INSTITUTE OF PHYSICS PUBLISHING
MEASUREMENT SCIENCE AND TECHNOLOGY
Meas. Sci. Technol. 16 (2005) 203–211
doi:10.1088/0957-0233/16/1/027
Combinatorial and high-throughput approaches in polymer science Huiqi Zhang, Richard Hoogenboom, Michael A R Meier and Ulrich S Schubert Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and Dutch Polymer Institute (DPI), PO Box 513, 5600 MB Eindhoven, The Netherlands E-mail:
[email protected]
Received 30 April 2004 Published 16 December 2004 Online at stacks.iop.org/MST/16/203 Abstract Combinatorial and high-throughput approaches have become topics of great interest in the last decade due to their potential ability to significantly increase research productivity. Recent years have witnessed a rapid extension of these approaches in many areas of the discovery of new materials including pharmaceuticals, inorganic materials, catalysts and polymers. This paper mainly highlights our progress in polymer research by using an automated parallel synthesizer, microwave synthesizer and ink-jet printer. The equipment and methodologies in our experiments, the high-throughput experimentation of different polymerizations (such as atom transfer radical polymerization, cationic ring-opening polymerization and emulsion polymerization) and the automated matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS) sample preparation are described. Keywords: combinatorial chemistry, high-throughput experimentation, atom
transfer radical polymerization, cationic ring-opening polymerization, emulsion polymerization, MALDI-TOF MS (Some figures in this article are in colour only in the electronic version)
1. Introduction Recently, combinatorial and high-throughput approaches have attracted great attention from both academic and industrial communities because of their significant advantages in increasing research productivity [1]. The essential parts of the combinatorial chemistry (CombiChem) and high-throughput experimentation (HTE) include design of experiment (DoE, library design), parallel chemical synthesis, high-throughput screening and data management. The impressive results of these approaches in pharmaceutical research [2], together with the fast development of automated synthesis and characterization workstations [3] as well as data handling and processing software [4] triggered the extension of these methods to many other research areas such as the 0957-0233/05/010203+09$30.00
discovery of new inorganic materials [5], catalysts [6] and polymers [7–9]. CombiChem and HTE seem to be perfectly suitable for polymer research due to the fact that many parameters can be varied during polymer synthesis (e.g., monomers, catalysts, initiators, solvents and reaction conditions), processing, blending and compounding. Recent years have witnessed a significant progress of these approaches in polymer synthesis [8, 9], coating formulations [10, 11] and polymer characterization [3, 7, 9]. A prominent example is the Symyx discovery tool for polyolefins [12, 13]. Recently, several reviews and feature articles on the application of HTE in polymer science have been published [7–11]. In this paper, we will mainly focus on our progress in introducing combinatorial and high-throughput approaches to polymer research.
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2. Equipment and methodologies
ka Pn-X + Cu(I)-Y/L
A commercially available automated parallel synthesizer Chemspeed ASW 2000 (Chemspeed Ltd, Switzerland) [14], a monomode microwave synthesizer EmrysTM Liberator (Personal Chemistry Ltd, Sweden) [15] and an AutoDrop dropon-demand printer (Microdrop, Norderstedt, Germany) [16] were utilized for the described experiments. 2.1. Automated parallel synthesizer [14] The automated parallel synthesizer Chemspeed ASW 2000 allows 20 to 80 reactions to be performed in parallel depending on the reaction vessels used (100 to 13 ml). Each reaction vessel can be heated or cooled through a jacketed oil bath and is equipped with a cold-finger reflux condenser. The temperature of the oil bath was controlled by a Huber Unistat 390 W Cryostat and can be varied from −70 to +150 ◦ C. The temperature of the reflux liquid was controlled by a Huber ministat and can be varied from −5 to +50 ◦ C. Reaction vessels were connected with a membrane pump, which could be utilized for inertization or evaporation processes. Stirring was performed by vortexing (0 to 1400 rpm). A glovebox ensured an argon atmosphere outside the reaction system. The automated synthesizer was equipped with an online size exclusion chromatograph (SEC) and an offline gas chromatograph (GC). A Gilson liquid handling system was used in the automated synthesizer. A program with all reaction procedures needs to be written to control the robot. It should be mentioned that an inertization process including several cycles of vacuum and argon filling under certain high temperature is required to remove oxygen and moisture from reaction vessels prior to the reactions for the controlled/living polymerizations such as atom transfer radical polymerization (ATRP) and cationic ring-opening polymerization (CROP). 2.2. Microwave synthesizer [15] Up to 120 reactions can be performed automatically in the monomode microwave synthesizer EmrysTM Liberator within a temperature range of 60 to 250 ◦ C at pressures of up to 20 bar. An IR-sensor is used to measure the reaction temperature, and a pressure sensor is used to detect the pressure inside the vials through the Teflon septum. There are two kinds of reaction vials, i.e., the round-bottom process vials for reaction volumes of 2–5 ml and conical process vials for reaction volumes of 0.5–2 ml. Magnets are used for the stirring of reaction mixtures. The EmrysTM Workflow Manager, the planning and execution software for Emrys Liberator, is available for the design of experiments. 2.3. Ink-jet printer [16] The AutoDrop drop-on-demand printer consists of a MDP-705-L xyz-stage, on which a holder for the print head is mounted. An AD-K-501 micropipette was used as print head (nozzle diameter 70 µm). This micropipette can aspirate a total sample volume of 25 µl by applying a slight underpressure. 204
