Spherical Particles - Matyjaszewski Polymer Group - Carnegie Mellon University

Spherical Particles

Spherical Particles: The functionalization of the surfaces of many solids; including silica (SiO2), gold, silver, germanium, PbS, carbon black, iron oxides and other metal oxide systems has been achieved, (many references are provided in reference 1 below) allowing for subsequent attachment of initiators for the ATRP of many monomers forming organic/inorganic hybrid nanoparticles containing an inorganic core and tethered glassy or rubbery homopolymers or copolymers.  


The consequence of radical-radical termination is more serious during the preparation of colloidal particles than in a normal ATRP.  In traditional ATRP reactions termination leads to coupling whereas with a particle with 1000’s of initiation sites this leads to crosslinking; as a result
Pc= 1/ {r(fa -1)(fb -1)}½  → 2/f   → 1/1000.
So gelation is predicted at 0.1% intermolecular coupling.

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Gelation can be avoided by running the reaction under high dilution conditions to low conversion, i.e. under conditions where there is a low concentration of active radicals and consequently slow propagation.[1]  During surface-initiated atom transfer radical polymerization (SI-ATRP) of styrene from the silica nanoparticles, thermal self-initiation of styrene produces unattached polymer chains. Size exclusion chromatography afforded a facile approach to quantify the mass of the unattached polymer, and provided an incentive to develop a substantial refinement to estimates of chain graft density beyond traditionally used approaches, such as thermogravimetry. This fraction of unattached polymer is still present in the solid polymer/hybrid even after post-polymerization work-up via precipitation and re-dissolution. Removal necessitates additional procedures, such as high speed centrifugation. Selection of a lower polymerization temperature, in concert with a more reactive Cu complex, significantly reduces the amount of unattached polystyrene impurity.[2]  Size exclusion chromatography was capable of detecting and quantifying the fraction of unattached polystyrene in the presence of the hybrid particle.  Under some circumstances the overestimate of grafting density coiuld be as high as 100% and therefor influenced the prediction and understanding of the properties of the hybrid particle. Changing from running the reaction at 90 0C with a diNbpy ligand to 70 0C and PMDETA reduced the % unattached polystyrene from 17.2% to 4.2% with the same degree of initiation efficiency and a decrease in reaction time from 47 to 33 hours.  The improved polymerization conditions and post-polymerization purification provide more refined polystyrene-grafted silica nanoparticles to clarify structure-property relationships of these coreshell hybrids.

Another approach, specifically tailored for the synthesis of hybrid particles with higher molecular weight grafted chains, is synthesis under high pressure. [3]

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AGET ATRP was used for the polymerization and it was possible to increase the overall rate of polymerization due to the fact that under high pressure the rate of propagation is enhanced while the rate of termination is decreased compared to ATRP at ambient pressure.  Two different sized particles were used for the reaction, one 20 nm in diameter and the other 120 nm and two different initiators were tethered to the surface by reacting respectively 1-chlorodimethylsilyl)propyl 2-bromoisobutyrate or 2-bromo-isobutyryloxyhexyltriethoxysilane  with the silanol groups on the silica particles surface. On the basis of elemental analysis, 20 nm functional silica particle had average ~2600 initiating sites (~ 2 Br/nm2), whereas 120 nm particle had 105 sites (also ~ 2 Br/nm2).  Lower initiator density (~600 initiating sites (0.5 Br/nm2) per 20 nm silica particle) was obtained when chlorotrimethylsilane was used in a mixture with 1-(chlorodimethylsilyl)propyl 2-bromoisobutyrate to block some silanol groups and reduce initiator density.  Several polymerizations  were carried out at 6 kbars pressure using the following reaction conditions: [MMA]0/[-Br]0/[CuBr2]0/[TPMA]0/[AsAc]0 = 10,000/1/2/25/25 in anisole (46 vol%) and DMF (8 vol%) at 22 °C, where TPMA is tris(2-pyridylmethyl)amine (ligand), AsAc is ascorbic acid (reducing agent) and DMF is dimethylformamide.  After the reaction was completed the polymers were detached from silica surface by dissolving silica with HF, and analyzed by SEC. The entire molecular weight distribution (MWD) moved smoothly towards higher MW, demonstrating good control over polymerization.  The grafting densities of polymer chains were calculated based on the known MW and the amount of polymer from thermogravimetric analysis.  The highest grafting density of 0.3 chain/nm2, for the brushes with Mn=1,600,000, suggests that brushes can be considered as “semi-diluted density” brushes. [4]

