A novel tridentate nitrogen donor as ligand in copper catalyzed ATRP of methyl methacrylate

A Novel Tridentate Nitrogen Donor as Ligand in CopperCatalyzed ATRP of Methyl Methacrylate

ANUJ MITTAL, SWAMINATHAN SIVARAM

Division of Polymer Science and Engineering, National Chemical Laboratory, Pune-411008, India

Received 8 March 2005; accepted 23 May 2005DOI: 10.1002/pola.20973Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A tridentate ligand, BPIEP: 2,6-bis[1-(2,6-diisopropyl phenylimino) ethyl]pyridine, having central pyridine unit and two peripheral imine coordination siteswas effectively employed in controlled/‘‘living’’ radical polymerization of MMA at90 8C in toluene as solvent, CuIBr as catalyst, and ethyl-2-bromoisobutyrate (EBiB)as initiator resulting in well-defined polymers with polydispersities Mw/Mn � 1.23.The rate of polymerization follows first-order kinetics, kapp ¼ 3.4 � 10�5 s�1, indicat-ing the presence of low radical concentration ([P*] � 10�8) throughout the reaction.The polymerization rate attains a maximum at a ligand-to-metal ratio of 2:1 in tol-uene at 90 8C. The solvent concentration (v/v, with respect to monomer) has a signi-ficant effect on the polymerization kinetics. The polymerization is faster in polarsolvents like, diphenylether, and anisole, as compared to toluene. Increasing themonomer concentration in toluene resulted in a better control of polymerization. Themolecular weights (Mn,SEC) increased linearly with conversion and were found to behigher than predicted molecular (Mn,Cal). However, the polydispersity remained nar-row, i.e., �1.23. The initiator efficiency at lower monomer concentration approaches avalue of 0.7 in 110 min as compared to 0.5 in 330 min at higher monomer concentra-tion. The aging of the copper salt complexed with BPIEP had a beneficial effect andresulted in polymers with narrow polydispersitities and higher conversion. PMMAobtained at room temperature in toluene (33%, v/v) gave PDI of 1.22 (Mn ¼ 8500) in48 h whereas, at 50 8C the PDI is 1.18 (Mn ¼ 10,300), which is achieved in 23 h. Theplot of ln kapp versus 1/T gave an apparent activation energy of polymerization as(DE=

app) 58.29 KJ/mol and enthalpy of equilibrium (DH0eq) to 28.8 KJ/mol. Reverse

ATRP of MMA was successfully performed using AIBN in bulk as well as solution.The controlled nature of the polymerization reaction was established through kineticstudies and chain extension experiments. VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part

A: Polym Chem 43: 4996–5008, 2005

Keywords: activation energy; apparent rate constant; atom transfer radical poly-merization (ATRP); 2,6-bis [1-(2,6-diisopropyl phenylimino) ethyl] pyridine (BPIEP);diphenylether; ethyl-2-bromoisobutyrate (EBiB); methyl methacrylate; polydispersityindex (PDI); toluene

INTRODUCTION

The transition metal catalyzed atom transfer rad-ical addition, ATRA, is an efficient method for

carbon–carbon bond formation in organic chemis-try.1 This methodology was extended to polymer-ization chemistry in 1995 and was termed asatom transfer radical polymerization (ATRP) byWang and Matyjaszewski2,3 who used copper astransition metal and bipyridine or substitutedbipyridines as ligands. Sawamoto4 and coworkersused ruthenium/organic phosphorous compounds

Correspondence to: S. Sivaram (E-mail: [email protected]).

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 4996–5008 (2005)VVC 2005 Wiley Periodicals, Inc.

4996

as catalyst systems for ATRP. Since its discovery,ATRP has become one of the most powerful toolsfor controlled polymer synthesis by radicalmethod.5,6 The method has been successfully per-formed in both bulk as well as in aqueous mediain emulsions, dispersions, and suspension. Recentstudies have been aimed at developing newligands and metals that increase the activity andselectivity of the catalyst.7

The key to tailoring the reactivity of the cata-lyst is the choice of the ligand.5 The ligand shouldcomplex strongly, yet, undergo facile dissociationfrom the coordination sphere and should allow itsexpansion8 to accept halogen atom from the dor-mant species. Catalytic activity is reduced whenthere is excessive steric hindrance around themetal center and also when the ligand used hadstrongly electron withdrawing substituents. Thecopper(I) catalytic systems based on bidentate(2,20-bipyridine derivatives)5,6 and multidentatenitrogen ligands5,6 have been shown to be veryeffective for producing relatively high molecularweight polymers and for controlling the polydis-persity. In general, Schiff based ligands are moreeffective for the polymerization of (meth)acry-lates9–11 in toluene or xylene solution in conjunc-tion with copper halides along with suitable alkylhalide initiators over a range of temperatures,even, as low as –15 8C (68% conversion in 116 h,Mn ¼ 10,200, Mw/Mn ¼ 1.28).12 In general, it hasbeen found that an increase in solvent polarityand the associated increase in the rate of reactionwould enable the catalyst level to be reduced inaddition to lowering the reaction temperature.13

