Direct Synthesis of Controlled Poly(styrene-co-acrylic acid)s of Various Compositions by Nitroxide-Mediated Random Copolymerization

Direct Synthesis of Controlled Poly(styrene-co-acrylic

acid)s of Various Compositions by Nitroxide-Mediated

Random Copolymerization

Laurence Couvreur,1 Bernadette Charleux,*1 Olivier Guerret,2 Stephanie Magnet2

1 Laboratoire de Chimie des Polymeres, UMR 7610, associee au CNRS, Universite Pierre et Marie Curie, Tour 44, 1er etage,4, place Jussieu – 75252 PARIS Cedex 05, FranceFax: 0033 1 44 27 70 89; E-mail: [email protected]

2 ATOFINA, Groupement de Recherches de Lacq, B.P. n8 34, 64170 LACQ, France

Received: May 12, 2003; Revised: September 3, 2003; Accepted: September 5, 2003; DOI: 10.1002/macp.200350065

Keywords: copolymerization; living polymerization; nitroxide; radical polymerization

Introduction

With the advent of controlled radical polymerization

(CRP),[1–3] the field of free-radical polymerization has con-

siderably extended towards the synthesis of well-defined

homopolymers and copolymers with various architec-

tures.[4–6] Because the free-radical polymerization process

is particularly attractive from the industrial viewpoint, the

expectation is that the CRP technique might replace, to

some extent, the ionic polymerizations that are the current

methods of choice to elaborate precise structures, but which

require drastic synthetic conditions.[7] More interestingly,

some monomers can only polymerize via a free-radical

process. This is particularly the case with hydrophilic and

ionic monomers, which find significant applications in the

synthesis of water-soluble and amphiphilic (co)polymers.

Well-defined macromolecular structures derived from such

monomers cannot be obtained by a direct ionic polymer-

ization but by various indirect synthetic strategies, inclu-

ding protection of the polar functionality or chemical

modification of a hydrophobic (co)polymer precursor.

These procedures are rather expensive and cannot always

Full Paper: Styrene and acrylic acid were copolymerizedunder controlled conditions, in 1,4-dioxane solution at 120 8Cand 2 bar, using an alkoxyamine initiator based on the N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl)nitroxide,SG1. A broad composition range from 90/10 to 10/90 wasinvestigated. With slightly different initiator concentrationsand a similar initial proportion of free SG1 (4.5 mol-% withrespect to the initiator) the polymerizations exhibited verysimilar rates, irrespective of the proportion of acrylic acid inthe comonomer mixture (80% conversion within 8 h). Inall cases, the copolymers presented number average molarmasses, Mn, that increased linearly with overall monomerconversion, and polydispersity indexes that ranged between1.2 and 1.4. Moreover, Mn followed the calculated values,based on the initial concentrations of monomers and initiator.The variation in the initiator concentration allowed to targetvarious molar masses, but some limitation appeared at lowinitiator concentration owing to chain transfer to 1,4-dioxane.From the kinetic data, the reactivity ratios were determined:rA¼ 0.27� 0.07 for acrylic acid and rS¼ 0.72� 0.04 forstyrene. Depending on the initial comonomer composition,chains exhibited no or small composition drift, and hence aslightly pronounced gradient structure.

Reactivity ratios for acrylic acid and styrene.

Macromol. Chem. Phys. 2003, 204, 2055–2063 2055

Macromol. Chem. Phys. 2003, 204, No. 17 DOI: 10.1002/macp.200350065 � 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

be applied. Thus, when possible, the best strategy remains

the direct (co)polymerization of the selected monomer(s).

