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European Polymer Journal 47 (2011) 486–496
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European Polymer Journal
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Feature Article Tailoring macromolecular architecture with imidazole functionality: A perspective for controlled polymerization processes
Matthew D. Green a, Michael H. Allen Jr. b, Joseph M. Dennis a, David Salas-de la Cruz c, ⇑ Renlong Gao b, Karen I. Winey c,d, Timothy E. Long b,
a Department of Chemical Engineering, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061, USA b Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061, USA c Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA d Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA
article info abstract
Article history: Controlled radical polymerization (CRP) allows for the design and synthesis of functional Received 21 July 2010 polymers with tailored composition and unique macromolecular architectures. Synthetic Received in revised form 23 September methods that are readily available for controlled radical polymerization include nitrox- 2010 ide-mediated polymerization, reversible addition–fragmentation chain transfer polymeri- Accepted 24 September 2010 zation, and atom transfer radical polymerization. N-Vinyl monomers that are typically Available online 14 October 2010 amenable to free radical methods are often difficult to synthesize in a controlled manner Dedicated to Professor Nikos Hadjichristidis to high molecular weight due to the lack of resonance stabilization of the propagating rad- in recognition of his contribution to polymer ical. However, recent advances in the field of CRP have resulted in successful controlled science. polymerization of various N-vinyl heterocyclic monomers including N-vinylcarbazole, N-vinylpyrrolidone, N-vinylphthalimide, and N-vinylindole. The incorporation of the Keywords: imidazole ring into homopolymers and copolymers using conventional free radical poly- Imidazole merization of N-vinylimidazole monomer is particularly widespread and advantageous Controlled radical polymerization due to facile functionalization, high thermal stability, and the relevance of the imidazole Heterocycles ring to many biomacromolecules. Copolymers prepared with methyl methacrylate Block copolymers Living polymerization displayed random incorporation according to differential scanning calorimetry and amor- phous morphologies according to X-ray scattering. Imidazole- and imidazolium-containing monomers have shown recent success for CRP; however, the controlled polymerization of N-vinylimidazole has remained relatively unexplored. Future efforts focus on the develop- ment of tailored imidazole-containing copolymers with well-defined architectures for emerging biomedical, electronic and membrane applications. Ó 2010 Elsevier Ltd. Open access under CC BY-NC-ND license.
1. Introduction trolled polymerization reactions typically produce polymers with predictable molecular weights and narrow polydisper- Controlled radical polymerization allows for the synthesis sity indices (PDI), as well as various architectures unattain- of functional copolymers with well-defined architectures, able with conventional free radical techniques, including controlled molecular weights, and tunable sequences. Con- star-shaped and block copolymers. The most widespread controlled polymerizations include nitroxide-mediated radi- cal polymerization (NMP) [1], reversible addition–fragmen- ⇑ Corresponding author. Address: Hahn Hall 2110, Department of Chemistry (0344), Virginia Tech, Blacksburg, VA 24061, USA. Tel.: +1 540 tation chain transfer polymerization (RAFT) [2,3],andatom 231 2480; fax: +1 540 231 8517. transfer radical polymerization (ATRP) [4–6]. The chemistry E-mail address: [email protected] (T.E. Long). of N-vinyl monomers presents a fundamental problem for
