Letters

Grafting Poly(n-butyl acrylate) from a FunctionalizedCarbon Black Surface by Atom Transfer Radical

Polymerization†

Tianqi Liu, Shijun Jia, Tomasz Kowalewski, and Krzysztof Matyjaszewski*

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue,Pittsburgh, Pennsylvania 15213

Rosa Casado-Portilla and James Belmont

Cabot Corporation, 157 Concord Road, Billerica, Massachusetts 01821-7001

Received February 8, 2003. In Final Form: May 5, 2003

The functionalization of a carbon black surface with atom transfer radical polymerization (ATRP) initiatingsites and subsequent ATRP of n-butyl acrylate (n-BA) from the surface of carbon black is reported. Thepolymerizations were carried out using CuBr/N,N,N′,N′,N′′-pentamethyldiethylenetriamine and CuBr2/4,4′-di(5-nonyl)-2,2′-bipyridine as the catalytic system at 70 °C. A significant amount of polymers wasattached to the carbon black as determined by thermogravimetric analysis and light scattering experiments.The molecular weights of the polymers grafted on carbon black were determined by detachment experimentsand measured by gel permeation chromatography. The presence of dense poly(n-BA) brushes aroundcarbon black aggregates was confirmed by atomic force microscopy. The resulting poly(n-BA)-bound carbonblacks formed stable dispersions in organic solvents for poly(n-BA).

Carbon black is composed of primary particles withdiameters of 10-75 nm that are fused into aggregatesthat range from 50 to 500 nm in size. It is widely used inthe rubber industry as a reinforcing agent and in theplastics industry for its antistatic, conductive, and UV-protective capacities.1-4 It is also an important pigmentused for coatings,5 plastics,1,3 packaging,6 ink,7 and inkjet8,9

and toner applications.10,11 These applications involvedispersing carbon black into either polymeric systems orsolvents. The control over interfacial properties of thesematerials is of great importance and still remains achallenge. It has been found that the dispersibility ofcarbon black in resins and solvents can be significantlyimproved by grafting polymers onto the surface of carbonblack.12,13

Two methods are generally used to form covalentpolymer-bound carbon black hybrids: “grafting from” and“grafting onto”. “Grafting onto” involves the reaction ofreactive macromolecules with surface groups. Initiallyattached polymers, however, hinder the approach of otherpolymers, leading to a limited density and coverage.“Grafting from” involves surface initiation and subsequentpolymer growth from the surface. It usually affords ahigher grafting density due to the much higher diffusionrate of small molecules (i.e., monomers) compared topolymers.

Anionic,14,15 cationic,16-18 and radical19-21 graft poly-merizations onto carbon black have been investigated.Radical polymerizations can tolerate many functionalgroups and can be performed under mild conditions. Sincecarbon black is known to be a strong radical scavengerdue to the presence of polycondensed aromatic rings andquinonic and phenolic oxygens, propagating polymericradicals can be easily trapped by carbon black surfacesduring polymerization. Preformed polymers can alsobe grafted onto carbon black if they can dissociate andform radicals. Tsubokawa and co-workers22 synthesizedTEMPO-terminated polystyrenes with controlled molec-

† Part of the Langmuir special issue dedicated to David O’Brien.(1) Donnet, J. B.; Bansai, R. C.; Wang, M. J. Carbon Black: Science

and Technology, 2nd ed.; Marcel Dekker: New York, 1993.(2) Wolff, S.; Wang, M. J. In Carbon Black, 2nd ed.; Donnet, J. B.,

Bansal, R. C., Wang, M. J., Eds.; Marcel Dekker: New York, 1993; p289.

(3) Funt, J. M.; Sifleet, W. L.; Tomme, M. In Carbon Black, 2nd ed.;Donnet, J. B., Bansal, R. C., Wang, M. J., Eds.; Marcel Dekker: NewYork, 1993; p 389.

(4) Donnet, J. B.; Voet, A. Carbon Black: Physics, Chemistry, andElastomer Reinforcement; Marcel Dekker: New York, 1976.

(5) Li, W.; Xie, Z.; Li, Z. J. Appl. Polym. Sci. 2001, 81, 1100.(6) Chi, A. Y. Pop. Plast. Packag. 1998, 43, 57.(7) Belmont, J.; Adams, C. U.S. Patent 5,713,988, 1998.(8) Belmont, J.; Johnson, J.; Adams, C. U.S. Patent 5,630,868, 1998.(9) Belmont, J.; Johnson, J.; Adams, C. U.S. Patent 5,571,311, 1996.(10) Hisashi, H. Kino Zairyo 2000, 20, 42.(11) Julien, P. C. In Carbon Black, 2nd ed.; Donnet, J. B., Bansal,

R. C., Wang, M. J., Eds.; Marcel Dekker: New York, 1993; p 409.(12) Ikeda, H.; Urashima, N.; Ando, N. European Patent Application

987303, 2000.(13) Tsubokawa, N.; Satoh, T.; Murota, M.; Sato, S.; Shimizu, H.

