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5614 J . Am. Chem. SOC.1995,117, 5614-5615

Control1ed"Living" Radical Polymerization. AtomTransfer Radical Polymerization in the Presence ofTransition-Metal ComplexesJin-Shan Wang and Krzysztof Matyjaszewski*

Department of C hemistry, Camegie-M ellon University4400 5th Avenue, Pittsburgh, Pennsylvania 15 213Received February 16, 99 5

Atom transfer radical addition, ATRA, is an efficient methodfor carbon-carbon bond formation in organic synthesis, andmany benefits of this method are now well recognized.' In someof these reactions, a transition-metal catalyst acts as a carrierof the halogen atom in a reversible redox process.2 Initially,the transition-metal species, Mtn, bstracts halogen a tom X fromthe organic halide, RX, o form the oxidized species, M('+'X,and the carbon-centered radical R'. In the subsequen t step, theradical R' participates in an inter- or intramolecular radicaladdition to alkene, Y, with the formation of the intermediateradical species, RY'. The reaction between M('+'X and RY'results in a target product, RYX, and regenerates the reducedtransition-metal species, M?, which further promotes a newredox process. The fast reaction between RY' and M:+'Xapparently suppresses bimolecular termination between alkylradicals and efficiently introduces a halogen functional groupX into the final product in good to excellent yields.2c-eThis communication reports that the aforementioned transi-tion-metal-catalyzed ATRA can be successfully used to controlradical polymerization. An alkyl chloride, 1-phenylethyl chlo-ride, 1-PEC1, is an efficient initiator, and a transition-metalhalide, CuCl, complexed by 2,2'-bipyridine, bpy, an efficientchlorine atom transfer promoter. This initiating system affordscontrolled polymers with predetermined molecular weight andnarrower molecular weight distribution, M w / M n 1.5, thanobtained by conventional free radical polymerization. Recently,several methods for controlled/"living" radical polymerizationh av e b een r ep ~ r t e d . ~A typical polymerization was carried out by heating a reddishbrown styrene, St, solution4of 1-PECl(O.01 molar equiv relativeto monomer), Cu Cl (1 molar equiv relative to 1-PECl), and bpy(3 molar equiv relative to CuCl) in a glass tube sealed undervacuum , at 130 "C. A linear increase of number averagemolecular weight, Mn,SEC,' versus monomer conversions up to95% was found after ca. 3 h (Figure 1).

* To whom all correspondence should be addressed.(1 ) For reviews of atom transfer methods in organic synthesis, see: (a )Curran, D. P. Synthesis 1988, 489. (b) Curran, D. P. In ComprehensiveOrganic Synthesis;Trost, B. M., Fleming, I., Eds.; Pergamon : Oxford, 199 1;Vol. 4, p 715.(2) For recent references to the formation of carbon-carbo n bonds bytransition metal-catalyzed chlorine atom transfer radical reactions, see: (a)Bellus, D. Pure Appl. Chem. 1985,57, 1827. (b) Hayes, T. K. ; Villani, R.;Weinreb, S. M . J . Am. Chem. SOC. 1988, 110, 5533. (c) Nagashima, H.;Wakamatsu, H.; Ozaki, N., Ishii, T.; Watanabe, M .; Tajima, T.; Itoh, K . J.Org. Chem. 1992, 57, 1682. (d) Nagashima, H.; Ozaki, N., Ishii, M.; Seki,K.; Washiyama, M.; Itoh, K . J. Org. Chem. 1993, 58, 464. (e ) Udding, J.H.; Tuijp, K. J. M .; van Zanden, M. N. A ,; Hiemstra, H.; Spec kamp, W. N.J . Org. Chem. 1994, 59, 1993.(3) (a) Otsu, T.; Yoshida, M. Makromol. Chem ., Rapid Commun. 1982,3, 127, 133. (b) Solomon, D. H.; Waverly, G.; Rizzardo, E. ; Hill, W.;Cacioli, P. U.S. Pat. 4,581,429, 1986. (c) Georges, M . K.; Veregin, R. P.N.; Kazmaier, P. M .; Hamer, G . K. Macromolecules 1993, 26 , 5316. (d)Wayland, B. B .; Poszmik, G.; Mukeqee, S. L.; Fryd, M. J. Am. Chem.SOC. 1994, 116, 7943. (e) Greszta, D.; Mardare, D. ; Matyjaszewski, K .Macromolecules 1994, 27 , 638. (0 Matyjaszewski, K.; aynor, S . ; Wang,J. S . Macromolecules 1995, 28 , 2093.(4) The reddish brown color of a slightly heterogeneous solution wasformed within 30 s at 130 OC, irrespective of the m ixing ord er of thecomponents.( 5 ) M",SEC alues were determined by size exclusive chromatographycalibrated using polystyrene standards.

