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Synthesis of Ultra-high Molecular Weight SiO2-g-PMMA Particle Brushes

Abstract

A simple route to synthesize ultra-high molecular weight particle brushes by surface-initiated atom transfer radical polymerization (SI-ATRP) from silica nanoparticles was developed. SiO2-g-PMMA and SiO2-g-PS particle brushes were prepared with different [SiO2–Br]0 concentration of initiating sites on the surface of the nanoparticles. Ultra-high MW (> 106) SiO2-g-PMMA particle brushes with narrow molecular weight distribution (< 1.3) and different grafting densities were synthesized. The grafting density of SiO2-g-PMMA particle brushes decreased with increasing target degree of polymerization. The same conditions were applied to the synthesis of SiO2-g-PS particle brushes. However, due to the lower propagation rate constant of styrene, coupling between SiO2-g-PS particle brushes occurred and also some fraction of unattached homopolystyrene was generated by the thermal self-initiation of styrene, preventing successful synthesis of ultra-high MW SiO2-g-PS particle brushes.

Introduction

Conventional radical polymerizations provides high molecular weight polymers, but slow initiation and fast termination limit access to polymers with predetermined molecular weights, narrow molecular weight distributions (MWDs), retained chain-end functionality, or block copolymers [1,2,3]. Conversely, reversible deactivation radical polymerization (RDRP) procedures allows for excellent control of molecular weight, and enables the synthesis of polymers with diverse architecture, including hyper branched, star, brush, and block copolymers, while also providing polymers with narrow and tunable MWDs [4,5,6,7,8,9,10,11,12]. However procedures for preparation of polymer nanocomposites with controlled molecular weights, well-defined architectures, and avenues to ultra-high molecular weight materials are limited. The relatively low molecular weights of the typical controlled radical polymerization products (< 150,000) result poor mechanical properties for the resulting hybrid materials. Applying RDRP techniques to achieve extraordinarily high chain lengths is challenging. The unavoidable side reactions generate a significant fraction of dead chains when the targeted molecular weights exceed 100,000. However, polymerizations targeting ultra-high molecular weight, defined here as molecular weight greater than 1 × 106, have been achieved through atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization [13, 14]. Some previous efforts targeting high molecular weight polymers employed very high pressure [15,16,17,18], long reaction times, heterogeneous conditions (i.e. emulsion polymerization) [19,20,21,22,23], low monomer conversion, and could be accompanied by a loss of control over the molecular weight distribution. In this contribution, we reveal a simple, low ATRP catalyst concentration route to prepare polymer grafted silica nanoparticles with degrees of polymerization (DPs) above 10,000.

Experimental Section

Materials

Monomers: styrene (S, 99%, Aldrich), methyl methacrylate (MMA, 99%, Aldrich) were purified by passing through a column filled with basic alumina to remove the inhibitor. Tris(2-dimethylaminoethyl)amine (Me6TREN, 99%, Alfa), 4,4′-dinonyl-2,2′-bipyridyne (dNbpy, 97%, Aldrich), anisole (99%, Aldrich), tetrahydrofuran (THF, 99%, VWR), methanol (99%, VWR), hexane (99%, VWR), acetone (99%, VWR), N,N-dimethylformamide (DMF, 99%, VWR), copper(II) bromide (CuBr2, 99%, Aldrich), copper(II) chloride (CuCl2, 99%, Aldrich),), tin(II) 2-ethylhexanoate (Sn(EH)2, 95%, Aldrich), hexane (Fluka), 48% hydrofluoric acid aqueous solution (HF, > 99.99%, Aldrich), ammonium hydroxide aqueous solution (NH4OH, 28.0–30.0%, Fisher), anhydrous magnesium sulfate (MgSO4, Fisher) were used as received without further purification. Copper(I) bromide (CuBr, 98%, Acros), was washed with glacial acetic acid to remove any soluble oxidized species, filtered, washed twice with anhydrous ethyl ether, dried and kept under vacuum. Silica nanoparticles, 30 wt% solution in methyl isobutyl ketone (MIBK-ST), effective diameter d ≈ 15.8 nm, were kindly donated by Nissan Chemical Corp. and used as received. The tethered ATRP initiator 1-(chlorodimethylsilyl)propyl 2-bromoisobutyrate and surface modified silica (SiO2–Br) were prepared using previous reported procedures [24, 25]. The surface initiator densities are moderated with a “dummy” initiator chlorotrimethylsilane (99%, Aldrich).

