Pieter-Jan
Voorter
ab,
Gayathri
Dev
ac,
Axel-Laurenz
Buckinx
ad,
Jinhuo
Dai
d,
Priya
Subramanian
d,
Anil
Kumar
c,
Neil R.
Cameron
be and
Tanja
Junkers
*a
aPolymer Reaction Design Group, School of Chemistry, Monash University, 19 Rainforest Walk, Building 23, Clayton, VIC 3800, Australia. E-mail: tanja.junkers@monash.edu
bDepartment of Materials Science and Engineering, Monash University, 14 Alliance Lane, Clayton, Victoria 3800, Australia
cDepartment of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India
dDulux Australia, 1956 Dandenong Road, Clayton, VIC 3168, Australia
eSchool of Engineering, Warwick University, Coventry CV4 7AL, UK
First published on 6th June 2023
A one-pass continuous flow strategy to form block copolymer nanoaggregates directly from monomers is presented. A key development towards such a sophisticated continuous flow setup is a significant improvement in continuous flow dialysis. Often impurities or solvent residues from polymerizations must be removed before block extensions or nanoaggregate formation can be carried out, typically disrupting the workflow. Hence, inline purification systems are required for fully continuous operation and eventual high throughput operation. An inline dialysis purification system is developed and exemplified for amphiphilic block copolymer synthesis from thermal and photoiniferter reversible addition fragmentation chain transfer (RAFT) polymerization. The inline dialysis system is found to be significantly faster than conventional batch dialysis and the kinetics are found to be very predictable with a diffusion velocity coefficient of 4.1 × 10−4 s−1. This is at least 4–5 times faster than conventional dialysis. Moreover, the newly developed setup uses only 57 mL of solvent for purification per gram of polymer, again reducing the required amount by almost an order of magnitude compared to conventional methods. Methyl methacrylate (MMA) or butyl acrylate (BA) was polymerized in a traditional flow reactor as the first block via RAFT polymerization, followed by a ‘dialysis loop’, which contains a custom-built inline dialysis device. Clearance of residual monomers is monitored via in-line NMR. The purified reaction mixture can then be chain extended in a second reactor stage to obtain block copolymers using poly(ethylene glycol) methyl ether acrylate (PEGMEA) as the second monomer. In the last step, nano-objects are created, again from flow processes. The process is highly tuneable, showing for the chosen model system a variation in nanoaggregate size from 34 nm to 188 nm.
Flow chemistry is known to improve control over polymerization and is often combined with RAFT polymerization. Moreover, flow chemistry also simplifies upscaling due to its efficient heat transfer properties, causing a uniform temperature distribution throughout the reactor.11–13 As an example, Baeten et al. synthesized tetra-block copolymers via RAFT polymerization in flow in different reactors connected in series.14 After extensive kinetic studies, they were able to assume complete conversion of the monomer and complete initiator usage after every block formation, ensuring good block copolymer synthesis. This approach was, however, limited to monomers with fast propagation rates to keep reactor residence reasonably low and suitable for flow chemistry.15 If a residual monomer was still present, statistical copolymers rather than block copolymers would be obtained. This is a significant drawback in the design because not all monomers can be used in this fast and facile method to create block copolymers. Gody et al. proposed the use of a looped flow RAFT polymerization setup to tackle this problem. Their setup allowed the looping of the reaction mixture through a heated reactor until complete conversion was reached, making the volume of the reactor independent of the reaction time. When full conversion had been reached, a new monomer and initiator were introduced to the reaction mixture.16 Thus, this allowed the reactivity issue to be mitigated, yet required constant monitoring of reactions and was limited with respect to scaling the reaction. A different approach would in contrast be to use an inline purification step between block formations (see Fig. 1). This gives the option to use the same reactor and setup also for less reactive monomers and to eliminate any initiator fragments or other side products that may be present in the reaction mixture at the end of the polymerization.17,18 Instead of avoiding residual monomer, monomer would be removed from the mixture, not unlike to what is standard in classical batch synthesis. However, polymer purification in flow is a less explored field compared to synthesis and characterisation and has to date not been realized.19 Removing the requirement of either offline isolation of polymers or the need to obtain practically complete conversion in flow to avoid residual monomers will enable the use of many other polymers in flow block copolymer synthesis, as less reactive monomers could be used without problems, such as methacrylates of styrene.