kd
Pn• + X-Cu(II)-Y/L kp
(X, Y = Br, Cl)
M
Pm•
2k t Pn+m /P n + Pm
Scheme 1
3. Progress in combinatorial and high-throughput polymer research 3.1. ATRP In the past decade, living radical polymerizations (LRPs) have drawn great attention for providing simple and robust routes to the synthesis of polymers with predetermined molecular weights, low polydispersities, specific functionalities and various architectures under relatively mild reaction conditions [17–20]. The extensively studied LRPs include nitroxidemediated living radical polymerization [17], ATRP [18, 19] and reversible addition-fragmentation chain-transfer (RAFT) polymerization [20]. Among these, ATRP is of special interest due to the easy availability of many kinds of catalysts, initiators and monomers [18, 19]. There exists a reversible dynamic equilibrium between the dormant species (alkyl halide or arenesulfonyl halide) and active radicals in ATRP (scheme 1), which determines the radical concentration in the system and subsequently the polymerization rate and the radical termination. Many parameters in ATRP, such as the utilized monomers, catalysts (metal salts/ligands), initiators, solvents, reactant ratios and reaction temperatures, can significantly influence the parameters of the equilibrium and thus the controllability of the polymerization, which makes the optimization of reaction conditions very time consuming, in particular when a new reaction system is investigated. Therefore, combinatorial and high-throughput approaches, which allow a fast and efficient optimization of the reaction conditions in an automated parallel synthesizer under comparable and reproducible conditions, seem to be perfectly suitable for this research direction. The successful application of combinatorial and highthroughput approaches in a specific experiment requires that each step in the entire experimental process must be highthroughput, so that the whole process is not hampered by bottlenecks at certain steps. One of the main experimental steps in ATRP is the separation of the obtained polymers and catalysts, which was usually carried out by manually passing polymer solutions through columns of aluminum oxide or silica. The development of a fast online purification method for the polymers is thus very important for the HTE of ATRP. Therefore, we firstly developed an automated purification method [21], where hand-made aluminum oxide columns in solid phase extraction (SPE) cartridges (length = 5.6 cm, diameter = 0.6 cm) including porous polyethylene frits and ASPEC caps (Chemspeed Ltd) were used (figure 1). A reaction mixture of ATRP was automatically transferred to the columns and tetrahydrofuran (THF) was used as the eluent to wash down the polymers. The efficient removal of the catalysts by using this technique was verified by UV-Vis and atomic absorption spectroscopy measurements as well as the optical appearance. Recently, we further optimized the
Combinatorial and high-throughput approaches in polymer science
a
a
b
b
Figure 1. The aluminum oxide column in the SPE set-up before the purification (a) and after the purification (b) of the polymers (from [21]).
automated purification process by utilizing different column materials, column lengths and eluent volumes [22]. The column materials used include standardized aluminum oxide, activated neutral aluminum oxide, activated basic aluminum oxide and silica gel. The column lengths were varied from 0.25 to 1.5 cm and the eluent volumes were changed from 1 to 4 ml. The molecular weights (Mn,SEC) and polydispersity indices (PDIs) of the purified polymers were hardly influenced by the utilized eluent volumes, column materials and column lengths. The effects of these parameters on the purification efficiency were systematically investigated and the optimal conditions were identified. The reproducibility of the automated reactions and their comparability with conventional laboratory experiments are also important issues and need to be tested in advance. For this purpose, we performed ATRPs in the automated synthesizer with different reactor volumes [21, 23]. Firstly, three parallel reactions for the CuCl-mediated ATRP of methyl methacrylate (MMA) with p-toluenesulfonyl chloride (TsCl) as the initiator and 2, 2 -bipyridine (bpy) or 4, 4 -dinonyl 2, 2 -bipyridine (dNbpy) as the ligand ([MMA]0/[TsCl]0/[CuCl]0/[bpy or dNbpy]0 = 150/1/1/2, MMA/p-xylene = 1/2 v/v) were carried out at 90 ◦ C in the automated synthesizer in 13 ml reaction vessels [23]. Figure 2 demonstrates the good reproducibility of the parallel reactions in the automated synthesizer, and the maximum differences of the experimental results were found to be within 6%. Most importantly, the results obtained from the automated synthesizer were also comparable with those obtained in conventional experiments. For instance, three parallel reactions of the ATRP of MMA (with TsCl as the initiator and CuCl/dNbpy as the catalyst) carried out in the automated synthesizer resulted in a CMMA of (67.5 ± 0.6)% at 510 min and polymers with a Mn,SEC of 11 790 ± 160 and PDI of 1.09 ± 0.01 (figure 2), while the same reaction conducted in the conventional laboratory set-up led to a CMMA of 61% at 480 min and polymers with a Mn,SEC of 11 000 and PDI of 1.13. Secondly, three parallel reactions for the CuBr-mediated ATRP of MMA with ethyl 2-bromoisobutyrate (EBIB) as the initiator and N-(n-hexyl)-2-pyridylmethanimine (NHPMI) as the ligand ([MMA]0/[EBIB]0/[CuBr]0/[NHPMI]0 = 150/1/1/3,