Miniemulsion: The first example of the successful synthesis of hybrid nanoparticles using multifunctional silica initiators in a miniemulsion ATRP was recently disclosed. The reaction was driven to higher conversion in a shorter time.[5]   This success with miniemulsion relies on the compartmentalization of the reaction media which segregates the reaction medium and therefore minimizes the effect of radical termination and macroscopic gelation.[6]  Since termination reactions should be limited to individual droplets the proportion of terminated chains should be relatively small and the degree of crosslinking is less than in bulk conditions. The confinement effect increases the deactivation rate and termination rate so control is better but reaction rate is also decreased.

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As shown in the AFM image below each silica particle is evenly coated with polymer and the individual particles are not crosslinked even though conversion was driven to greater than 60%.  The particles were prepared with a 200:1 monomer to initiator ratio and each n-BA tethered chain has an average Mn=15,900 g/mol.

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SiO2-g-pSt hybrid nanoparticles with tethered polystyrene possessing molar masses in the range of Mn = 5,000 to 33,000 g/mol were prepared using commercially available silica nanoparticles as colloidal initiators, which greatly facilitated scale-up synthesis. The hybrid particles were characterized both in the solid state and in solution using transmission electron microscopy (TEM) and dynamic light scattering (DLS) respectively.  TEM images of the SiO2-g-pSt colloids revealed the formation of (sub)monolayer patches with interparticle spacing that increased with an increase in the molar mass of the tethered polystyrene.   Comparison of the hydrodynamic radii (Rh) of hybrid nanoparticles of varying size determined by DLS in toluene, versus the molar mass (Mn) of the polystyrene chains cleaved from colloids, determined by SEC, revealed a linear relationship.  Such a linear dependence of Rh vs. Mn is a strong indication that when the particles are dispersed in toluene, the tethered chains adopt highly chain extended conformations, presumably due to steric interactions caused by the high grafting density.[7]
AFM examination of spherical brushes formed by grafting n-butyl acrylate from silica particles deposited on mica surfaces show that the swollen brush has collapsed and that the hard silica core is surrounded by the soft grafted chains.

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Since the polymerization from the particle surface was being conducted by ATRP, it is simple task to isolate the particles and add them to a fresh monomer solution and form tethered block copolymers.
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Tri-phasic separation was clearly observed by AFM examination of a particle with tethered poly(St-b-nBA) block copolymer chains.  A hard polystyrene core surrounding the silica particle and the soft acrylate shell strongly adsorbed on the mica substrate are observed forming three distinct observable phase separations.
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As noted above, conducting the “grafting from” reaction in miniemulsion systems provide some additional benefits.  The reactions follow first order kinetics and can be driven at a higher rate to higher conversion without excessive production of coupled particles. Equivalent polymerization rates were observed for miniemulsion reactions initiated by alkyl halides, regardless of whether the initiators were attached to particle surfaces or free in solution. Therefore, the procedure provides a viable commercial approach to novel, functionally-designable materials with properties that can be pre-selected to target many specific applications.[8]