For similar monomer conversion, greater thepolarity of the solvent, lower is the experimentalmolecular weight. Munakata14 suggested that thestructures of CuI halide/ bpy complexes in solu-tion depend on the polarity of the solvent. Inaddition to solvent, temperature has a significanteffect on thermodynamic parameters like activa-tion energy (DE=

app) as well as equilibrium con-stant (Keq) and molecular weights.15

In this paper, we examine a tridentate ligandpossessing bulky aryl groups, namely, 2,6-bis[1-(2,6-diisopropyl phenylimino) ethyl] pyridine(BPIEP), copper (I) bromide as the catalyst, andethyl-2-bromo isobutyrate (EBiB) as the initiatorfor the controlled polymerization of methylmethacrylate (MMA). Similar ligands havefound extensive use with late transition metalsin ethylene polymerization by Brookhart16 andGibson17 respectively. Gobelt et al.18 used di-alkylimino- and dialkylaminopyridine complexesof CuBr and FeBr2 in ATRP of MMA, styrene,

and methylacrylate in anisole at 90 8C with2-bromo propionitrile as initiator. However, thepolymerization of MMA was uncontrolled withdialkyl imino-/FeBr2 catalyst (42% conversion in9 h, Mn ¼ 8400, Mw/Mn ¼ 1.68) and somewhatbetter controlled when used with CuBr as cata-lyst (68% conversion in 3.5 h, Mn ¼ 13,600, Mw/Mn ¼ 1.23). The features of the new catalystsystem are discussed with respect to polymeriza-tion rate, initiation efficiency, conversion, andpolydispersity of the obtained PMMA.

EXPERIMENTAL

Reagents

The reagents employed in the synthesis ofligands16,17 were distilled/crystallized before use.Methyl methacrylate (Aldrich, 99%), Toluene,and other solvents (Anisole, DMF, and diphenyl-ether) were distilled using standard procedures.CuIBr (Aldrich, 98%) was purified according tothe method of Keller and Wycoff.19 CuBr2,CuCl2, p-toluene sulfonylchloride (pTsCl), and3-methyl-2-butanone were purchased from Aldrichand used directly. The initiator ethyl-2-bromo-isobutyrate (EBiB, Aldrich) was vacuum distilledbefore use. 2,20-Azobisisobutyronitrile (AIBN),N-bromosuccinimide (NBS), benzoylperoxide (BPO),CCl4, CaCl2, and others were purchased fromMerck (India) and used after purification.

Polymerization

CuBr, ligand, toluene, and monomer were addedto the flask with stirring using either syringe orcannula. Three freeze-pump-thaw cycles were per-formed to remove the molecular oxygen. The flaskwas immersed in an oil bath at the required tem-perature. When reaction contents attained thereaction temperature, required amount of initiatorwas added. After a given time, the flask wascooled, opened, and a certain amount of THF wasadded to quench the reaction completely. Kineticpick-outs were withdrawn at regular intervals,quenched, and stored at 0 8C. The slope of the plotbetween ln([M]0/[M]t) versus time gives apparentrate constant, kapp, of polymerization.

Characterization

All manipulations were performed using stand-ard Schlenk or syringe techniques under anatmosphere of dinitrogen. Molecular weight dis-tributions were measured using GPC-TQ withone linear column (60 cm) and one 100-A mixed

NOVEL TRIDENTATE NITROGEN DONOR AS LIGAND 4997

column (60 cm) connected in series. The detectorin GPC is being RI and UV, tetrahydrofuran asan eluent with 1 mL min�1 flow rate and mono-dispersed commercially available PMMA stand-ards from PSS Germany were used for polymeranalysis. Monomer conversions were determinedby Gas Chromatography using PerkinElmer XLgas chromatograph equipped with FID detectorand a BP1 column.

RESULTS AND DISCUSSION

Kinetic Analyses of the Polymerization

ATRP of MMA in toluene (66% v/v) at 90 8C wereperformed using CuBr as catalyst, EBiB as ini-

tiator, and a tridentate ligand (I) bearing a cen-tral pyridine unit and two peripheral imine co-ordination sites. A tridentate ligand (Scheme 1),NPEPEA: {(N-((1E)-1-{6-[(1E)-N-phenylethanimi-doyl]pyridine-2-yl}ethylidene)aniline,(II),was alsoexplored. II differed from I in the extent of stericcrowding around the metal center.

