Article pubs.acs.org/Macromolecules
Monodisperse Conjugated Polymer Particles via Heck CouplingA Kinetic Study to Unravel Particle Formation in Step-Growth Dispersion Polymerization Sibel Ciftci and Alexander J. C. Kuehne* DWI − Leibniz Institute for Interactive Materials, RWTH Aachen University, Forckenbeckstrasse 50, 52074 Aachen, NRW, Germany S Supporting Information *
ABSTRACT: Palladium-catalyzed cross-coupling dispersion polymerizations follow a step-growth mechanism and deliver highly monodisperse particles. This is surprising, considering the high polydispersity usually inherent to classical polycondensations at high conversion. Here we present a novel Heck-type dispersion polymerization yielding fluorescent submicrometer particles with an extremely narrow size distribution. We investigate the kinetics of the dispersion polymerization to unravel the mechanism behind the formation of monodisperse particles. We find that, at medium conversion, only one type of oligomer forms nuclei for the particles. These seed particles phase separate from solution at a critical molecular weight. Oligomers, reaching the critical molecular weight for dissolution after this initial nucleation event, condensate onto the existing nuclei, leading to uniform growth of the particles. Higher molecular weight material is obtained toward high conversion by coupling reactions inside of the particles. The comprehension of the mechanism behind particle formation will enable adaptation of other reactions to the dispersion polymerization protocol. The monodisperse polymer particles produced here are ideal materials for self-assembled organic photonics, optoelectronics, or fluorescent biomedical probes and markers.
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INTRODUCTION Classical polymer colloids prepared by emulsion and dispersion polymerization find widespread application ranging from paints to adsorbents and cosmetic formulations.1,2 Monodisperse sets of such particles self-assemble into regular colloidal crystals upon drying via van der Waals interactions and capillary forces, enabling more topical applications of colloids for example in self-assembled photonics.3−5 By contrast, conjugated polymers are a potent class of materials with high quantum yields, broad biocompatibility and facile processability.6 The fluorescence spectra of conjugated polymers can be easily tuned by varying the amount and type of respective comonomers. Colloidal particles prepared from conjugated polymers fully exploit the advantages of both worlds and several methods for the preparation of conjugated polymer particles have been presented.7,8 Among them are reprecipitation processes, where a conjugated polymer solution is injected into a bad solvent and finely dispersed particles are produced.9 Alternatively, particles can be produced by Pd-catalyzed cross coupling in emulsion templated systems.10 For example, emulsion polymerization can be applied, when the monomers are liquid;11 otherwise miniemulsion processes can be applied, where a monomer solution is emulsified in water.12−16 However, these processes deliver polydisperse particles preventing applications in self-assembled colloidal photonics and complicating utilization in particle based cellular and biomedical imaging.14,17−21 Microfluidic processes haven been © XXXX American Chemical Society
developed to gain greater control over the particle size distribution.22,23 However, the throughput in such devices is low, obviating the production of large amounts of monodisperse conjugated polymer colloids. We have recently presented an easily scalable dispersion polymerization protocol for Suzuki−Miyaura and Sonogashira coupling reactions yielding conjugated polymer particles with precise monodispersity.24,25The dispersion polymerization mechanism and the kinetics of particle formation have been extensively studied for chain-growth polymerization, where monomers add to an active polymer chain-end one after the other.26−28 Here, the growth rate for a single polymer chain is linear. One example for such a mechanism is free radical polymerization of monomers like MMA and styrene.29 However, there exist only few examples of dispersion polymerization following a step-growth mechanism.29,30 Here, the growing polymer chains couple among each other in a condensation reaction, leading to an exponential growth rate of the polymer chains. In stepgrowth polycondensation high degrees of polymerization can only be achieved at high conversion p as expressed by the Carothers equation Xn = 1/(1 − p). At high conversions, the molecular weight distribution becomes very broad leading to the formation of polydisperse Received: September 2, 2015 Revised: October 15, 2015
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DOI: 10.1021/acs.macromol.5b01932 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules particles in dispersion polycondensations.29 Pd-catalyzed crosscoupling polycondensations represent ideal candidates to study the kinetics for externally catalyzed step-growth dispersion polymerizations. With respect to the formation of monodisperse conjugated polymer colloids, the mechanism remains completely unknown. There is a need to understand why the process affords such monodisperse particles despite being a polycondensation and how to control the crucial processes of nucleation and growth to transfer the approach to other stepgrowth dispersion polymerizations. Here, we aim to resolve this lack of understanding by investigating a new type of dispersion polycondensation utilizing Heck cross-coupling as the growth-step. We determine the kinetics of the step-growth dispersion polymerization to unravel the mechanism behind particle formation and their narrow size distribution.
