Synthesis of Polymer Grafted Magnetite Nanoparticle with the Highest Grafting Density via Controlled Radical Polymerization
© to the authors 2009
Received: 9 March 2009
Accepted: 26 May 2009
Published: 14 June 2009
The surface-initiated ATRP of benzyl methacrylate, methyl methacrylate, and styrene from magnetite nanoparticle is investigated, without the use of sacrificial (free) initiator in solution. It is observed that the grafting density obtained is related to the polymerization kinetics, being higher for faster polymerizing monomer. The grafting density was found to be nearly 2 chains/nm2for the rapidly polymerizing benzyl methacrylate. In contrast, for the less rapidly polymerizing styrene, the grafting density was found to be nearly 0.7 chain/nm2. It is hypothesized that this could be due to the relative rates of surface-initiated polymerization versus conformational mobility of polymer chains anchored by one end to the surface. An amphiphilic diblock polymer based on 2-hydroxylethyl methacrylate is synthesized from the polystyrene monolayer. The homopolymer and block copolymer grafted MNs form stable dispersions in various solvents. In order to evaluate molecular weight of the polymer that was grafted on to the surface of the nanoparticles, it was degrafted suitably and subjected to gel permeation chromatography analysis. Thermogravimetric analysis, transmission electron microscopy, and Fourier transform infrared spectroscopy were used to confirm the grafting reaction.
The use of material in the nanoparticles form offers many advantages due to the large surface-to-volume ratio . Magnetite nanoparticles (MNs) is one of the most popular nanomaterial known for its biomedical applications because of its low toxicity for living cells and in the view of possibility of selected targeting of tumor area, through external magnetic field. MNs, especially in the size range of 10 nm, is interesting because of its superparamagnetic nature, as it does not retain its residual magnetism after the magnetic field is removed. The superparamagnetic iron oxide nanoparticles are used in a number of biomedical areas such as magnetic resonance imaging , targeted drug delivery [3, 4], gene delivery systems, and gene therapy  as well as targeted hyperthermia of cancers . In all the above applications, it is preferable that MNs are encapsulated with a polymer of interest in order to avoid its agglomeration for various biomedical applications. This is in view of the tendency of nanoparticle to agglomerate, as a result of van der waals attractive forces. The two common modes of preventing the agglomerization and stabilizing the nanoparticles are: (1) electrostatic stabilization and (2) steric stabilization. The electrostatic stabilization results from the coulombic repulsion between the particles caused by the electrical double layer, which inturn is formed by ions adsorbed on the particle surface. The citrate ion is commonly used as the reducing agent as well as an electrostatic stabilizer for gold nanoparticles [7, 8]. The stabilization thus brought about is kinetic stabilization and is applicable only to dilute systems . Thus to overcome this disadvantage, steric stabilization is introduced in which the coordination of sterically demanding organic molecules, surfactants, and polymers can act as protecting shields for the steric stabilization of metal colloids. Steric stabilization provides a thermodynamically stable system. Among the stabilizers, polymers are considered to be better steric stabilizing agents .
There are two ways of attaching polymer layers to nanoparticulate surfaces namely, “grafting from” and “grafting to”. The shape of the semiflexible polymer chain, in solution, is a sphere. The adsorption or “grafting to” of polymer to a surface produces a monolayer of “spherical” polymer chains. Further adsorption is restricted since the surface concentration is much higher than solution concentration (diffusion barrier) and in addition the “entropic” penalty for stretching away from the surface is high . For example, in a recent publication “click chemistry” was used to anchor an oligomer to silica particle wherein a grafting density of 0.34 chains/nm2. In contrast in the “grafting from” technique, polymer chains are grown from the surface-attached initiator by in situ polymerization via thermal or photochemical means  in which the optimum control over the structure of the composite can be achieved with the nanomaterial core and a dense polymer shell. Thus the surface-initiated polymerization i.e., polymerization from a nanoparticle with an active initiator, helps to form a uniform surface coating of the polymer chains on the surface of the particles.