0014-3057 Ó 2010 Elsevier Ltd. Open access under CC BY-NC-ND license. doi:10.1016/j.eurpolymj.2010.09.035 M.D. Green et al. / European Polymer Journal 47 (2011) 486–496 487 their use in controlled polymerizations: the formation of a time, as shown in Scheme 1 [15]. The amphiphilic block highly reactive N-vinyl propagating radical, due to the lack copolymers consisted of a poly(acrylic acid) star modified of resonance stabilization, increases the likelihood for chain with xanthate end groups for subsequent RAFT polymeri- transfer and chain termination events. Thus, achieving high zation with NVC. Due to the success of the controlled poly- molecular weights, which is desired for optimum thermome- merization of NVC, recent publications have focused on the chanical performance, is often challenging. Despite the synthesis of block copolymers to optimize the photosensi- apparent difficulty for controlled polymerizations, N-vinyl tizing properties of NVC as well as the synthesis of supra- heterocyclic monomers have functioned well in controlled molecular structures for drug delivery applications [16,17]. radical polymerizations due to recent advances in the field An additional N-vinyl monomer for use with controlled as described below (Fig. 1). radical polymerization techniques, N-vinylpyrrolidone Polymers with photoactive applications including pho- (NVP), is attractive for the pharmaceutical, food, and textile tovoltaic devices, photorefractive materials, and photo- industry industries due to its reported biocompatibility copiers typically contain the monomer N-vinylcarbazole [18,19]. Devasia et al. [20] and Wan et al. [21] first reported (NVC) due to its attractive high hole-transporting capabil- the controlled polymerization of poly(NVP) using RAFT ity and high charge carrier properties [7,8]. Interest in the techniques. The polymerization of Devasia et al. used a controlled radical polymerization of NVC began with the dithiocarbamate chain transfer agent and showed pseu- work of Fukuda et al. [9] who successfully formed NVC- do-first order kinetics with a linear increase in Mn with containing block copolymers with styrene using NMP; conversion. Wan et al. chose a xanthate-based chain trans- however, homopolymerization of NVC failed. Later, fer agent dissolved in fluoroalcohol solvents for tacticity
Schmidt-Naake et al. [10] effectively homopolymerized control. The results demonstrated narrow PDIs and Mn NVC using NMP, however, molecular weights proved showed a linear increase with monomer conversion as uncontrollable and narrow polydispersities were not re- shown in Fig. 2. The polymers also contained high syndio- ported. The synthesis of a novel NVC block copolymer tacticity (60%) due to the use of fluoroalcohol solvents, ver- using NMP for potential photosensitizer applications, ifying simultaneous control of molecular weight and poly(sodium styrenesulfonate-b-NVC), required acetic tacticity during polymerization. Later studies confirmed anhydride as a rate-accelerating additive, but contained xanthates as the chain transfer agent necessary to obtain only 5 mol.% NVC [11]. Although the formation of the block controlled molecular weight and narrow PDIs when homo- copolymer occurred successfully, the PDI of the block polymerizing NVP [22–24]. These studies utilized xan- copolymer remained unreported as well as the ability to thate-mediated RAFT polymerization to synthesize novel control molecular weight effectively. architectures of poly(NVP) forming linear, star, and block ATRP of NVC led to the first reported homopolymeriza- copolymers. Novel amphiphilic block copolymers formed 0 tion of a narrow PDI (Mw/Mn = 1.33) with C60Cln/CuCl/2,2 - with NVP include poly(NVP-b-vinyl acetate)[23,25] and bipyridine (bpy) as the catalyst [12]. Ultimately, using a C60 poly(ethylene glycol-b-NVP) [26]. Targeted applications in- core with multiple initiation sites caused the formation of cluded materials for coating catheters or drug encapsula- star-like architectures; the size exclusion chromatograms tion as well as stabilizers in emulsion polymerizations. showed a bimodal nature, further suggesting the formation Recently, Pound et al. [27] discovered side reactions oc- of nonlinear architectures. Brar et al. [13] optimized the curred when employing xanthate-mediated RAFT poly-
ATRP reaction of NVC with Cu(I)Cl/Cu(II)Cl2/bpy catalyst merization, including dimerization of NVP in bulk and in in toluene, which produced the homopolymer in 76% yield. solution. In addition, the NVP unit adjacent to the xanthate
