was hypothesized that some of the limitations previously encountered in the application of Diels−Alder chemistry to butyl rubber could be attributed to the incomplete reactions and mixtures of products that were obtained during the preparation of the diene, as well as C

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Figure 1. 1H NMR spectra (CDCl3, 400 MHz): (a) 9; (b) 5; (c) 10; (d) 11.

Scheme 4. Butyl Rubber Functionalization by Diels−Alder [4 + 2] Cycloaddition

indicated ∼75% conversion, and a PEO content of 35 wt %, very close to the theoretical value of 42% obtainable from the isoprene content of 2 mol %. As previously reported, it was not possible to characterize the molecular weights of these graft copolymers by SEC due to their anomalous elution behavior.15,31 Although the production of graft copolymers by reaction with maleimide-derivatized polymers may offer some advantages, the high cost and the limited commercial availability of these polymers can limit the scope of this approach. The preparation of a cycloaddition adduct with readily available maleic anhydride (MAn) followed by alcoholysis with hydroxyl functionalized polymers, can extend the scope of this chemistry considerably. However, it was not previously possible to

unoptimized reaction conditions for polymer grafting.32 The preparation of butyl rubber−PEO graft copolymers has been of ongoing interest due to the properties of these materials, including their resistance to protein adsorption and cell adhesion when cast as films and their ability to act as surfactants and form stable micelles in solution.15,43 As shown in Scheme 4, treatment of 5 with maleimide functionalized poly(ethylene glycol) monomethyl ether having a molecular weight (MW) of 2000 g/mol at room temperature for 16 h afforded the corresponding graft copolymer 10. Excess ungrafted PEO was removed by precipitation of the graft copolymer in acetone as confirmed by SEC and differential scanning calorimetry (DSC) (Supporting Information). As shown in Figure 1c, analysis of the 1H NMR spectrum D

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Table 2. Ring-Opening of PIB−MAn Cycloadduct (11) with Hydroxyl- and Amine-Functionalized Polymersa,b

a On the basis of 1H NMR following removal of ungrafted PEO and or PS by precipitation; Other conditions were also investigated (see Supporting Information). bSigns of gel formation were observed (perhaps due to interchain anhydride formation).

4000−5000 g/mol, it was possible to obtain a PS content of 49 wt %. It was even possible to prepare graft copolymers bearing a mixture of PEO and PS arms using 0.6 equiv of each polymer relative to the bicyclic anhydride. The resulting polymer contained 54 wt % butyl rubber, 12 wt % PEO, and 34 wt % PS. In each case, the content of the graft copolymers was characterized by 1H NMR spectroscopy and the absence of free homopolymer was confirmed by thermal analysis and SEC (Supporting Information). Being clean, rapid and mild, this methodology provided an efficient and versatile method for the preparation of butyl rubber graft copolymers. To demonstrate how the preparation of butyl rubber graft copolymers using Diels−Alder chemistry can be used to tune their properties, the mechanical properties of copolymers 10, and 12−15 were studied and compared to both uncross-linked butyl rubber 402 (2 mol % isoprene) and SIBS, containing 20 wt % polystyrene. As shown in Table 3, all of the materials

optimize this sequence of reactions to make the synthesis of graft copolymers by this approach practical.32 Given the ability to more cleanly prepare the requisite diene by the current approach, this strategy was revisited. Indeed, treatment of diene 5 with MAn for 3 h at room temperature led cleanly to the bicyclic anhydride adduct 11 (Scheme 4, Figure 1d). Next, various conditions were investigated for the ring-opening of the anhydride with hydroxyl and amine functionalized polymers (Table 2). First, PEO was investigated for the reasons described above. An added benefit of this approach is that at the same time as introducing PEO to the butyl rubber, carboxylic acid moieties are also introduced along the polymer backbone. These carboxylic acids can potentially be used for further chemical modification of butyl rubber. In addition, on their own they may introduce new properties such as enhanced adhesion of rubber to metal substrates such as the stainless steel vascular stents or other medical implants.44 They may also impart some degree of hydrogen-bond-mediated cross-linking that may alter the mechanical properties of the rubber. In addition to PEO, PS was also investigated. The preparation of butyl rubber-PS graft copolymers is of interest due to the thermoplastic properties imparted by the PS blocks, as in the SIBS materials used in biomedical applications.3 There are no previous reports of butyl rubber-PS graft copolymers to the best of our knowledge. As shown in Table 2, the reaction proceeds slowly in absence of catalyst, but after 2 days it was possible to obtain graft copolymers with 21 wt % PEO (∼50% conversion). Although the anhydride ring-opening under methanesulfonic acid catalysis led to poor conversions (Table 2 and Supporting Information), the use of DMAP as a nucleophilic catalyst/base led to much better conversions. The presence of DMAP likely drives this otherwise reversible ring-opening reaction to completion by irreversible salt formation, then the carboxylate is subsequently neutralized upon acid treatment during the work-up. In the case of hydroxyl-terminated PEO (PEO−OH), under conditions involving refluxing toluene for 1 day, it was possible to obtain graft copolymers with 39 wt % PEO, very close to the theoretical value and even higher than what was obtained from the direct Diels−Alder reaction with the maleimide derivative. Similar results were obtained with amine terminated PEO (PEO−NH2) but at room temperature. Using a hydroxyl terminated PS (PS−OH) with a MW of