The thickness of the grafted polymer layer increases with increasing polymerization time for a controlled radical polymerization, at fixed monomer concentration. When polymer chains are densely grafted to a surface, steric crowding can force the chains to stretch away from the surface to form a brush. Under this condition, the thickness of the polymer layer should be larger than the radius of gyration of the equivalent free polymer in solution [14, 15]. This results in high grafting density as well as the formation of a stable dispersion of the particle in the solvent of interest.
ATRP of methyl methacrylate from the various nanoparticle
Various anchoring chemistry
Grafting density after polymerization (chain/nm2)
Initiators with a phosphonic acid anchoring moiety are quite interesting since they are known to form self-assembled monolayer (SAM) by strong covalent binding with the surface hydroxyl (–OH) group on metal oxide (titania and zirconium oxide) surfaces . In comparison to other SAMs, the phosphonic acid-based anchoring is preferable for its three advantages: (1) higher hydrolytic stability  under physiological conditions due to chemisorption, (2) easily anchored by sonication , and (3) retention of particle property , especially in the case of magnetite, after anchoring.
The recent developments in living radical polymerization techniques such as atom transfer radical polymerization (ATRP) , nitroxide-mediated polymerization (NMP) , and reversible addition-fragmentation transfer polymerization (RAFT)  have been considerably applied in the surface modification of the nanoparticles. ATRP is a versatile technique to precisely control the chain length and polydispersity of the polymer, and can be used to synthesize well-defined block copolymers with a range of functionalities since the end-groups remain active  at the end of the polymerization. If the ATRP reaction conditions used are mild, a wide range of monomers and macromolecular structures can be used for grafting . Thus, atom transfer radical polymerization (ATRP)  that can be performed at ambient temperature is less prone to side reactions and chain transfer, resulting in better control over molecular weight and polydispersion index (PDI) thus enabling the facile synthesis of a wide variety of hybrid materials [34, 37].
The synthesis of polystyrene grafted MN nanoparticles, without the addition of sacrificial initiator is reported in the literature  but the estimation of grafting density is not reported. The nitroxide-mediated polymerization of styrene  was carried out at 125 °C, from a phosphonic acid anchored MN surface. This results in lower grafting density of 0.2 polystyrene chain/nm2, where the density of surface initiator was 0.73 chain/nm2. It can therefore be concluded that 27% of initiator on the magnetite surface participated in the polymerization, with the addition of sacrificial initiator. This low initiator efficiency could be due to the termination between free chains formed in solution (because of the addition of sacrificial initiator) and a surface-bound polymer [39, 40]. In comparison, the ambient temperature ATRP of methyl methacrylate from a phosphonic acid anchored magnetite surface results in a grafting density of an initiator 1 chain/nm2 for an initiator grafting density of 2 molecules/nm2. Thus 50% of the surface initiating groups on the magnetite surface participated in the polymerization . This was carried out without sacrificial initiator as well as without the initial addition of Cu(II). The high grafting density obtained in this case could be due to the faster polymerization in comparison to conformational relaxation of the growing chain. In order to explore this hypothesis in detail, we have chosen monomers that polymerize faster as well as slower in comparison with methyl methacrylate and report the results obtained. In this work, the ATRP of benzyl methacrylate at ambient temperature as well as that of styrene at 100 °C is carried out from the magnetite nanoparticle surface, without using the sacrificial initiator  to compare its grafting density with that of methyl methacrylate, which was polymerized from the surface of MNs, at ambient temperature . Surface-Initiated polymerization without the use of sacrificial initiator could offer certain advantages such as the elimination of the step associated with the removal of unattached polymer that is formed from the sacrificial initiator. In addition, it could proceed at a faster rate thus facilitating simultaneous growth from the surface sites. A disadvantage of this method is that it is relatively less controlled. To investigate the effect of rate of polymerization on the graft density of polymer chains grown from the MNs surface, the ATRP of benzyl methacrylate, styrene, and MMA were carried out without the addition of sacrificial initiator from a tertiary bromide ATRP initiator anchored to the surface through phosphonic acid anchoring group. In addition, the results from this study are compared with one case where the ATRP initiator is anchored to MNs through carboxylic acid based anchoring group.