Polym. Adv. Technol. 2001, 12, 596.

(14) Tsubokawa, N. J. Macromol. Sci., Chem. 1987, A24, 763.(15) Tsubokawa, N.; Tsuchida, H.; Kobayashi, K. Nippon Gomu

Kyokaishi 1993, 66, 124.(16) Tsubokawa, N.; Takeda, N.; Kanamaru, A. J. Polym. Sci., Polym.

Lett. Ed. 1980, 18, 625.(17) Tsubokawa, N.; Takeda, N.; Kanamaru, A. Carbon 1980, 18,

378.(18) Tsubokawa, N.; Jian, Y.; Sone, Y. J. Polym. Sci., Part A: Polym.

Chem. 1988, 26, 2715.(19) Hayashi, S.; Handa, S.; Tsubokawa, N. J. Polym. Sci., Part A:

Polym. Chem. 1996, 34, 1589.(20) Hayashi, S.; Naitoh, A.; Machida, S.; Okazaki, M.; Maruyama,

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10.1021/la034219d CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 05/31/2003

ular weights and low polydispersities and grafted themonto carbon black by thermal dissociation of a C-ON bondfollowed by trapping of the polymeric radicals by carbonblack. Despite the advantage of grafting well-definedpolymers onto carbon black, the percentage of graftingwas low (<20%) due to inefficient trapping of polymericradicals and unavoidable chain-chain coupling. Alter-natively, peroxyesters,21 diazo groups,23 and alcoholichydroxyl groups5,24 have been introduced onto carbon blacksurfaces to grow polymers by conventional radical or redox-initiated polymerization methods. However, these poly-merization methods cannot offer control over molecularweights and molecular weight distributions of polymersgrown onto carbon black surfaces.

It would be ideal to have both well-defined polymers oncarbon black and a desired percentage of polymer grafting.Therefore, both “grafting from” and controlled radicalpolymerization need to be considered. Atom transferradical polymerization (ATRP)25,26 has evolved as one ofthe most promising controlled/living radical polymeriza-tion methods and has been successfully applied to preparenot only polymers with interesting functionalities27-30 andarchitectures31-33 but also polymer brushes on organic/inorganic substrates.34-44 In this communication,wereport

using ATRP to grow polymers from functionalized carbonblack surfaces. A hydrophobic monomer, n-butyl acrylate,was polymerized from a carbon black surface bearingR-bromoisobutyrate groups. For the first time, to ourknowledge, the morphology of the polymer-bound carbonblack was visualized by atomic force microscopy (AFM).The polymers grown on carbon black were detached bycleavage of ester bonds and characterized by gel perme-ation chromatography (GPC) to determine the polymerchain length.

An aqueous dispersion of Monarch 700 carbon black(CB) with attached C6H4COONa groups (CB-C6H4-COONa) was used as the starting material.45,46 Theparticles in the aqueous dispersion are stabilized elec-trostatically. ATRP initiator containing an R-bromoestermoiety was attached to the carbon black as shown inScheme 1.

n-Butyl acrylate (n-BA) was used as a model hydro-phobic monomer for ATRP initiated from the carbonblack surface. The bulk polymerization was run at amonomer-to-initiator ratio of 375:1 and was catalyzedby CuBr/N,N,N′,N′,N′′-pentamethyldiethylenetriamine(PMDETA)47 at 70 °C. Two control experiments were alsoperformed with a small-molecule initiator, methyl 2-bromopropionate (MBP), which bears a similar initiatingmoiety. One was homogeneous ATRP of n-BA, and theother was ATRP of n-BA in the presence of unfunction-alized CB. Three percent CuBr2/4,4′-di(5-nonyl)-2,2′-bipyridine (dNbpy) relative to CuBr was also added to thesystem to provide enough deactivators to achieve con-trolled polymerization from the surface.40,48,49 Ten percentCuBr2/dNbpy50 relative to CuBr was also tried, but the

(22) Yoshikawa, S.; Machida, S.; Tsubokawa, N. J. Polym. Sci., PartA: Polym. Chem. 1998, 36, 3165.