The mole ratios of the components utilizedin present study are [MMA]:[EBiB]:[CuBr]:[L]¼ 100:1:1:2. Table 1 shows the results of poly-merization of MMA as a function of ligand struc-ture. In case of polymerization with CuBr/BPIEPand CuCl/BPIEP, the concentration of growingradicals ([P*] � 10�8) is constant at 90 8C as ob-served by linear plot of ln([M]0/[M]t) versus time(t). From Table1 the apparent rate constant ofthe polymerization (kapp) was found to be in theorder, CuBr/II > CuCl/I > CuBr/I. The reactionusing the less steric bulky ligand, II, was highlyuncontrolled as seen by obtained Mn,SEC andvery low initiator efficiencies. This shows thatthe initiator, EBiB, employed with II/CuBr sys-tem was incapable of giving controlled polymer-ization. The result shows that steric crowdinghas a pronounced effect on controlled polymer-ization of MMA.

The molecular weight and molecular weightdistribution of resulting PMMA with both cata-lysts, BPIEP/CuX, were narrow. The PDIincreased slightly with conversion but remained

Table 1. Effect of Ligand Structure and Catalyst in Atom Transfer RadicalPolymerization of MMAa in Toluenef at 90 8C

Ligands/CuXTime(min) Mn,SEC PDI Ieff

b(�10�5, s�1)

kappc Conv (%)d

I/CuIBr 45 5,400 1.16 0.20 3.40 8120 6,600 1.18 0.30 16195 7,900 1.20 0.30 27330 10,200 1.23 0.60 55

I/CuICl 45 4,100 1.18 0.30 4.02 11120 6,100 1.17 0.43 26195 7,600 1.18 0.50 38330 9,300 1.15 0.6 50

IIe/CuICl 45 18,600 1.54 0.10 6.82g 18120 19,100 1.56 0.15 28195 17,800 1.60 0.20 35330 18,400 1.63 0.23 43

a [MMA]0 ¼ 3.12 M; [MMA]:[EBiB]:[CuBr]:[L] ¼ 100:1:1:2.b Ieff ¼ Mn,Cal/MSEC.c Slope of ln([M]0/[M]t) vs time plot.d From GC.e N-((IE)-1-{6-[(1E)-N-phenylethanimidoyl]pyridin-2-yl}ethylidene) aniline.f 66% (v/v) with respect to monomer.g From nonlinear curve fit, kapp and kt, ¼ 1.13 � 10�3 s�1.

Scheme 1.

4998 MITTAL AND SIVARAM

�1.24. The PMMA molecular weights increasedlinearly with conversion but were much higherthan predicted, indicating lower efficiency of theinitiator system employed in this work. This isprobably due to the persistent radical effect.20

The calculated initiator efficiencies were 0.5–0.6throughout the polymerization.

Since the ln([M]0/[M]t) versus time (t) plot islinear, low initiator efficiency must be caused by

the consumption of some of the initiator due toside reactions that could have originated frommore labile C��Br bond end group21 at thebeginning of polymerization.

Although both polymerization reactions arecatalyzed at 90 8C, the monomer conversion islower at early stage of reaction catalyzed byCuIBr and higher at the end of reaction in com-parison to CuICl. In addition, CuICl was foundto be a better catalyst than CuIBr in terms ofgiving a higher reaction rate (kapp ¼ 4.02 � 10�5

s�1 for CuICl vs. 3.4 � 10�5 s�1 for CuIBr) andhigher Mn,SEC at 50% conversion (Table 1). Incase of CuICl a curvature was observed in firstorder plot for longer reaction times (> 4 h), i.e,indicating occurrence of unavoidable termina-tion reactions. Therefore, the I/ CuIBr catalystwas employed in all further studies.

Influence of Molar Ratio of [BPIEP]/[CuBr]

The dependence of the rate of polymerizationin ATRP of MMA with catalyst concentrationwas investigated. The plot of ln kapp versus[BPIEP]0: [CuBr]0 (Fig. 1) shows the dependenceof the apparent rate constant of polymerization asa function of [ligand]:[CuIBr] ratio. The mostfavorable ratio was found to be 2:1 (Fig. 2) withthe rate of polymerization increasing significantlywith increasing ligand concentration. ATRP is

Figure 1. Semi logarithmic kinetic plots for theATRP of MMA at 90 8C in toluene (66%, v/v). [MMA]¼ 3.12 M. [EBiB]: [CuBr]:[BPIEP] ¼ 100:1:1:2.