The inhibitor present in methyl methacrylate (MMA), benzyl methacrylate (BnMA), styrene, and 2-hydroxylethyl methacrylate (Lancaster) were removed by passing through a basic alumina column. The monomer was used immediately after purification. Copper(I) bromide (Aldrich, 99.98%),N,N,N′,N″,N″′-pentamethyldiethyltriamine (Aldrich, 99%), aluminum oxide (activated, basic, for column chromatography, 50–200 μm) were used without purification. Anisole, ethyl methyl ketone, 1-propanol, and DMF (SRL India) were used as received.
Synthesis of ATRP Initiator and its Anchoring to MNs
The synthesis of magnetite nanoparticles as well as the ATRP initiator, 2-bromo-2-methyl-propionic acid 2-phosphonooxy-ethyl ester (1) and its anchoring to the magnetite nanoparticle to get ATRP initiator immobilized magnetite nanoparticle (2) has been reported by us in the literature .
Surface-Initiated ATRP of Benzyl Methacrylate from Magnetite Surface
The polymerization was carried out with CuBr (0.01 mmol) and 25 mg of magnetite-ATRP initiator, (2), in a dry Schlenk flask equipped with a magnetic pellet and a rubber septum. Initially, the mixture was subjected to dynamic vacuum for 1 h. This was followed by the addition of the degassed benzyl methacrylate (17.04 mmol) (50% v/v of anisole) such that the mole ratio of [BnMA]:[Initiator]:[CuBr]:[PMDETA] is 1900/1/1/1. The mixture was purged with argon for 15 min. and finally, pentamethyldiethyltriamine ligand (0.01 mmol) was added and the mixture was stirred, at 30 ± 1 °C. After the required time, the polymerization was stopped by diluting the reaction mixture with THF. This was followed by precipitation in excess of hexane. It was then redispersed in ~5 ml of THF and was centrifuged to remove any homopolymer to obtain the hybrid material, (3). This was then analyzed by TGA, TEM, and GPC (after degrafting the polymer from the surface).
Surface-Initiated ATRP of Styrene from Magnetite Surface
CuBr (0.070 mmol) and 50 mg of magnetite-ATRP initiator, (2), were added to a dry Schlenk flask equipped with a magnetic pellet and rubber septum. Initially, the mixture was subjected to dynamic vacuum for 1 h. This was followed by the addition of the degassed styrene (25.6 mmol) (50% v/v of DMF) and PMDETA ligand (0.070 mmol) such that the molar ratio of [Styrene]:[Initiator]:[CuBr]:[PMDETA] is 1400/1/4/4. Then, the flask was purged with argon and the contents were stirred in an oil bath maintained at 100 °C, for the required period, toward the preparation of polymer chains of various molecular weights. At the end of the required period, the polymerization was stopped by dilution with THF and precipitated into excess methanol. The precipitate was redispersed in ~5 ml of THF and centrifuged to remove any homopolymer to obtain the hybrid material, (4). This was characterized by FT–IR, TGA, TEM, and GPC (after degrafting the polymer from the surface) from measurements.