(23) Fujiki, K.; Tsubokawa, N.; Sone, Y. Polym. J. 1990, 22, 661.(24) Tsubokawa, N.; Fujiki, K.; Sone, Y. J. Macromol. Sci., Chem.

1988, A25, 1159.(25) Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117,

5614.(26) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T.

Macromolecules 1995, 28, 1721.(27) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921.(28) Coessens, V.; Pintauer, T.; Matyjaszewski, K. Prog. Polym. Sci.

2001, 26, 337.(29) Davis, K. A.; Matyjaszewski, K. Adv. Polym. Sci. 2002, 159, 2.(30) Matyjaszewski, K. Chem.sEur. J. 1999, 5, 3095.(31) Patten, T. E.; Matyjaszewski, K. Adv. Mater. (Weinheim,

Germany) 1998, 10, 901.(32) Qiu, J.; Charleux, B.; Matyjaszewski, K. Prog. Polym. Sci. 2001,

26, 2083.(33) Patten, T. E.; Xia, J.; Abernathy, T.; Matyjaszewski, K. Science

1996, 272, 866.(34) Beers, K. L.; Gaynor, S. G.; Matyjaszewski, K.; Sheiko, S. S.;

Moeller, M. Macromolecules 1998, 31, 9413.(35) Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436.(36) Xiao, D. Q.; Zhang, H.; Wirth, M. Langmuir 2002, 18, 9971.(37) Kickelbick, G.; Holzinger, D.; Brick, C.; Trimmel, G.; Moons, E.

Chem. Mater. 2002, 14, 4382.(38) Bontempo, D.; Tirelli, N.; Masci, G.; Crescenzi, V.; Hubbell, J.

A. Macromol. Rapid Commun. 2002, 23, 418.

(39) Bottcher, H.; Hallensleben, M. L.; Nuss, S.; Wurm, H.; Bauer,J.; Behrens, P. J. Mater. Chem. 2002, 12, 1351.

(40) Huang, W. X.; Kim, J. B.; Bruening, M. L.; Baker, G. L.Macromolecules 2002, 35, 1175.

(41) Perruchot, C.; Khan, M. A.; Kamitsi, A.; Armes, S. P.; von Werne,T.; Patten, T. E. Langmuir 2001, 17, 4479.

(42) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813.(43) Guerrini, M. M.; Charleux, B.; Vairon, J. P. Macromol. Rapid

Commun. 2000, 21, 669.(44) Pyun, J.; Matyjaszewski, K.; Kowalewski, T.; Savin, D.; Patter-

son, G.; Kickelbick, G.; Huesing, N. J. Am. Chem. Soc. 2001, 123, 9445.(45) Belmont, J. A.; Amici, R. M.; Galloway, C. P. U.S. Patent

5,851,280, 1998.(46) Belmont, J.; Amici, R.; Galloway, C. U.S. Patent 6,042,643, 2000.(47) Xia, J.; Matyjaszewski, K. Macromolecules 1997, 30, 7697.(48) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.;

Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.;Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716.

Scheme 1. Functionalization of Carbon Black

Letters Langmuir, Vol. 19, No. 16, 2003 6343

resulting polymerization rate was too slow (<2% con-version after 40 h). As shown in Figure 1, linear first-order kinetic plots were observed for all three cases,indicating a stable radical concentration in the systems.The polymerization of n-BA initiated by MBP was quitefast, reaching 70% conversion after 8 h. When thepolymerization was performed in the presence of carbonblack, the polymerization rate decreased by a factor of 10.The drop in rate is attributable to carbon black’s knownradical-scavenging capacity. When the initiator wasimmobilized on the carbon black surface, the polymeri-zation rate was even slower, suggesting that other factorsbesides typical radical trapping may be at play. Since theinitiators are on the surface instead of being free insolution, the chance of radical coupling between neighborsis greatly increased. More efficient termination of growingradicals by surface quinone, phenol or polycondensed ringsis also expected. The initiator density on the carbon blackwas determined by elemental analysis; however, some ofthe initiator sites may be inaccessible to the catalyst/liganddue to the structure characteristics of carbon black andthe formation of agglomerates in the reaction system.During the polymerization, a significant increase inviscosity was observed at as early as 10% monomerconversion, most likely due to the interaction betweenthe chains attached to carbon particles. A similar phe-

nomenon was observed when functionalized colloidal silicawas used to polymerize methyl methacrylate (MMA) andstyrene in bulk.51 After the polymerization was stopped,unreacted monomer and catalyst/ligand were removed bycentrifugation in tetrahydrofuran (THF)/MeOH (1:3 v/v).The poly(n-BA)-bound carbon black (CB-PBA) could beeasily dispersed in any good solvent for poly(n-BA). WhenCB-PBA was dispersed in THF and centrifuged at 39000gforce for 3 h at 4 °C, only a small amount of carbon blackwas found to deposit in the tube and the majority of thematerial stayed dispersed.