This is now theoretically achievable with CRP, because

free-radicals are tolerant to a great variety of functionalities,

including ionic and polar ones.[8,9]

An example of monomer that is particularly important

for industry is acrylic acid (A), for which only free-radical

polymerization is appropriate. The incorporation of poly-

(acrylic acid) sequences in a controlled copolymer (i.e.

predictable molar mass, narrow molar mass distribution and

precise architecture) was initially done by polymerizing

tert-butyl acrylate anionically, followed by hydrolysis of

the ester groups.[10] Although this might appear unneces-

sary, such an approach was also applied with CRP.[11–15]

Indeed, early works on CRP of acrylic acid showed that this

monomer was actually rather reluctant in both ATRP (atom

transfer radical polymerization) and NMP (nitroxide-

mediated polymerization).[8] With ATRP, some incompat-

ibility with the transition metal complex catalyst was

suspected;[16] with NMP, the acidic group was supposed to

be involved in side reactions with the nitroxide,[17] although

no mechanism has ever been suggested. Even though

unwanted chemical reactions were usually invoked, it is

worth noting that acrylic acid also exhibits a very high rate

constant of propagation in free-radical polymerization[18]

together with a propensity for self-initiation,[19] both cap-

able of ruining the quality of control.

Nevertheless, successful CRP of this monomer was

reported in a small number of papers[8] with the synthesis of

well defined homopolymers and/or various types of

copolymer architectures.

So far, RAFT (reversible addition-fragmentation trans-

fer)[20] was the only CRP technique able to yield controlled

homopolymers of acrylic acid as well as block copolymers

with poly(acrylic acid) sequences. The first example of

homopolymerization, performed in dimethylformamide

solution at 60 8C, was reported by Chiefari et al.,[20] but

no detailed information was provided. They also briefly

reported the synthesis of poly(butyl acrylate)-block-poly-

(acrylic acid) block copolymers by the same technique.[21]

Ladaviere et al.[22] examined a series of RAFT agents to

control the free-radical polymerization of acrylic acid in

alcohol or water solution, and they came to the conclusion

that phenoxyxanthates and trithiocarbonates were particu-

larly well-suited. Recently, the same group completed this

work and showed, as an application, that low polydispersity

poly(acrylic acid)s were very efficient dispersants of inor-

ganic particles.[23] They also described the application of

RAFT to the synthesis of controlled poly(butyl acrylate)-

block-poly(acrylic acid) block copolymers, and their use as

stabilizers in emulsion polymerization.[24] The controlled

homopolymerization of acrylic acid was also performed at

room temperature in the presence of dibenzyltrithiocar-

bonate, under 60Co irradiation.[25] The MADIX process

(macromolecular design via the interchange of xanthates)

was successfully used in aqueous solution to prepare a well-

defined poly(acrylic acid) homopolymer and hydrophilic

copolymers based on acrylamide and acrylic acid.[26] A few

other examples of copolymers bearing a poly(acrylic acid)

sequence and synthesized using the RAFT technique can be

found in the literature.[27–30]

Graft copolymers with poly(sodium acrylate) branches

were prepared via cyanoxyl-mediated free-radical poly-

merization of sodium acrylate starting from polystyrene

precursors with pendant arene-diazonium salts.[31]

Random copolymers of acrylic acid and butyl acrylate

were prepared in bulk via nitroxide-mediated polymeriza-

tion using the 2,2,5-trimethyl-4-phenyl-3-azahexane-3-

nitroxide.[32] The copolymerizations were controlled below

50 mol-% of the acidic comonomer, whereas for larger

proportions the control became quite poor. For instance, the

polydispersity index increased from 1.26 to 1.55 when the

proportion of Awas raised from 20 to 50 mol-%. Also using

NMP, sodium acrylate was polymerized in water-solution,

using poly(sodium 4-styrenesulfonate) macroinitiators

end-capped with water-soluble nitroxides, but the con-

trolled behavior was not demonstrated.[33]

The purpose of this article is to demonstrate that, under

selected conditions, acrylic acid can be copolymerized with

styrene (S) in a controlled way, using nitroxide-mediated

polymerization. The nitroxide used was the N-tert-butyl-

N-(1-diethylphosphono-2,2-dimethylpropyl), also called

SG1, which proved to be better suited than TEMPO

(2,2,6,6-tetramethylpiperidinyl-1-oxy) to control the poly-

merization of monomers other than styrene and deriva-

tives.[34,35] A broad composition range was investigated

from 10/90 to 90/10 A/S molar ratios. The effect of

composition on control over molar mass and distribution

and on kinetics was studied and discussed. It is the first time

that such controlled copolymers with broad composition

range are synthesized in a direct way, which is very pro-

mising for a variety of applications. This study is part of a

more general work devoted to SG1-mediated polymeriza-

tion of acrylic acid and we recently reported the controlled

homopolymerization and its limitation.[36,37]

