RAFT polymerization of styrenic-based phosphonium monomers and a new family of well-defined...

RAFT Polymerization of Styrenic-Based PhosphoniumMonomers and a New Family of Well-Defined Statisticaland Block Polyampholytes

RANWANG, ANDREW B. LOWE

The Department of Chemistry and Biochemistry, 118 College Drive No. 5043, The University of Southern Mississippi,Hattiesburg, Mississippi 39406-5043

Received 13 January 2007; accepted 8 February 2007DOI: 10.1002/pola.22009Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: We describe herein the first example of the controlled reversible addition-fragmentation chain transfer (RAFT) radical homo- and copolymerization of phospho-nium-based styrenic monomers mediated with a trithiocarbonate-based RAFT chaintransfer agent (CTA) directly in aqueous media. In the case of homopolymer synthesesthe polymerizations proceed in a controlled fashion yielding materials with predeter-mined molecular characteristics as evidenced from the narrow molecular mass distribu-tions (MMD) and the excellent agreement between the theoretical and experimentallydetermined molecular masses (MM). We also demonstrate the controlled nature of thehomopolymerization of 4-vinylbenzoic acid with the same CTA in DMSO. We subse-quently prepared both statistical and block copolymers from the phosphonium/4-vinyl-benzoic acid monomers to yield the first examples of polyampholytes in which the cati-onic functional group is a quaternary phosphonium species. We show that the kineticcharacteristics of the statistical copolymerizations are different from the homopolymeri-zations and proceed, generally, at a significantly faster rate although there appears tobe a composition dependence on the rate. Given the inherent problems in characterizingsuch polyampholytic copolymers via aqueous size exclusion chromatography we havequalitatively proved their successful formation via FTIR spectroscopy. Finally, in a pre-liminary experiment we qualitatively demonstrate the ability of such pH-responsiveblock copolymers to undergo supramolecular self-assembly. VVC 2007 Wiley Periodicals, Inc.

J Polym Sci Part A: Polym Chem 45: 2468–2483, 2007

Keywords: block copolymers; kinetics (polym); polyampholytes; reversible addition-fragmentation chain transfer (RAFT); stimuli-sensitive polymers; water-soluble polymers

INTRODUCTION

The ability to prepare polymeric materials withcontrollable molecular characteristics has under-gone significant advances in the past decade.1 Forexample, since its disclosure in the open litera-ture in 1998 by researchers at CSIRO,2,3 reversi-

ble addition-fragmentation chain transfer (RAFT)radical polymerization (Scheme 1) has proved tobe an extremely versatile synthetic techniquethat facilitates the controlled polymerization of awide range of functional monomers under a broadrange of experimental conditions.4–8 One particu-larly useful feature of RAFT relates to its applica-tion to the synthesis of water-soluble/dispersible(co)polymers (WSPs) under homogeneous condi-tions in either organic or directly in aqueousmedia.4,5,9 For example, RAFT has been success-fully employed for the synthesis of (co)polymers

Correspondence to: A. B. Lowe (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 2468–2483 (2007)VVC 2007 Wiley Periodicals, Inc.

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based on the (meth)acrylamido,10,11,12–17 (meth)acrylic,15,18 and styrenic15,19–21 families of mono-mers containing neutral,10,12,13,18,21–27 ani-onic,17,19,28–30 cationic,19,20,31–33 and zwitterionic(betaine)15,34–36 hydrophilic functionality. Asidefrom monomeric substrates, a wide range ofRAFT-mediating agents37,38 (RAFT CTAs) havelikewise been evaluated for the preparation ofWSPs.4 All the major families of RAFT CTAshave been employed, including derivatives of tri-thiocarbonates, dithioesters, xanthates, anddithiocarbamates.4

Within the family of WSPs, cationic, or poten-tially cationic, materials are particularly interest-ing because of their varied aqueous solution prop-erties and potential applications.39 To date, awide range of cationic or amine-containing sub-strates from various monomer families have beenpolymerized via RAFT in both aqueous and nona-queous media.4 For example, one of the earliestreports outlining the controlled polymerization ofamine- or ammonium-containing substrates wasby Mitsukami et al.,19 who described the synthe-sis of AB diblock copolymers of N,N-dimethylben-zylvinylamine (DMBVA) and the permanentlycationic species (ar-vinylbenzyl)trimethyl-ammo-nium chloride. More recently, Mitsukami andcoworkers20 reported the synthesis of a series ofthe same styrenic-based amine/ammonium blockcopolymers and conducted detailed aqueous solu-tion studies of these materials as a function of so-

lution pH and clearly demonstrated the effect ofcopolymer composition and architecture (block vs.statistical structures) on the size of the pH-induced nanosized supramolecular assemblies.Sumerlin et al.21 reported the synthesis and aque-ous solution properties of pH-responsive copoly-mers comprised of N,N-dimethylacrylamide withDMBVA and likewise demonstrated the ability ofsuch copolymers to undergo reversible pH-induced supramolecular self-assembly as well asthe ability to form novel core-crosslinked polymeraggregates. Styrenic derivatives are not the onlytypes of amine/ammonium monomers that havesuccessfully polymerized in a controlled mannerunder RAFT conditions. (Meth)acrylamido and(meth)acrylate substrates, such as 2-(dimethyl-amino)ethyl methacrylate (DMAEMA)33 and N-[3-(dimethylamino)propyl]-methacrylamide,31 havealso been successfully homo- and co-polymerized inboth aqueous and nonaqueous media. What isclear, however, is that all cationic monomers thathave so far been polymerized via RAFT have con-tained tertiary amine and/or quaternary ammo-nium functional group(s).