concentrations the determined polydispersity indices are smaller than 0.05 as represented by the error bars in Figure 1b. Monodispersity of the particles is clearly visible in the SEM images, where the particles assemble into hexagonally arranged crystallites (see Figure 1c and inset as well as Supporting Information for further SEM images). At higher concentrations the particles become bidisperse as a result of secondary nucleation. To further investigate the particle formation mechanism we conduct the dispersion polymerization without the polymeric stabilizer to avoid interference in GPC and 1H NMR analysis. This synthesis results in particles, which exhibit a slightly increased dispersity (see SEM image in Supporting Information). We perform synthesis in 80 mL of 1-propanol aiming for 1.2 μm particles. This batch is about 10 times larger than previously reported syntheses. We find that in larger batches the desired particle size and monodispersity are more reproducible, because detrimental factors such as nucleation sites at the surface of the flask are reduced. We start the polymerization by injecting the base in ca. 14 mL of solvent at 80 °C and take aliquots of 3 mL every 120 s and later after every few minutes. The aliquots are quenched in 2 mL of cold 1-propanol, which stops the polymerization but does not dissolve the seeds or generated particles. We then analyze the samples using dynamic light scattering and SEM to determine the particle size (see Figure 2a). A rapid growth phase of the particles is visible between 10 and 25 min, which correlates well with the observation of turbidity made by eye. To transform the time domain into the conversion domain, we measure 1H NMR of the aliquots and analyze the disappearance of the terminal vinylic hydrogen (see red marked hydrogen in Figure 1a) as a measure for conversion against an internal standard, namely the aliphatic hydrogens of the fluorene moiety. The obtained data is fitted with the polymerization kinetics of an externally catalyzed polycondensation (see inset in Figure 2a):
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RESULTS AND DISCUSSION To develop the Heck-type dispersion polymerization we adapt the previously reported Pd-crosscoupling dispersion polymerization methods to Heck conditions.24,25,31 We apply diiodo9,9-dioctylfluorene and divinylbenzene (a mixture of the m-/pisomers) together with Pd(OAc)2 as catalyst and NEt3 as base in 1-propanol (see Figure 1a). We apply a mixture of poly(vinylpyrrolidone-co-vinyl acetate) (Mn ≈ 50 kDa) and Triton X-45 to stabilize the colloidal dispersion upon formation
p=1−
1 1 + kappt
(1)
The experimental data correspond well with the theoretical model with 0.1 min−1 ≤ kapp ≤ 0.15 min−1, allowing us to examine the growth of the particle size versus conversion (see Figure 2b). Two striking observation can be made: First, the increase in particle size follows d ∝ 3 p , which is in agreement with growth of a spherical object at constant feed. Second, nucleation occurs at 66% conversion (p = 0.66) meaning that here the critical molecular weight for dissolution Mcrit is reached. According to the Carothers equation the mean degree of polymerization at 66% conversion is Xn = 2.9. This would mean that di- and trimers phase separate to form nuclei of the resulting particles. However, it seems counterintuitive that the observed spherical particles could be the result of such small oligomers. We perform GPC to determine the molecular weight Mn of the final particles and find Mn = 15 kDa (7.1 kDa after correction (see Figure S2)), corresponding to Xn = 28, which apparently contradicts our efforts to determine Mcrit theoretically. To resolve this inconsistency, we monitor the evolution of molecular weight over time by taking aliquots between 2 and 190 min after initiation. The aliquot is subjected to GPC analysis without previous purification. The aliquots contain the dissolved as well as the precipitated polymer content for samples taken after nucleation. Eight distinct peaks or bands can be identified in the gel permeation chromato-
Figure 1. (a) Reaction scheme for Heck polymerization. The hydrogen marked in red can be followed in 1H NMR to determine the conversion at different points in time. (b) Evolution of particle size with increasing monomer loading. The error bars indicated the particle dispersity. In the gray shaded area only bidisperse sets of particle could be obtained. The inset shows the reaction mixture before (left) and after dispersion polymerization (right) under UV illumination. (c) SEM image showing the monodispersity of the particles produced. Scale bar represents 10 μm. The inset shows a close up of the particles in (c) with a smooth surface. Scale bar represents 1 μm.