This approach is currently being applied to other multifunctional initiators, including multi-arm star molecules, molecular brushes, and other well-defined polymers with complex architectures.  Indeed molecular brushes were successfully synthesized in a miniemulsion system via AGET ATRP.[9]  Macroscopic gelation was observed for bulk ATRP but was not detected in miniemulsion.  The side-chain polymers grew from backbones rapidly in miniemulsion droplets and high monomer conversion was reached in relatively short time. Molecular visualization by AFM proved that some cross-linking did occur in miniemulsion droplets when the conversion was high (84%). However, this cross-linking showed no effect to the miniemulsion stability and fluidity, and therefore, the synthesized materials can be easily processed for further uses/applications.
The miniemulsion procedure has also been extended to functionalizing CdS quantum dots with poly(n-butyl acrylate) using activators generated by electron transfer (AGET) ATRP.[10]  The quantum dots were first modified by complexation with a phosphorous-containing ligand, which was then further modified to contain an ATRP initiating group. Bromine-containing functionalities degraded the QDs, but a chlorine-functionalized initiating group slowed the degradation enough that 3-4 nm QDs were obtained post-modification. The polymerization of n-butyl acrylate further protected the QDs from degradation via chemical reaction, and the resulting materials were both well-distributed on the nanoscale and possessed the optical properties expected from quantum dots of their size.
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A one-pot synthesis of thermally stable core/shell gold nanoparticles (Au-NPs) was developed via surface-initiated atom transfer radical polymn. (ATRP) of Bu acrylate (BA) and a dimethacrylate-based cross-linker.[11]  
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The higher reactivity of the cross-linker enabled the formation of a thin cross-linked polymer shell around the surface of the Au-NP before the growth of linear polymer chains from the shell. The cross-linked polymer shell served as a robust protective layer that prevented the dissociation of linear polymer brushes from the surfaces of Au-NPs, and provided excellent thermal stability to the Au-NPs at elevated temp. (e.g., 110 DegC for 24 h).
This synthetic method could be easily expanded for preparation of other types of inorganic/polymer nanocomposites with significantly improved stability.

1.    Pyun, J., et al., Synthesis and Characterization of Organic/Inorganic Hybrid Nanoparticles: Kinetics of Surface-Initiated Atom Transfer Radical Polymerization and Morphology of Hybrid Nanoparticle Ultrathin Films. Macromolecules, 2003. 36(14): p. 5094-5104.
2.    Tchoul, M.N., et al., Enhancing the fraction of grafted polystyrene on silica hybrid nanoparticles. Polymer, 2012. 53(1): p. 79-86.
3.    Pietrasik, J., et al., Silica-Polymethacrylate Hybrid Particles Synthesized Using High-Pressure Atom Transfer Radical Polymerization. Macromol. Rapid Commun., 2011. 32(3): p. 295-301.
4.    Choi, J., et al., Flexible Particle Array Structures by Controlling Polymer Graft Architecture. J. Am. Chem. Soc., 2010. 132(36): p. 12537-12539.
5.    Bombalski, L., et al., Preparation of Well-Defined Hybrid Materials by ATRP in Miniemulsion. Macromolecules, 2007. 40(21): p. 7429-7432.
6.    Kagawa, Y., et al., Studies on suspension and emulsion: part CCLXXXI. Compartmentalization in atom transfer radical polymerization (ATRP) in dispersed systems. Macromolecular Theory and Simulations, 2006. 15(8): p. 608-613.
7.    Pyun, J., et al., Synthesis and surface attachment of ABC triblock copolymers containing glassy and rubbery segments. Macromolecular Chemistry and Physics, 2004. 205(4): p. 411-417.
8.    Matyjaszewski, K., et al., Atom transfer radical polymerization in the presence of a reducing agent, in PCT Int. Appl. 2005, (Carnegie Mellon University, USA). WO 2005087819. p. 96 pp.
9.    Min, K., et al., High Yield Synthesis of Molecular Brushes via ATRP in Miniemulsion. Macromolecules, 2007. 40(18): p. 6557-6563.
10.    Esteves, A.C.C., et al., Polymer grafting from CdS quantum dots via AGET ATRP in miniemulsion. Small, 2007. 3(7): p. 1230-1236.
11.    Dong, H., et al., One-Pot Synthesis of Robust Core/Shell Gold Nanoparticles. J. Am. Chem. Soc., 2008. 130(39): p. 12852-12853.