PolymerChemistry

PAPER

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Institut Charles Gerhardt Montpellier UMR

Ingenierie et Architectures Macromolecula

Place Eugene Bataillon, 34095 Montpellier

univ-montp2.fr; Fax: +33-467144028; Tel: +

Cite this: Polym. Chem., 2013, 4, 3676

Received 2nd April 2013Accepted 16th April 2013

DOI: 10.1039/c3py00426k

www.rsc.org/polymers

3676 | Polym. Chem., 2013, 4, 3676–

RAFT polymerization of dimethyl(methacryloyloxy)-methyl phosphonate and its phosphonic acid derivative:a new opportunity for phosphorus-based materials

Benjamin Canniccioni, Sophie Monge,* Ghislain David and Jean-Jacques Robin

The RAFT polymerization of dimethyl(methacryloyloxy)methyl phosphonate (MAPC1) using dithioester

chain transfer agents is reported for the first time to our knowledge in the literature.

Poly(dimethyl(methacryloyloxy)methyl phosphonate) (PMAPC1) was synthesized in DMF at 70 �C with a

good control over the molecular weight, the latter ranging from 8000 to 24 000 g mol�1. Polymers

were characterized by 1H and 31P NMR, and size exclusion chromatography using both refractive index

and triple detection. The effect of the solvent was also investigated. We demonstrated that polarity had

an effect on the control of the polymerization as low polarity led to low polymerization rate and

termination reactions whereas very high polarity resulted in high polymerization rate but also transfer

reactions. Additionally, we showed that it was possible to restart the RAFT polymerization from a

PMAPC1 macrochain transfer agent. PMAPC1 was hydrolyzed to afford poly((methacryloyloxy)methyl

phosphonic acid) (hPMAPC1), whose pKa values were determined. Finally, we achieved the controlled

RAFT polymerization of the (methacryloyloxy)methyl phosphonic acid (hMAPC1). The easy obtaining of

PMAPC1 and hPMAPC1 opens the way to the synthesis of complex polymer architectures.

Introduction

These last years, phosphorus-containing polymers have attrac-ted considerable attention due to their very interesting prop-erties which led to their development for various applications.Indeed, such kind of materials were already notably employedas ame-retardants,1,2 corrosion inhibiting agents,3 to bindmetals,4–7 or in biomedical eld,8 and proved to be very efficient.In general, these polymers are mainly synthesized by UV9 orradical (co)polymerization,10,11 or by the introduction of aphosphorated moiety during a post-polymerization reaction.12

On the reverse, less examples dealt with the “living” radicalpolymerization (LRP) of phosphorus-based monomers whereasthese polymerization techniques allow the synthesis of well-dened polymers with tunable structure, compositions andproperties.13 Among all possible chemical environments ofphosphorus atom, some monomers containing phosphate,phosphorylcholine, phosphinic, and phosphonated functionswere evaluated. Polymerization of monoacryloxyethyl phos-phate (MAEP) and 2-(methacryloyloxy)ethyl phosphate (MOEP)by the reversible addition-fragmentation transfer (RAFT)process was successfully achieved, enabling the synthesis ofblock copolymers by association with a poly((2-acetoacetoxy)-

5253 CNRS-UM2-ENSCM-UM1 - Equipe

ires, Universite Montpellier II cc1702,

, France. E-mail: sophie.monge-darcos@

33-467144158

3685

ethyl methacrylate) segment.14 Atom transfer radical polymeri-zation of deprotonatedMOEP15 and dimethyl(1-ethoxycarbonyl)-vinyl phosphate16 was also carried out, leading to polymerbrushes formation on a gold surface and to block copolymers,respectively. 2-(Meth)acryloyloxyethyl phosphorylcholine (MPC)was polymerized by RAFT17–19 and ATRP.20 In the latter case, awide range of well-dened, biocompatible MPC-based diblockcopolymers were prepared in protic medium. RAFT polymeri-zation of a phenylphosphinic acid monomer led to diblockcopolymers which self-assembled in water21 or to phenyl-phosphinic acid-functionalized polystyrene microspheres by anemulsion process.22

Controlled radical polymerization of phosphonated/phos-phonic acid-based monomers was also studied. Among phos-phonated vinyl monomers, works reported in the literaturealmost exclusively dealt with the “living” radical polymerizationof dialkyl paravinylbenzyl phosphonate monomers. Indeed,diethyl23,24 and diisopropyl25,26 paravinylbenzyl phosphonatewere polymerized by ATRP, enabling the obtaining of protonconducting copolymers. Nitroxide mediated polymerization(NMP) was also successfully achieved on diethyl27 or dimethyl28

paravinylbenzyl phosphonate, in the presence of stable 2,2,5-trimethyl-4-phenyl-3-azahexane-2-nitroxide (TIPNO) or 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radicals, respectively.Polymerization of vinylphosphonic acid (VPA) was controlled inwater using an O-ethyl xanthate transfer agent, representing therst example of reversible deactivation radical polymerizationof a monomer bearing an unprotected phosphonic acid