Figure 2. Plot of kapp as a function of increasing ligand concentration (BPIEP) atconstant CuBr concentration for the solution ATRP of MMA in toluene at 90 8C.[EBiB] ¼ [CuBr] ¼ 0.0312 M.


usually carried out with 2 times molar excess ofSchiff base ligand with respect to CuIX, irrespec-tive of the coordinating ability of the ligand exceptwith linear branched multidentate amines likePMDETA (N,N,N,N0,N@-penta methyldiethylene-triamine)5 and Me6TREN (tris(dimethylami-noethyl) amine)5 where lower concentration givesbetter results. This mole ratio employed is consis-tent with X-ray crystal structures in which CuI

prefers a tetrahedral geometry, which can beachieved by two moles of bidentate ligands.22

However, the solution structures may differ fromthose in solid state. The reason behind thedecrease of the rate constant in the presence ofhigher ligand concentration ([C]:[L] ¼ 1:2.5) is notclear. Further coordination of the ligand to themetal center may hinder the activity of the cata-lyst, resulting in a slight decrease in the rate ofpolymerization. Different geometries5,6 of theactive complex that are in equilibrium are alsoinferred by the change of color during the reac-tion. The color of CuBr/BPIEP changes from paleyellow at room temperature to orange in freeze-dried state and to reddish-brown upon heating.The obtained Mn,SEC increased linearly with con-version. However, the Mn,SEC were higher thanthe Mn,Cal values. The polydispersity of theobtained PMMA remained narrow, i.e., Mw/Mn �1.23. The controlled nature of the CuBr/BPIEPsystem was confirmed by adding a fresh aliquotof monomer in a chain extension experiment.Thus, the isolated PMMA (Mn,SEC ¼ 14,600, Mw/Mn ¼ 1.21) was further utilized as macroinitiatorto initiate polymerization of fresh MMA at 90 8Cin toluene, using CuBr as catalyst and BPIEP asligand for 5.5 h. The GPC of the PMMA obtainedfrom this study is shown in Figure 3. The forma-tion of high molecular weight PMMA (Mn,SEC ¼90,000 Mw/Mn ¼ 1.10) clearly establishes thatthe reaction is well controlled and that essen-tially all the end groups of the macroinitiator areavailable for chain extension.

Influence of Concentration of Solvent and Initiator

It is known that polar solvents such as anisoleor phenyl ether produce high activity for ATRPof MMA because of their ability to form a homo-geneous solution during the entire polymeriza-tion period. Pascual et al.23 found that the sol-vent like N,N-dimethylformamide (DMF, "DMF

¼ 37.6) makes the system homogeneous and alsoacts as coordination solvent to the catalyst andmonomer.

In general for controlled polymerization, ki� kp

24 and the rate of initiation (ki) is symbol-ized by the yield and initiation efficiency. Cham-bard et al.25 reported that besides its effect oncatalyst solubility, the solvent influences activa-tion rate constant, kact, in case of styrene andbutyl acrylate, through its polarity and coordi-nating ability. Matyjaszewski5 found that use ofethylene carbonate for ATRP of n-butyl acrylateor DMF for ATRP of styrene enhanced the solu-bility of both the CuI and CuII/bpy complexes inthe reaction mixture. The polarity of solventsmay affect the induction period as observed incase of lauryl methacrylates.26 We also founddiphenylether and anisole to be the most suit-able solvent among the common organic sol-vents. ATRP of MMA was conducted in fourdifferent solvents27 i.e., "DMF (37.6, 20 8C),"anisole (4.33, 25 8C), "diphenylether (3.87, 25 8C),and "toluene (2.39, 20 8C) of varying polarity,keeping toluene as the reference solvent.

(The result of the batch studies of the ATRPof MMA in different solvents with different con-centration is shown in Table 2). In toluene, thesystem is heterogeneous at all reaction condi-tions, whereas, in other solvents the reactionbecomes homogeneous at reaction temperature.As shown in Figure 4, ln([M]0/[M]t) versus time(t) remained linear indicating that polymeriza-

Figure 3. Gel Permeation chromatograms ofPMMA-Br (before and after chain extension) by ATRPinitiated by EBiB with CuBr/BPIEP as ligand intoluene. [MMA] ¼ 3.12 M. [MMA]: [EBiB]:[CuBr]:[BPIEP] ¼ 100:1:1:2.


tion was first order with respect to, and that theconcentration of active species during the poly-merization was constant. The molecular weights(Mn) obtained increased linearly with respect toconversion but were higher, in case of 66% (v/v)as compared to 33% (v/v) solvent, than Mn,Cal

(Fig. 5). The polydispersities remained constant

(�1.23) at higher monomer concentration. Thiscould be attributed to the fact that the reactionin 33% (v/v) solvent concentration (toluene andanisole) was very fast, thereby, increasing theviscosity of the solution within 3 h. Thus, kineticstudy was conducted for 1.83 h instead for 5.5 h(Table 3).