Synthesis

Procedure for synthesis of SiO2-g-PMMA/PS particle brushes via activators regenerated by electron transfer (ARGET) ATRP.

Initiator (SiO2–Br nanoparticles), monomer (MMA/S), solvents (anisole, DMF), CuBr2, and Me6TREN were mixed thoroughly in a sealed Schlenk flask. A stock solution of Sn(EH)2 in anisole was prepared. Both mixtures were degassed by nitrogen purging, then the Sn(EH)2 solution was injected into the Schlenk flask to activate the catalyst complex and the flask was immediately put into an oil bath. The MW of the polymer was measured by SEC.

Characterization

Transmission electron microscopy (TEM) was carried out using a JEOL 2000 EX electron microscope operated at 200 kV. The spatial distribution, radius and inter-particle distances of the SiO2 nanoparticles were determined from statistical analysis of the TEM micrographs using ImageJ software.

Thermogravimetric analysis (TGA) with TA Instruments 2950 was used to measure the fraction of SiO2 in the hybrids. The data were analyzed with TA Universal Analysis. The heating procedure involved four steps: (1) jump to 120 °C; (2) hold at 120 °C for 10 min; (3) ramp up at a rate of 20 °C/min to 800 °C; (4) hold for 2 min. The TGA plots were normalized to the total weight after holding at 120 °C.

Dynamic light scattering (DLS). DLS, using a Malvern Zetasizer Nano ZS, was performed to confirm results obtained from TEM. It was also employed to determine volume-weighted average hydrodynamic diameters and distribution.

Grafting density was calculated using formula (1) [26,27,28].

$$\sigma_{\text{TGA}} = \frac{{ \left( {1 - f_{{{\text{SiO}}2}} } \right)N_{\text{Av}} \rho_{{{\text{SiO}}2}} d}}{{6 f_{{{\text{SiO}}2}} M_{\text{n}} }}$$
(1)

where fSiO2 is the SiO2 fraction measured by TGA, NAv is the Avogadro number, ρSiO2 is the density of SiO2 nanoparticles (2.2 g/cm3), d is the average diameter of SiO2 nanoparticles (15.8 nm), Mn is the overall number-average MW of the cleaved polymer brushes.

Number-average molecular weights (Mn) and MWDs were determined by size exclusion chromatography (SEC). The SEC was conducted with a Waters 515 pump and Waters 410 differential refractometer using PSS columns (Styrogel 105, 103, 102 Ǻ) in THF as an eluent at 35 °C and at a flow rate of 1 mL/min. Linear PS and PMMA standards were used for calibration. Conversion was calculated by gravimetric analysis.

Determining the initiating sites density for SiO2–Br nanoparticles

The concentration of initiating sites on the surface of silica nanoparticles was determined by model reactions (i.e. polymerization of SiO2-g-PMMA, [MMA]0/[SiO2–Br]0/[CuBr2]0/[Me6TREN]0/[Sn(EH)2]0 = 2000:1:1:10:8 with 45 vol% anisole, 5 vol% DMF at 60 °C) with certain amount of SiO2–Br nanoparticles (e.g. with 100 mg). After purification, SEC and TGA were conducted to characterize the grafting density of the particle brushes. The particles used in current study had grafting density ~ 0.45/0.15/0.052 nm−2, the –Br (initiating site) concentration on the surface was assumed the same. Based on the average radius of nanoparticles, 7.9 nm, density of silica, 2.2 g/cm3, the average molar mass of SiO2–Br, per Br, are 7500 g/mol (high grafting density), 23,250 g/mol (medium grafting density), and 67,000 g/mol (low grafting density).

Determining the initiation efficiency of the reactions

The initiation efficiency was determined by two methods.

Efficiency-1 was calculated by Eq. 2

$$\text{Efficiency-1} = {{M_\text{n,theo} } \mathord{\left/ {\vphantom {{M_{\text{n,theo}} } {M_{\text{n,SEC}} }}} \right. \kern-0pt} {M_\text{n,SEC} }}$$
(2)

Efficiency-2 was calculated by Eq. 3

$$\text{Efficiency-2} = {\sigma \mathord{\left/ {\vphantom {\sigma {\sigma_{0} }}} \right. \kern-0pt} {\sigma_{0} }}$$
(3)

in where, σ is the grafting density of particle brushes, \(\sigma_{0}\) is the concentration of initiating sites on the surface of SiO2–Br nanoparticles, which are 0.45 nm−2 (high grafting density), 0.15 nm−2 (medium grafting density), 0.052 nm−2, (low grafting density), respectively.