(see Supporting Information for synthetic procedure). The mixture is heated to 80 °C and stirred vigorously. The solution is completely transparent before synthesis, as all components are dissolved. However, clouding with a blue hue representing scattering from very small particles becomes apparent after about 15 min, while after 30 min of polymerization the mixture is turbid and exhibits fluorescence (see inset in Figure 1b and the video in the Supporting Information). The final particle size can be adjusted between 250 nm to about 1.2 μm by varying the initial monomer loading. The particles sizes are determined using dynamic light scattering or by SEM image analysis by averaging over at least 100 particles. Up to 8 mM monomer B
DOI: 10.1021/acs.macromol.5b01932 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (a) Evolution of the particle size versus reaction time. The inset depicts the relation between the conversion p versus reaction time t. The data is obtained from 1H NMR and the cruve represents the fit using kinetics for an externally catalyzed polymerization. (b) The particle diameter versus the conversion. The data is fitted in accordance with the growth of the diameter of a sphere at constant volume feed. Nucleation occurs at p = 0.66.
Figure 3. (a) GPC traces of the combined liquid and solid phase at various reaction times from blue =2 min (p = 0.2) to gray =190 min (p = max). (b) Probability Pi of finding a molecule composed of i monomer units versus the conversion p. The gray shaded area represents the two phase reaction conditions after nucleation has occurred at 66% conversion. (c) GPC traces of only the separated particles versus the reaction time from blue = 28 min (p = 0.73) to gray =190 min (p = max). (d) Probability Pi for only the particles indicates that the condensation is also occurring in the solid phase. Black data points represent monomer, blue = dimer, cyan = trimer. All of the before mentioned molecules contain one fluorenyl unit. The broader peaks are grouped in accordance with the amount of incorporated fluorenyl units. Two fluorenes: green = 3′-5mer. Three fluorenes: olive = 5′-7mer. Four fluorenes: orange = 7′-9mer. Five fluorenes: red = 9′-11mer. Six and more fluorenes: brown = 11′mer−polymer. Molecular weights are determined by performing GPC against polystyrene standards followed by correcting the obtained data using a correction curve derived from Figure S1. The correction curve is shown in Figure S2.
trimer can be composed of one fluorenyl and two vinylstyryl units or vice versa.) We verify the respective peak assignments by cross checking with GPC traces of the fluorene monomer and a bis(vinylstyryl)fluorene trimer (see Figure S1 in the Supporting Information). We can now integrate under these peaks and bands and obtain relative values, which are proportional to the probability Pi of finding the respective species i in the reaction mixture. When we monitor Pi versus p, it becomes apparent that the monomer concentration monotonously decreases, while dimer and trimer seem to reach a low but relatively constant concentration. These can be
grams (see Figure 3a). While the first three peaks are narrow and appear to be composed of individual species, the higher molecular weight signals are separated into broader bands. We attribute this banding to the eluotropic character of the fluorenyl unit resulting in subpopulations with the same amount of fluorene units but different amounts of vinylstyryl units (see Figure 3a and Figure S1 in the Supporting Information).32 Following this rationale, we can assign the different peaks and bands, with the respective oligomer species. Oligomers with the same amount of monomer units but different composition are marked with prime symbols. (A C
DOI: 10.1021/acs.macromol.5b01932 Macromolecules XXXX, XXX, XXX−XXX
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and monomer can diffuse inside of the particles and take part in coupling. Finally, we conclude that our reaction is in agreement with Carothers theory, which only describes the mean degree of polymerization. At p = 0.66 the most common species in the reaction mixture is still the monomer, followed by oligomers leading to a low mean degree of polymerization Xn but to a small amount of oligomers above this value, which form the initial nuclei for the monodisperse particles. Even though the polydispersity Đ = 1.66, when p = 0.66, only the highest molecular weight species phase separates at Mcrit, allowing nucleation into monodisperse particles and uniform growth. Growth occurs via condensation of oligomers with steadily growing molecular weights as their solubility and Mcrit increase. Further coupling can occur inside of the particles, leading to a very broad final molecular weight distribution (see Figure 3c). The final size of the particles is governed by the amount of nuclei formed and the initial monomer loading (see Figure 1b). In the future we will investigate the role and the amount of coupling events inside of the particles. We have developed a novel Heck-type dispersion polymerization and we have for the first time elaborated why and how dispersion polycondensations can yield monodisperse particles. Such monodisperse conjugated polymer particles may find application in selfassembled Bragg mirrors and laser resonator or as biomedical contrast agents for imaging and theranostics.34−36 The insight gained into the dispersion polycondensation kinetics will alleviate constraints for the development of monodisperse particles for a variety of available polycondensates.