The number-average molecular weight (Mn) versus percent mediator underwent elimination at high temperatures conversion resulted in a linear plot, and the polymers ob- (>70 °C). These side reactions affected polymerization tained exhibited low PDIs (1.01–1.38), suggesting a con- kinetics and molecular weight control, which led to the trolled polymerization. Mori et al. [14] demonstrated growth of other controlled polymerization methods for excellent control of NVC homopolymerization using RAFT NVP. Ray and coworkers demonstrated organostibine- polymerization methods with xanthate-based chain trans- mediated living radical polymerization as a highly effective fer agents. The reported Mn ranged from 3000 to 48,000 g/ method to synthesize well-defined poly(NVP) and its block mol with PDIs between 1.15 and 1.20. This optimized RAFT copolymers [28]. As shown in Fig. 3, organostibine media- polymerization produced poly(NVC) four-arm star poly- tors resulted in high molecular weight polymers with low mers and amphiphilic star block copolymers for the first PDIs (1.07–1.29), as well as molecular weight that
Fig. 1. Structures of N-vinyl heterocyclic monomers available for controlled radical polymerization. 488 M.D. Green et al. / European Polymer Journal 47 (2011) 486–496
Scheme 1. Synthesis of an NVC-containing amphiphilic star block copolymer. Copyright 2005 American Chemical2005 Society
Fig. 2. Mn values (d), Mw/Mn values (N), and SEC curves of poly(NVP) obtained with AIBN/[1-(O-ethylxanthyl)ethyl]benzene in (CF3)3COH at 20 °C under UV irradiation: [NVP]0/[CTA]0/[AIBN]0 (150/1/0.28). Reprinted with permission from [21].
exhibited a PDI of 1.5–1.6 with a Mn of 15,000–16,000 g/ mol and formed spherical micelles in aqueous solution. Re- cently, Huang et al. [33] synthesized NVP block copolymers with styrene or methyl methacrylate (MMA) using RAFT polymerization of NVP and ATRP of the latter monomers with a chloroxanthate inifer as shown in Fig. 4. The well- controlled polymerization produced high molecular
weight polymers (Mn >50,000 g/mol) with low PDIs (1.3– 1.4). This combination of RAFT and ATRP with chloroxant- hate inifers allowed access to a wide variety of monomers Copyright 2006 American Chemical Society. 2006 to form block copolymers with NVP. N-Vinylphthalimide (NVPI) and N-vinylindole (NVIn), Fig. 3. The effect of NVP concentration in bulk polymerizations on the Mn and PDI of the resulting homopolymer of NVP using an organostibine two additional N-vinyl heterocyclic monomers, have also mediator and AIBN at 60 °C. Reprinted with permission from [28]. undergone controlled radical polymerization. Numerous biological, photochemistry, and electroactive polymer applications incorporate phthalimide and indole groups increased linearly with increasing NVP concentration. [34–37]. Most homopolymerization and copolymerizations Organotellurium- [29] and organobismuthine-mediated with these monomers involved free radical methods. Maki reactions [30] also successfully polymerized NVP in a con- et al. [38] reported the first controlled polymerization of trolled fashion. The PDIs obtained using organotellurium- NVPI following the same methods utilized in the controlled mediated polymerizations remained lower than the PDIs polymerization of NVP. Xanthate-mediated RAFT polymer- reported using xanthate-mediated RAFT polymerization. ization produced low molecular weight polymers (Mn Another controlled polymerization study utilized for <3700 g/mol, PDI 1.19–1.35). After polymerization, NVP polymerization combined RAFT with either NMP or hydrazinolysis in dioxane/methanol deprotected the ATRP. Bilalis et al. [31] employed the combination of phthalimide group forming poly(vinyl amine). This NMP and RAFT to form NVP block copolymers containing demonstrated the feasibility of the controlled polymeriza- either styrene or 2-vinylpyridine. Both methods produced tion of a primary amine-containing polymer, allowing for an NVP block with PDIs between 1.4 and 2.2 that was subsequent modification. The same RAFT method polymer- indicative of a broad molecular weight distribution and ized block copolymers containing NVPI and N-isopropylac- uncharacteristic of controlled polymerizations. Hussain rylamide [39] to create an amphiphilic block copolymer. et al. [32] combined ATRP and RAFT to synthesize poly(sty- Maki et al. [40] also applied xanthate-mediated RAFT poly- rene-b-NVP). Hussain et al. used ATRP to create the poly- merization to the controlled polymerization of NVIn and styrene macroinitiator and subsequently converted the its derivatives. As shown in Fig. 5, the polymerization of bromide endgroup to a xanthate endgroup for the RAFT 2-methyl-N-vinylindole proceeded with control of molecu- polymerization of NVP. The resulting block copolymer lar weight and resulted in a polymer having a narrow PDI. M.D. Green et al. / European Polymer Journal 47 (2011) 486–496 489 Copyright 2008 American Chemical2008 Society.