Figure 4. Semi logarithmic kinetic plots for the ATRP of MMA at 90 8C in tolueneand anisole. [MMA] ¼ 3.12 M. [MMA]:[EBiB]:[CuBr]:[BPIEP] ¼ 100:1:1:2.

Figure 5. Dependence of molecular weight and polydispersity on conversion in thesolution ATRP of MMA at 90 8C with [EBiB] ¼ 0.0312 M. Open symbols representpolydispersities and filled symbol represents Mn-(GPC).


Although the reaction was controlled in bothsolvents, it was observed that kapp of the reac-tion in 33% (v/v) toluene was 1.2 times fasterthan 33% (v/v) anisole, resulting in an increasedviscosity of the solution. Samples from the reac-tion mixture could not be withdrawn after 3 h.This behavior is just the opposite of what isexpected, considering that, as polymerizationconditions approaches almost bulk reactionconditions, an increase of initiator and catalystconcentration will occur and, as a result, poly-merization rate should be faster.5 A similartrend was observed when butyl acrylate waspolymerized using initiated RBr/CuIBr /bpy inethylene carbonate by Matyjaszewski et al.28

The authors stated that reduction of the amountof solvent gives rise to a decrease in the polarityof the reaction mixture, which results in a lowerconcentration of CuI in solution and, conse-quently, the rate of polymerization is reduced.

Apparently, the solubility of CuII is not stronglyaffected since the polydispersity indices arelower when less proportion of solvent withrespect to monomer is used. However, the reac-tion in 66% (v/v) toluene possesses lower Ieff andconversion as compared to anisole, which couldbe attributed to slower initiation in the former.Thus, solvent polarity plays an important rolein controlling the polymerization reaction.

The rate of polymerization was higher underbulk conditions (kapp ¼ 26 � 10�5 s�1), but witha relatively broader Mw/ Mn of 1.28 and 90%conversion. In general, the propagation rate wasfound to be directly proportional to the concen-tration of EBiB for the solution ATRP of MMA(kapp ¼ 3.4 � 10�5 s�1) for [CuIBr]:[EBiB] ¼ 1:1versus 2.6 � 10�5 s�1 for [CuIBr]:[EBiB] ¼ 1:0.5.The plot of ln kapp against ln[EBiB]0 showed therate of polymerization is pseudo order (0.5) withrespect to concentration of initiator. Thus, the

Table 2. Study on the Effect of Solvents in Atom Transfer Radical Polymerizationof MMAa at 90 8C

Solvent(%, v/v) Mn,sec PDI Ieff

b Conv (%)c

Toluene 66 10,600 1.24 0.60 5550 14,000 1.27 0.60 9033 12,400 1.22 0.73 90

DMF 66 4,000 1.20 0.50 2050 4,600 1.21 0.70 3233 5,800 1.21 0.80 53

Anisole 66 9,600 1.30 0.80 8050 11,700 1.24 0.76 9033 10,500 1.24 0.85 90

Diphenyl ether 66 9,900 1.29 0.80 8250 12,800 1.22 0.70 9033 11,300 1.23 0.80 90

a [MMA]0 ¼ 3.12 M and reaction time ¼ 5.5 h; [MMA]:[EBiB]:[CuBr]:[BPIEP] ¼ 100:1:1:2.b Ieff ¼ Mn,Cal/MSEC.c From gravimetry.

Table 3. Kinetic Study of the Effect of Solvents in ATRP of MMAaat 908C

Solvent (33%, v/v) Time (min) Mn,SEC PDI Ieffb Conv (%)c kapp (�10�5 s�1)

Toluene 110 8,200 1.20 0.70 55 12.97DMF 330 5,800 1.21 0.80 53d nde

Anisole 110 8,000 1.20 0.70 51 10.96Diphenyl ether 330 11,300 1.23 0.80 90d nde

a [MMA]0 ¼ 3.12 M; [MMA]:[EBiB]:[CuBr]:[BPIEP] ¼ 100:1:1:2.b Ieff ¼ Mn,Cal/MSEC.c From GC.d From gravimetry.e Not determined.


1:1 ratio of [CuIBr]: [EBiB] was found to beoptimum ratio for the solution ATRP of MMA intoluene at 90 8C.