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function.29 Aside from vinyl monomers, RAFT polymerizationof diethyl-2-(acrylamido)ethylphosphonate acrylamide-typemonomer (DAAmEP) proved to be controlled, using both2-cyano-2-propyl dodecyl trithiocarbonate or 2-(dodecylth-iocarbonothioylthio)-2-methylpropionic acid.30 Finally,controlled radical polymerization of dialkyl((meth)acryloyloxy)-alkyl phosphonate was rarely evaluated. ATRP of dimethyl(me-thacryloyloxy)methyl phosphonate (MAPC1) was performed butresulting polymers showed only low molecular weight polymers(less than 5000 g mol�1).3 The only example of RAFT polymer-ization of phosphonated (meth)acrylate was the terpolymeriza-tion of dimethyl-2-methacryloxyethylphosphonate (MAPC2)31 oranother phosphonated monomer with urethane linkage namedMAUPHOS12 with vinylidene chloride (VC2) and methyl acrylate(MA). In all cases, the phosphonated monomer was introducedin low concentrations in comparison with other monomers andas a result incorporation of the phosphonated moiety waslimited.

To date and to the best of our knowledge, a successful RAFThomopolymerization of a phosphonated (meth)acrylate mono-mer was never reported in the literature. Some results publishedon the polymerization of MAPC1 by RAFT or reverse iodinetransfer polymerization (RITP) in acetonitrile showed that RAFTpolymerization of this monomer was sensitive to very low level ofoxygen (limiting the conversion and causing the degradation ofthe dithioester moiety) and in any case only low monomerconversion was obtained. Additionally, even if reverse iodinetransfer polymerization (RITP) led to higher monomer conver-sion, a relatively low amount of living polymer (55% by 1H NMR)was obtained.32 This poor control was attributed to the presenceof the phosphonated group as the homopolymerization ofMAPC1 initiated by AIBN demonstrated that the kp/(kt)

1/2 wasalmost one order of magnitude lower than for the methyl meth-acrylate under the same conditions (0.04 and 0.17 L1/2mol 1/2 s1/2

for MAPC1 and MMA, respectively).33 This result suggested thatthe addition of an initiating radical (derived from the AIBN or thechain transfer agent) onto the MAPC1 should be more difficultthan on other methacrylate derivatives. As the addition rateconstant is relatively slow, a competitive reaction with traces ofoxygen in the medium is expected, producing hydroperoxidesand thus leading to the RAFT degradation. This side-reaction isenhanced by the low reactivity of MAPC1. This explanation wasconsistent with the limited conversions and the difficulties toprepare high molecular weight poly(dimethyl(methacryloyloxy)-methyl phosphonate) (PMAPC1) even in conventional radicalpolymerization.

In the present contribution, we report for the rst time in theliterature the controlled radical polymerization by RAFT of thedimethyl(methacryloyloxy)methyl phosphonate (MAPC1), amonomer whose synthesis requires less hazardous and cheaperreactants than most of the other phosphonated monomers.Homopolymerization was deeply investigated varying differentexperimental conditions. The effect of temperature, molecularweight, chain transfer agent, and solvent was determined.Phosphonated ester moieties were also hydrolyzed, leading tophosphonic acid groups. In parallel, the (methacryloyloxy)-methyl phosphonic acid (hMAPC1) was also polymerized by

This journal is ª The Royal Society of Chemistry 2013

RAFT. Well-dened poly(dimethyl(methacryloyloxy)methylphosphonate) and hydrolyzed derivatives open the way to thesynthesis of promising materials that could be used for manyapplications.

Experimental sectionMaterials

1,4-Dioxane, N,N-dimethylformamide (DMF), N,N-dimethylace-tamide (DMAc), dimethylsulfoxide (DMSO) were of organicsynthesis grade and used as received. Cyanoisopropyl dithio-benzoate (CIDB) and 4-cyanopentanoic acid dithiobenzoate(CPAD) were purchased from Sigma Aldrich and used asreceived. Dimethyl(methacryloyloxymethyl) phosphonate(MAPC1) and (methacryloyloxy)methyl phosphonic acid(hPMAPC1) were purchased from Specic Polymers (SP41-003,and SP41-007, respectively). 2,20-Azobisisobutyronitrile (AIBN)(Sigma Aldrich) was recrystallized twice from methanol.

Analytical techniques1H NMR spectra were recorded using a Bruker AC 200 withacetone-d6, deuterated water or chloroform as the solvent. Theconversion was measured by 1H NMR in deuterated chloroformcomparing the signals of the reactive double bond (5.62 and6.13 ppm) to the signals of the methylene in a-position ofthe phosphonated ester group at 4.14–4.47 ppm. For thedetermination of molecular mass distributions, a size exclusionchromatography (SEC) system (Varian 390-LC) comprising anauto-injector and a guard column (ResiPore, 50 � 7.5 mm)followed by two linear columns (ResiPore, 300 � 7.5 mm, 3 mmparticle size) and a differential refractive index detector wasemployed. DMF (+0.1% wt LiBr) was used as the eluent with aow rate of 1 mL min�1. The column temperature was set to60 �C. The SEC system was calibrated using narrow poly(methylmethacrylate) (PMMA) standards ranging from 690 to 1 944 000g mol�1 (EasyVial - Agilent). Same apparatus (Varian 390-LC)was used for the SEC triple detection analyses. This instrumentcontains the following three detectors: dual angle (15 and 90�)light scattering detector, four capillary bridge viscometer, anddifferential refractive index detector. Data acquisition andcalculations were performed using Cirrus Multi GPC/SEC so-ware. pH-metric titrations were performed in water with a EutecpH-meter (Instruments Ion 510) by the addition of an aqueous0.1 N sodium hydroxide (NaOH) solution.