A more specialized—and complex—family ofionic materials are polyampholytes (PAMs),40–43

which are polyzwitterions that contain, or poten-tially contain, both cationic and anionic residueslocated on different repeat units, in contrast tothe other major family of polyzwitterions—thepolybetaines.34,35,43,44 A review of the literature

Scheme 1. A simplified RAFT mechanism.

RAFT POLYMERIZATION OF PHOSPHONIUM MONOMERS 2469

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

indicates that the cationic building block in allreported synthetic polyampholytes has likewisebeen either an amine or ammonium species whilethe anionic building block may be a carboxylateor sulfonate species, for example. Synthetic exam-ples of such materials have been known since the1950s,43 but even today there are relatively fewexamples of controlled-structure statistical orblock PAMs, and many of these are attainableonly after recourse to either functional group pro-tection/deprotection protocols or the postpolyme-rization modification of suitably functional pre-cursor materials.43,45–47 The direct synthesis ofwell-defined polyampholytes, that is withoutresorting to either protecting group chemistry orpostpolymerization modification, has only beenreported several times previously. Armes andcoworkers48 described the nitroxide-mediated po-lymerization of sodium styrenesulfonate in anethylene glycol/water mixture in which theresulting homopolymer was employed as a macro-initiator for the block copolymerization with vari-ous comonomers including the 2-vinylpyridine(2VP) and DMBVA to yield the correspondingblock PAMs. Polymerization yields were very lowfor the 2VP copolymerization but significantlybetter in the case of DMBVA comonomer. Morerecently, Xin et al.49 reported the RAFT synthesisof AB diblock copolymers of DMAEMA with so-dium acrylate (NaA). DMAEMA was polymerizedfirst in the presence of cumyl dithiobenzoate withAIBN in anisole to yield homopolymers with well-controlled molecular masses and narrow molecu-lar mass distributions. The polyDMAEMA homo-polymers were then employed as macro RAFTagents for the block copolymerization with NaAdirectly in water. Block copolymer formation wasconfirmed via a combination of aqueous sizeexclusion chromatography, and NMR and FTIRspectroscopies. The aqueous solution properties ofthe resulting block polyampholytes were alsobriefly examined.

In light of these limited reports, especially withPAMs prepared in a direct fashion via CRP meth-ods, we decided to examine the feasibility ofemploying RAFT as a synthetic technique for thedirect synthesis of new examples of controlled-structure statistical and block PAMs. We reportherein our results concerning the homopolymeri-zation of styrenic-based cationic phosphoniummonomers and 4-vinylbenzoic acid in aqueousand nonaqueous media using an acid-functional,water-soluble trithiocarbonate CTA whose syn-thesis we reported recently.50 We demonstrate

the controlled nature of the homopolymerizationsof the monomeric substrates, as evidenced by theexperimentally determined molecular character-istics. Subsequently we show the ability to pre-pare statistical PAMs with kinetic characteristicsdifferent from those of the homopolymerizationsof the cationic or 4-vinylbenzoic acid monomers.Finally, we employ phosphonium macro CTAs forthe block copolymerization with 4-vinylbenzoicacid to yield the first examples of PAMs with acationic phosphonium building block, and onlythe second example describing the direct synthe-sis of such materials via RAFT.

EXPERIMENTAL

All reagents were purchased from the AldrichChemical Company and used as received unlessstated otherwise. 2-(2-Carboxyethylsulfanylthio-carbonylsulfanyl) propionic acid was preparedaccording to our recently reported procedure.50

2,20-Azobis(2-methylpropionitrile) (AIBN) was re-crystallized from methanol and stored in a freezeruntil needed. 4,40-Azobis(4-cyanovaleric acid) (V-501) was purchased from Wako Chemicals, re-crystallized from methanol, and stored in afreezer until needed.

Synthesis of 4-Vinylbenzyl(trimethylphosphonium)Chloride (M1)

To a 500-mL conical flask equipped with a mag-netic stir-bar was added 4-vinylbenzyl chloride(15.25 g, 0.1 mol) and 100 mL of trimethylphos-phine solution (1.0 M in THF). The mixture wasthen stirred at room temperature for 3 days,yielding a white precipitate. The precipitate wascollected by Buchner filtration, washed with THF,and dried in vacuo overnight at room tempera-ture. Yield: ca. 85%.

Synthesis of 4-Vinylbenzyl(triphenylphosphonium)Chloride (M2)

To a 500-mL conical flask equipped with a mag-netic stir-bar was added 4-vinylbenzyl chloride(15.25 g, 0.1 mol), triphenylphosphine (26.20 g,0.1 mol) and benzene (100 mL). The mixture wasthen stirred at room temperature for 3 days,yielding a white precipitate. The precipitate wascollected by Buchner filtration, washed with ace-tone, and dried in vacuo overnight at 40 8C. Yield:ca. 80%.

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Synthesis of 4-Vinylbenzoic Acid (M3)

To a 500-mL round-bottomed flask equippedwith a magnetic stir-bar was added a-bromo-p-toluic acid (19.0 g, 0.088 mol), triphenylphos-phine (26.2 g, 0.10 mol), and acetone (200 mL).The flask was then immersed in a preheated oilbath at 60 8C and stirred overnight. Subse-quently, the resulting precipitate was isolatedby Buchner filtration. To a 1-L round-bottomedflask equipped with a magnetic stir-bar wasadded the isolated solid along with formalde-hyde (360 mL of a 37 wt % aqueous solution),distilled water (170 mL), and sodium hydroxide(28.0 g, 0.70 mol) dissolved in 170.0 mL of deion-ized water. The mixture was stirred vigorouslyfor ca. 3 h after which the solution was filteredto remove the precipitated triphenylphosphineoxide. The filtrate was subsequently acidifiedwith conc. HCl and the resulting precipitate iso-lated as the crude product by Buchner filtration.The crude product was washed with distilledwater and subsequently dried in vacuo at roomtemperature overnight. Yield: 80%.