Effect of Structure of Initiator

Various �-bromo active organic compounds, viz.,EBiB, MBP3 (methyl-2-bromopropionate) andMBB29 (3-bromo-3-methyl-2-butanone), havingstructural similarities to the of growing polymerchain were used as initiators for ATRP of MMA.Jiang et al. has recently utilized NBS as initia-tor for ATRP of MMA in anisole30 at 90 8C. Thepolymerization resulted in lower polydispersity(1.13) in 2.5 h and 34% conversion, but the ini-tiator efficiency was only 0.17. This lower valueis a consequence of the fact that molecular bro-mine produced undergoes side reaction, i.e., itbrominates the monomer. In fact, the use ofNBS as initiator for MMA polymerization wasfirst introduced by Otsu31 with reduced nickeland later by Percec32, but all produced higherpolydisperisities (� 1.8) and negligible initiatorefficiencies (0.017). Similarly, Matyjaszewski33

and Percec34 also reported the fast initiationof ATRP of MMA using arenesulfonyl chloridesgiving polydispersities close to 1.10. Thus bothinitiators, namely, NBS and p-toluenesulfonylchlkoride (pTsCl), were employed in the pre-sent study. The mole ratios of various com-ponents used were [MMA]:[Initiator]:[CuBr]:[BPIEP] ¼ 100:1:1:2.

Table 4 shows the results of different initia-tors used for polymerizing MMA. The polymer-ization was faster with �-bromoalkyl initiatorsin comparison with pTsCl and NBS. This can be

explained on the basis of difference in bondenergies of C��Br and C��Cl bond. The reactionwas controlled with EBiB, MBB, and pTsCl,whereas, uncontrolled with MBP and NBS.This result is in accordance with prior reportswherein EBiB/CuBr couple exhibited a higherpolymerization rate as compared to pTsCl/CuBr.35 The polymer conversion was in therange of 60–70% but was lowest with NBS. Onthe other hand, polydispersity remained verylow with pTsCl as compared to 1.2 with that ofother initiators. Also, the initiator efficiency var-ied as pTsCl > MBP > EBiB > MBB > NBS.Thus, to prepare PMMA with high chain-endfunctionalities as starting material for the syn-thesis of block copolymer, the pTsCl/CuBr coupleappear to be the best choice.

Effect of Catalyst Concentration

The polymerization conducted with various cata-lyst to initiator mole ratios in ATRP of MMA at90 8C in toluene (66%, v/v) are [MMA]:[EBiB]:¼ 100; [CuBr]:[BPIEP] ¼ 1:2. The concentrationof catalyst was increased in steps of 0.5, thereby,keeping the initiator concentration constant. Asseen from the Table 5, polymerization was slowerwith lower catalyst concentration, resulting inhigher PDI with low initiator efficiency. On theother hand, when [C]:[I] ¼ 2, higher conversionwas obtained with higher initiator efficiency butbroader molecular weight distribution.

Aging of the BPIEP/ CuBr Complex

The conversion obtained in ATRP of MMA in tol-uene at 90 8C after 2 h of reaction was �20%.The lower value was attributed to the slow rateof formation of the required complex or the slowequilibrium dynamics actually required to give

Table 4. Various Initiator in ATRP of MMA inToluene (66%, v/v)a at 908C

Initiator Time (h) Mn,SEC PDIConv(%)b Ieff

c

EBiB 5.5 9,300 1.23 65 0.70MBP 5.5 13,400 1.82 74 0.74MBBd 5.5 9,300 1.26 75 0.81NBS 5.5 4,300 1.17 5 0.12pTsCl 5.5 5,400 1.12 46 0.85

a [MMA]0 ¼ 3.12 M; [MMA]:[I]:[CuBr]:[BPIEP] ¼100:1:1:2.

b From gravimetry.c Ieff ¼ Mn,Cal/MSEC; EBiB; ethy-2-bromoisobutyrate,

MBP: methyl-2-bromopropionate.d MBB: 3-bromo-3-methyl-2-butanone, reaction at 50% (v/

v) with respect to monomer, NBS: N-bromosuccinimide,pTsCl: p-toluene sulfonyichioride.

Table 5. ATRP of MMAa in Toluene (66%, v/v) forDifferent Catalyst Concentration

[C]/[I]b Time (h) Conv (%)c Mn,SEC PDI Ieffd

0.5 5.5 50 7,600 1.33 0.661.0 5.5 60 8,300 1.21 0.721.5 5.5 95 11,600 1.24 0.822.0 5.5 95 11,100 1.28 0.86

a [MMA]0 ¼ 3.12 M.b [MMA]:[EBiB] ¼ 100; [CuBr]:[BPIEP] ¼ 1:2.c From gravimetry.d Ieff ¼ Mn,Cal/MSEC.


controlled polymerization. Therefore, attemptswere made to form the complex in situ prior toreaction. A 2:1 M ratio of ligand to copper chlor-ide was aged at 25 8C/ 12 h, followed by heatingto 90 8C for an additional 1.5 h. The reddish-brown colored catalyst complex was cooled toroom temperature followed by addition of mono-mer, degassing three times using freeze-pump-thaw, and initiation of polymerization at 90 8C.The results are shown in Table 6. Higher conver-sions of the polymer were obtained with goodcontrol over molecular weight and polydisper-sities. This particular effect was more pro-nounced when solvent concentration of 33% (v/v)was used. Thus, aging of the complex has a posi-tive effect on initiator efficiency as well as poly-mer conversion.