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Figure 1. Dependence of molecular weights and polydispersities onconversion in bulk polymerization of styrene at 13 0 "C with [l-PEC l]o= 0 .1 m o m , [CuCl Io = 0 .1 m o m , [ b p y ] ~ 0 .3 mom.

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M W h .Figure 2. Correlation of experimental and theoretical (eq 1) molecularweights and polydispersities in bulk polymerization of styrene at 130"C with [l-PECl]d[CuCl]d[bpy]o = 1:1:3.

Th e M n , s ~ cs very close to the theoretical one, Mn,th,calculated by means of eq l e6

This indicates that 1-PECl acts as an efficient initiator andthe number of chains is constant. The molecular weightdistribution is fairly narrow (MJMn = 1.3-1.45). A linear plotof In( [M]d[M]) versus polym erization time indicates that theconcentration of growing radicals remains constant duringpropagation and termination is not significant. Both of theseresults suggest a "living" polymerization process with anegligible amount of irreversible transfer and termination.Additionally, a series of experiments has been carried out at13 0 OC, sing various [M]d[l-PECl], ratios and a constant[l-PEC l]d[CuC l]d[bpy]o ratio of 11113. Figure 2 compares the

Mn,SEC and Mn,th calculated based on eq 1.A linear plot is observed in a molecular weight range from3.8 x lo3 o 1.05 x lo5. The slope of the straight line is 0.93,indicating a high initiator efficiency. The polydispersities ofall polymers obtained also remain low and are smaller than ina conventional radical polymerization, Le., M J M n < 1.5.Table 1 summarizes the results of styrene polymerizationunder various experimental conditions. When neither 1 PECl,( 6 ) MI0 and [l-PECl]o represent the initial concentrations of monomer,St, and 1 -PE Cl, respectively, and (MW)o is the molecular weight ofmonomer.

0 1995 American Chemical Society

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Communications to the Editor J . Am. Chem. SOC., ol . 117, No. 20, 1995 5615Table 1. Results of Styrene B ulk Polymerizat ion at 130 "C

[Stlo conv [ l -PECl Io [CuCl Io [bpy]~(mmol) (%) (mmol) (mmol) (mmol) Mn.ha M,.SEC Mw/M,4.375 15 0 0 0 132100 1.764.375 52 0 0.182 0.54 134700 1.954.375 40 0 .0528 0.0455 3400 68700 3.104.40 45 0.0528 0.135 4100 76500 2.10

a Calculated on the basis of eq 1.Scheme 1

Initiation:R - CI + Cu'L, 5 R' + CI - Cu"L, I$+M ki 1 +M

R-M-CI + Cu'L, [ R-M' + CI -Cu"L,]Propagation:

Pi - CI + CU'L,s - [ Pi' + CI - Cu=L, I&kp

CuC1, nor bpy is used, the polymers have ill-controlledmolecular weight and broader molecular weight distribution.Furthermore, similar initiating systems based on various alkylhalides/transition metal species/coordinative ligands can besuccessfully used for the controlled polymerization of variousvinyl monomers, such as acrylics, styrenics, and diene^.^Moreover, block copolymers of St and (meth)acrylates havebeen produced using the same technique as described forhomopolymerization of styrene. Heating of chlorine atom end-capped polystyrenes (OSg, M,, = 4000, Mw/M,, = 1.45) and2-fold excess MA (1.0 g) in the presence of 1 molar equiv ofCuCl and 3 molar equiv of bpy (both relative to polystyrene)at 1 30 "C results in MA block polymerization to form a desiredPSt-b-PMA block copolymer (yield: 95%, Mn= 13 000, Mw/