Experimental Part

Materials

Styrene (S, Aldrich, 99% purity) was distilled under vacuumbefore use. Acrylic acid (A, purest grade, Atofina, stabilizedwith 200 ppm of hydroquinone) was stored at roomtemperature and used without further purification. Thealkoxyamine initiator (SG1-based alkoxyamine derived frommethyl acrylate, CH3–O–C( O)–CH(CH3)–SG1, Monams,96% purity) and the N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide (SG1, 86.5% purity) were kindlysupplied by Atofina. The solvent 1,4-dioxane (from SDS,synthesis grade) was used as received. The methylation agent,

2056 L. Couvreur, B. Charleux, O. Guerret, S. Magnet

Macromol. Chem. Phys. 2003, 204, No. 17 www.mcp-journal.de � 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

trimethylsilyldiazomethane (2 M solution in hexane) wassupplied by Aldrich and used as received.

Solution Copolymerizations of Styrene and Acrylic Acid

The copolymerization reactions were carried out in a Parrreactor of 300 mL, thermostatted at 120 8C, under a 2 barnitrogen atmosphere. The stirring rate was 300 rpm. In a typicalrecipe (Expt. 3), the Monams (1.731 g, 4.5� 10�3 mol) andSG1 (0.0610 g, 2.1� 10�4 mol, 4.5 mol-% with respect toMonams) were dissolved in styrene (30.01 g, 0.289 mol) andsolvent (1,4-dioxane, 143 mL). Then, acrylic acid (20.70 g,0.287 mol) was added to the mixture. Deoxygenation wasperformed by nitrogen bubbling for 30 min. Afterwards, thepolymerization solution was transferred into the reactor(already heated at 120 8C) and a 2 bar pressure of nitrogenwas applied. Time zero of the reaction was arbitrarily setwhen the mixture reached 110 8C. Samples were periodicallywithdrawn (on the top of the reactor) and cooled in an icedwater bath to stop the polymerization. Polymers were reco-vered by drying in a ventilated oven at 35 8C for 2 d. Aftermodification (next section) they were analyzed by sizeexclusion chromatography (SEC) in THF solution. Allperformed experiments are collected in Table 1.

Chemical Modification of the Copolymers

For size exclusion chromatography, the polymers weremodified by methylation of the carboxylic acid groups, usingtrimethylsilyldiazomethane.[38] In this way, 50 mg of eachsample was dissolved in a mixture of THF and water (overallvolume¼ 10 mL; the proportion of water was increased withthe amount of acid functions, to get solubilization at roomtemperature). The yellow solution of trimethylsilyldiazo-methane was added dropwise at room temperature into thecopolymer solution. Upon addition, bubbles appeared and thesolution became slowly colorless. Addition of the methylationagent was continued until the solution remained yellow andstopped bubbling. Then, an excess of methylation agent wasadded and the solution was stirred for 3 more hours at roomtemperature. As verified by 1H NMR analysis of the copoly-mers, methyl ester formation was always quantitative.

Analytical Techniques

The overall conversions were determined by gravimetry usingan automatic thermic balance (Mettler Toledo, HG53) at125 8C (evaporation till constant weight, approximately for15 min). This technique gave access to a weight conversion(xwt), that can be written as a function of the individualconversions xA and xS and of the weight fraction of eachmonomer in the initial mixture (wA0 and wS0 for acrylic acidand styrene respectively) (Equation (1)).

xwt ¼ xA � wA0 þ xS � wS0 ð1Þ

For the largest conversions however (above approximately50%) the samples were never perfectly dry even when dryingconditions were improved (remaining monomers were alwaysdetected by NMR); therefore only weight conversions obtainedfrom gravimetry below 50% were used in this work.