Fig. 4. The synthesis of NVP block copolymers using difunctional haloxanthate inifers under RAFT and ATRP conditions. A bromoxanthate inifer resulted in the formation of the NVP dimer that led to the use of a chloroxanthate inifer. Reprinted with permission from [33].
Recently, Chikhaoui-Grioune et al. [41] demonstrated tion is displayed in Scheme 2, and a common solvent is not successful RAFT polymerization of NVPI using a thiocar- specified due to striking solubility differences. The poly- bonate chain transfer agent. The molecular weight of the merization of 4VIm becomes heterogeneous as poly(4VIm) polymer increased linearly with time and exhibited narrow precipitates from solution during the reaction. The poly- PDIs (<1.2). Electrospinning of this polymer occurred from merization of NVIm however follows pseudo-first order a DMF solution, and high polymer concentrations kinetics as observed using in situ fourier transform infra- (>70 wt.%) were required to obtain uniform, micron-scale red spectroscopy (FTIR), and possesses an observed rate (1.2–2.2 lm) fibers. constant of 1.63 10 5 s 1, an order of magnitude faster Recent fundamental developments in controlled radical than styrene [44]. We found the resulting polymers offer polymerization have accelerated the impact of the N-vinyl significantly different thermal properties. Poly(NVIm) pos- monomers discussed above. However, the controlled sesses a glass transition temperature (Tg) of 176 °C and a polymerization of N-vinylimidazole (NVIm), which is a thermal degradation temperature of 350 °C or higher, potentially versatile monomer possessing a nitrogen-con- while poly(4VIm) has a Tg of 220 °C, and a Tg for poly(2- taining aromatic ring for use in non-viral DNA delivery VIm) was not observed. The increase in Tg is presumably therapeutics and in electromechanical actuator materials, due to the hydrogen bonding in poly(4VIm), increasing currently remains relatively unexplored [42,43]. Conven- intermolecular interactions and restricting backbone seg- tional free radical polymerization of NVIm is extensively mental motion. All monomers are protonatable and are reported in the literature providing a foundation for stim- therefore soluble in acidic media, and despite the lack of uli-responsive polymers with tunable structures through amphotericity in NVIm polymers, it remains soluble in ba- chemical modification and high thermal stability from sic media similar to 4VIm and 2VIm polymers. 4VIm is syn- multiple resonance contributors. This manuscript details thesized through a decarboxylation reaction of urocanic recent advances in the polymerization of NVIm and the po- acid, as reported by Overberger et al. [45]. Recently, Ihm tential for emerging applications. Moreover, a perspective et al. [46,47] utilized poly(4VIm) as a potential non-viral for the utility of controlled radical polymerization methods gene delivery vector with some success. Others have also is presented to catalyze future studies in this important studied 4VIm due to the pendant imidazole functionality field. resembling the amino acid histidine, and the potential with regards to non-viral gene delivery and stimuli- responsive behavior related to the amphoteric nature of 2. Conventional free radical polymerization of N- imidazole [48]. Lawson et al. [49] reported the synthesis vinylimidazole and polymerization of 2VIm, and copolymers prepared with MMA, acrylonitrile, and styrene. The literature has Previous literature has extensively explored the con- yet to explore controlled polymerizations of 2VIm and ventional free radical polymerization of NVIm monomers. 4VIm. The isomers, 2-, and 4-vinylimidazole (2VIm and 4VIm, In our laboratory, investigations into the copolymeriza- respectively) have received significantly less attention, tion of NVIm with two monomers separately, n-butyl acry- although they offer resonance stabilization for the propa- late (nBA) and MMA have provided insight into the gating radical, potentially enabling more controllable poly- copolymerization behavior of NVIm. Scheme 3 displays merizations. One immediate and prominent difference the synthetic strategy for the preparation of imidazole- between the monomers is the presence of hydrogen bond based copolymers with MMA. Conventional free radical donors in 2VIm (m.p. 128 °C) and 4VIm (m.p. 84 °C). As a polymerization yielded the copolymers, which were subse- result, these monomers are crystalline solids, whereas quently alkylated through a facile SN2 substitution with NVIm remains a liquid above 50 °C. As observed in our the alkyl halide 1-bromobutane. The bromide counteran- laboratory, the homopolymerization of the isomers in solu- ion was subsequently exchanged to the tetrafluoroborate 490 M.D. Green et al. / European Polymer Journal 47 (2011) 486–496
Scheme 2. Polymerization of various vinylimidazole isomers (NVIm, 2VIm, and 4VIm) using conventional free radical polymerization methods.