RAFT polymerization of MAPC1 (Mn targeted ¼ 8000 g mol�1

at 80% conversion)

Dimethyl(methacryloyloxymethyl) phosphonate (MAPC1) (2 g,9.6 mmol), cyanoisopropyl dithiobenzoate (44.2 mg, 0.2 mmol),and 2,20-azobisisobutyronitrile (AIBN) (10.9 mg, 0.07 mmol)were added to DMF (7 mL) in a Schlenk tube under nitrogen.Themixture was degassed by four freeze–pump–thaw cycles andthen heated at 70 �C under nitrogen in a thermostatted oil bathfor an appropriate time. Samples were taken periodically forconversion and molecular weight analyses. The reaction wasstopped by quenching the solution in liquid nitrogen. The

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Scheme 1 Reaction pathway for (i) the RAFT polymerization of the dime-thyl(methacryloyloxy)methyl phosphonate (MAPC1) followed by the hydrolysis ofphosphonate into phosphonic acid groups and (ii) the RAFT polymerization of(methacryloyloxy)methyl phosphonic acid (hPMAPC1).

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resulting poly(dimethyl(methacryloyloxy)methyl phosphonate)(PMAPC1) was precipitated in cold hexane.

1H NMR (acetone-d6, 200 MHz) d (ppm): 8.0–7.2 (Haromatic),4.4–4.2 ppm (CH2–P), 4–3.7 ppm (OCH3), 2.2–1.9 (CH2 back-bone), 1.2–0.8 (CH3).

31P NMR (acetone-d6, 200 MHz) d (ppm): 21 (P(O)(OCH3)2).

RAFT polymerization of MAPC1 (Mn targeted ¼ 16 000 gmol�1 at 80% conversion)

The procedure was the same as for PMAPC1 (Mn targeted 8000 gmol�1) described above with the following quantities: dime-thyl(methacryloyloxymethyl) phosphonate (MAPC1) (2 g, 9.6mmol), cyanoisopropyl dithiobenzoate (22.1 mg, 0.1 mmol),2,20-azobisisobutyronitrile (AIBN) (5.47 mg, 0.03 mmol), andDMF (7 mL).

RAFT polymerization of MAPC1 (Mn targeted ¼ 24 000 gmol�1 at 80% conversion)

The procedure was the same as for PMAPC1 (Mn targeted 8000 gmol�1) described above with the following quantities: dime-thyl(methacryloyloxymethyl) phosphonate (MAPC1) (2 g, 9.6mmol), cyanoisopropyl dithiobenzoate (14.7 mg, 0.067 mmol),2,20-azobisisobutyronitrile (AIBN) (3.64 mg, 0.022 mmol), andDMF (7 mL).

RAFT polymerization of MAPC1 (Mn targeted ¼ 8000 g mol�1

at 80% conversion) using PMAPC1 as the macrochain transferagent

The procedure was the same as for PMAPC1 (Mn targeted 8000 gmol�1) described above with the following quantities: dime-thyl(methacryloyloxymethyl) phosphonate (MAPC1) (2 g,9.6 mmol), PMACP1 macrochain transfer agent (PMAPC1,Mn,NMR ¼ 8200 g mol�1) (1.64 mg, 0.2 mmol), 2,20-azobisiso-butyronitrile (AIBN) (10.9 mg, 0.07 mmol), and DMF (7 mL).

RAFT polymerization of hMAPC1 (Mn targeted ¼ 8000 g mol�1

at 80% conversion)

The procedure was the same as for PMAPC1 (Mn targeted 8000 gmol�1) described above with the following quantities: (meth-acryloyloxy)methyl phosphonic acid (hMAPC1) (1 g, 5.55 mmol),cyanoisopropyl dithiobenzoate (22.1 mg, 0.1 mmol), 2,20-azo-bisisobutyronitrile (AIBN) (5.47 mg, 0.03 mmol), and DMF(7 mL).

Typical procedure for the hydrolysis ofpoly(dimethyl(methacryloyloxymethyl) phosphonate)(PMAPC1), leading to the poly(methacryloyloxy)methylphosphonic acid (hPMAPC1)

Trimethylsilyl bromide (TMSBr) (5.47 mL, 37 mmol) was addedto a solution of poly(dimethyl(methacryloyloxymethyl) phos-phonate) (2 g, 0.24 mmol,Mn,NMR ¼ 8200 g mol�1) in anhydrousdichloromethane (20 mL). Aer stirring for 3 hours at roomtemperature, the mixture was concentrated under reducedpressure. Methanol (100 mL) was added and the mixture wasstirred for 1 hour at room temperature. The solvent was

3678 | Polym. Chem., 2013, 4, 3676–3685

evaporated and the product was dried to a constant weightunder vacuum.