Homopolymerization of 4-Vinylbenzyl(trimethylphosphonium) Chloride (M1)

To a 20-mL round-bottomed flask equipped witha magnetic stir-bar was added 4-vinylbenzyl(tri-methylphosphonium) chloride (4.0 g, 1.7513 10�2 mol), 2-(2-carboxy-ethylsulfanylthiocar-bonylsulfanyl) propionic acid (CTA1, 34.0 mg,1.339 3 10�4 mol), V-501 (4.0 mg, 1.429 3 10�5

mol) (target Mn ¼ 30,000, [CTA]:[I] ¼ 10) andD2O (8.0 g). The flask was immersed in an ice-bath and stirred for �1 h to ensure complete dis-solution of all components. Subsequently thecontents were split equally between eight smallvials that were sealed with rubber septa. Eachvial was then purged with N2 for ca. 30 minwhile immersed in an ice-bath. After purging,all vials were immersed in a preheated oil bathat 80 8C. Vials were removed from the oil-bath atregular time intervals and polymerization ter-minated via immediate exposure to air andquenching with liquid nitrogen.

Homopolymerization of 4-Vinylbenzyl(triphenylphosphonium) Chloride (M2)

4-Vinylbenzyl(triphenylphosphonium) chloride washomopolymerized according to the procedure de-scribed above forM1.

Homopolymerization of 4-Vinylbenzoic Acid (M3)

To a 20-mL round-bottomed flask equipped with amagnetic stir-bar was added 4-vinylbenzoic acid(4.0 g, 2.70 3 10�2 mol), CTA1 (34.0 mg, 1.343 10�4 mol), AIBN (4.0 mg, 1.43 3 10�5 mol)(Target Mn ¼ 30,000, [CTA]:[I] ¼ 10, 50 wt %), andd6-DMSO (8.0 g, 0.103 mmol). The flask wasimmersed in an ice-bath and stirred for �1 h toensure complete dissolution of all species. Subse-quently, the contents were split equally betweeneight small vials which were sealed with rubbersepta. Each vial was then purged with N2 for ca.30 min while immersed in an ice-bath. After purg-ing, all vials were immersed in a preheated oil bathat 80 8C. Vials were removed at predeterminedtime intervals and terminated via immediate expo-sure to air and quenching with liquid nitrogen.

Statistical Copolymerization of 4-Vinylbenzoic Acidand 4-Vinylbenzyl(trimethyl-phosphonium) Chloride

To a 20-mL round-bottomed flask equipped with amagnetic stir-bar was added 4-vinylbenzyl(tri-methyl-phosphonium) chloride (0.607 g, 2.063 10�3

mol), 4-vinylbenzoic acid (0.393 g, 2.67 3 10�3

mol), CTA1 (8.0 mg, 3.15 3 10�5 mol), V-501 (1.0mg, 3.57 3 10�6 mmol) (Target Mn ¼ 30,000,[CTA]:[I] ¼ 10, at 10 wt %), sodium carbonate(0.28 g, 2.67 3 10�3 mol), and D2O (10.0 g,0.50 mol). The flask was immersed in an ice-bathand stirred for�1 h to ensure complete dissolution.Subsequently the contents were split equally be-tween eight small vials that were sealed with rub-ber septa. Each vial was then purgedwithN2 for ca.30 min while immersed in an ice-bath. After purg-ing, all vials were immersed in a preheated oil bathat 80 8C. Vials were removed at predetermined timeintervals and terminated via immediate exposureto air and quenchingwith liquid nitrogen.

Block Copolymerization of 4-Vinylbenzyl(trimethylphosphonium) Chlorideand 4-Vinylbenzoic Acid

To a 50-mL round-bottomed flask equipped with amagnetic stir-bar was added 4-vinylbenzyl(tri-methyl-phosphonium) chloride (2.0 g, 8.76 3 10�3

mol), CTA1 (17.0 mg, 6.70 3 10�5 mol), V-501 (4.0mg, 1.43 3 10�5 mol) (target Mn ¼ 30,000,[CTA]:[I] ¼ 5, at 50 wt %), and distilled water (4.0g). The mixture was stirred while being purgedwith nitrogen for ca. 1 h after which it wasimmersed in a preheated oil bath at 80 8C. After



40 min, the polymerization was stopped by imme-diate exposure to air and quenching in liquidnitrogen. The mixture was subsequently dialyzedagainst distilled water for 2 days with dailychanges of water. Following this, the macro CTAwas isolated via lyophilization. To a 100-mLround-bottomed flask equipped with a magneticstir-bar were added the macro CTA (1.2 g, 5.253 10�3 mol), 4-vinylbenzoic acid (0.39 g, 2.633 10�3 mol), sodium carbonate (0.279 g, 2.633 10�3

mol), V-501 (2.0 mg, 7.14 3 10�6 mol), and D2O(4.0 g, 0.2 mol). The mixture was then stirred for1 h while submersed in an ice-bath to ensure com-plete dissolution. Following this, the solution wastransferred to five separate vials; each vial wascapped with a rubber septum and then purgedwith nitrogen for ca. 30 min while immersed in anice-bath. All vials were then immersed in a pre-heated oil bath at 80 8C. Vials were removed fromthe oil bath at various time intervals and thepolymerization terminated via immediate expo-sure to air and immersion in liquid nitrogen. Thecopolymer solution was then dialyzed againstdeionized water for 2 days prior to being isolatedby lyophilization.