polymers were obtained at high conversions. Ideal copoly- merization occurs when the product of the reactivity ra-
tios, r1 and r2, is equal to one, r1r2 =1. Fig. 6 displays the copolymer composition and the Tg as a function of NVIm incorporation. However, the copolymers prepared in our laboratory with nBA and NVIm did not appear to form ran- dom copolymers. The copolymer composition shifted sig- Copyright 2007 American Chemical Society. 2007
Ó nificantly relative to the monomer feed ratio, and
multiple, weak, Tg’s were observed using DSC. This behav- ior suggested that NVIm formed more ideal copolymers with alkyl methacrylates than with alkyl acrylates. The determination of reactivity ratios for these two comono- mers would provide evidence to validate this hypothesis. We further investigated the morphological properties of the NVIm–MMA copolymers and quaternized copolymers using X-ray scattering over a wide range of scattering an- gles, Fig. 7. The lack of higher order peaks in these scatter- ing profiles indicated completely amorphous morphologies in both the NVIm–MMA and 1-butyl-3-vinylimidazolium bromide–MMA (BVIm–MMA) copolymers. For MMA homopolymers, the primary source of scattering was a re- Fig. 5. (a) Time-conversion (d) and first-order kinetic (j) plots for the sult of correlations between neighboring polymer back- polymerization of 2-methyl-N-vinylindole (2MNVIn) with AIBN in the bones. The scattering peaks in the uncharged NVIm and presence of O-ethyl-S-(1-phenylethyl)dithiocarbonate (CTA) in 1,4-diox- charged BVIm homopolymers correspond to two correla- ane at 60 °C (b) Mn (d) and PDI (j) as a function of conversion. (c) Evolution of SEC traces with conversion. Reprinted with permission from tion lengths in the amorphous structure, specifically, the [40]. distance between pendant groups along the polymer back- bone at 16 nm 1 and the distance separating the neigh- 1 (BF4) anion in acetonitrile where NaBr precipitated from boring polymer backbones at 4–8 nm . Between these the reaction. The resulting copolymer was precipitated into homopolymers, the amorphous morphologies of the water, and washed copiously to remove excess salt. The copolymers vary systematically. As the comonomer copolymer composition was analyzed using 1H NMR spec- (NVIm) content increases in the NVIm–MMA copolymers, troscopy, and the thermal properties were determined the backbone-backbone spacing increases slightly (peaks using differential scanning calorimetry (DSC) which exhib- shift to lower values of nm 1). The increase in the back- ited a single Tg for the neutral copolymers. We found the bone–backbone spacing is considerably greater for the copolymers formed with NVIm and MMA appeared ran- BVIm–MMA copolymers, because the comonomer is dom, based on their adherence to the theoretical Fox equa- charged and significantly larger. The pendant–pendant tion that predicts the Tg for random copolymers. Also, the correlations are evident between the imidazole functional copolymer composition relative to the feed indicated a groups in the NVIm–MMA copolymers when the comono- nearly ideal copolymer system, compared to a specific mer content is 80% or higher. The pendant–pendant ideal copolymerization where r1 = r2 = 1, with an azeotro- correlations are evident at lower copolymer contents pic monomer feed occurring at 45% NVIm, although the (50%) between imidazolium rings in the BVIm–MMA M.D. Green et al. / European Polymer Journal 47 (2011) 486–496 491
Scheme 3. The preparation of MMA–NVIm copolymers with subsequent alkylation and anion exchange.