Synthesis of Block Copolymer from Polystyrene Grafted-Magnetite Surface
The polymerization was carried out with the initial addition of CuBr (0.140 mmol), 50 mg of polystyrene-magnetite, (4), ethyl methyl ketone, and 1-propanol (in 70/30% v/v) to a dry Schlenk flask equipped with magnetic pellet and rubber septum. This was followed by the addition of the degassed 2-hydroxyethyl methacrylate (27.86 mmol) and purging with argon for 15 min. Finally, pentamethyldiethyltriamine ligand (0.140 mmol) was added and the mixture was stirred, at 30 ± 1 °C, for 48 h. The polymerization was stopped by opening the septum and diluting the reaction mixture with DMF. This was followed by precipitation in 200 ml of hexane to remove the unpolymerized monomer. It was then vacuum dried, (5), and characterized by FT–IR and TGA analyses.
Surface-Initiated ATRP of MMA from Carboxylic Acid Based Magnetite Surface
The anchoring of 2-bromoisobutyric acid, which is a carboxylic acid based ATRP initiator, to magnetite nanoparticles was performed according to the reported literature [43, 44] to get carboxylic immobilized magnetite-ATRP initiator (6). CuBr (0.070 mmol), PMDETA ligand (0.070 mmol), and 50 mg of magnetite-ATRP initiator (6) were added to a dry Schlenk flask equipped with a magnetic pellet and rubber septum. It was degassed using the vacuum line. This was followed by the addition of the degassed methyl methacrylate (27.86 mmol) (50 v/v of anisole) such that the molar ratio of [MMA]:[Initiator]:[CuBr]:[PMDETA] is 335/1/1/1. Then, the flask was purged with argon, and was stirred in an oil bath, maintained at 30 °C. After the desired time, the polymerization was stopped by opening the septum and diluting the reaction mixture with THF. This was followed by precipitation in excess of hexane (200 ml). The precipitate was redispersed in ~5 ml of THF and centrifuged to remove any homopolymer, to obtain the hybrid material, (7). This was characterized by FT–IR, TGA, and GPC analyses.
Thermal analysis was performed using a Mettler Toledo STARe(Switzerland) thermal analysis system under flowing nitrogen atmosphere. The number average molecular weights and polydispersity indices of the degrafted polymer were determined by Waters GPC system. A Waters GPC system with 515 pump (New Jersey, USA; with styragel columns HR3, HR4, HR5) along with Millennium v 2.15 data analyses package was used for the determination of number average molecular weight (M n ) and polydispersity index (PDI). THF was used as an eluent (at a flow rate of 1 ml/min) and narrow molecular weight polystyrene standards were used as the standard. All the measurements were carried out at room temperature. Sample detection was done using a Waters 2414 refractive index detector. Transmission electron microscopy was carried out using a JEOL100CX transmission electron microscope at an acceleration voltage of 100 KeV. Samples were prepared by applying a drop of the nanoparticle solution in THF, to a carbon coated copper grid and imaged after drying. Nicolet 6400 instrument was used for FT–IR analysis. Measurement of magnetization was carried out with a vibrating sample magnetometer (EC&G PARC VSM 155).
Results and Discussion
Surface-Initiated Polymerization of BnMA
ATRP of benzyl methacrylate at ambient temperature
M n × 103(g/mol)
% Weight lossa
Surface-Initiated Polymerization of Styrene and 2-HEMA Block Copolymer
ATRP of styrene at 100 °C
M n × 103(g/mol)
% Weight lossa
An amphiphilic diblock polymer based on 2-hydroxylethyl methacrylate was synthesized from the polystyrene monolayer (Fig. 1). This was done to asses the livingness of the PS synthesized via ATRP. The CuBr/PMDETA catalyst of relatively higher concentration was taken, in comparison to the initiating sites, in order to ensure faster initiation in comparison with propagation. This should help in synthesizing the hybrid material with some control over the ratio of hydrophobic to hydrophilic blocks (the block copolymer synthesized will have hydrophobic character due to styrene and hydrophilic character due to 2-hydroxyethyl methacrylate).