1H NMR (D2O, 200MHz) d (ppm): 8.1–7.3 (Haromatic), 4.25–3.8(CH2–P), 2.3–1.4 (CH2 backbone), 1.15–0.5 (CH3).

31P NMR (D2O, 200 MHz) d (ppm): 15.4 (P(O)(OH)2).

pH-metric titration

Hydrolyzed poly(dimethyl(methacryloyloxymethyl) phospho-nate) (hPMAPC1) (125 mg) was dissolved in water (50 mL).Titration of the resulting solution is achieved using a 0.1 NNaOH solution. pH values were plotted as a function of thevolume of NaOH and pKa values were determined at the halfequivalence point for each titration.

Results and discussion

As previous results demonstrated the low reactivity of MAPC1,32

RAFT polymerization of MAPC1 was carried out in DMF as theuse of a polar solvent proved to enhance the rate of polymeri-zation in some cases, especially in the case of the ATRP process,due to ionic intermediates.34,35 Even if the mechanism of RAFTpolymerization is different and does not have charged transi-tion states, the solvent used proved to have an inuence,notably on the life-time of the intermediate radicals and/or oncross-termination reactions.36 More generally, polarity of thesolvent can inuence the free radical polymerization reactions,as shown for instance in the case of methyl methacrylate.37 Thereaction was performed at three different temperatures: 65, 70and 75 �C using rst cyanoisopropyl dithiobenzoate (CIDB) asthe chain transfer agent (CTA) in the presence of 2,20-azobisi-sobutyronitrile (AIBN) and the [CTA]/[AIBN] ratio was equal to3 (Scheme 1). The theoretical molecular weight targeted was8000 g mol�1.

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Table 1 Conversion, molecular weight data, and apparent rate constant (kapp) for the polymerization of dimethyl(methacryloyloxy)methyl phosphonate (MAPC1) fordifferent temperatures, molecular weights, and solvents. The chain transfer agent was cyanoisopropyl dithiobenzoate (CIDB), except for entries 6 and 7

EntryTemp.(�C) Solvent

Time(min)

Conv.a

(%) Mn,theob Mn,exp

c SEC/RI Mn,expd SEC/DLS Mn,exp

a 1H NMR Ðckapp(10�5 s�1)

1 65 DMF 510 75 7500 7900 8300 7900 1.25 4.982 70 DMF 400 76 7600 10 800 8500 8200 1.23 6.403 75 DMF 300 86 8600 10 600 8700 8900 1.37 11.084 70 DMF 460 77 12 300 15 600 13 050 13 100 1.33 5.705 70 DMF 520 73 17 500 23 100 18 800 18 300 1.35 4.776e 70 DMF 420 75 7500 9400 — — 1.28 6.327f 70 DMF 240 56 11 400 13 000 — 11 900 1.30 —8 70 DMAc 400 78 7800 10 300 8700 8200 1.34 6.789 70 1,4-Dioxane 480 50 5000 8100 — 5200 1.27 —10 70 DMSO 360 76 7600 7200 — 8400 1.33 —11 70 Water 270 90 9000 7300 — 8300 1.31 —

a Determined by 1H NMR. b Mn,theo¼ ([M]0/[I]0�Mw of themonomer� conv.)/100. c Estimated by PMMA-calibrated SEC at 60 �C in DMF (+LiBr 0.1%weight). d Determined by triple detection SEC at 60 �C in DMF (+LiBr 0.1% weight). e Experiment achieved using 4-cyanopentanoic aciddithiobenzoate (CPAD) as the chain transfer agent. f Experiment achieved using poly(dimethyl(methacryloyloxy)methyl phosphonate) (PMAPC1,Mn,SEC ¼ 9100 g mol�1; Mn,NMR ¼ 8200 g mol�1) as the chain transfer agent.

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Obtained results are gathered in Table 1. Samples wereperiodically taken from the reaction mixture to follow thecontrol of the polymerization. The conversion was measuredfrom 1H NMR in deuterated chloroform by using the signalintensities of both the reactive double bond (5.62 and 6.13 ppm)and the methylene protons in a-position of the phosphonatedgroup (4.14–4.47 ppm). Molecular weights and dispersity (Ð)were determined from size exclusion chromatography (SEC) inDMF at 60 �C.

Fig. 1 shows (i) the evolution of the ln([M]0/[M]) as a functionof time and (ii) the evolution of the molecular weight and thedispersity as functions of the conversion. RAFT polymerizationof MAPC1 gave rst order kinetic plots at all temperatures(Table 1, entries 1–3), and high conversions were reached in allcases, between 75 and 86%.

As expected, the reaction went on faster at highertemperature, the apparent rate constant (kapp) being equal to11.08 � 10�5, 6.40 � 10�5, and 4.98 � 10�5 s�1 at 75, 70, and65 �C, respectively. These values were similar to the kapp

Fig. 1 (Left) Pseudo-first order kinetic plot and (right) number-average moleculardimethyl(methacryloyloxy)methyl phosphonate (MAPC1) in DMF at different tempertransfer agent and AIBN as the initiator. [MAPC1]/[CIDB]/[AIBN] ¼ 137/3/1.