As can be seen from Table 1, poly(n-BA) was grafted tocarbon black using ATRP (entries 1-4) and the volatilecontent detected for all “grafted from” systems was above50%,corresponding tomore than100%graftingpercentage(calculated by (weight of polymer/weight of carbon black)× 100%). As a result, these carbon blacks formed stabledispersions in THF and it was difficult to collect them bycentrifugation. Instead, THF/MeOH (1:2 v/v) (entries 2Aand 3A) was used as the solvent to collect the particles;however, the upper layer still contained some carbon blackwhich did not settle down after hours of centrifugation.Presumably these particles had a higher polymer contentthan those that settled down under these conditions.Indeed, when the amount of methanol, which is a poorsolvent for poly(n-BA), was increased in the THF/methanolsolvent pair (entries 2B and 3B), carbon black particlesfrom the upper layer settled down in the centrifuge.Correspondingly, the volatile content of these particlesmeasured by thermogravimetric analysis (TGA) washigher than for those collected from the THF/MeOH (1:2v/v) mixture. The particle size measurements confirmedthe formation and good dispersibility of poly(n-BA)-boundcarbon black in THF as shown by the size increasecompared to the original carbon black dispersed in water.As a comparison, when unfunctionalized carbon black waspresent in the ATRP of n-BA (entry 5 in Table 1), thevolatile content was much lower and the sample could beeasily collected by centrifugation in THF. This confirmedthat the “grafting from” strategy is superior to the “graftingonto” strategy in terms of producing stable dispersedcarbon black with high polymer content. It is also notedthat the molecular weight of the polymers grafted oncarbon black was far greater than what was expectedtheoretically based on the monomer-to-initiator ratio andconversion, which was attributed to a low initiatingefficiency due to the reasons described before.

One of the most distinct structural features of carbonblack is that the basic dispersible units are aggregates,which are composed of covalently bonded primary particles

(49) Jeyaprakash, J. D.; Samuel, S.; Dhamodharan, R.; Ruhe, J.Macromol. Rapid Commun. 2002, 23, 277.

(50) Matyjaszewski, K.; Patten, T. E.; Xia, J. J. Am. Chem. Soc. 1997,119, 674.

(51) Savin, D. A.; Pyun, J.; Patterson, G. D.; Kowalewski, T.;Matyjaszewski, K. J. Polym. Sci, Part B: Polym. Phys. 2002, 40, 2667.

Table 1. Characterizations of Poly(n-BA)-Bound Carbon Black

sampleBr content in

CB-Br (mmol/g) conv (%)volatile

contenta (%)volatile

contentb (%)particle

sizec (nm) Mn (theor) Mn (PDI)d

1 0 0 6.7 121 (in H2O)2Ae

0.35 17 74 64.7 302 8160 48450 (1.33)2Bf 65.2 3653Ae

0.49 10 70 56.9 349 4800 40220 (1.83)3Bf 66.7 3064g 0.49 10 70 64.6 303 4800 39600 (1.86)5h 0 62 27.7 nondispersible 29760

a Calculated by [(weight of monomer × conv %)/(weight of monomer × conv % + weight of carbon black)] × 100%. b Determined by TGAunder N2. c Measured in THF. d Measured by GPC using THF as the eluent and toluene as the internal standard at a flow rate of 1 mL/minat 25 °C; Mn was measured relative to polystyrene standards. e Solid sample collected from centrifugation of the crude polymerizationproduct in THF/methanol (1:2 v/v). f Solid sample obtained after the supernatant liquid from (e) was concentrated, redissolved in THF/methanol (1:3 v/v), and centrifuged. g Solid sample collected by centrifugation of the crude polymerization product in THF/methanol (1:3v/v). h Carbon black (Monarch 700) was put into the polymerization system only as a radical scavenger.

Figure 1. Kinetics of ATRP of n-BA initiated by MBP (9),initiated by MBP in the presence of carbon black ((), andinitiated by functionalized carbon black (2). [n-BA]/[initiator]/[CuBr/PMDETA]/[CuBr2/dNbpy] ) 375:1:2:0.06. The sameamount of carbon black was added in the last two cases. Thereactions were run at 70 °C.