Estimation of Grafting Density of Polymer Grafted MNs using Phosphonic Based Anchoring Group
which is given in the literature . Here, W60–730 °C is the weight loss in percentage of immobilized molecules on MNs after grafting, Wmagnetite is the weight loss in percentage for MNs before grafting, M is molar mass of the immobilized molecules on magnetite and S is the surface area of MNs as measured using BET (Brunauer–Emmett–Teller) adsorption isotherms method (found to be 115 m2/g). The ATRP of methyl methacrylate from MNs with the use of sacrificial initiator was reported by Marutani et al. . The use of sacrificial initiator results in the generation of sufficient concentration of the persistent radical, which enables better control of the surface-initiated ATRP. They reported a grafting density of 0.7 chain/nm2, but the big disadvantage associated with this method is the need to remove free polymer, which is formed due to the addition of sacrificial initiator in the polymerization system (by Soxhlet extraction) . The ATRP of poly(ethylene glycol) methyl ether methacrylate from MNs, without the use of sacrificial initiator was reported by Hu et al. . They reported a grafting density of 0.7 chain/nm2. However, they did not report about the control obtained in the polymerizations. The atom transfer radical polymerization of methyl methacrylate from MNs with the initial addition of Cu(II) was reported by Garcia et al. . Cu(II) addition is expected to bring about control in the surface-initiated polymerization by the persistent radical effect. They reported a grafting density of 0.1 chain/nm2. This could be due to the polymerization kinetics being much slower than the conformational rearrangement of the chains at the interface, which may not permit the growth of new chains by restricting the access of the monomer to the initiating sites. One way of testing this hypothesis is to study the grafting density of surface-initiated ATRP, involving monomers with varying rate of propagation. Therefore, we choose to study the grafting density of surface-initiated ATRP involving three different monomers, viz., benzyl methacrylate, styrene, and methyl methacrylate . The results from these study are summarized below.
Summary of grafting density results from MNs
Initiator anchoring chemistry
Grafting density in chain(s)/nm2
30 °C, ATRP CuBr/PMDETA
30 °C, ATRP CuBr/PMDETA
100 °C, ATRP CuBr/PMDETA
Dispersion of Phosphonic Acid Based Polymer Stabilized MNs
Surface-Initiated Polymerization of MMA from Carboxylic Acid Based Surface Anchored Initiator
ATRP of methyl methacrylate at ambient temperature
M n × 10 (g/mol)
% Weight lossa
Comparison of Initiator Efficiency with Methyl Methacrylate Polymerization from MNs
ATRP of methyl methacrylate from MNs—comparison of grafting density for various anchoring chemistry
Grafting density after immobilizing initiator (molecules/nm2)
Grafting density after polymerization of MMA (chain/nm2)
Average initiator efficiency after polymerization
Polymer brushes [P(BnMA), PS and P(S-b-2-HEMA)] were grown from the surface of magnetite nanoparticles using ATRP. ATRP from the surface was enabled by initiator with phosphonic acid as well as carboxylic acid anchoring groups. It was inferred that phosphonic acid anchoring system can play a better role in modifying the surface when compared with carboxylic acid anchoring system. To synthesize the polymer brush of the highest grafting density, it is preferable to use the fastest polymerization system i.e., benzyl methacrylate polymerization at ambient temperature. Block copolymerization of 2-hydroxylethyl methacrylate was carried out from the polystyrene monolayer, without using sacrificial initiator, and this confirms the controlled “living” nature of the polymerization. The polymer grafted nanoparticles (stabilized by phosphonic acid anchoring moiety) form stable dispersions in various solvents of interest. Thus the surface-initiated polymerization from the magnetite nanoparticles, without the addition of the sacrificial initiator as well as without the initial addition of Cu(II), results in high grafting density provided the fastest polymerizing system is used. This suggests that under the conditions of the experiment, a polymer brush with higher grafting density can be obtained if polymerization kinetics are faster than conformational rearrangement associated with the grafting chain. This result requires detailed modeling and the same is under study.
(See supplementary material 1)
We thank the Council for Scientific and Industrial Research (CSIR) for sanctioning the project.
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