Block Copolymerization of 4-Vinylbenzoic Acid and4-Vinylbenzyl(triphenylphosphonium) Chloride

Block copolymers of 4-vinylbenzoic acid and 4-vinylbenzyl(triphenylphosphonium) chloride wereprepared by a method identical to that describedabove for the 4-vinylbenzoic acid/4-vinylbenzyl(trimethylphosphonium) chloride block copolymer.

Methylation of Poly(4-vinylbenzoic acid)Homopolymers

To a 250-mL round-bottomed flask equipped witha magnetic stir-bar was added poly(4-vinylben-zoic acid) (0.5 g, 3.38 3 10�3 mol), sodium carbon-ate (0.5 g, 4.72 3 10�3 mol), methyl iodide (0.973g, 1.003 10�2 mol), and DMF (30.0 mL). The mix-ture was subsequently stirred overnight at 50 8C.After cooling to room temperature, distilled water(200 mL) was added to the reaction flask and theresulting precipitate isolated by Buchner filtra-tion. The precipitate was washed with additionaldistilled water and then dried in vacuo.

Characterization Techniques

1H (300 MHz) and 13C (75 MHz) NMR spectrawere recorded on a Bruker 300 53mm spectrometer

in appropriate deuterated solvents or solvent mix-tures. FTIR spectra were recorded on a ThermoNicolet Nexus 470 FTIR spectrometer equippedwith a Smart Orbit. Polymer molecular masses,molecular mass distributions, and polydispersityindices were determined by aqueous size exclusionchromatography (ASEC) in 0.1 M Na2SO4/1 vol %acetic acid flow rate of 0.20 mL min�1 at ambienttemperature. The system comprised a ViscotekVE1122 pump, Viscotek VE3580 RI detector, Visco-tek T60 dual viscosity/right angle laser light scat-tering detector, a CATSEC 1000 7l (50 3 4.6 mm)guard column followed by a series of two CATSECcolumns (CATSEC 1000 7l 2503 4.6 mm þ 100 5l250 3 4.6 mm) with a theoretical linear molecularmass range of 200–2000,000 g/mol. The dn/dc forthe homopolymers derived from M1 was deter-mined to be 0.150. Data were analyzed with theOmnisec Interactive GPC software package. Or-ganic SEC was conducted on a Waters system com-prised of a Waters 515 HPLC pump, Waters 2487Dual k absorbance detector, Waters 2410 RI detec-tor with a PolymerLabs PLgel 5 lm MIXED-C col-umn, in THF stabilized with BHT at a flow rate of0.5 mL/min. The column was calibrated with a se-ries of narrow molecular mass distribution poly(methyl methacrylate) standards.

RESULTS AND DISCUSSION

One of the most remarkable features of RAFT isits broad applicability with respect to the generalmonomer classes that are capable of being poly-merized in a controlled fashion coupled with itshigh functional group tolerance. Indeed, the con-trolled polymerization of monomers bearing ani-onic, cationic, zwitterionic, and neutral function-ality from various monomer families can—andhas—been readily achieved in both organic andaqueous environments under both homogeneousand heterogeneous conditions.4 Interestingly, ofall the functional materials which have so farbeen prepared via RAFT, little attention has beenpaid to the preparation of zwitterionic materials,and especially to PAMs.49 Indeed, while examplesof synthetic PAMs have been known for over 50years, there are, even today, few reports describ-ing the preparation of well-defined, controlledstructure PAMs with, for example, block architec-tures prepared by any polymerization tech-nique.43 Given the high monomer/functionalgroup tolerance of RAFT, one might expect it to be

2472 WANG AND LOWE


the ideal technique to facilitate the direct synthe-sis of PAMs without recourse to protection/depro-tection protocols or postpolymerization modifica-tion reactions. Currently, there is only one reportin the open literature detailing the direct synthesisof PAMs via RAFT.49 In light of the sparse litera-ture detailing the RAFT synthesis of PAMs wedecided to explore the possibility of preparing awholly new family of such materials, namely PAMsin which the cationic building block is a perma-nently charged phosphonium species. Currently,phosphonium-based PAMs are unknown, and tothe best of our knowledge, an evaluation of the po-lymerization of phosphonium monomers, by anycontrolled radical polymerization technique hasnot been conducted. Given this, we decided to pre-pare two examples of styrenic-based phosphoniummonomers and evaluate their homopolymerizationcharacteristics via RAFT with a carboxylic acid-functional trithiocarbonate CTA directly in aque-ous media.

4-Vinylbenzyl(trimethylphosphonium) chloride(M1) and 4-vinylbenzyl(triphenyl-phosphonium)chloride (M2) (Figure 1) were prepared from thereaction of 4-vinylbenzyl chloride and trimethyl-phosphine or triphenylphosphine respectively,in high yields. 4-Vinylbenzoic acid (M3) was pre-pared via a Wittig reaction as detailed above. Werecently disclosed the synthesis of CTA1.50

Given that the RAFT polymerization ofphosphonium substrates has not been previ-ously reported, we first examined the homo-polymerization characteristics of both M1 andM2 (Table 1) with CTA1 under homogeneous,aqueous conditions to ensure the substratescould be polymerized in a controlled fashion.