6344 Langmuir, Vol. 19, No. 16, 2003 Letters

(transmission electron microscopy (TEM) shown in Sup-porting Information). The starting water-dispersiblecarbon black particles are irregular in shape and exist inseparate aggregates around 120 nm in size due toelectrostatic repulsions. The morphology of poly(n-BA)-bound carbon black was studied with the aid of tappingmode AFM. Figure 2 shows a height image of the carbonblack particle with poly(n-BA) chains (Table 1, entry 2)forming a dense “brush” extending as far as ∼130 nmbeyond the ∼150 nm diameter carbon black core. Heightprofile analysis indicated that in the aggregate depositedon the mica surface the poly(n-BA) chains conformed tothe contours of the ∼40 nm tall carbon black core,presumably due to the polymer’s low Tg. Some freeindividual polymer chains are also visible in the image,which is probably caused by polymers formed in thesolutionphase initiated fromphysicallyabsorbed initiatorson carbon black due to its large surface area. An AFMimage showing agglomerates with polymer brushes isincluded in the Supporting Information. In comparison,when poly(n-BA) was “grafted onto” carbon black as shownin entry 5, Table 1, no polymer chains were found to extendout from the carbon black core although some amount ofthe polymer was attached to the surface (AFM imageshown in Supporting Information). It is consistent withTsubokawa’s observations that carbon black preferentiallytraps low molecular weight polymers. An AFM image ofa physical mixture of carbon black with poly(n-BA)(Supporting Information) did not reveal any polymerchains extending from carbon black particles, whichindicates that physical absorption cannot lead to theassembly of polymer chains around carbon black cores.The AFM image of the polymer-grafted carbon blackparticle described in Table 1, entry 3, is shown in Figure3. The carbon black core has similar dimensions comparedto the one shown in Figure 2 and is also surrounded bythe corona of polymer chains. However, in this case thelengths of polymer chains appear to be much more broadlydistributed. This observation is consistent with the highpolydispersity index (PDI) of the detached polymermeasured by GPC.

Poly(n-BA)-bound carbon blacks were subjected todetachment experiments in order to determine the mo-lecular weight of the polymer covalently attached to thecarbon black. Detachment was made possible by acid-catalyzed transesterification with alcohol of ester linkages

between poly(n-BA) chains and carbon black.52 Beforedetachment, CB-PBA was dispersed in THF and cen-trifuged. The supernatant layer was concentrated toensure that there was no free polymer detectable by GPC.Then it was refluxed in THF/butanol in the presence ofconcentrated H2SO4 for several days. n-Butanol waschosen because it can maintain the structure of the poly-(n-BA) side chains. After centrifugation, the upper super-natant layer was concentrated. Detachment of poly(n-BA) from the surface was confirmed by 1H NMR. Themolecular weight of the detached polymer was measuredby GPC (results are shown in Table 1). Interestingly, thelength of poly(n-BA) chains attached to carbon black asdetermined by GPC and AFM was much larger thanpredicted from the consumed monomer-to-initiator ratio.This discrepancy can be explained by a limited initiatingefficiency of this system due to radical coupling andtrapping.

In conclusion, the “grafting from” strategy has beensuccessfully employed to grow poly(n-BA) from a func-tionalized carbon black surface in a controlled fashionusing ATRP. Dense polymer brushes with high graftingpercentages were formed around carbon black cores asdemonstrated by AFM and TGA. These hybrid particlesalso displayed excellent dispersibility and stability inorganic solvents which act as good solvents for poly(n-BA).

Acknowledgment. T. Liu thanks Dr. Jeffrey Pyunfor suggestions and discussions and Joseph Suhan(Carnegie Mellon University) for conducting the TEMmeasurements. Financial support from the Cabot Cor-poration (Billerica, MA) and the National Science Foun-dation (DMR 00-90409) is appreciated.

Supporting Information Available: Experimentalprocedures of functionalization of carbon black with ATRPinitiator and ATRP of n-BA from the functionalized carbon blacksurface, and TEM and AFM images of original and modifiedcarbon black. This material is available free of charge via theInternet at http://pubs.acs.org.

LA034219D

(52) Boerner, H. G.; Beers, K.; Matyjaszewski, K.; Sheiko, S. S.;Moeller, M. Macromolecules 2001, 34, 4375.

Figure 2. Tapping mode AFM image of poly(n-BA)-boundcarbon black (entry 2, Table 1).

Figure 3. Tapping mode AFM image of poly(n-BA)-boundcarbon black (entry 3, Table 1).

Letters Langmuir, Vol. 19, No. 16, 2003 6345