Influence of Reaction Temperature onthe Polymerization

The effect of reaction temperature was studiedin toluene (33%, v/v) at 30, 50, and 90 8C. Fig-ure 6 shows a good linear relationship betweenln([M]0/[M]t) and polymerization time. The rateof polymerization increases with increasingtemperature, and the induction period becomeszero when the polymerization temperature was90 8C. The effect of temperature on the molecu-lar weight and polydispersity of the obtainedPMMA by ATRP is given by Figure 7. The sta-tionary concentration of the propagating radi-cals during the solution ATRP of MMA can beestimated by combination of values of kapp in eq1 and rate constant of radical propagation for

Table 6. Aging of the BPIEP/CuBr Complex Using Different Concentration ofSolvent in ATRP of MMAa at 90 8C

Solvent(%, v/v) Time (h) Mn,SEC PDI Ieff

b Conv (%)c

Toluene 66 5.5 9,300 1.23 0.70 6533 5.5 13,800 1.18 0.69 95

Anisole 33 5.5 13,800 1.23 0.73 95Diphenyl ether 33 5.5 12,400 1.23 0.73 90

a [MMA]0 ¼ 4.68 M; [MMA]:[EBiB]:[CuBr]:[L] ¼ 100:1:1:2.b Ieff ¼ Mn,Cal/MSEC.c From gravimetrv.

Figure 6. First order plots on the effect of the polymerization temperature onATRP of MMA in toluene. [MMA] ¼ 3.12 M [MMA]:[EBiB]:[CuBr]:[BPIEP] ¼100:1:1:2


MMA calculated according to the eq 2. The dataare shown in Table 7.

Rp ¼ kapp½M� ¼ kp½M�½P�� ð1Þ

ln kp ¼ 14:8 L mol�1s�1 � 22:36 K J mol�1=RT36

ð2Þ

As shown in Table 7, the rate of polymeriza-tion apparently increases with increasing poly-merization temperature because of increase inboth the rate constant of propagation and atomtransfer equilibrium constant.

�H�eq ¼ �E6¼

app ��E6¼prop ð3Þ

The Arrhenius plot, of ln kapp versus 1/T, forCuBr/BPIEP catalyzed polymerization of MMAis plotted in Figure 8. Based on the slope, anapparent energy of activation, DE=

app ¼ 50.96KJ mol�1, is calculated. According to eq 3, withDE=

prop ¼ 22.15 KJ mol�1 for MMA, DH8eq¼ 28.8 KJ mol�1 can be calculated for Br-medi-ated ATRP of MMA. This value is much lowerthan the corresponding values for MMA (DE=

app

from 62.9 KJ mol�115 and 52.8 KJ mol37)reported previously by Haddleton et al.38 Thepolymerization rate constant in toluene usingCuBr/BPIEP is about four times higher than therate constant reported by Matyjaszewski39 usingCuCl/bipyridyloctylamine system in diphenyl-ether and about 1.5 times higher than that

Figure 7. Temperature effect on the molecular weight and polydispersity withrespect to conversion in the solution ATRP of MMA at 90 8C with [EBiB] ¼ 0.0312M. Open symbols represent polydispersities and filled symbol represents Mn-(GPC).

Table 7. Kinetic Data for ATRP of MMAa in Toluene (33%, v/v) at Different Polymerization Temperatures

Temperature (8C) Time (h) kappb (�10�5, s�1) kp

c (�103, mol L-1) Mn,SEC PDI [P*]d (�10�8, mol L�1)

30 46 0.474 0.374 8,500 1.22 1.2730 23 1.640 0.647 10,200 1.18 2.5430 1.84 13.10 1.620 8,200 122 8.27

a [MMA]0 ¼ 3.12 M; [EUiB]: [CuBr]:[BPIEP] ¼ 100:1:1:2.b Slope of ln([M]0[M]) vs. time plot.c Calculated according to reference 36.d [P*] ¼ kapp/kp.


reported by Haddleton using CuBr/ N-alkyl-2-pyridylmethanimine system in xylene.10

Effect of [M]/[I] Ratios

The effect of monomer to initiator concentrationon the ATRP of MMA using CuBr/BPIEP systemwas examined. The results obtained are tabu-lated in Table 8. The polymerization reactionwas very slow. The conversion decreased whileinitiator efficiency increased with higher [M]/[I]values. The polydispersity remains narrow.