Figure 2 shows the pseudo-first-order rateplots for the homopolymerization of M1 and M2conducted in D2O at 50 wt % monomer, 80 8C,and at two different values of [CTA1]:[V-501](the monomer conversion were determinedusing 1H NMR spectroscopy). It is evident thatfor both monomers the plots are essentially lin-ear. It should be noted that in the case of RAFTpolymerizations this only indicates that the po-lymerization is operating under steady-stateconditions. However, the fact that the linearityis observed up to high conversions does indicatethe effective suppression of the Trommsdorffeffect. In the case of M1 [Fig. 2(A)], the homopo-lymerizations proceed rapidly with, for example,83% conversion being reached in 60 min for[CTA1]:[V-501] ¼ 5. There is no evidence of aninduction period, which can be observed for cer-tain RAFT CTA/monomer combinations, and isparticularly apparent in certain dithioester-mediated RAFT polymerizations,51 but which isless common/absent in the case of trithiocarbon-ate-mediated systems. From the kinetic plots,we can readily determine the apparent rate con-stant of propagation, kapp ¼ kp[R

�], where kp isthe rate constant of propagation and [R�] is theradical concentration. For these homopolymeri-zations, kapp is equal to 1.8 and 1.1 h�1 in thecase of M1 for [CTA1]:[V-501] ¼ 5 and 10 respec-tively. Additionally, the effect of [CTA1]:[V-501]is as expected with the higher ratio of reagentsresulting in slower overall rate of polymeriza-tion which is consistent with previous reports onthe effect of [CTA]:[I].3

M1 homopolymers were analyzed by aqueoussize exclusion chromatography (ASEC) to deter-

Figure 1. Chemical structures of monomers and RAFT chain transfer agent used inthese studies.



mine their molecular masses (MMs) and molecu-lar mass distributions (MMDs). As a representa-tive example, Figure 3 shows the ASEC traces (RIsignal) for aliquots withdrawn from an M1 homo-polymerization conducted at 80 8C, 50 wt % mono-mer, and with [CTA1]:[V-501] ¼ 10. In all instan-ces the experimentally determined chromato-grams are unimodal, symmetric, and narrow (Mw/Mn � 1.10) with no visible evidence of either highor low MM impurities. Additionally, the system-atic shift to lower retention times with increas-ing conversion is consistent with a controlledpolymerization.

While both the kinetic and ASEC results sug-gest a controlled polymerization, perhaps a betterindicator is the plot of number-average molecularmass (Mn) versus conversion. Figure 4 shows acomposite Mn versus conversion plot for twohomopolymerizations of M1 at [CTA1]:[V-501]¼ 5 and 10 along with the evolution of Mw/Mn fortarget MMs at quantitative conversion of 30,000.In both instances the evolution of MM is linear,passes through the origin, and is in extremelyclose agreement with the theoretical value at allgiven fractional conversions. Such linearity isentirely consistent with a controlled polymeriza-

Figure 2. Pseudo-first-order kinetic plots for the homopolymerization of M1 and M2at 50 wt % monomer in aqueous media with CTA1 at two different ratios of [CTA1]:[V-501].

Table 1. Summary of Polymerization Conditions and Molecular Characteristics (Mn andMw/Mn) forM1 andM3homopolymers

MonomerPolymerisation

SolventConcentration(wt % Monomer)

Temperature(8C) CTA1:I Mn,theory Mn,expt

a Mw/Mna

M1 D2O 50 80 5:1 7,500 9,600 1.05M1 D2O 50 80 5:1 12,900 15,200 1.07M1 D2O 50 80 5:1 18,800 20,700 1.06M1 D2O 50 80 5:1 24,600 28,000 1.06M1 D2O 50 80 10:1 3,800 5,400 1.03M1 D2O 50 80 10:1 7,600 8,400 1.03M1 D2O 50 80 10:1 11,400 12,950 1.04M1 D2O 50 80 10:1 14,700 16,500 1.02M1 D2O 50 80 10:1 22,100 23,600 1.05M3 DMSO 50 80 10:1 11,500 12,500 1.24M3 DMSO 50 80 10:1 18,400 19,900 1.18M3 DMSO 50 80 10:1 26,400 28,300 1.15M3 D2O 10 80 10:1 21,000 95,000 1.32

aAs determined by size exclusion chromatography.

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tion. Additionally, in both instances the resultingpolydispersities (Mw/Mn) are very low with mea-sured values �1.10.

Having established that the homopolymeriza-tion of M1 proceeds in a controlled fashion wenext conducted similar experiments for M2.

Figure 2(B) shows the pseudofirst order kineticplots for the homopolymerization of M2 underidentical conditions to those reported above forM1. A direct comparison of M1 with M2 indicatesthatM1 polymerizes faster thanM2 under identi-cal conditions at both [CTA1]:[V-501] ratios. For

Figure 3. Aqueous size exclusion chromatographic traces (RI signal) for the homopo-lymerization of M1 in aqueous media at 10 wt % monomer demonstrating the evolutionof molecular mass as a function of conversion.

Figure 4. Plots of MnASEC and Mw/Mn versus conversion for a homopolymerizationofM1 at [CTA1]:[V-501] ¼ 5:1 and 10:1.



example, M1 reaches ca. 84% conversion after100 min whereas M2 reaches ca. 55% conversionafter the same period of time for [CTA1]:[V-501]¼ 10. While M2 polymerizes at a slower rate thanM1, presumably due to its enhanced steric bulk,the kinetic plots are, however, linear to high con-version and exhibit the same general trends asM1. Unfortunately, the subsequent determina-tion of the MM and MMD could not be accom-plished for the M2 homopolymers due to the verylimited solubility of the materials in the ASECeluent at room temperature.