Fig. 6. The observed copolymer compositions relative to monomer feeds plotted versus a unique case for an ideal copolymerization when r1 = r2 = 1, and Tg for MMA–NVIm copolymers relative to the Fox equation determined using 1H NMR and DSC. copolymers, because the charged comonomers provide with molecular weights varying significantly from targeted greater scattering contrast. In both copolymers, the posi- values. Additionally, successful copolymerization with NI- tion of the pendant-pendant scattering peak is indepen- PAm provided seemingly well-controlled block copolymers dent of copolymer composition, because the distance is with PDIs below 1.4 also attributed to the NIPAm precur- set by the local p–p stacking of the rings. sor. Finally, Nakamura et al. [52] prepared homopolymers of NVIm in a non-traditional manner using UV irradiation of a tellurium-based chain transfer agent, affording con- 3. Perspective for future studies: controlled radical trolled polymerization with a PDI of 1.14. In our laboratory, polymerization of imidazole-containing monomers homopolymers of NVIm were recently prepared using nitr- oxide-mediated polymerization conditions utilizing Bloc- Various sources report the synthesis of NVIm using con- builderÒ with number-average molecular weights varying trolled polymerization conditions. Closer analyses of the from targeted values as displayed in Fig. 8. The targeted findings reveal that the polymerization processes most molecular weights were significantly higher than those ob- likely do not proceed in a controlled fashion. Liu et al. served from aqueous size exclusion chromatography mea- [50] prepared block copolymers of N-isopropylacrylamide surements, and large PDIs (>1.4) indicated a lack of control (NIPAm) with NVIm using RAFT conditions obtaining PDIs over the polymerization process. Also, repeated attempts of approximately 1.2 resulting from the controlled synthe- at second block addition to form well-defined styrene sis of the NIPAm block. The copolymers displayed unique and nBA multiblock copolymers yielded only starting solution properties and formed temperature-dependent material. micelles. Endo et al. [51] polymerized NVIm quaternized In addition to controlled polymerization using NMP with various substituents, and reported PDIs above 1.26 conditions, our laboratory also investigated anionic 492 M.D. Green et al. / European Polymer Journal 47 (2011) 486–496
literature shows the ionic liquid monomer, 2-(1-butylimi- dazolium-3-yl) ethyl methacrylate tetrafluoroborate (BIMT), possesses imidazolium functionality in a methacrylate monomer for a more controlled radical polymerization. Free radical polymerizations of the imi- dazolium–methacrylate monomers did not exhibit control of molecular weight [53,54]. Ding et al. [55] first polymer- ized BIMT in a controlled fashion using ATRP with a both Cu(I)Br and Cu(I)Cl catalyst. Polymerizations with the Cu(I)Br catalyst proceeded rapidly with little molecular weight control, whereas polymerizations with the Cu(I)Cl catalyst showed a linear dependence of molecular weight with conversion and resulted in polymers with narrow PDIs. RAFT polymerization of the imidazolium-containing methacrylate derivatives, however, has not shown similar success. Yang et al. [56] first reported RAFT polymerization of the BIMT monomer to synthesize the block copolymer poly(N-2-thiazolylymethacrylamide-b-BIMT) for magnetic property characterization. However, the authors did not perform molecular weight or kinetic analysis to confirm controlled polymerization of a novel block copolymer. Vi- jayakrishna et al. [57] reported the synthesis of BIMT homopolymers and other synthetic analogues with RAFT polymerization as well as block copolymers containing methacrylic acid. However, the polymerizations proceeded slowly (50% conversion required >150 h) and polymeriza- tion kinetics and PDI were not determined for these compositions. Lack of reliable control and the inability to form block copolymers under nitroxide-mediated polymerization con- ditions of NVIm led to the investigation of 1-(4-vinylben- Fig. 7. X-ray scattering of (a) NVIm–MMA copolymers and (b) BVIm– zyl)imidazole (VBIm) in our laboratories. This styrenic MMA copolymers. derivative is readily synthesized from 1-(4-vinylben- zyl)chloride (VBC) [58] and provides propagating radical polymerization as a means of polymerizing NVIm. However, stabilities similar to styrene which accommodate facile con- in the presence of anionic initiators such as n-butyllithium, trolled polymerization. This monomer was explored for the the proton on the C2 position of the imidazole ring was preparation of block copolymers previously, where Shen sufficiently acidic for deprotonation, effectively quenching et al. [59] and Pei et al. [60] investigated the synthesis and the initiator and leading to premature termination. Fig. 9 polymerization of the charged monomer – to avoid catalyst shows a study performed with NVIm in deuterium oxide, coordination – and obtained PDIs below approximately 1.5 where the C2 proton slowly exchanges to deuterium with using ATRP. Waymouth et al. [61,62] also prepared block a half-life of approximately 2.6 h. The acidity of the C2 pro- copolymers of VBIm, but used NMP to grow a second block ton led to premature termination during the anionic poly- of VBC from a polystyrene precursor and subsequently func- merization process. tionalized with imidazole. These copolymers were investi- Although the efforts for controlled radical polymeriza- gated for their solution behavior and micellar formation in tion have proven difficult for N-vinylimidazole, recent solution. Current research in our laboratory is focused on
Fig. 8. Illustration of nitroxide-mediated polymerization of NVIm and obtained molecular weight data using aqueous size exclusion chromatography. M.D. Green et al. / European Polymer Journal 47 (2011) 486–496 493
Fig. 9. Deuterium exchange study of the C2 proton on NVIm to investigate anionic polymerization inhibition.
Scheme 4. Homopolymerization of VBIm using BlocbuilderÒ to afford nitroxide-mediated polymerization.
Fig. 10. Copolymerization of VBIm and nBA confirmed using 1H NMR spectroscopy and copolymer composition relative to monomer feed plotted versus a unique case for an ideal copolymerization when r1 = r2 = 1, and random incorporation confirmed with Tg’s using DSC which followed the Fox equation. the preparation of controlled homopolymers of VBIm. VBIm 20.0 kg/mol was targeted, and a molecular weight of was polymerized under NMP conditions using the initiator 18.3 kg/mol was obtained, as determined using end group Ò 1 Blocbuilder , as shown in Scheme 4. For example, Mn of analysis with H NMR spectroscopy. 494 M.D. Green et al. / European Polymer Journal 47 (2011) 486–496
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Polymer neering in 2007 from Virginia Tech. Matthew 2010;51(12):2447–54. [43] Green MD, Long TE. Designing imidazole-based ionic liquids and is currently coadvised by Dr. Richey M. Davis ionic liquid monomers for emerging technologies. Polym Rev in the Department of Chemical Engineering 2009;49(4):291–314. and Dr. Timothy E. Long in the Department of [44] Odian G. Principles of polymerization. Hoboken, NJ: John Wiley & Chemistry at Virginia Tech. He was awarded a Sons, Inc.; 2004. NSF IGERT fellowship in 2009 investigating [45] Overberger CG, Vorchheimer N. Imidazole-containing polymers. the interface between macromolecular struc- Synthesis and polymerization of the monomer 4(5)-vinylimidazole. tures and life sciences. His research focuses on the design and synthesis of J Am Chem Soc 1962;85:951–5. charged imidazolium-containing block copolymers and homopolymers 496 M.D. Green et al. / European Polymer Journal 47 (2011) 486–496 with applications ranging from electromechanical devices to therapeutic Karen I. Winey is Professor of Materials Sci- delivery vehicles. ence and Engineering and Chemical and Bio- molecule Engineering at the University of Pennsylvania. Her group designs and fabri- Michael H. Allen, Jr. holds a B.Sc. degree in cates polymer nanocomposites containing Chemistry, Biology, and Biochemistry from carbon nanotubes and metal nanowires with DeSales University. He is currently a Ph.D. the aim of understanding how to improve candidate in Chemistry at Virginia Tech in their