3268

Kinetic Analysis of the Atom Transfer RadicalPolymerization of Butyl Acrylate Mediated by Cu(I)Br/N,N,N9,N9,N99-Pentamethyldiethylenetriamine

Magdalena Reyes, Xiang Yu, Devon A. Shipp*

Department of Chemistry and Center for Advanced Materials Processing, Clarkson University,Potsdam, New York 13699-5810, USAFax: +1 (315) 268-6610; E-mail: shippda@clarkson.edu

Keywords: atom transfer radical polymerization (ATRP); catalysis; kinetics (polym.);

IntroductionRecent examples of living radical polymerization techni-ques, such as atom transfer radical polymerization(ATRP – see Scheme 1),[1, 2] nitroxide-mediated polymer-ization[3, 4] and reversible addition-fragmentation chaintransfer polymerization,[5] rely on, inter alia, an activa-tion/deactivation cycle of the polymer chains with favor-able kinetics. This allows growth to occur in a steadyfashion, resulting in well-defined molecular weights, lowpolydispersities, and functionalized polymers.

For ATRP there have been several investigations intothe apparent orders of reactants, typically using differentmonomers and/or catalyst species. One unifying theme ofthese studies has been that the apparent orders have been(usually) non-integer, ranging from 0.2 to approximatelyunity for initiator and catalyst. Initial studies suggestedthat the apparent orders of activator (catalyst) and initia-tor were first-order.[6, 7] This was in agreement with the

rate of polymerization as given by Equation (1). Note thatEquation (1) was derived for a simplified version of thereaction in Scheme 1, one that excluded terminationevents.[6] This deficiency was used to explain the non-lin-ear response of the deactivator concentration. Morerecently, experimental studies have found fractionalapparent orders,[6–11] and simulations[12, 13] have shownthat because of the persistent radical effect (PRE),[14, 15] itis not expected that the apparent orders of the reactants

Full Paper: The kinetics atom transfer radical polymeri-zation (ATRP) of n-butyl acrylate, using the Cu(I)Br/N,N,N 9,N 9,N99-pentamethyldiethylenetriamine catalyticsystem and methyl-2-bromopropionate as initiator, is stud-ied. The orders of the initiator, activator and deactivatorare estimated for two systems – one close to bulk condi-tions (5 vol.-% toluene added), and another with 10vol.-% N,N-dimethylformamide added. The former sys-tem gave non-integer orders for initiator and activator,while the latter gave orders of unity for both of these reac-tants. The second system gave a non-linear response forthe deactivator.

Macromol. Chem. Phys. 2001, 202, No. 17 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/1711–3268$17.50+.50/0

Dependence of the apparent propagation rate constant (k app)on [Cu(I)Br], [MeBrP] and [Cu(II)Br2] (with 10 vol.-%DMF).

Macromol. Chem. Phys. 2001, 202, 3268–3272

Scheme 1. Atom transfer radical polymerization. Pn–X =(polymeric) alkyl halide; X,Y = halogen; Mtn–Y/L = metalhalide ligand.

Kinetic Analysis of the Atom Transfer Radical Polymerization ... 3269

be simply first-order (for initiator, activator and mono-mer) and inverse first-order (for deactivator).

Rp ¼ kp½M�½RX�0ka½MtnY=L�

kd½XMtnþ1Y=L� ¼ kapp½M� ð1Þ

The PRE describes how radical–radical terminationevents lead to an increase in deactivator concentration,which then results in the activation/deactivation equili-brium being shifted toward the dormant species. Thus, thepolymerization slows down as termination events occurthroughout the polymerization, and monomer consump-tion should not be simply first-order. Both Fischer[15] andFukuda et al.[16] have separately shown that under idealconditions the persistent radical effect should result inapparent orders of reactants of less than unity for mono-mer, initiator and activator. However, most publishedATRP kinetic data indicate that monomer consumption is(very nearly) first-order. Deviations from ideal PRE beha-vior come from a number of sources, such as the potentialof a non-negligible amount of deactivator being present inthe activator at the beginning of the polymerization,[13, 16]

the fact that the termination rate will decrease as conver-sion (and therefore chain length) increases, and the poten-tial heterogeneity of the system.[13]

It is apparent that, especially from an experimentalpoint of view, the kinetics of ATRP are not well under-stood. To rectify this situation it is necessary to obtainfundamental data regarding how the rate of polymeriza-tion varies with reactant concentrations. This will notonly further our understanding of the reactions takingplace but also allow better reaction conditions to bedesigned. Towards this end, this paper reports a kineticstudy of the ATRP of butyl acrylate (BA), using theCu(I)Br/N,N,N 9,N 9,N 99-pentamethyldiethylenetriamine(PMDETA) catalyst system,[17] and methyl-2-bromopro-pionate (MeBrP) as initiator. This system is of particularinterest as the copper salt and ligand are readily obtain-able and relatively cheap, and thus it has been widelyused.