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obtained in the case of the polymerization of methyl meth-acrylate in DMF at 60 �C, using CIDB and AIBN as the chaintransfer agent and initiator, respectively. Indeed, in this lattercase, the apparent rate constant was equal to 3.75 � 10�5 s�1,for Mn ¼ 64 000 g mol�1.38 The induction period was observedat 65 and 70 �C, as oen reported in the case of RAFT poly-merization.36 This was explained by both slow fragmentation39

and/or intermediate radical40 model(s).41 The induction periodlogically decreased with the increase of the temperature. Thenumber-average molecular weights showed a relatively goodagreement with those expected from the correspondingmonomer/chain transfer agent ratio. The relative differencebetween both theoretical and experimental values was attrib-uted to the use of size exclusion chromatography in DMF(+0.1% wt LiBr) with PMMA calibration. Fig. 2 shows themolar mass evolution at various reaction times at 70 �C.Whatever the conversion, monomodal peaks were obtained,conrming that polymerization was perfectly controlled atthis temperature.

weight and dispersity (Ð) versus conversion plot for the RAFT polymerization ofatures (65, 70, and 75 �C) using cyanoisopropyl dithiobenzoate (CIDB) as the chain

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Fig. 2 Size exclusion chromatography traces of dimethyl(methacryloyloxy)methyl phosphonate (MAPC1) polymerization in DMF at 70 �C at different timesof the reaction. [MAPC1]/[CIDB]/[AIBN] ¼ 137/3/1. SEC was performed in DMF,using PMMA standards.

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Aer purication by precipitation in cold hexane, the molec-ular weight of resulting poly(dimethyl(methacryloyloxymethyl)phosphonate) (PMAPC1) was determined by SEC using bothrefractive index detector and triple detection combining dualangle (15 and 90�) light scattering detector, viscometer, anddifferential refractive index detector. The dispersities were quitenarrow, Ð varying from 1.23 to 1.37, depending on the tempera-ture.Molecularweights determinedby triple detectionwere closeto theoretical ones. In addition, a very good agreement wasobtained when molar mass was calculated from the 1H NMRspectrum, by comparison of protons of the aromatic ring comingfrom the cyanoisopropyl dithiobenzoate chain transfer agent and

Fig. 3 1H NMR spectra of poly(dimethyl(methacryloyloxy)methyl phosphonate)(PMAPC1) (in acetone-d6) (A) and the resulting hydrolyzed PMAPC1, namely thepoly((methacryloyloxy)methyl phosphonic acid) (hPMAPC1) (in deuterated water)(B). [MAPC1]/[CIDB]/[AIBN] ¼ 137/3/1.

3680 | Polym. Chem., 2013, 4, 3676–3685

the methyl ester group of the phosphonate function, at 8.0–7.2and 4–3.7 ppm, respectively (Fig. 3A).

The 1H NMR spectroscopy was carried out in deuteratedacetone. Broad peaks at 2.2–1.9 and 0.8 ppm were attributed toprotons of the polymer backbone (CH2 and CH3 groups,respectively) whereas signals at 4.4–4.2 ppm corresponded tothe methylene in a-position of the phosphorus atom. 31P NMRin deuterated acetone showed a broad signal at 21 ppm, char-acteristic of a phosphonate ester group.

All experimental results permitted us to conclude that poly-merization of dimethyl(methacryloyloxymethyl) phosphonatewas controlled. 70 �C was the most appropriate temperature asit allowed a quite rapid polymerization (76% conversion aer400 minutes) with a low induction period (25 minutes). Themolecular weight was controlled as we managed to preparePMAPC1 with experimental Mn, determined by 1H NMR, equalto 8200 g mol�1 whereas the theoretical value was 7600 g mol�1.The dispersity was low (Ð ¼ 1.23).

Then, predetermined experimental conditions (i.e. cyanoi-sopropyl dithiobenzoate as the chain transfer agent, in DMF at70 �C) were used to prepare higher molar masses of poly-(dimethyl(methacryloyloxymethyl) phosphonate). Two differentmolecular weights were targeted: 16 000 and 24 000 g mol�1, at80% conversion. In both cases, high conversion was reached,around 75% aer 7–8 hours (Table 1, entries 4 and 5). Apparentrate constant logically decreased with the increase of themolecular weight but remained reasonable compared to thekapp obtained for the RAFT polymerization of MMA under verysimilar conditions.38 The induction period was similar to theone observed for lower molecular weight, around 25 minutes(Fig. 4, le). The evolution of the molecular weight versusconversion (Fig. 4, right) was linear, indicating that all chainsgrew simultaneously.

PMAPC1 was precipitated in cold hexane. Experimentalmolecular weights were close to theoretical ones and the dis-persity was low (1.33 and 1.35 for PMAPC1Mn,theo ¼ 16 000 and24 000 g mol�1, respectively). So, it was possible to prepare poly-(dimethyl(methacryloyloxymethyl) phosphonate) of differentmolecular weights with a very good control, as shown in Fig. 5,reporting the size exclusion chromatography traces of the puri-ed PMAPC1 of Mn ranging from 8000 to 24 000 g mol�1.