MMA/p-xylene = 1/2 v/v) were also carried out at 90 ◦ C in 75 ml reaction vessels [21]. The results obtained from the automated reactions were again highly reproducible and they were also comparable with those from the conventional experiments. With these results in hand, we performed the homogeneous CuBr/NHPMI-mediated ATRP of MMA in 75 ml reaction vessels in the automated synthesizer [24]. Three different kinds of initiators, namely EBIB, (1-bromoethyl)benzene (BEB), and TsCl were utilized to initiate the polymerization. EBIB was revealed to be the best initiator for the studied system in terms of molecular weight control and PDIs of the obtained polymers, while BEBinitiated polymerization provided polymers with PDIs close to 1.6 and Mn,SEC much higher than the theoretical ones. The solvents used (i.e., toluene, p-xylene, and n-butylbenzene) showed a strong influence on the polymerization. The reactions in toluene and p-xylene were well controlled and proceeded at almost the same rates. However, a dramatic increase in the polymerization rate was observed in n-butylbenzene and polymers with higher polydispersities were obtained. This phenomenon needs further investigation. The initiator and Cu(I) concentrations had a positive effect on the polymerization rate (figure 3). In the meantime, all the reactions were well controlled and polymers with predetermined molecular weights and low PDIs ( MeOTs > MeI > BB). However, polymerizations initiated with MeOTf or MeOTs were much more sensitive to residual moisture or other small contaminations, resulting in loss of control over the polymerization. 3.4. Automated matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS) sample preparation MALDI-TOF MS has emerged as one of the most powerful techniques for the fast and accurate determination of a number of polymer characteristics such as molecular weights, molecular weight distributions and end-groups [35]. MALDI is a ‘soft’ ionization technique in which the energy from the laser is spent in volatilizing the matrix rather than in degrading the polymer. Preparation of an appropriate polymer/matrix mixture is one of the critical limiting factors for the general application of MALDI to synthetic polymers. In our laboratory, we developed some new automated MALDI sample preparation methods based on a multiple layer approach (from bottom to top: polymer layer, salt additive layer and matrix layer) [36]. The multiple layer approach offers the ability to prepare complex samples without the requirement of premixing the different components, which makes it possible to obtain and spot easily a large number of samples during polymerizations and also supports the integration of MALDI sample preparation into the workflow of combinatorial polymer research. In order to implement the multiple layer MALDI sample preparation method into the automated synthesizer, the liquid handling system of the synthesizer was utilized (figure 8) [37]. The solutions of polymer, salt additive and matrix were aspirated and subsequently spotted onto a defined position on the MALDI target using the Chemspeed ASW 2000 liquid handling system. This automated spotting technique was evaluated with polystyrene standards and also applied to the 209
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Figure 8. Left: spotting of matrix solution onto the MALDI target in the custom-made rack with a needle attached to the robotic arm of the automated synthesizer. Right: comparison of automatically (A) and manually (B) spotted samples (from [37]).
suitable data-handling and data-mining tools. It is expected that all known polymerizations could possibly be performed in an automated parallel way with the continuing development of new robot systems, even for very viscous polymerizations. We believe that these activities will allow a much higher level of fundamental understanding in polymer science in the future.
Acknowledgments The authors thank the Dutch Polymer Institute (DPI) and the Fonds der Chemischen Industrie for financial support.
References Figure 9. Photograph of the ink-jet printer used in this study (left). The microtitre plate for stock solutions as well as MALDI sample target on the stage of the ink-jet printer are shown top right. Right-bottom: close-up view of the MALDI target (from [38]).
screening of the CROP of EtOx in the automated synthesizer. The results from this spotting method and the manual spotting method were in good agreement in terms of molecular weights and polydispersities of the polymers. Recently, ink-jet printing was also applied to prepare MALDI samples based on the multiple-layer approach in our group [38] (figure 9). This technique allows fast and accurate deposition of matrix, additive and analyte solutions and offers improved analysis possibilities. This new sample preparation method was evaluated with synthetic polymers including poly(ethylene glycol) and poly(methyl methacrylate) standards and the optimized settings for both polymers were provided.
4. Conclusion The combinatorial and high-throughput approaches have had a remarkable impact on the research in a large number of areas including polymer science and more significant progress can be envisioned in the near future. As far as our research is concerned, we have successfully performed ATRP, CROP, emulsion polymerization and automated MALDI sample preparation using an automated synthesizer, a microwave synthesizer and an ink-jet printer. Our current efforts focus on the automated anionic polymerization, free radical co- and ter-polymerizations, RAFT polymerization and ring-opening polymerization of L-lactides as well as the development of 210
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