The raw polymerization media with added deuteratedacetone were analyzed by proton NMR spectroscopy at regulartime intervals. Analyses were performed in 5 mm tubes at roomtemperature using a Bruker AC200 apparatus, operating at afrequency of 200 MHz. The chemical shift scale was calibratedon the basis of the solvent peak (deuterated acetone at2.04 ppm). The overall molar conversions (xmol) and theindividual conversions of each monomer (xA and xS) weredetermined by integrating the peaks corresponding to thevinyl protons of the monomers, using the broad peak between6.5 and 7.5 ppm as an internal reference (5 aromatic H forstyrene and polystyrene, and 1 vinylic H for the styrenemonomer that was subtracted before calculation). The overallmolar conversion is a function of the individual conversions xA

and xS and of the molar fraction of each monomer in the initialmixture (fA0 and fS0 for acrylic acid and styrene respectively)(Equation (2)).

xmol ¼ xA � fA0 þ xS � fS0 ð2Þ

Overall molar conversions from NMR were used for kineticanalysis (for instance conversion versus time plots). Molarmasses and polydispersity indices were always plotted as afunction of the overall weight conversion. The latter wasderived from the direct gravimetric analysis (below 50%) orwas calculated from the NMR individual conversions (overthe whole conversion range) using Equation (1). This leads to

Table 1. Experimental conditions for the SG1-mediated copolymerizations of styrene and acrylic acid in 1,4-dioxane solution at 120 8C.

Expt. Symbol [S]0 [A]0 fA0a) [Monams]0 [SG1]0 rb) hkpi � [P�]

mol �L�1 mol �L�1 mol �L�1 mol �L�1 s�1

1 * 2.70 0.30 0.101 0.0274 0.0012 0.044 8.8� 10�5

2 ~ 2.25 0.77 0.255 0.0262 0.0012 0.044 1.1� 10�4

3 & 1.47 1.47 0.499 0.0235 0.0011 0.045 1.4� 10�4

4 ^ 0.75 2.25 0.749 0.0218 0.0010 0.046 1.5� 10�4

5 ~ 0.30 2.69 0.899 0.0205 0.0009 0.046 1.2� 10�4

6 * 1.48 1.47 0.498 0.0117 0.0006 0.047 –7 & 1.49 1.47 0.497 0.0058 0.0003 0.045 –

a) fA0: molar fraction of A in the initial monomer feed.b) r¼ [SG1]0/[Monams]0.

Direct Synthesis of Controlled Poly(styrene-co-acrylic acid)s . . . 2057


linear theoretical relationships between the number averagemolar mass, Mn, and conversion (ideally, for wA0 in Equation(1) we should consider the additional contribution of themethyl ester group introduced by methylation, but the diffe-rence was usually small enough to be neglected).

Composition of the comonomer mixture (fA, the molarproportion of acrylic acid) was also determined from 1HNMR analysis at regular time intervals, by integrating thepeaks corresponding to the vinyl protons of the residualmonomers.

The molar mass and molar mass distribution of themethylated copolymers were obtained by SEC in THF with a1 mL �min�1 flow rate. The apparatus is composed of adegasser (Viscotek, VE7510 GPC), a Waters 515 HPLC pump,an auto-sampler (Viscotek,VE5200 GPC), 3 linear columns(3 PSS SDV linear) in an oven (Croco-cilTM) thermostatted at40 8C and two detectors: differential RI (LDC Analytical,refractoMonitor IV) and UV at 254 nm (Waters 484). Themolar masses were derived from a calibration curve based onpolystyrene standards (162 to 1 090 000 g �mol�1, fromPolymer Standards Service).

Results and Discussion

Selection of Suitable Experimental Conditions

SG1-mediated copolymerization of styrene and acrylic acid

was carried out using a SG1-based unimolecular alkox-

yamine initiator, CH3–O–C( O)–CH(CH3)–SG1, also

called Monams. Because of too fast polymerization and

poorly controlled behavior, bulk copolymerization was

discarded. Consequently, an important point was the choice

of an appropriate solvent, in which the monomers and

the copolymers remained soluble throughout the reaction.

The purpose here was to work under homogeneous condi-

tions: any macrophase separation would actually alter the

quality of control by changing the local concentrations.