The same polymerization reaction was carried out withanother chain transfer agent, namely the 4-cyanopentanoic aciddithiobenzoate (CPAD) (Table 1, entry 6). Polymerization wasachieved in DMF, at 70 �C, with a monomer/CPAD ratio equal to3. The targeted molecular weight was 8000 g mol�1. Experi-mental results were similar to those obtained with the cyanoi-sopropyl dithiobenzoate (CIDB), with a similar apparent rateconstant (kapp ¼ 6.32 � 10�5 s�1 and 6.40 � 10�5 s�1 usingCPAD and CIDB, respectively). Experimental molecular weight,determined by SEC in DMF, was close to the theoretical one anddispersity was low (Ð ¼ 1.28). To conclude, no signicantdifferences were noticed in the control of the polymerizationusing CPAD or CIDB. The interest of the 4-cyanopentanoic aciddithiobenzoate is the presence of the acid function that can beused for further functionalization of the polymer. Additionally,it is interesting to notice the inuence of the solvent used on the

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Fig. 4 (Left) Pseudo-first order kinetic plot and (right) number-average molecular weight and dispersity (Ð) versus conversion plot for the RAFT polymerization ofdimethyl(methacryloyloxy)methyl phosphonate (MAPC1) in DMF at 70 �C using cyanoisopropyl dithiobenzoate (CIDB) as the chain transfer agent and AIBN as theinitiator. Three different molecular weights were targeted:Mn,theo ¼ 8000, 16 000, and 24 000 g mol�1 at 80% conversion, [MAPC1]/[CIDB]/[AIBN]¼ 137/3/1, 320/3/1, and 480/3/1, respectively.

Fig. 5 Size exclusion chromatography traces of poly(dimethyl(methacryloyloxy)methyl phosphonate) (PMAPC1) with three different molecular weights targeted:Mn,theo¼ 8000, 16 000, and 24 000 gmol�1 at 80% conversion, [MAPC1]/[CIDB]/[AIBN] ¼ 137/3/1, 320/3/1, and 480/3/1, respectively. SEC was performed inDMF, using PMMA standards.

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control of the RAFT polymerization of the MAPC1 monomer inthe presence of CPAD as the chain transfer agent. Indeed,polymerization in acetonitrile was limited (only 24% conver-sion)32 whereas 75% conversion was reached in DMF.

To attempt to understand such a result, the effect of thesolvent on the polymerization of MAPC1 was deeply investi-gated. Polymerization of dimethyl(methacryloyloxymethyl)phosphonate was achieved in different polar solvents to ensurethat the rate of polymerization would remain high. Fourdifferent solvents were evaluated: N,N-dimethylacetamide(DMAc), 1,4-dioxane, dimethylsulfoxide (DMSO), and water(Fig. 6). The polymerization reaction was achieved at 70 �C in allcases, and the theoretical molecular weight was equal to 8000 gmol�1 at 80% conversion (Table 1, entries 8–11).

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Different behaviors were obtained as a function of thesolvent. RAFT polymerization in DMAc was quite similar to theone in DMF. Conversion of 78% was obtained aer 400minutes, and the induction period was equal to 25 minutes.Molecular weights increased linearly as a function of theconversion and dispersities remained low during the RAFTprocess, between 1.28 and 1.35. Aer precipitation in coldhexane, the molecular weight of the obtained PMAPC1 deter-mined by 1H NMR was 8200 g mol�1, i.e. close to the theoreticalone, equal to 7800 g mol�1, and the dispersity was 1.34. Theseresults were not surprising as DMAc shows a similar polarity toDMF, due to its very close chemical structure. Noteworthily,different behaviors were obtained with the other chosensolvents. When 1,4-dioxane was used, no problem of solubilitywas noticed and polymerization went on slowly in comparisonwith DMF or DMAc and the ln([M]0/[M]) decreased from 50%conversion, indicating that termination reactions occurred.Thus, in this case, polymerization was stopped at 50% conver-sion, leading to PMAPC1 with control of the molecular weightand a low dispersity. Nevertheless, we can assume that it isimpossible to synthesize high molecular weight polymers usingsuch solvents. RAFT polymerization of MAPC1 in DMSO andwater led to the same conclusions for both solvents. The reac-tion proceeded faster than for previously considered solvents(DMF, DMAc, 1,4-dioxane), the highest rate of polymerizationbeing obtained in water. Unfortunately, both solvents led totransfer reactions, as shown in the evolution of the molecularweight versus conversion plot. As a consequence, molecularweights were not controlled. Nevertheless, dispersities were stillreasonable aer precipitation.

The differences of kinetics and control obtained in theconsidered solvents could be explained by their polarity. It hasbeen shown in the literature that polarity could enhance therate of polymerization, in the case of free radical or telomeri-zation processes.37,42 But a study on RAFT polymerization ofmethyl methacrylate using trithiocarbonate chain transferagents demonstrated that solvent polarity had no observable

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Fig. 6 (Left) Pseudo-first order kinetic plot and (right) number-average molecular weight and dispersity (Ð) versus conversion plot for the RAFT polymerization ofdimethyl(methacryloyloxy)methyl phosphonate (MAPC1) in different solvents at 70 �C using cyanoisopropyl dithiobenzoate (CIDB) as the chain transfer agent and AIBNas the initiator. [MAPC1]/[CIDB]/[AIBN] ¼ 137/3/1.