Experimental PartBA was obtained from Acros, run through an alumina col-umn to remove inhibitor and distilled from CaH2 underreduced pressure. PMDETA was obtained from Acros anddistilled under reduced pressure before use. MeBrP wasobtained from Acros and used as received. Cu(I)Br wasobtained from Acros and purified by stirring in acetic acidovernight, filtering and washing with ethanol. Toluene andN,N-dimethylformamide (DMF) were obtained from Fisherand used as received.

Gas chromatography (GC) was performed using a Hew-lett-Packard 5890 Series II GC equipped with a 30m HP-5column. Gel permeation chromatography (GPC) was carriedout on a system comprised of a Waters 515 pump, Rheodyne

7725i injector, 3 Styragel columns (HR2, HR4, HR5), and aViscotek LR40 refractive index detector. Tetrahydrofuran(THF) was used as the eluent. Data were analyzed using apolystyrene calibration curve.

Cu(I)Br (and Cu(II)Br2, if included) were added to a10-mL or 25-mL Schlenk flask equipped with a stirring barand subjected to 3 vacuum/N2 cycles. All liquids werebubbled with N2 for at least 20 min. BA (5 mL), PMDETA,and toluene (0.25 mL) or DMF (0.5 mL) were added to theflask via N2-washed syringes, in that order. The solids wereallowed to dissolve, and the solution was subjected to 3freeze–pump–thaw cycles, then placed under a N2 atmo-sphere, and then the flask was immersed in a 608C oil bath.Once the solution was homogeneous, MeBrP was added tothe flask via a syringe. Samples were taken periodicallyusing N2-washed syringes and diluting with THF, and ana-lyzed using a gas chromatograph for monomer conversion.Each sample then had the copper removed by runningthrough an alumina column, and was then filtered (0.2 lm)and analyzed using GPC.

The green precipitate was isolated from two ATRP experi-ments, one with toluene and the other with DMF, after reach-ing high monomer conversions (l80%). This was done bythe repeated addition of benzene to the Schlenk flask and thedrawing off of the supernatant liquid via a syringe. Thesolids were filtered, dissolved in acetonitrile and their UV/visible spectrum measured. Each spectrum showed a weakand broad band between 500 and 800 nm (kmax approximately650 nm; d–d transition). This is typical of Cu(II) complexedwith a N-based ligand.[18]

Results and DiscussionThe ATRP of BA was systematically carried out to allowthe kinetics of the polymerization to be analyzed. Reac-tant concentrations were varied from polymerization topolymerization to allow the apparent orders of the reac-tant to be obtained. This was done under close-to-bulkconditions, with small amounts of either toluene or DMFadded. In general, molecular weights were kept low(l a20000) in order to reduce the effect of chain transferto monomer.[19]

Figure 1 shows the first-order kinetic plot for monomerconsumption during the ATRP of BA with toluene addedas an internal standard (see also Table 1). These polymer-izations were homogeneous before the addition of theinitiator, but a green Cu(II) precipitate formed during theearly stages of the polymerization (a20% conversion).For each polymerization shown in Figure 1 there tends tobe a slight increase in the rate of the polymerization, i.e.,the first-order kinetic plot has a slight upwards curvature.This may be due to the decrease in deactivator solubilityas monomer is consumed, therefore forcing the equili-brium toward the right (Scheme 1), and increasing radicalproduction and thus the rate of polymerization.

From each of the first-order kinetic plots in Figure 1 anapparent propagation rate (k app) was obtained from a lin-

3270 M. Reyes, X. Yu, D. A. Shipp

ear fit. This is related to the overall rate of polymerizationby Equation (1). Figure 2 shows a plot of the form of aln(k app) vs. ln([reagent]) plot (where the reagent is eitherCu(I)Br or MeBrP). Both sets of data result in linearplots, with slopes of 0.68 and 0.82 for Cu(I)Br andMeBrP, respectively. These numbers represent the appar-ent orders of the reactants. If all rate constants in Equa-tion (1) were in fact constant and termination did notoccur, these orders should be unity.[6, 12] Since this is notthe case, it is reasoned that these parameters are varyingduring the reaction. Indeed, this would be expected as thetermination produces deactivator throughout the experi-ment, and the termination rate itself would vary withchain length and viscosity (and therefore conversion).[13]

These apparent orders-of-reactant data are in qualitativeagreement with several other experimental studies[6–11]

and simulations[12, 13] – typically, a value of less than unityhas been obtained, ranging from approximately 0.2 to 1.