Mn= 1.35).By analogy with transition-metal-catalyzed ATRA 2C-e.9 hepresent results can be explained by the plausible mechanismshown in Scheme 1 . It constitutes a number of ATRA processesand therefore can be called atom transfer radical polymerization,ATRP.The catalyst CUI acts as a carrier of the chlorine atom in aredox reaction between CUI and C U ~ ~ . ~ ~ - ~ ~ ~he coordinationof the bidentate nitrogen ligand to CUI increases the solubility(7 ) Matyjaszewski, K.; Wang, J. S. U.S. pat. pending.(8) Polystyrene was prepared as follows: heating of styrene (52.5 mmol),1-PECI (1.66 mmo l), CuCl (1.66 mmol), and bpy (4.98 mmol) in a sealedtube with stirring at 13 0 "C for 5 h. It was then dissolved in THF an dprecipitated in MeOH (three times), filtered, and finally dried at 60 "C undervacuum for 48 h. Yield: 95%.(9) Kochi, J. K. Organometallic Mechanisms and Catalysis; AcademicPress: New York , 1978 and references therein.

of the inorganic salt and can also affect the position of the redoxequilibrium to facilitate the abstraction of a chlorine from theinitiato r, 1-PEC1, and the dormant specie s, Pi-Cl, with theformation of initiating and growing radicals, respectively. Th ereversible conversion of alkyl radical, R' nd Pi', to alky l halide ,R-C1 and Pi-C1, may involve a direct atom transfer r e a c t i ~ n ~ . ' ~or oxidative additiodreductive elimination with the formationof the organocopper(1II) intermediate^.^-" If the concentrationof growing radicals is low and the redox reaction is fastcompared to bimolecular reactions of the radicals, the extentof the termination reactions is minim ized, resulting in a "living"process. Moreover, if the rate of reversible conversion of Pi'to Pi-Cl is comparable to that of propagation, the molecularweight should be defined by eq 1 and the molecular weightdistribution remains narrow.

Two observations support the participation of free radicalsin ATRP. First, the tacticity of the polymers is similar to thetacticity of polymers sy nthesized by typical radical initiators.I2Therefore, the organocuprate(III) species, if it exists, sh ould notreact directly with monomer; otherwise som e effect on tacticitywould be expected. Second, addition of 1.5 molar equiv ofgalvinoxyl relative to 1 PECl effectively inhibits the polymer-ization, and no styrene polymerization was found within 18 h.It must be stressed that the present transition-metal-promoted

ATRP, in which the molecular weight linearly increases withmonomer co nversion, is very different from typical redox radicaltelomerization promoted by transition-metal specie^,'^^'^ inwhich the molecular weigh t does not increase with conversion.In conclusion, we have shown that, when an alkyl chloride,1-PECl, and a C uC Ihp y complex were used as an initiator anda catalyst, respectively, styrene polymerized by repetitive atomtransfer radical additions to yield well-defined high molecularweight polymers with narrow molecular weight distributions.Compared to o ther "living" radical system^,^ ATRP representsa simple, inexpensive, and more general method for controlledradical polymerization?

JA950538I(10) (a) Asscher, M.; Vofsi, D. J. Chem. Soc.,Phys. Org. 1968 ,947 . (b)(1 1) (a) Orochov. A.: Asscher. M .: Vofsi. D. J . Chem. Soc.. Perkin Trans.Cohen, H.; M eyerstein, D. Znorg. Chem. 1974, 13 , 2434.2 1973,'lOoO. b) Mitani, M.; Kato, I. ; Koyama, K. J. Ah. Chem. SOC.1983.105. 6719.. -, .-, ..(12) For exa mple, the tacticity of poly(m ethy1 methacry late) ( M , =35 400, Mw/Mn= 1.40) synthesized using 1-PECVCuClBpy (1/1/3 molarratio) initiator system at 130 "C is rr/mr(rm)/mm: 53/38/9. These valuesare very close to the one prepared using a typical radical initiator, BPO, atthe sam e temperature.(13) Boutevin, B.; Pietrasanta, Y. In Comprehensive Polymer Science;Allen, G.,Agganval, S. L. , Russo, S., Eds.; Pergamon: Oxford, 1991; Vol._3, p 185.(14) Bamford, C. H. In Comprehensive Polymer Science (First Supple-ment); Allen, G., Agganval, S. L., Russo, s., Eds.; Pergamon: Oxford,1991; p 1.