7498 J. Am. Chem. Soc., Vol. 123, No. 31, 2001
Von Werne and Patten
Scheme 1. Synthetic Steps for Forming Hybrid Polymer-inorganic Nanoparticles
trolled polymerization from flat surfaces.15,18 The lack of
molecular weight control was manifested as a large jump in
film thickness at short polymerization times and was a conse-
quence of the low concentration of initiator in the system. The
nanoparticles using ATRP.28 We found that the polymerization
of styrene from the surface of 75-nm silica particles exhibited
the diagnostic criteria for a controlled/“living” polymerization:
an increase in the molecular weight of the pendant polymer
chains with monomer conversion and a narrow molecular weight
distribution for the grafted chains. In this report, we elucidate
the chemistry of these grafting reactions and how changes in
monomer type and particle size affect the polymerization
kinetics and molecular weight control. The data are examined
in the context of other ATRP studies from low surface area
substrates.
18
addition of deactivator and the addition of “sacrificial” free
initiator15 to the polymerization system were two methods
examined for inducing molecular weight control. The addition
of free initiator to the polymerization medium served to lower
the initial monomer-to-initiator ratio and to increase the overall
initiator concentration, thereby allowing some radical coupling
in solution to build up the concentration of deactivator. The
addition of the CuBr2/2 equiv of ligand complex in copper(I)-
catalyzed ATRP systems mitigated the insufficient formation
of deactivator from the small initial concentrations of initiator
and copper(I) catalyst. Both studies showed that it was possible
to use controlled/“living” radical polymerizations to prepare a
thin film of well-defined polymer chains covalently anchored
to an inorganic surface.
Results and Discussion
To study the grafting of polymer chains from inorganic
nanoparticles, we broke the problem down into two steps
(Scheme 1): (1) depositing a monolayer of polymerization
initiators on a nanoparticle surface and (2) conducting poly-
merizations using the nanoparticle as a macroinitiator and
examining the effect of varying synthetic parameters, such as
monomer type and nanoparticle diameter, upon the polymeri-
zation reaction. For this research, silica (SiO2) nanoparticles
were employed, because their preparation and surface chemistry
are well-understood. Spherical silica particles were prepared
The chemistry of grafting polymers to and from the surfaces
of nanoparticles is also a relatively new area. Gold nanoparticles
2
2
were coated with alkanethiol-containing initiators for ROMP
2
3
and isocyanate polymerizations and used to initiate living
polymerizations. Latexes functionalized with initiators for ATRP
were used as initiators for the polymerization of water-soluble
acrylates and methacrylates.24 Silica and silver nanoparticles
2
9,30
using the St o¨ ber process:
the hydrolysis and condensation
have been coated with polymer using emulsion polymeriza-
2
5
of tetraethoxysilane in an ammonia/ethanol solution. This
synthesis yields a high volume fraction of silica particles with
a uniform size distribution, and the size of the particles can be
controlled through variations in the initial concentrations of
reagents. Accessible surface silanol groups can be derivatized
using organosiloxanes, providing a means for covalent attach-
ment of polymerization initiators to the nanoparticle surface.
Among various polymerization methods, we chose to use ATRP,
in particular, because it affords the molecular weight control of
a living polymerization method, can be used to polymerize a
range of vinyl monomers, does not require stringent conditions,
and is tolerant of functional groups and impurities that are
detrimental to living anionic, cationic and some transition-metal-
tions, and composite materials of silica colloidal crystals and
26
polymer have been prepared by in situ polymerizations. Also,
various polymers have been attached to silica1 and CdS27
nanoparticle surfaces by chemisorption of reactive polymer chain
ends.
,2
Recently we reported a technique for conducting controlled/
“
living” radical polymerizations from the surface of silica
(18) Matyjaszewski, K.; Miller, P.; Shukla, N.; Immaraporn, B.; Gelman,
A.; Luokala, B.; Siclovan, T.; Kickelbick, G.; Vallant, T.; Hoffmann, H.;
Pakula, T. Macromolecules 1999, 32, 8716-8724.
(
19) Ma, H.; Davis, R. H.; Bowman, C. N. Macromolecules 2000, 33,
3
31-335.
20) de Boer, B.; Simon, H. K.; Werts, M. P. L.; van der Vegte, E. W.;
Hadziioannou, G. Macromolecules 2000, 33, 349-356.
21) Kim, N. Y.; Jeon, N. L.; Choi, I. S.; Takami, S.; Harada, Y.; Finnie,
(
(
3
1
mediated polymerizations.
K. R.; Girolami, G. S.; Nuzzo, R. G.; Whitesides, G. M.; Labinis, P. E.
Macromolecules 2000, 33, 2793-2795.
Tandem ATRP initiators/organosiloxanes were prepared via
the hydrosilation of allyl ester group-containing ATRP initiators
with dimethylethoxysilane. The use of a monosiloxane group
provided a site for single attachment to the nanoparticle surface,
as opposed to di-and trisiloxanes, which can form complex
(
22) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem.
Soc. 1999, 121, 462-463.
(23) Huber, D. L.; Carlson, G.; Gonsalves, K.; Seery, T. A. P. The
Formation of Polymer Monolayers: From Adsorption to Surface Initiated
Polymerizations. In Interfacial Aspects of Multicomponent Polymer Materi-
als; Lohse, D. J., Russell, T. P., Sperling, L. H., Eds.; Plenum Press: New
York, 1997; pp 107-122.
(
24) Guerrini, M. M.; Charleux, B.; Vairon, J.-P. Macromol. Rapid
(28) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 7409-
Commun. 2000, 21, 669-674.
7410.
(
25) Quaroni, L.; Chumanov, G. J. Am. Chem. Soc. 2000, 122.
(29) Philipse, A. P.; Vrij, A. J. J. Colloid Interface Sci. 1989, 128, 121-
136.
(26) Jethmalani, J. M.; Sunkara, H. B.; Ford, W. T.; Willoughby, S. L.;
Ackerson, B. J. Langmuir 1997, 13, 2633-2639.
27) Carrot, G.; Scholz, S. M.; Plummer, C.; Hilborn, J.; Hedrick, J.
Chem. Mater. 1999, 11, 3571-3577.
(30) Bogush, G. H.; Tracy, M. A.; Zukowski, C. F., IV. J. Noncryst.
Solids 1988, 104, 95-106.
(31) Patten, T. E.; Matyjaszewski, K. AdV. Mater. 1998, 10, 1-15.
(