While M3 has been both homo- and co-poly-merized previously via RAFT15,19 it has not beenpolymerized under conditions mediated by a tri-thiocarbonate-based RAFT CTA, and as such wefelt it prudent to additionally examine the homo-polymerization of M3 as described above for M1and M2. Kinetic evaluations for the homopolyme-rization of M3 were conducted in DMSO since aninitial polymerization in water yielded a homopoly-mer with an experimentally determined Mn

significantly higher than the theoretical (Mn,expt

¼ 93,300 and Mw/Mn ¼ 1.36, whereas Mn,theory

¼ 21,000, see Table 1). The only differencebetween the M1/M2 and M3 homopolymeriza-tions in water was the need for added base to aidin the dissolution of M3. While a weak base wasemployed in a stoichiometric amount based onM3 we cannot dismiss the possible occurrence ofloss of CTA1 via base hydrolysis. Indeed,dithioesters have been demonstrated to be sus-ceptible to base hydrolysis and are, in fact, morestable under acidic conditions.52 Figure 5 shows

the pseudo-first-order rate plots for the homopoly-merization of M3 with [CTA1]:[V-501] ¼ 5 and10, at 50 wt % monomer and 80 8C. Consistentwith the use of CTA1 as a mediating agent in theM1 and M2 homopolymerizations, M3 exhibitslinear pseudo-first-order kinetics. In the case ofM3, kapp at the two different ratios of CTA1/V-501 are 1.3 and 0.75 h�1 respectively. As such, M3also appears to polymerize at a slower rate thanM1, although polymerizes faster than M3. How-ever, direct apparent rate comparisons are diffi-cult given the different solvent conditions.

The determination of the MM and MMD forM3 homopolymers also could not be achieveddirectly via ASEC since our instrument was con-figured specifically for the analysis of cationicpolymers. Under such ASEC eluent conditionsthe M3 homopolymers are protonated and thushydrophobic and as such cannot be analyzed.However, in contrast to the M2 homopolymers,M3 homopolymers can be chemically modified tofacilitate their analysis via organic SEC. PolyM3can be readily methylated using CH3I in DMF toyield the methyl carboxylic ester derivative thatis readily soluble in THF. While not ideal, sincepostpolymerization chemical modification of a(co)polymer rarely results in quantitative deriva-tization, it can be accomplished to an extent thatfacilitates analysis via SEC in THF. For example,Figure 6 shows the experimentally measuredSEC traces (RI signal) for methylated polyM3samples from the previously described homo-polymerization. Gratifyingly, the chromato-grams are unimodal and symmetric indicating

Figure 5. Pseudo-first-order kinetic plot for thehomopolymerization of M3 at 50 wt % monomer inDMSO with CTA1 at two different ratios of [CTA1]:[V-501].

Figure 6. Size exclusion chromatographic traces (RIsignal) for the homopolymerization ofM3, after methyl-ation with CH3I, recorded in THF.

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a high level of esterification as well as demon-strating the controlled nature of the original M3homopolymerization.

With the MM and MMD values readily avail-able it is possible to examine the evolution of Mn

and MMD as a function of conversion. Figure 7shows the plot of Mn, as determined by SEC withTHF as the eluent, versus conversion with thetheoretical Mn line-adjusted to take account ofthe increase in MM assuming quantitative meth-ylation. We see that the evolution of MM is linearand agrees almost perfectly with the theoreticalvalue at a given fractional conversion. Addition-ally, the polydispersity index decreases withincreasing conversion with Mw/Mn falling from1.24 to 1.15. Given these observations we canconclude that the homopolymerization of M3under these conditions with CTA1 is likewisecontrolled.

Having established that M1-M3 polymerize ina controlled fashion with CTA1 directly in wateror DMSO we next examined the ability to preparestatistical and block copolymers of M1/M2 withM3 with the aim of directly synthesizing a newfamily of well-defined polyampholytic materials.Initially we focused on the statistical copolymer-ization of M1 with M3 at molar ratios of 1:1, 1:3,and 3:1. Copolymerizations were conducteddirectly in aqueous media at 10 wt % monomer,80 8C and with [CTA1]:[V-501] ¼ 5 and 10 in thepresence of an equimolar concentration of sodiumcarbonate, based on M3, to aid in its dissolution.Copolymerizations were conducted at 10 ratherthan 50 wt % because of solubility issues—theM1/M3 combination for example was not homo-geneous at 50 wt % monomer. An evaluation ofthe pseudo-first-order kinetic plots shows that in

the case of the 1:1 copolymerization of M1 andM3 [Fig. 8(A)], the general trends are the same asthose observed for the homopolymerizations,namely linear plots which pass through the originwith a clear, and expected, effect of [CTA1]:[V-501]. However, the statistical copolymerization issignificantly faster than any of the homopolyme-rizations even though the [M] was only 10 wt %compared to the 50 wt % in the case of the homo-polymerization experiments. For example, in thecase of the copolymerization at [CTA1]:[V-501]¼ 5, 82% conversion is achieved in 30 min com-pared with M1 and M3 homopolymerizationswhich reached 60 and 48% conversion respec-tively, after a similar time period even though the[M] was five times greater. The calculated kappvalues for the M1/M3 copolymerizations are 3.4and 1.8 h�1 for the [CTA1]:[V-501] ¼ 5 and 10respectively. One possible cause for this differencebetween the M1, M2, M3 homo- and M1/M3 co-polymerizations can be attributed to the possibleoccurrence of ion-pairing. Even though eachmonomer has an associated counterion, it is possi-ble that there exists, in solution,M1/M3 ion-pairs.Indeed, such species are well known and there isoften, but not always, a tendency toward alterna-tion in the resulting structure since the ion-pairmay polymerize as a discrete ‘‘dimeric’’ species.Such monomer pairing, or dimerization, and thecorresponding enhancement in polymerizationrate is well documented for monomers capable offorming, for example, hydrogen-bonded monomerpairs such as those that exist between (meth)acrylic acid, or 2-(hydroxyethyl) (meth)acrylate.53