To overcome the problems associated with the hetero-geneity of the polymerization mixture in either bulk orwith a small amount of toluene present, we examined asimilar ATRP system with the addition of 10 vol.-%DMF. In these reactions, the solutions were homogeneousto the eye for most of the polymerization, with some

green Cu(II) precipitate forming at conversions greaterthan l70%. The first-order kinetic plots from these reac-tions are shown in Figure 3 (see also Table 2). Theseplots are mostly linear, with any curvature being down-wards, corresponding to a slowing the of polymerizationrate. The decrease in the rate is indicative of loss of activesites and/or an increase in the deactivator concentration,which result(s) in the equilibrium shown in Scheme 1moving to the left. This is, as outlined above, the basisfor the persistent radical effect.[14–16] The curvature is notas pronounced as would be expected from the PRE model– probably a result of either the termination rate coeffi-cient changing as a function of conversion,[12] or a non-zero concentration of deactivator being present at thebeginning of the reaction.[16] Similar kinetic data fromexperiments were obtained where Cu(II)Br2 was added(data not shown in Figure 3).

The k app was determined for each polymerization with10 vol.-% DMF, and again presented in the form of aln(k app) vs. ln([reagent]) plot (where the reagent isCu(I)Br, MeBrP or Cu(II)Br2) in Figure 4. In this case,

Figure 1. First-order kinetic plots from the ATRP of BA with5 vol.-% toluene (see Table 1 for experimental conditions).

Table 1. Initial reactant concentrations and apparent rate con-stant (k app) for the ATRP of BA (5 mL) at 60 8C with 5 vol.-%toluene.

Reaction name ½MeBrP�0m

½CuðIÞBr�0m

kapp

sÿ1

MR25 0.266 0.023 6.95610–4

MR18 0.133 0.023 4.53610–4

MR15 0.066 0.023 3.11610–4

MR17 0.033 0.023 1.45610–4

MR23 0.013 0.023 5.98610–5

MR22 0.066 0.035 4.15610–4

MR24 0.066 0.012 1.70610–4

MR21 0.066 0.005 1.07610–4

Figure 2. Dependence of the apparent propagation rate con-stant (kapp) on [Cu(I)Br] and [MeBrP] (with 5 vol.-% toluene).

Figure 3. First-order kinetic plots from the ATRP of BA with10 vol.-% DMF (see Table 2 for experimental conditions).

Kinetic Analysis of the Atom Transfer Radical Polymerization ... 3271

the orders of reactants for both Cu(I)Br and MeBrP areunity, whereas the data for Cu(II)Br2 are non-linear butindicate that the addition of Cu(II)Br2 does not have agreat affect on the polymerization rate (at least over theconcentration range studied here).

The apparent orders of reactants found here (with 10vol.-% DMF) concur with earlier data from kinetic inves-tigations of the ATRP of styrene and methyl methacrylateusing the Cu(I)Br (or Cu(I)Cl)/4,49-di-(5-nonyl)-2,29-bipyridine catalyst system (i.e., the rate shows a first-order dependence on [MeBrP] and [Cu(I)Br]).[6, 7] How-ever, while the results for [MeBrP] and [Cu(I)Br] agreewith Equation (1), the affect of [Cu(II)Br2] on the poly-merization rate is not so straightforward as the log–log

plot in Figure 4 is non-linear. The inability to use Equa-tion (1) to completely describe the kinetics of ATRP canbe attributed to the presence of the PRE. However,experimental kinetic data generally do not follow the rateequation derived from the PRE model either.[15, 16] There-fore, most (homogeneous) ATRP experiments fallbetween two extremes; one extreme where no terminationtakes place and therefore Equation (1) holds, and a sec-ond extreme where termination occurs at a consistentrate, and thus the polymerization occurs under ‘ideal’PRE conditions.