Table 3. Mechanical Properties of Butyl Rubber and Its Copolymers with PEO and PS polymer butyl rubber 402 SIBS (20 wt % PS) 10 12 13 14 15

tensile strength at break (MPa)

Young’s modulus (MPa)

% elongation at break

0.07 ± 0.04

0.6 ± 0.1

739 ± 198

15.6 ± 0.3

3.7 ± 0.3

621 ± 18

2.3 1.6 1.6 9.8 3.0

± ± ± ± ±

0.1 0.1 0.2 0.7 0.1

0.7 0.3 0.3 2.6 1.2

± ± ± ± ±

0.1 0.1 0.1 0.4 0.1

673 1028 1220 664 575

± ± ± ± ±

51 46 58 48 59

exhibited high % elongation at break, a characteristic property of elastomers. The introduction of PEO in copolymers 10, 12, and 13 led to a significant increase in tensile strength relative to butyl rubber, but no significant changes in the Young’s modulus. The introduction of PS in copolymer 14 led to an even greater increase in tensile strength and a significant increase in Young’s modulus, with values approach those of SIBS, a linear analogue of this polymer with somewhat lower styrene content. As expected, copolymer 15, containing both PEO and PS had properties intermediate between those E

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Scheme 5. Synthesis of Cross-Linked Rubber 16

of only one of the Diels−Alder adducts on the polymer (Supporting Information). This suggests that the cycloreversion reaction went to completion. However, a mixture of alkene products was observed, likely a mixture of exo and endo dienes as well as possibly other products resulting from alkene rearrangements. In addition, the SEC analysis showed a small decrease in the molecular weight and a small increase in PDI, indicating some degree of degradation.

containing just PEO or just PS with butyl rubber. Representative stress−strain curves are shown for each polymer in the Supporting Information. Overall, these results indicate that the grafting of linear polymers using Diels−Alder chemistry can be used to tune the tensile properties of the rubber for applications. Thermoreversible Cross-Linking via Diels−Alder Chemistry. The development of new mild and efficient cross-linking reactions for butyl rubber are of ongoing interest as the current curing approaches are not optimal for applications such as biomedical materials, where the leaching of toxic molecules from the cross-linked rubber is a significant concern. On the basis of the above successes in the grafting of both small molecules and macromolecules, Diels−Alder chemistry should provide an opportunity to cross-link butyl rubber in an efficient manner with the added benefit that Diels−Alder reactions are typically reversible at high temperatures. This provides a mechanism to thermally reverse the cross-linking, which could be of interest for some applications. Indeed the reaction of diene functionalized rubber 5 with 0.5 equiv of N,N′-(1,4-phenylene)dimaleimide (per diene) in toluene at room temperature afforded the corresponding insoluble cross-linked rubber 16. Acetone washings after the reaction did not contain any detectable dimaleimide. To characterize the cross-linked rubber 16, the gel content and volume swelling ratio were calculated. A sample of 16 (0.10 g) was extracted using toluene. The weight of unextracted rubber was measured before and after vacuum drying to a constant weight. The gel content was calculated as the weight percentage of insoluble rubber and it was found to be 97%. The volume swelling ratio45 of the insoluble rubber was calculated according to eq 1, where qw is the ratio of the wet weight to the initial weight of the rubber, ρ is the density for butyl rubber (0.92 g/cm3) and for toluene (0.87 g/cm3). The volume swelling ratio of rubber 16 was found to be 2.96. This volume swelling ratio is relatively low and combined with the high gel content suggests that a highly cross-linked rubber was produced. qv = 1 +