In a further demonstration of this kinetic dif-ference, Figure 8(B) shows the pseudo first orderrate plots of an M1 homopolymerization and anM1/M3 (3:1 M ratio) statistical copolymerizationperformed under identical conditions, that is at10 wt % monomer with [CTA1]:[V-501] ¼ 10. Thecopolymerization proceeds at approximately twicethe rate of the analogous homopolymerization ofM1 with kapp & 0.5 h�1 for the copolymerizationand 0.25 h�1 for the M1 homopolymerization.Interestingly however, the 3:1 statistical copoly-merization proceeds at a significantly slower ratethan the analogous 1:1 statistical copolymeriza-tion (kapp ¼ 1.8 h�1 and 0.5 h�1 respectively).Assuming that monomer pairing is responsiblefor the enhanced rate of polymerization, this isnot surprising since in the 3:1 copolymerization,there will exist a combination of monomer pairsand free M1. As a consequence, the overall rate ofpolymerization would be predicted to be interme-

Figure 7. Plots of MnSEC and Mw/Mn versus conver-sion for a homopolymerization of M3 as determined inTHF after methylation of the precursorM3 polymer.



diate between the M1 homopolymerization andthe equimolar, that is 1:1, M1/M3 copolymeriza-tion, which is the case.

Having successfully prepared novel phospho-nium-based statistical PAMs we next evaluated thepossibility of preparing the corresponding blockPAMs. Block copolymers were prepared using either

M1 or M2 homopolymers as macro CTAs for thesubsequent polymerization of M3. In the samemanner as described above, the kinetic profiles forthe block copolymerizations were evaluated. Figure9 shows the pseudo-first-order rate plots for twoseparate block polymerizations. Figure 9(A) showsthe kinetic profile for the block PAM prepared

Figure 8. Pseudo first-order kinetic plots for (A) the statistical copolymerization ofM1 with M3 at a molar ratio of 1:1 at 10 wt % monomer in water and two different[CTA1]:[V-501] ratios, (B) the statistical copolymerization of M1 with M3 at a molarratio of 3:1 at 10 wt % monomer in water at [CTA1]:[V-501] ¼ 10 and the correspondinghomopolymerization ofM1 under identical conditions.

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using anM2macro CTA at 10 wt %, and 80 8C. Theplot is clearly linear with a kapp of 0.5 h�1. Simi-larly, Figure 9(B) shows the rate plot for the blockcopolymerization of M3 employing an M1 macroCTA which has a measured kapp of 0.6 h�1. Thenear identical kapp values is not surprising since inthe case of the block copolymerizations, employing

either M1 or M2 macro CTAs under the same con-ditions, still leads to what amounts to a simple M3‘‘homopolymerization.’’

Unfortunately, determining the MMs andMMDs for polyampholytic copolymers, with ei-ther statistical or block architectures, via SEC, isextremely problematic especially for materials

Figure 9. Pseudo first-order kinetic plots for (A) the block copolymerization of M3employing a polyM1 macro CTA at a target molar ratio of 1:1 at 10 wt % in water with[CTA1]:[V-501] ¼ 10, and (B) the block copolymerization of M3 employing a polyM2macro CTA at a target molar ratio of 1:1 at 10 wt % in water with [CTA1]:[V-501] ¼ 10.



such as these where one building block has a pH-dependent aqueous solubility. The insolubility ofM3 residues at low pH does not facilitate analysesunder those conditions used for the permanentlycationic M1 or M2 homopolymers. Similarly, it isnot possible to easily modifyM1 orM2 residues tofacilitate analysis by organic SEC.

Given the inherent difficulty in analyzing thepolyampholytic materials via either organic oraqueous SEC, FTIR spectra were recorded for theM1–M3 homopolymers as well as examples of sta-tistical and block copolymers to demonstrate,qualitatively, the successful formation of thesenew polyampholytic materials. Figure 10 shows

the FTIR spectra of an M1 and M3 homopolymer(A and B) as well as examples of the statistical (C)and block (D) PAMs. Consider first 10(A): a polyM1 homopolymer. The key absorptions here arethose centered at ca. 3350, 3000–2900, 1700,1450–1350, and 950 cm�1, which can be attrib-uted to ��OH, aromatic C��H, aromatic combina-tion and overtones bands, PCH2�� and ��PCH3

(1450–1350 cm�1), and ��PCH3 (950 cm�1),although the PCH2�� and PCH3 (1450–1350cm�1) overlap with various absorptions associ-ated with carboxylic acid dimers/carboxylateanions which may also be present as end-groups.The key absorption for diagnostic purposes is the

Figure 10. FTIR spectra of (A) a polyM1 homopolymer, (B) a polyM3 homopolymer,(C) a poly(M1-M3) statistical copolymer, and (D) a poly(M1-M3) block copolymer.