Reverse ATRP of MMA

Since more active catalytic systems are gener-ated by less oxidatively stable CuI complexes,reverse ATRP, using more stabilized CuII com-plexes, is an alternative approach to controlledradical polymerization. Wang and Matyjaszew-ski reported that ‘‘living’’/ controlled radical pol-ymerization is also observed when a conven-tional radical initiator (e.g., AIBN) is associatedwith a transition metal compound of higher oxi-dation state (e.g., CuCl2 or CuBr2) complexedwith suitable ligands (e.g., 2,20-bipyridine).40,41

The process is termed as reverse ATRP. Matyjas-zewski and coworkers42 reported that BPO/CuBr/dNbpy could be used as a reverse ATRP

initiating system for bulk polymerization of sty-rene at higher temperature (110 8C). Morerecently, Wang and Yan developed a new proce-dure for reverse ATRP of MA styrene usingAIBN/CuII/bpy as initiating systems under het-erogeneous conditions. Later, Yan and Jerome43

reported that the polymerization of MMA usingBPO/ CuIBr in butanone gave Mn ¼ 35,000, PDI¼ 1.20 with good initiator efficiency (0.92). How-ever, the reaction time was 100 h. In addition,iron complexes have been used in reverse ATRPof MMA. The general conditions for successfulreverse ATRP are Cu(II)0/[AIBN]0 � 1 andrather high polymerization temperature forrapid decomposition of initiator provided the

Figure 8. Plot of ln kapp versus 1/T for ATRP of MMA initiated by EBiB withCuBr/BPIEP as ligand in toluene. [MMA] ¼ 3.12 M. [MMA]:[EBiB]:[CuBr]:[BPIEP]¼ 100:1:1:2

Table 8. ATRP of MMAa in Toluene (66%, v/v) atDifferent DP Values

[M]/[I]bTime(h)

Conv(%)c Mn,SEC PDI Ieff

d

100 5.5 60 8,300 1.21 0.70200 5.5 60 14,600 1.21 0.82400 5.5 40 16,200 1.19 0.98800 5.5 30 26,500 1.20 0.91

a [MMA]0 ¼ 312 M.b [CuBr]:[BPIEP]:[EBiB]:[MMA] ¼ 1:2:X:100.c From gravimetry.d Ieff ¼ Mn,Cal/MSEC.


half-life of AIBN (t1/2 ¼ 4.8 h at 70 8C and 74 hat 50 8C44) at that temperature is known. FromTable 9, it is seen that when the reaction wascarried out in presence of CuBr2/BPIEP, thecolor of initially green solution of cupric complexchanged to reddish-brown, typical for cuprousspecies, whereas the color was dark green incase of CuCl2/BPIEP in bulk. The nature of colorproves that the dynamics of reverse ATRP isfollowed. However, in toluene the copper (I)complex is insoluble, which is detrimental to theoccurrence of redox reaction and formation ofdormant and active species in equilibrium.Therefore, no controlled polymerization wasobserved. The reaction showed good controlwhen reaction was performed in polar solventlike anisole. Thus, the reverse ATRP of MMAwas successful in solution as well as bulk.

CONCLUSIONS

The diiminoarylpyridine ligand, BPIEP, was suc-cessfully employed in ATRP of MMA. In additionto CuBr-mediated ATRP, it was used in combi-nation with CuCl forming an active catalyst forthe polymerization of MMA. The polymerizationfollowed first-order kinetics. Changing the elec-tronic environment and reducing the steric bulkof the ligand proved to be detrimental for con-trolled polymerization of MMA. It was foundthat �-bromoisobutyrate (EBiB), an �-bromoke-tone (MBB), and arenesulfonyl chloride (pTsCl)gave controlled polymerization. The initiatorefficiency was highest with pTsCl/CuBr couple.The optimum condition for BPIEP ligand wasfound to be 100:1:1:2 ([MMA]:[EBiB]:[CuBr]:[BPIEP]) ratio at 90 8C in anisole as well as tol-uene (33%, v/v), which yielded a well definedPMMA with a narrow molecular weight distribu-tion of 1.22. The aging of the copper complexresulted in improved initiator efficiencies and

conversion. PMMA ([M]/[I] ¼ 800) with Mn,SEC

¼ 26,500 could be prepared in 5.5 h with PDI ¼1.20 and initiator efficiency of 0.9. The presentstudy utilizes effectively a very bulky ligand forATRP of MMA, giving low apparent activationenergy (DE=

app ¼ 50.96 KJ mol�1) and veryhigh rate of polymerization. The reverse ATRPof MMA using CuII/AIBN system showed goodcontrol in solution as well as bulk. In addition,the controlled nature of the polymerization wasconfirmed by the formation of high molecularweight PMMA (Mn,SEC ¼ 90,000 Mw/Mn ¼ 1.10)using a predefined PMMA macroinitiator in achain extension experiment.

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A novel tridentate nitrogen donor as ligand in copper catalyzed ATRP of methyl methacrylate

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