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CONCLUSIONS In summary, we have explored here the conversion of epoxide functionalized butyl rubber to various diene products via allylic alcohol intermediates. Using this strategy it was possible to cleanly convert butyl rubber to the exo-diene 5, which is an ideal substrate for the Diels−Alder [4 + 2] cycloaddition reaction. Using this diene, it was possible to efficiently prepare graft copolymers for the first time by either grafting maleimide terminated polymers or by forming the bicyclic anhydride adduct with maleic anhydride, followed by ring-opening with hydroxyl or amine terminated polymers. This approach has the advantage that carboxylic acid moieties are introduced concomitantly with the cyclic anhydride ring-opening, and it was also possible to simultaneously graft multiple different polymers. These graft copolymers exhibited tunable mechanical properties. Furthermore, the Diels−Alder reaction was used to prepare a cross-linked rubber under mild conditions by reaction with a dimaleimide, and this cross-linking could be reversed upon heating to ∼140 °C. Overall, the versatility of this chemistry and the ability to prepare multifunctional materials via simple, additive and byproduct free reaction sequences opens prospects for new applications of butyl rubber and its derivatives. The use of this approach to graft other polymers and small molecules as well as studying the mechanical and adhesion properties of the new PIB-graft copolymers is currently under investigation in our group.

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Experimental procedures, characterization data, 1H NMR spectra of the polymers, SEC traces, DSC traces, and stress− strain curves. This material is available free of charge via the Internet at http://pubs.acs.org.

(qw − 1)ρpolymer ρsolvent

ASSOCIATED CONTENT

S Supporting Information *

(1)

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Finally, it was possible to reverse the cross-linking process by refluxing 16 in xylenes for three days. The insoluble crosslinked material gradually became soluble. Characterization of this material by 1H NMR spectroscopy indicated that no maleimide was present, which would result from the reversion

AUTHOR INFORMATION

Corresponding Author

*E-mail: (E.R.G.) [email protected]. F

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Notes

(29) Morgan, D. L.; Storey, R. F. Macromolecules 2010, 43, 1329− 1340. (30) Kurian, P.; Zschoche, S.; Kennedy, J. P. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3200−3209. (31) Yamashita, S.; Kodama, K.; Ikeda, Y.; Kohjiya, S. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2437−2444. (32) McLean, J. K.; Guillen-Castellanos, S. A.; Parent, J. S.; Whitney, R. A.; Resendes, R. Eur. Polym. J. 2007, 43, 4619−4627. (33) McLean, J. K.; Guillen-Castellanos, S. A.; Parent, J. S.; Whitney, R. A.; Kulbaba, K.; Osman, A. Ind. Eng. Chem. Res. 2009, 48, 10759− 10764. (34) Parent, J. S.; Malmberg, S.; McLean, J. K.; Whitney, R. A. Eur. Polym. J. 2010, 46, 702−708. (35) Wu, W.; Karamdoust, S.; Turowec, B.; Gillies, E. R. Submitted for publication. (36) Baldwin, F. P.; Rae, J. A.U.S. Patent 4,068,051, 1978. (37) Canter, N. H.; Kennedy, J. P. Butyl Rubber Graft Copolymers. U. S. Patent 3,646,166, 1972. (38) Makoto, A. U. S. Patent Application 20040010089, 2004. (39) Malmberg, S. M.; Parent, J. S.; Pratt, D. A.; Whitney, R. A. Macromolecules 2010, 43, 8456−8461. (40) Murata, S.; Suzuki, M.; Noyori, R. J. Am. Chem. Soc. 1979, 101, 2738−2739. (41) Parent, J. S.; Thom, D. J.; White, G.; Whitney, R. A.; Hopkins, W. J. Polym. Sci. Part. A: Polym. Chem. 2001, 39, 2019−1026. (42) Kitahara, T.; Matsuoka, T.; Kiyota, H.; Warita, Y.; Kurata, H.; Horiguchi, A.; Mori, K. Synthesis 1994, 7, 692−694. (43) Karamdoust, S.; Bonduelle, C. V.; Amos, R. C.; Turowec, B.; Ferrari, L.; Gillies, E. R. J. Polym. Sci., Part A: Polym. Chem. 2013, DOI: 10.1002/pola.26733. (44) Sahoo, R. R.; Biswas, S. K. J. Colloid Interface Sci. 2009, 333, 707−718. (45) Caykara, T.; Bozkaya, U.; Kantoglu, O. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 1656−1664.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank LANXESS Inc. and the Natural Sciences and Engineering Research Council of Canada for funding this work. Aneta Borecki is thanked for acquiring SEC data. Inderpreet Sran is thanked for help in measuring the mechanical properties. The Charpentier group is thanked for providing access to their tensile testing system.

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dx.doi.org/10.1021/ma401087v | Macromolecules XXXX, XXX, XXX−XXX