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strong ��PCH3 absorption at 950 cm�1 that willclearly be associated only with the M1 residues.Figure 10(B) shows the spectrum for a polyM3homopolymer. Similarly, key absorptions includethe broad ��OH band centered at ca. 3400 cm�1,with a weak aromatic C��H next to it at �3000cm�1, and the triplet of strong bands at ca. 1650–1400 cm�1 which are attributed to the symmetricand asymmetric bands associated with the car-boxylate functionality and also the C��O��H inplane bend. It is this grouping of three bandswhich will serve as the key diagnostic featureassociated with the M3 residues. Figure 10(C,D)represents examples of M1–M3 statistical andblock polyampholytes respectively. Since we areunable to distinguish architecture from the spec-tra we would anticipate that the FTIR spectra ofthese two materials be essentially identical,which appears to be the case. Gratifyingly, inboth instances we can clearly identify those‘‘unique’’ absorptions associated with the individ-ual M1 and M3 species such as the distinctive��PCH3 band at ca. 950 cm�1 confirming the pres-ence ofM1 and the triplet of bands from ca. 1650–1400 cm�1 associated with M3. The presence ofabsorption bands associated with the M1 and M3residues in Fig. 10(C,D), thus qualitatively con-firms successful synthesis of both the novel statis-tical and block PAMs.

It should be noted that the presence of the thio-carbonyl end-groups is difficult to confirm viaFTIR spectroscopy since the characteristic C��Sand C¼¼S absorptions are both weak and can bevariable (C��S), occur in the fingerprint region,and overlap with other more intense absorptionsincluding the C��O stretch for example.

While M1 and M2 are permanently charged,M3 has a readily accessible pKa and is thus easilyreversibly ionized. Also, M3 is an example of a‘‘smart’’ building block in the sense that in thefree acid form it is hydrophobic whereas in theionized state it is hydrophilic. Such readily tuna-ble hydrophilicity/hydrophobicity can be exploitedin the preparation of stimulus-induced nanosizedself-assemblies in aqueous media. Indeed,M3 hasbeen previously employed in such a capacity.19,48

While we have not, at this time, conducted a thor-ough evaluation of the aqueous solution pro-perties of these new PAMs we have performedsome preliminary NMR spectroscopic experiments.NMR spectroscopy, and especially 1H NMR spec-troscopy, has proven to be a very powerful anduseful tool for evaluating the relative solvation ofcomponent building blocks of ‘‘smart’’ copolymers

in aqueous solution as a function of applied stim-ulus, including (but not limited to) changes in pH,temperature, and salt concentration. Unfortu-nately, 1H NMR spectroscopy proved to be of littleuse for these styrenic-based copolymers sincethere are no distinct resonances associated withthe M3 block, which can be conveniently moni-tored with changes in the solution pH. As such weexamined the 13C NMR spectra. However, this isalso problematic given its lower sensitivity andproblems associated with being able to prepare anaqueous solution of a copolymer at a sufficientlyhigh concentration to facilitate straightforwardanalysis. Figure 11 shows the 13C NMR spectra ofan M3/M1 block copolymer (molar ratio 1:1)recorded at pH 10.0 (A) and pH 2.0 (B) plottedbetween d ¼ 200 and d ¼ 100 ppm. Two points areworth noting. Firstly, the C¼¼O resonance associ-ated with the carboxylate is clearly evident inspectrum A at ca. d ¼ 175 ppm under conditionsof high pH when we would expect theM3 residuesto be ionized and hence hydrophilic and solvated.In contrast, at pH 2.0 (B), when the M3 residuesare fully protonated, the resonance associatedwith the C¼¼O are completely absent. Addition-ally, changing the solution pH from 10.0 to 2.0results in a broadening of the resonances associ-ated with the aromatic carbons. Both of these fea-tures are entirely consistent with a hydrophilic-to-hydrophobic phase transition association withthe M3 residues, and given the block structure itis reasonable to assume that this results in self-assembly yielding nanosized polymer aggregatessuch as micelles.

Figure 11. 13C NMR spectrum of a 1:1 M ratio ABdiblock copolymer of M1 with M3 recorded in water atpH ¼ 10.0 (A) and pH ¼ 2.0 (B).



SUMMARY AND CONCLUSIONS

Herein we have described the first example ofthe controlled, aqueous radical polymerizationof phosphonium, styrenic-based monomers byRAFT, indeed by any controlled-radical tech-nique, employing a water-soluble trithiocarbon-ate that we described recently. We have shownthat the characteristics of the M1 and M2 homo-polymerizations are entirely consistent with themproceeding in a controlled fashion as judged fromthe experimentally determined molecular charac-teristics of the resulting homopolymers. Addition-ally, M3 was also shown to polymerize in a con-trolled fashion in DMSO. Statistical copolymer-ization of M1 with M3 was readily achieved inwater at 10 wt % monomer yielding the firstexamples of statistical PAMs with a cationic phos-phonium building block. Also, the use of eitherpolyM1 or polyM2 macro CTAs facilitated thesubsequent block copolymerization of M3 to yieldthe first examples of block PAMs with a phospho-nium building block and only the second examplein which block PAMs have been prepareddirectly by RAFT and one of only a handful ofexamples in which materials have been preparedwithout the need for either protection/deprotec-tion chemistries or postpolymerization modifica-tion. The successful formation of the statisticaland block PAMs was proven qualitatively byFTIR spectroscopy. Finally, we demonstrated thepH-responsive nature of one of the AB diblockPAMs using 13C NMR spectroscopy.

A.B. Lowe thanks the US Department of Energy (DE-FC26-01BC15317) for financial support for this re-search and the MRSEC program of the National Sci-ence Foundation (DMR-0213883) and Avery-Dennisonfor funds enabling the purchase of the aqueous SEC.

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