Preliminary experiments were performed in tert-butylben-

zene and in toluene. The copolymerization medium

remained homogeneous when the acrylic acid molar ratio

was rather low (10 mol-% for instance). However, phase

separation occurred when this ratio was raised above

25 mol-%. A solvent that actually met the required con-

ditions was 1,4-dioxane, in which the copolymers were

fully soluble, irrespective of the composition and of the

monomer conversion. In this work, the initial monomer

concentration was kept constant and equal to 3 mol �L�1;

only the relative proportions of styrene and acrylic acid

were changed. For Expts. 1–5, the initial concentration of

Monams was adjusted in order to target molar masses

close to 11 000 g �mol�1 (in the acidic form). In two other

examples (Expts. 6 and 7) Monams concentration was

divided by a factor of respectively 2 and 4, to achieve larger

molar masses. A small initial concentration of free SG1 was

introduced (r¼ 4.5 mol-% with respect to Monams), as was

previously proposed for butyl acrylate, to moderate the

polymerization rate and reach the best compromise

between fast reaction and quality of control.[35] Indeed, it

should be kept in mind, as mentioned above, that acrylic

acid, similarly to butyl acrylate, exhibits a very high pro-

pagation rate constant.[18] Finally, the polymerization

temperature was 120 8C, which is the optimal value for

Monams decomposition as well as for styrene homopoly-

merization.[39] Details on the experimental parameters can

be found in Table 1.

Effect of Monomer Feed Ratio

Molar Mass and Molar Mass Distribution

One of the features that a polymer should exhibit

when synthesized under controlled conditions is the

linear increase of molar mass with monomer conversion,

along with the decrease of the polydispersity index.[1–3]

They reveal that polymerization takes place with a cons-

tant chain concentration and that all chains grow simulta-

neously with the same rate of monomer incorporation.

Figure 1 and Figure 2 show that these criteria were

actually fulfilled by the copolymerizations 1 to 5, over the

whole conversion range, indicating that they actually took

place via a nitroxide-mediated controlled free radical

process.

The Mn values of the methylated copolymers followed

the theoretical line (calculated from the initial concentra-

tions of monomers and initiator), pointing out that all

growing chains were indeed generated by the alkoxyamine

initiator (i.e. concentration of chains equals the initial con-

centration of Monams). Simultaneously, the polydispersity

indices decreased with increasing conversion from approxi-

mately 1.6 to 1.2 for Expt. 1 to 3. Such final values indicate a

narrow molar mass distribution that could not be reached

under classical free-radical copolymerization conditions.

This behavior is illustrated in Figure 3, which displays the

continuous shift of the SEC curves towards the higher molar

masses, obtained for the methylated copolymers of Expt. 2.

The situation was slightly different when the proportion of

acrylic acid was increased; all the other parameters remain-

ing the same (Expt. 4 with 75 mol-% of A; Expt. 5 with

90 mol-% of A). At first, the initial polydispersity indices

were far above the 1.6 value observed for the previous

experiments, but decreased down to 1.2–1.3 in the 40–

60 wt.-% conversion range, and slightly increased at larger

conversions, particularly for the experiment with fA0¼0.90. The shift of the molar mass distribution with

conversion can be clearly seen in Figure 4 for Expt. 5, but

in contrast to the previous experiments with a lower

proportion of acrylic acid, a tailing was observed on the low

molar mass side, as an indication of the possible existence

of irreversibly terminated chains (by bimolecular macro-

radical termination or more likely by chain transfer to

solvent as previously shown for SG1-mediated homopoly-

merization of acrylic acid).[36,37]



Evolution of the Composition with theOverall Conversion

Proton NMR was used to follow the evolution of the

composition of the comonomer mixture (fA, the molar

proportion of acrylic acid) with the overall molar conver-

sion. The experimental data are displayed in Figure 5. They

clearly indicate the existence of an azeotropic composition

between fA0 ¼ 0.25 and 0.50. From these data, and using the

terminal model, the two reactivity ratios (rA for acrylic acid

and rS for styrene) were determined by simultaneously

fitting all the experimental points (for five initial composi-

tions) with the best theoretical curves, using a non linear

least square method, with the assumption of constant

relative error.[40] For each initial composition the theore-

tical curve was the integrated copolymerization equation

(overall conversion as a function of fA),[41] which was

numerically converted into the corresponding fA versus

Figure 1. Mn (from SEC, polystyrene calibration) of themethylated copolymers versus overall weight conversion (fromgravimetry up to 50% and from NMR over the whole range usingEquation (1) to recalculate the overall weight conversion) forExpts. 1–5 (see Table 1 for experimental conditions); the linesrefer to the corresponding theoretical curves for the methylatedcopolymers.