considered transient, as they are produced and then consumed when reacting to higher value oligomers. At p = 0.66 the amount of the 5′-7mer population with three fluorenyl units begins to rise (see olive data in Figure 3b). This coincidence of nucleation and onset of the oligomer increase reflects that the Mcrit is crossed and 5′-7mers appear to form the initial nuclei. If we consider the extinction coefficients and scattering cross sections to be similar among all monomer, oligomers and polymer we can assume their GPC detector signals to be similar. This allows comparison of the respective peak intensities and their integrals at p = 0.66 and thus determination of Xn (see Figure 3b). The experimentally determined degree of polymerization Xn = 2.3 corresponds reasonably well with the theoretical value of Xn = 2.9. However, as assumed earlier this value does not reflect the degree of polymerization for the species responsible for nucleation. Only the species with the highest molecular weight in the relatively broad distribution of species reaches Mcrit and phase separates. Since the mixture of oligomers is still dominated by large amounts of monomer, the Xn will always underestimate the molecular weight of the nucleating species. Nucleation is synonymous with phase separation, meaning that nucleated material is no longer part of the liquid phase, where polymerization occurs. The withdrawal of material increases the solubility of higher molecular weight species, leading also to a rising Mcrit over time. This fact is reflected by the rise of higher molecular weight bands for p > 0.66. Beyond 80% conversion we see a sharp increase for polymer chains with Mn > 10 kDa (see brown data in Figure 3b). To understand how this high molecular weight material is formed we also take a look at the molecular weight distribution of only the particle fraction over the course of the dispersion polymerization (see Figure 3c). We obtain analyzable samples with sufficient solid content for t > 28 min. The particles are dissolved in chloroform and analyzed by GPC. While the lower molecular weight content decreases over time, a broad polymer band appears with time. When we examine the individual oligomer bands and plot their integral versus the conversion, we see that the content of oligomers smaller than nonamers decreases, while the polymer content of the particles increases (see Figure 3d). We have previously established, that the particles grow in size for about 60 min corresponding to 85% conversion, and after that the diameter remains constant (see Figure 1a). However, the molecular weight of the particles increases further even after 60 min (>85% conversion), indicating that coupling may also occurs inside of the phase separated particles as a consequence of the confined space effect (see Figure 3, parts c and d). Coupling can only occur if oligomers together with Pdcatalyst have phase separated. To test whether the polymer chains possess sufficient mobility for coupling to occur, we perform DSC of the polymer particles and find the onset of the glass transition at Tg of 79 °C (see Figure S3 in the Supporting Information). This value is similar to previously reported Tgs for arylene vinylene polymers.33 The particles will swell with residual small oligomers and monomers and the low Tg will sustain limited mobility allowing for Heck-coupling events to occur inside of the particles. To prove that indeed Pd-catalyst phase-separates together with the oligomers and polymers we perform metal analysis via ICP-OES and find small amounts of Pd in the particles as well as in the supernatant. This indicated that indeed limited coupling events may occur inside of the particles. This could mean that oligomers couple to form higher molecular weight species or low molecular weight oligomers
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01932. Synthesis and characterization of the prepared particles (PDF) Video of the dispersion polymerization (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support. This work was in part performed in part at the Center for Chemical Polymer Technology (CPT), which is supported by the EU and the federal state of North Rhine-Westphalia (Grant No. EFRE 30 00 883 02).
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DOI: 10.1021/acs.macromol.5b01932 Macromolecules XXXX, XXX, XXX−XXX