For all polymerizations the molecular weights (M—

ns)increased linearly with conversion and the polydispersi-ties remained low (typically, M

—w/M

—n a 1.3). Figure 5

shows these data for selected experiments, but these aretypical for all polymerizations. Some differences betweenthe observed M

—n and the predicted M

—n based on the mono-

mer-to-initiator ratio could be a result of the use of poly-styrene standards to calibrate the GPC instrument, or the

Table 2. Initial reactant concentrations and apparent rate con-stant (k app) for the ATRP of BA (5 mL) at 60 8C with 10 vol.-%DMF.

Reaction name ½MeBrP�0m

½CuðIÞBr�0m

kapp

sÿ1

MR30 0.252 0.032 4.13610–4

MR29 0.126 0.032 2.28610–4

MR28 0.063 0.032 1.18610–4

XY1 0.032 0.032 3.36610–5

MR32 0.013 0.032 2.24610–5

MR35 0.063 0.047 1.34610–4

MR34 0.063 0.016 4.81610–5

MR36 0.063 0.0063 1.76610–5

XY13a) 0.063 0.032 1.57610–5

XY3b) 0.063 0.032 2.89610–5

XY2c) 0.063 0.032 2.73610–5

XY6d) 0.063 0.032 2.20610–5

XY7e) 0.063 0.032 3.13610–5

a) [Cu(II)Br2]0 = 6.67610–3m.

b) [Cu(II)Br2]0 = 1.63610–3m.

c) [Cu(II)Br2]0 = 3.17610–3m.

d) [Cu(II)Br2]0 = 4.90610–3m.

e) [Cu(II)Br2]0 = 0.65610–3m.

Figure 4. Dependence of the apparent propagation rate con-stant (k app) on [Cu(I)Br], [MeBrP] and [Cu(II)Br2] (with 10vol.-% DMF).

Figure 5. Number-average molecular weight (M—

n) and polydis-persity (M

—w/M

—n) changes with monomer conversion for experi-

ments with changing [MeBrP]0 (with 10 vol.-% DMF). M—

n =closed symbols; M

—w/M

—n = open symbols; lines represent theoreti-

cal molecular weights based on M—

ntheo = 128.26D[M]/[MeBrP]0.

Figure 6. GPC traces of reaction XY6 (see Table 2). Percentconversion, M

—n and M

—w/M

—n are given (in that order) for each

sample within the legend.

3272 M. Reyes, X. Yu, D. A. Shipp

occurrence of chain transfer to monomer.[19] One trend inthe data presented in Figure 5 is that the polymers ofhigher molecular weights tend to have lower polydisper-sities compared with those of lower molecular weights atequivalent monomer conversions.

Lower polydispersities (e.g., a1.15) for the lowermolecular weight polymers can be obtained by the addi-tion of Cu(II)Br2 to the reaction mixture. An example ofthis can be seen in Figure 6, where the GPC traces of sev-eral samples taken from a reaction with 15.3% Cu(II)Br2

added to the reaction (relative to Cu(I)Br). Here, atapproximately 25% conversion the polydispersity is 1.11,whereas for the equivalent reaction (MR28) without theadded Cu(II)Br2, the polydispersity at 25% conversion is1.24. Figure 6 also shows the movement of the wholemolecular weight distribution, as a function of monomerconversion, towards higher molecular weights.

ConclusionsIn this work we studied the kinetics of BA polymerizationusing the ATRP catalyst system of Cu(I)Br/PMDETA.The results indicate that the apparent orders of the initia-tor and catalyst are non-unity for near-bulk conditions,while these apparent orders are unity when a smallamount of polar solvent (10 vol.-% DMF) is added. Thedifferences between these two systems are probably aresult of the heterogeneity of the former case. In terms ofusing the data presented here to design ATRP experi-ments, under the near-bulk conditions the rate of poly-merization will not change proportionally with a changein either initiator or catalyst concentration. Conversely,under homogeneous conditions the rate will change pro-portionally with a change in either initiator or catalystconcentration. It is difficult, however, to draw anydetailed mechanistic conclusions from the current data,other than that most ATRP experiments are performedunder conditions where neither Equation (1) nor the PREcan be exclusively applied.

Acknowledgement: We thank Ms. Jennifer Eppstein and Ms.Sarah Ebeltoft for help with the GPC and GC measurements, the

McNair Scholars program at Clarkson University (to MR) andthe New York State Science and Technology Foundation and theCenter for Advanced Materials Processing (CAMP) at ClarksonUniversity for financial support.

Received: April 10, 2001Revised: August 20, 2001

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