Results and Discussion

15 nm SiO2 nanoparticles were surface modified with an ATRP initiator, 1-(chlorodimethylsilyl)propyl 2-bromoisobutyrate, using previously reported procedures [29]. Moderating the concentration of initiator on the particle surface was accomplished by use of a “dummy” initiator chlorotrimethylsilane, to provide SiO2–Br nanoparticles with three different grafting densities, 0.45/0.15/0.052 chain/nm2, i.e. densely/intermediately/sparsely grafted. A low concentration catalyst system (25 ppm CuII/Me6TREN) was chosen to carry out ARGET ATRP. In ARGET ATRP, a pre-catalyst CuII/L is added, or formed in situ, at the beginning of the polymerization. A fraction of the stable CuII complex is continuously reduced to an active CuI/L species by a reducing agent. [30]. The advantage of ARGET ATRP is that the amount of copper in the polymerization can be significantly reduced, since CuII/L complexes formed via oxidation or via radical termination are constantly transformed to active CuI/L species by the reducing agent [31,32,33,34,35]. Tuning the targeted molecular weight and reaction times was expected to provide particle brushes with ultra-high molecular weight and different grafting densities (Fig. 1).

Fig. 1
figure1

Particle brush synthesis with different grafting densities

As shown in Fig. 2, compared to a linear homopolymer system with the same concentration of initiators in the system, localization of initiator sites on the surface of nanoparticles significantly alter the homogeneity of the reaction solutions, as the higher the grafting density of the particle brushes introduces a greater heterogeneity to the system. Termination in ATRP is a bimolecular reaction, which requires the collision between two radicals [36, 37]. Due to placement of the initiators on a particle, the size and viscosity effects, the probability of collision between particle brushes (nanoparticles) is much lower than between linear homopolymers (small initiator molecules). Less termination could lead to preparation of higher molecular weight polymer ligands.

Fig. 2
figure2

Same initiator concentration with linear homopolymer and particle brushes

Controlled polymerization of methyl methacrylate (MMA) from the surfaces of SiO2 nanoparticles was conducted using a low Cu catalyst ATRP procedure with different initiating sites densities and different [SiO2–Br]0, where SiO2–Br served as the initiator and CuBr2/Me6TREN as the catalyst. The polymerizations produced PMMA brushes with low dispersities (Table 1). To avoid gelation of the particle brushes, the conversion of MMA was limited to below 10%. As shown in Table 1, the obtained Mn values were consistently higher than theoretically predicted values as the target molecular weight increased, which is due to limited initiation efficiency. This also resulted in reduced grafting density of the particle brushes which diminished with the target degree of polymerization. The initiation efficiency of the reactions were calculated by both Mn,SEC versus Mn,theo and grafting density versus the concentration of initiating sites (Table 1), and the results were very close. Ultra-high molecular weight PMMA particle brushes with different grafting densities (Mn = 1.7 × 106, Đ = 1.31, 0.094 nm−2, Mn = 1.0 × 106, Đ = 1.31, 0.016 nm−2, Mn = 1.1 × 106, Đ = 1.24, 0.018 nm−2) were successfully synthesized within 21 h.

Table 1 Result of syntheses of SiO2-g-PMMA particle brushes with different grafting densities

TEM was applied to study the morphology of the particle brushes and Fig. 3 confirmed successful grafting of PMMA from silica nanoparticles. In the densely grafted particle systems (Fig. 3a–d), no apparent aggregation/particle couplings were observed. On the other hand, clear aggregations were seen in intermediately/sparsely grafted particle systems (Fig. 3e–l), which indicated more inter-particle interactions in those reactions. Agglomeration/couplings between particles could result from termination between polymer ligands or interactions between bare particle surfaces. The clear/non-agglomeration TEM images from densely grafted particle brushes system suggest large heterogeneity which was introduced by the localization of initiators on the surface of nanoparticles can efficiently reduce the terminations between particle brushes. As compared to linear homopolymers, in the particle brush system the probability of effective collisions (and radical termination) between particles is much lower than between linear homopolymer chains. The hydrodynamic sizes of the particle brushes in THF solutions were measured by DLS. Figure 4a shows the DLS traces of SiO2-g-PMMA particle brushes with different DP and grafting densities, where the increase in hydrodynamic size with DP and grafting density can be clearly observed. Figure 4b shows more detailed measurement results for each sample, the trend of hydrodynamic size increases with DP and can be observed for all three sets of samples. It should be noted that the increase of particle brush size with DP was not linear due to the change of grafting density.