mechanical, thermal, and especially Blacksburg, Virginia. He is currently advised electrical properties. More recently this work by Dr. Timothy E. Long. He was awarded an has expanded to include simulations of elec- NSF IGERT Fellowship in 2009. His current trical conductivity and polymer dynamics in research focuses on the design and charac- the presence of nanoparticles. Winey’s group terization of ionic liquid based polyelectro- has pioneered the use of HAADF STEM to probe the nanoscale morphol- lytes for biological and material applications. ogy in ion-containing polymers. Now, their effort focuses on correlating the structures in these materials, including block copolymers, with transport properties. She has published over 110 journal articles and holds 5 patents. Winey earned her B.S. in materials science and engi- neering from Cornell University and her M.S. and Ph.D. in polymer science and engineering from the University of Massachusetts, Amherst. Her David Salas-de la Cruz obtained a Bachelor of honors and awards include NSF Young Investigator Award (1994 – 1999), Science degree in Chemical Engineering from Fellow of the American Physical Society (2003), and NSF Special Creativity the University of Puerto Rico-Mayaguez and a Award (2009 – 2011). In addition to serving on various advisory boards, Master’s of Science in Chemical Engineering Winey chaired the 2010 Polymer Physics Gordon Research Conference from Villanova University. Currently, he is and is currently an Associate Editor for Macromolecules and Chair-Elect completing a doctorate degree in Chemical of the Division of Polymer Physics of the American Physical Society. and Biomolecular Engineering at the Univer- sity of Pennsylvania. In 2006 he was appoin- ted President of the Engineering Graduate Timothy Long received his B. S. in 1983 from Student Council at Villanova University, and St. Bonaventure University, followed by his in 2007 he was invited to be a member of Phi Ph.D. in 1987 from Virginia Tech. He spent Kappa Phi, the United States’ oldest and most several years as a research scientist at East- selective collegiate honor society for all academic disciplines. Over the man Kodak Company before returning to last two years he has received the Fontaine Society and the Carl Storm Virginia Tech as a professor in chemistry. He Underrepresented Minority Fellowships, and he serves on the Board of has been a faculty member in the department Directors of the American Institute of Chemical Engineers, Delaware of chemistry since 1998 and recently served Valley. His current research focuses on understanding the fundamental as Associate Director of Interdisciplinary relationship between morphology and ionic conductivity in polymerized Research and Education, Fralin Life Science ionic liquids. He is a renewable energy and biofuels enthusiast. David Institute at Virginia Tech. He serves currently resides in Philadelphia with his wife and young son. as the Associate Dean for Strategic Initiatives in the College of Science at Virginia Tech. He has received many presti- gious honors in his field of polymer chemistry including the American Renlong Gao is currently a graduate student Chemical Society (ACS) Cooperative Research Award in 2011, Virginia in the Department of Chemistry at Virginia Tech’s Alumni Award for Research Excellence (AARE) in 2010, 2009 ACS Tech. He received his B.S. (2007) in polymer Fellow, invited organizer of the Gordon Research Conference – Polymers, chemistry from University of Science and and Chair, ACS Polymer Division. He has also assembled a successful Technology of China. He is now pursuing his interdisciplinary research group and has been awarded $30M in Ph.D. degree in polymer science under the research funding during his time with Virginia Tech. His group’s contin- supervision of Professor Timothy E. Long. His uing research goal is to integrate fundamental research in novel macro- research interests focus on the synthesis and molecular structure and polymerization processes with the development characterization of well-defined ion-contain- of high performance macromolecules for advanced technologies. ing block and segmented copolymers.