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effect on the rate of polymerization and over the control ofmolecular weight distribution.43 In the present contribution, itappeared that polarity had an inuence on both the rate and thecontrol of the polymerization. Polarity increases in thefollowing order: 1,4-dioxane < N,N-dimethylformamide,N,N-dimethylacetamide < dimethylsulfoxide < water. A lowerrate of polymerization was obtained in the less polar solvent(1,4-dioxane), then polymerization of MAPC1 was controlled inDMF and DMAc with good apparent rate constants, and nally,in the case of the highly polar solvents (DMSO and water), thereaction took place very rapidly but transfer reactions occurred.This means that RAFT polymerization of dimethyl(methacry-loyloxymethyl) phosphonate was controlled when a polarsolvent was used but polarity need not have to be too high tokeep the control.

Fig. 7 Number-average molecular weight and dispersity (Ð) versus conversionplot for the RAFT polymerization of dimethyl(methacryloyloxy)methyl phospho-nate (MAPC1) in DMF at 70 �C using poly(dimethyl(methacryloyloxy)methylphosphonate) (PMAPC1) as the macrochain transfer agent and AIBN as theinitiator (Table 1, entry 6). PMAPC1 macrochain transfer agent: Mn,SEC ¼ 9100 gmol�1, Ð ¼ 1.31;Mn,NMR ¼ 8200 g mol�1. [MAPC1]/[PMAPC1]/[AIBN] ¼ 137/3/1.

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The “living” character of the synthesized poly(dimethyl-(methacryloyloxymethyl) phosphonate) had to be checked. Toensure that the polymer was end-functionalized by a dithioesterfunction coming from the chain transfer agent, PMAPC1(Mn,SEC ¼ 9100 g mol�1; Mn,NMR ¼ 8200 g mol�1) was used as amacrochain transfer agent. RAFT polymerization was achievedin DMF at 70 �C. RAFT polymerization of MAPC1 gave a rstorder kinetic plot (Table 1, entry 6), and conversion was equal to56% aer four hours. Fig. 7 shows the evolution of the molec-ular weight and the dispersity (Ð) as functions of theconversion.

The evolution of the molecular weight versus conversion waslinear, indicating that all the chains grew simultaneously.Molecular weight (determined by 1H NMR) and dispersity(determined by SEC) of the precipitated PMAPC1 “copolymer”were 11 900 g mol�1 and 1.30, respectively. Experimental molarmass was close to the theoretical one, indicating that thePMAPC1 macrochain transfer agent reacted with the MAPC1

Fig. 8 Size exclusion chromatography traces of the PMAPC1 macrochaintransfer agent and the resulting PMAPC1 “copolymer” after RAFT polymerizationof MAPC1 in DMF at 70 �C. SEC was performed in DMF, using PMMA standards.

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Fig. 9 Regression curves allowing the determination of both pKa (pKa1 ¼ 2.75and pKa2 ¼ 8.2). [MAPC1]/[CIDB]/[AIBN] ¼ 137/3/1. Mn,theo ¼ 8000 g mol�1 at80% conversion, Mn,NMR ¼ 8200 g mol�1.

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monomer. This point was conrmed by SEC analysis as thePMAPC1 “copolymer” showed a monomodal trace (Mn,SEC ¼13 000 g mol�1) (Fig. 8). This result was of prime importance asit indicated that it was possible to synthesize complex archi-tectures such as diblock copolymers, for instance, from a poly-(dimethyl(methacryloyloxymethyl) phosphonate) macrochaintransfer agent.

Hydrolysis of the phosphonated ester groups was alsocarried out as phosphonic acid-containing polymers proved toexhibit important properties.31 PMAPC1 prepared in DMF withcyanoisopropyl dithiobenzoate as the chain transfer agent washydrolyzed in the presence of trimethylsilyl bromide rst, forthree hours at room temperature, followed by addition ofmethanol, as already reported in the literature.44,45 Resultinghydrolyzed poly(dimethyl(methacryloyloxymethyl) phospho-nate) (hPMAPC1) was characterized by 1H NMR (Fig. 3B) indeuterated water which showed the disappearance of the signalcorresponding to the protons of the methyl associated tothe phosphonated ester groups. Protons of the methylene in

Fig. 10 (Left) Pseudo-first order kinetic plot, number-average molecular weightacryloyloxy)methyl phosphonic acid (hMAPC1) in DMF at 70 �C using cyanoisopropytargeted molecular weight was Mn,theo ¼ 8000 g mol�1 at 80% conversion, [hMAPCresulting hPMAPC1. SEC was performed in DMF, using PMMA standards.

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a-position of the phosphorus atom were found at 4.25–3.8 ppmin the phosphonic acid form. Other chemical shis remainedunchanged. In particular, we noticed the presence of aromaticprotons between 7.3 and 8.2 ppm due to the chain transferagent moiety, indicating that the hydrolysis was selective, aspreviously reported in the literature, not affecting the thioesterfunction.46 31P NMR also proved that hydrolysis was successfulas a broad signal was obtained at 15.4 ppm, corresponding tothe chemical shi of the phosphorus atom in the phosphonicacid group. The pH titration of hydrolyzed poly(dimethyl-(methacryloyloxymethyl) phosphonate) enabled determinationof the dissociation constant of the phosphonic diacid group.The values of pKa1 and pKa2, associated to the rst and thesecond acidity, respectively, corresponded to the half equiva-lence point for each titration measurement. Titration permittedus to determine that pKa1 and pKa2 were equal to 2.6 and 8.2,respectively. These values enabled us to plot the correspondingregression curves showing the relative amount of each phos-phonic acid dissociation state as a function of the pH (Fig. 9).