Figure 2. Polydispersity indexes, Mw=Mn (from SEC, polystyr-ene calibration) of the methylated copolymers versus overallweight conversion (from gravimetry up to 50% and from NMRover the whole range using Equation (1) to recalculate the overallweight conversion) for Expts. 1–5 (see Table 1 for experimentalconditions).

Figure 3. SEC curves (refractive index detector) for Expt. 2 (seeTable 1 for experimental conditions).



conversion curve. Indeed, conversion was selected as the

independent variable, whereas fA was the corresponding

dependent variable. The 95% joint confidence interval was

determined using the F-test.[40] The calculated reactivity

ratios were rA¼ 0.27� 0.07 and rS¼ 0.72� 0.04 and the

azeotropic composition was fA(azeo) ¼ 0.28. These results

are in correct agreement with the values reported long ago

in the literature for copolymerization of the same mono-

mers in 1,4-dioxane at 50 8C, using AIBN as an initiator:

namely rA¼ 0.13 and rS ¼ 0.75.[42] However, at this stage,

it is difficult to discuss the comparison between data from

CRP and from classical free-radical copolymerization,

since they were not obtained under strictly the same

conditions.

For copolymerizations carried out at low fA0 (typically

below fA(azeo)), a very small composition drift was obser-

ved; this indicates that copolymer composition was close to

that of the monomer feed and remained uniform all along

the polymer chains. For copolymers prepared from a

monomer mixture with larger proportion of acrylic acid

(fA0> fA(azeo)), the comonomer mixture and hence the

copolymer enriched in acrylic acid. Because in CRP all

chains are created within a short conversion interval at the

beginning of the polymerization, they all grow simulta-

neously with the same rate of monomer incorporation.

Thus, in copolymerization they should undergo the same

composition drift from the a-end to the o-end (gradient

structure) and exhibit a narrow composition distribution.

Consequently, the controlled poly(styrene-co-acrylic acid)

copolymers with a proportion of acrylic acid above fA(azeo)

should present such a gradient structure, although remain-

ing slightly pronounced.

Copolymerization Kinetics

The copolymerization kinetics were followed for Expts. 1–

5. The overall molar conversions from 1H NMR (xmol) are

represented versus time in Figure 6.

The first interesting point is that all reactions reached

large conversions, typically 80 mol-%, within 8 h. The

second point is that no significant difference in polymer-

ization rate could be seen between the various reactions.

The initial slopes of the ln (1/(1� xmol)) versus time curves

giving hkpi � [P�] are reported in Table 1 (the plots were

linear during about 3 h reaction time and then exhibited a

slight downward curvature). It indeed appears that these

values were not affected by the comonomer composition.

hkpi is the average rate constant of propagation that depends

on comonomer feed ratio (see appendix), and [P�] is the

overall concentration of propagating radicals. In nitroxide-

mediated controlled free-radical polymerizations, [P�] is

Figure 4. SEC curves (refractive index detector) for Expt. 5 (seeTable 1 for experimental conditions).

Figure 5. Proportion of acrylic acid in the comonomer mixture,fA, as a function of the overall molar conversion (from NMR). Thelines refer to the best fit of the theoretical curves and allow toestimate the reactivity ratios: rA¼ 0.27� 0.07 and rS¼ 0.72�0.04. (Results from two experiments are superimposed for theinitial compositions fA0 ¼ 0.90 and fA0¼ 0.75).

Figure 6. Overall molar conversion versus copolymerizationtime for Expts 1–5 (see Table 1 for experimental conditions).



given by the activation-deactivation equilibrium relation-

ship (Equation 3; see Scheme 1).[43]

½P�� ¼ Kh i � ½P--SG1�½SG1� ¼ Kh i � ½Monams�0

½SG1� ð3Þ

In this equation the concentration of SG1-capped macro-

molecular chains, [P–SG1], was replaced by the initial

concentration of Monams (owing to the good match

between experimental and calculated Mn’s, which indicates

a fast and complete initiation). The concentration of free

SG1 is a function of the initial concentration and of the

amount released owing to the persistent radical effect.[44]

With fairly high initial concentration of SG1 (close to

10�3 mol �L�1 for Expts. 1–5) one can simplify Equation 3

and consider that [SG1] remained close to the initial

concentration [SG1]0 (Equation 4).