Fig. 3
figure3

TEM image of SiO2-g-PMMA particle brushes: ad HGM-1–4, eh MGM-1–4, il LGM-1–4, scale bar 200 nm

Fig. 4
figure4

DLS traces a SiO2-g-PMMA particle brushes, b hydrodynamic diameter of SiO2-g-PMMA particle brushes versus DP

SiO2-g-PS particle brushes were prepared using the same method and same reaction conditions as developed for SiO2-g-PMMA particle brushes (Table 2). The major differences between styrene (S) and methyl methacrylate (MMA) include: (i) a much lower kp value for styrene than for MMA [38,39,40]; (ii) the termination mechanisms for MMA and S are different, i.e. in the former termination by disproportionation dominates, where as in the latter combination dominates and coupling between particle brushes could lead to formation of aggregates and gelation [41, 42]; and (iii) styrene undergoes spontaneous (thermal) polymerization [43, 44].

Table 2 Result of syntheses of SiO2-g-PS particle brushes with different grafting densities

As shown in Table 2, ultra-high MW particle brushes cannot be obtained in a SiO2-g-PS system under the same reaction conditions that were successful for SiO2-g-PMMA. The highest Mn is around 200,000 and with high dispersity (Mw/Mn > 2). This can be attributed to termination reactions and thermal-self initiation in the styrene system. Unlike SiO2-g-PMMA, coupling in SiO2-g-PS system would cause non-uniform structures, agglomeration of particles and gelation, as shown in the TEM images (Fig. 5). The heterogeneity introduced by the agglomeration and increase of viscosity could lead to a further increase in the dispersity. One feature that was observed in the attempted synthesis of ultra-high MW SiO2-g-PMMA particle brushes is that the grafting density of the particle brushes (initiation efficiency of the reactions) decreased with the increasing target DP. However, in the SiO2-g-PS system, the apparent grafting density/initiation efficiency first decreased but then increased with the target DP. The difference originates from contribution of thermal-self initiation in styrene polymerization, which increases with reaction temperature and reaction time. The apparent high initiation efficiency (> 100%, calculated from the entire polystyrene same i.e. attached to the particles and unattached formed via self-initiation) shown in Table 2 indicated the presence of a large amount of linear PS homopolymers in the systems.

Fig. 5
figure5

TEM image of SiO2-g-PS particle brushes: ad HGS-1–4, eh MGS-1–4, il LGS-1–4, scale bar 200 nm

Conclusions

In conclusion, ultra-high MW (> 106) SiO2-g-PMMA with narrow molecular weight distribution (< 1.3) and variable grafting densities of particle brushes were synthesized through ARGET-ATRP under low [SiO2–Br]0 condition within 21 h. The grafting density of SiO2-g-PMMA particle brushes decreased as the target DP was increased. The same conditions were applied on the synthesis of SiO2-g-PS particle brushes; however, the ultra-high MW SiO2-g-PS cannot be obtained. This is due to the lower propagation rate constant of styrene, more significant coupling between SiO2-g-PS particle brushes and the unattached homopolymers formed by the self-thermal initiation of styrene.

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Acknowledgements

This work was supported by NSF (DMR 1501324 and DMR 1410845), the Department of Energy (DE-EE0006702), the National Science Center (Grant UMO-2014/14/A/ST5/00204), as well as the Scott Institute for Energy Technologies at Carnegie Mellon University.

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Correspondence to Krzysztof Matyjaszewski.

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Wang, Z., Liu, T., Lin, K.C. et al. Synthesis of Ultra-high Molecular Weight SiO2-g-PMMA Particle Brushes. J Inorg Organomet Polym 30, 174–181 (2020) doi:10.1007/s10904-019-01289-8

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Keywords

  • Ultra-high molecular weight
  • Particle brush
  • SI-ATRP