Finally, RAFT polymerization of the hydrolyzed MAPC1,namely the (methacryloyloxy)methyl phosphonic acid(hMAPC1), was performed, using predetermined experimentalconditions. Thus, the reaction was carried out in DMF at 70 �C,using cyanoisopropyl dithiobenzoate (CIDB) as the chaintransfer agent (CTA) and 2,20-azobisisobutyronitrile (AIBN) andthe [CTA]/[AIBN] ratio was equal to 3 (Scheme 1). The theoreticalmolecular weight targeted was 8000 g mol�1. Fig. 10 (le) shows(i) the evolution of the ln([M]0/[M]) as a function of time and (ii)the evolution of the molecular weight and the dispersity (Ð) asfunctions of the conversion. RAFT polymerization of hMAPC1gave a rst order kinetic plot, and 79% conversion was reached.The induction period, equal to 50 minutes, was higher than forthe RAFT polymerization of MAPC1.

The number-average molecular weights showed a goodagreement with those expected from the corresponding mono-mer/chain transfer agent ratio. Aerpuricationbyprecipitationin cold hexane, the molecular weight of resulting poly-((methacryloyloxy)methyl phosphonic acid) was determined by

and dispersity (Ð) versus conversion plot for the RAFT polymerization of (meth-l dithiobenzoate (CIDB) as the chain transfer agent and AIBN as the initiator. The1]/[CIDB]/[AIBN] ¼ 167/3/1. (Right) Size exclusion chromatography traces of the

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SEC in DMF (+0.1% wt LiBr) with PMMA calibration using arefractive index detector and by 1H NMR comparing the protonsof the aromatic ring coming from the cyanoisopropyl dithio-benzoate chain transfer agent and the protons of the methylenein a-position of the phosphorus atom, at 8.0–7.2 and 4.25–3.8ppm, respectively (Fig. 3B). Molar masses determined by bothtechniques were similar (Mn,SEC ¼ 8100 and Mn,NMR ¼ 8000 gmol�1) whereas dispersity was equal to 1.32. Size exclusionchromatography trace was monomodal (Fig. 10 right), conrm-ing the good control of the polymerization. Finally, 31P NMR indeuterated water showed only one signal at 15.4 ppm, i.e. atthe same shi as the one obtained in the case of thehydrolyzed PMAPC1prepared in two steps (RAFT polymerizationofMAPC1 followed by the hydrolysis). This result proved that thephosphonic acid function was stable during the RAFT process.So, we were able to prepare well-dened poly((methacryloyloxy)-methyl phosphonic acid) in one step. This result is valuable dueto the strong acidity of the phosphonic acid groups whichnotably leads to an interesting self-etching property, forinstance.47

Conclusions

In the present contribution, we reported for the rst time in theliterature the RAFT polymerization of a phosphonated methac-rylate, namely the dimethyl(methacryloyloxymethyl) phospho-nate (MAPC1). Different experimental parameters were studied.The reaction was rst carried out in DMF at different tempera-tures. 70 �C was chosen as it allowed a good control over themolecular weight, with a relatively high rate of polymerizationand a short induction period. Two chain transfer agents weresuccessfully used: cyanoisopropyl dithiobenzoate and 4-cyano-pentanoic acid dithiobenzoate. Then, we also proved that it waspossible to synthesize PMAPC1 of different molecular weights,ranging from 8000 to 24 000 g mol�1. The effect of solvent wasevaluated. Among the other polar solvents used, we demon-strated that it was possible to control the polymerization in N,N-dimethylacetamide whereas 1,4-dioxane led to terminationreactions, and dimethylsulfoxide andwater to transfer reactions.This result was explained by the polarity of the different solvents.To be well controlled, polarity has to be high enough to ensure agood rate of polymerization without being too high to avoidtransfer. We checked the “living” character of the producedpoly(dimethyl(methacryloyloxymethyl) phosphonate). Wedemonstrated that the polymer end-group was a dithioesterfunction, which allowed growth of the macromolecular chainsfrom the PMAPC1 macrochain transfer agent. This point is ofprime importance because it indicates that it is possible toprepare complex architectures as block copolymers. Finally,PMAPC1 was also hydrolyzed to afford phosphonic acid func-tions that can be useful for many applications and, in parallel,RAFT polymerization of (methacryloyloxy)methyl phosphonicacid was also successfully performed.

To conclude, these well-dened poly(dimethyl(methacryloy-loxymethyl) phosphonate) (PMAPC1) and poly((methacryloy-loxy)methyl phosphonic acid) (hPMAPC1) could be associatedwith other polymer segments. This possibility opens the way to

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numerous chemical structures, and thus, to the synthesis ofinnovative materials.

Acknowledgements

The authors would like to thank the French Ministry ofEducation and Research for a grant (B. C.).

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