½P�� ¼ Kh i � ½Monams�0½SG1�0

¼ Kh ir

ð4Þ

In contrast to homopolymerization where the activation-

deactivation equilibrium constant K only depends upon

temperature for a given monomer (Scheme 1), in copoly-

merization hKi should be an apparent value that also

depends upon comonomer composition.

From the kinetic analysis and Equation (4), it is then

possible to estimate hkpi � hKi¼ hkpi � [P�] � r. However, it

should be kept in mind that using r in this equation might

lead to underestimated values (use of [SG1]0 instead of

‘‘true’’ [SG1]). Nevertheless, we do not intend to give

accurate data, but simply discuss the results qualitatively.

The consequence of a rather constant hkpi � [P�] with an

increasing proportion of acrylic acid in the comonomer

mixture, together with similar r values, is that hkpi � hKiremained rather constant too. However, the propagation

rate constant was reported to be much larger for acrylic acid

than for styrene. Indeed, the propagation rate constant of

acrylic acid, kpA, is in the order of 105 L �mol�1 � s�1,[18]

whereas it is kpS¼ 2 000 L �mol�1 � s�1 for styrene at

120 8C.[45] As a consequence, hkpi is likely to increase with

the increase of fA0 (see appendix) and hence, hKi should

decrease, leading to a decrease of the overall concentration

of propagating radicals. This trend indicates that the

activation-deactivation equilibrium constant for the SG1-

mediated homopolymerization of acrylic acid, KA, might

be significantly lower than that for styrene homopoly-

merization, KS. Indeed, the latter was reported to be

KS¼ 6� 10�9 in bulk at 120 8C,[39] whereas KA was shown

to be close to 10�10 or even below, as derived from our

kinetic study of SG1-mediated homopolymerization of

acrylic acid.[36,37]

Effect of the Alkoxyamine Initiator Concentration

For a given molar composition of the comonomer feed, the

variation in alkoxyamine initiator concentration should

lead to a change in the average molar mass. As reported

in Table 1, two copolymerizations similar to Expt. 3(fA0 ¼ 0.5) were carried out, with Monams concentration

divided by a factor of 2 (Expt. 6) and by a factor of 4

(Expt.7). The initial concentration of free SG1 was adjusted

so as to keep r constant. As anticipated, the change in the

experimental conditions affected the average molar masses

as displayed in Figure 7. Indeed, it appeared that the

experimental Mn’s followed the theoretical lines, and that

the polydispersity indices remained in the 1.2–1.4 range.

These results fully support the controlled character of the

copolymerization reaction. However, for the lowest initia-

tor concentration ([Monams]0¼ 0.0058 mol �L�1, Expt. 7),

the variation of Mn with conversion did not remain linear

throughout the reaction but deviated from linearity above

�40% conversion. At this stage the polydispersity indices

were slightly above those found for the other two experi-

ments (i.e. 1.4 instead of 1.2–1.3). This behavior was

assigned to chain transfer to 1,4-dioxane, which creates new

short chains, as was also found for SG1-mediated homo-

polymerization of acrylic acid,[36,37] as well as for the

RAFT polymerizations performed in the same solvent.[23]

This result shows that our system actually undergoes molar

mass limitation at low initiator concentration, which is

however less pronounced in copolymerization of A with

styrene than in homopolymerization.[36,37] Additionally, it

is likely that the chain transfer constant (and hence the

impact of transfer on molar mass) depends on the como-

nomer composition.

Conclusion

Free-radical copolymerization of styrene and acrylic acid

was carried out under controlled conditions in 1,4-dioxane

solution at 120 8C, using an alkoxyamine initiator based on

Scheme 1. Activation (rate constant kd) – deactivation (rateconstant kc) equilibrium in SG1-mediated controlled free-radicalpolymerization. In homopolymerization, the equilibrium constantis K ¼ kd

kc¼ ½P��½SG1�

½P�SG1� , where P� represents the propagating macro-radical and P-SG1 is the dormant chain with alkoxyamine end-group.



the N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpro-

pyl) nitroxide, SG1. A broad composition range from 90/10

to 10/90 was investigated. The kinetic study showed that the

polymerizations performed with slightly different initiator

concentrations and a similar initial proportion of free SG1

of 4.5 mol-% with respect to the initiator exhibited very

similar rates, (80% conversion within 8 h) irrespective of

the proportion of acrylic acid in the comonomer mixture.

Actually, the increase in acrylic acid proportion did not

enhance the copolymerization rate (as should be expected

from the much larger rate constant of propagation than that

of styrene) because the activation-deactivation equilibrium

constant that controls the overall concentration of propa-

gating radical was shown to decrease concomitantly.

In all cases, the copolymers exhibited average molar

masses (Mn) that increased linearly with overall monomer

conversion, and polydispersity indices that ranged between

1.2 and 1.4. The experimental Mn’s followed the calculated

ones, based on the initial concentrations of monomers and

initiator. The variation in the initiator concentration allow-

ed to target various molar masses, but some limitation

appeared at low initiator concentration owing to chain

transfer to 1,4-dioxane.

From the kinetic analysis, the reactivity ratios were deter-

mined: rA¼ 0.27� 0.07 for acrylic acid and rS¼ 0.72�0.04 for styrene. They showed that chains exhibited no or

small composition drift, and hence a slightly pronounced

gradient structure.

This system enabled us to synthesize, in a direct and

simple way, well-defined copolymers of styrene and acrylic

acid, which exhibited a narrow molar mass distribution

together with a narrow composition distribution.

Appendix

For a copolymerization system, the average propagation

rate constant hkpi is a function of the comonomer compo-

sition (fA, the molar proportion of acrylic acid and fS, the

molar proportion of styrene) and of the reactivity ratios.

Here, in the absence of strong information on the rate

constant of propagation of acrylic acid and on the kinetic

features of the copolymerization under classical conditions,

the terminal model is considered for qualitative interpreta-

tion. We are aware that this model, although able to describe

the copolymer composition quite correctly, is rather inade-

quate to predict the copolymerization rate.[46] It would

be better replaced by the penultimate unit model,[46,47] in

particular, the implicit penultimate unit model.[46,48] How-

ever the latter requires the knowledge of two additional

radical reactivity ratios, which have never been determined

for styrene and acrylic acid free radical copolymerization.[48]

According to the terminal model, the variation of hkpi can

be written as Equation (5):[46–48] it is a simple function of

the two reactivity ratios, of the two homopropagation rate

constants (kpA, for acrylic acid, and kpS for styrene), and of

the comonomer composition.

kp

� �¼ rA � f 2

A þ 2 � fA � fS þ rS � f 2S

rA � fA

kPA

þ rS �fS

kpS

ð5Þ

From this equation, it can be calculated that, when fA0 is

below 75 mol-%, hkpi slightly increases with initial fA0

and remains little affected by the very large rate constant

of homopropagation of acrylic acid. Above this proportion,

the increase in hkpi is much more pronounced, but the

values exhibit large uncertainty owing to the lack of

accuracy on kpA.

Acknowledgement: The authors wish to thank C. Bui, M. Save,B. Coutin, L. Dubreucq and J. Belleney for their kind helpthroughout this work. Atofina is also acknowledged for financialsupport of LC, as well as for supplying SG1 and Monams.

Figure 7. Number average molar masses, Mn, and polydisper-sity indexes, Mw=Mn, (from SEC, polystyrene calibration) of themethylated copolymers versus overall weight conversion (fromNMR, using Equation (1) to recalculate the overall weightconversion) for Expts. 3, 6 and 7 with different Monamsconcentrations (see Table 1 for experimental conditions); thelines refer to the corresponding theoretical curves for themethylated copolymers.



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Documents

Direct Synthesis of Controlled Poly(styrene-co-acrylic acid)s of Various Compositions by Nitroxide-Mediated Random Copolymerization