In batch, the purification of a particular mixture where a miscible solvent or small molecule – such as a monomer – has to be removed, is typically performed via precipitation or in cases where the polymer is not a solid or is difficult to precipitate via dialysis. With dialysis, a cellulose membrane is typically used with a pore size small enough (molecular weight cut-off in this project is ∼3500 Da) that will not let the species of interest (e.g. polymers) diffuse across the membrane but allows small molecules and solvents to pass.20,21 The smaller molecules will pass through the membrane via osmosis until an equilibrium in concentration is reached on both sides of the membrane. This concentration gradient across the membrane is the driving force behind diffusion.22–25 Batch dialysis is often a time-consuming process taking more than 24 hours and is typically limited to rather small amounts of product since the polymer needs to be filled into small dialysis bags. It is conventionally not scalable. The solvent must be renewed several times to keep the concentration gradient across the membrane high enough to promote the diffusion and removal of all the solvents or smaller molecules. In flow, it has been shown that the concentration gradient can be kept high at all times, accelerating the dialysis. Verstraete et al. demonstrated this when purifying block copolymer nanoaggregates from residual organic solvent in continuous flow.22 In principle, such an inline purification system can be directly coupled to any continuous flow system, including multiblock synthesis. This eliminates the limitation of using highly reactive monomers in a flow process. Schuett et al. were the first to automate batch dialysis using simple robotics for the multi-step synthesis of block copolymers.26 In their approach, a traditional batch dialysis method is used and the dialysis is followed via NMR to track the monomer removal over time. This setup was later updated with an extra pump that gives circulation to the solvent outside of the dialysis bag to promote a higher concentration gradient and therefore faster dialysis.27 Terzioğlu et al. demonstrated, shortly after, the strength of the dialysis method in an automated setup for high throughput screening for synthesising a polymer library using sophisticated parallel synthesis equipment.28
For this work, a simple, comparatively cheap and reusable setup was built to produce multiblock copolymers via RAFT polymerization in a continuous flow process without the disturbing interference of residual monomers affecting the block copolymer fidelity. This approach solves the issues previously encountered by Baeten et al.,14 yet is much faster than any other described technique to date. Reusability is achieved by allowing dialysis membranes to be reused for various runs. Acrylate or methacrylate monomers are polymerized in a conventional flow reactor. As a major improvement, the block copolymer formation is entirely independent of the obtained conversion of the monomer in the first block polymerization. The macroRAFT agent is purified via inline dialysis using a custom designed flow dialysis block (see Fig. 2). The setup makes it possible to synthesise different block copolymers in a single day as dialysis is significantly accelerated. It further allows nanostructures such as micelles to be created in a similar fashion, also continuously, allowing us to quickly change the composition of a block copolymer and to study – in principle – compositional influences on the nanoaggregate size (see Fig. 3).29 In principle, synthesis from the monomer stage to nanoparticles can be achieved in a single setup without intermediate isolation or other interruption. This is unprecedented to date and can also be applied to monomers such as methacrylates, which so far have been difficult to use in block copolymers in flow synthesis. Also, morphological studies of nanoaggregates are possible in principle, yet this was not at the centre of this investigation. It should hereby be noted that the term micelle is often loosely used in the literature, and nanoaggregates, even if spherical and small in size might not be micelles in the stricter sense of the word.
MMA was polymerized via photoiniferter polymerization in continuous flow at 90 °C (see Scheme 1). In photoiniferter polymerization the RAFT agent or RAFT polymers themselves can act as a radical source under UV and visible light irradiation.32 A blue light source is used to initiate the polymerization controlled by a trithiocarbonate via the interplay of the iniferter mechanism and the classical thermal RAFT polymerization scheme. This combination of light initiation and thermal activation of propagation means that after 20 minutes high conversions are obtained for methacrylates. Typically, batch reaction times of 8–24 hours are required for high conversions for this monomer class due to the slower propagation of the methacrylate macroradicals. With our high temperature photoiniferter polymerization, monomer conversion was found to be typically around 90%.
Via inline NMR spectroscopy, we followed the removal of the monomer. The integrals of the vinyl peaks of the monomer are observed in comparison to the backbone peaks of the polymer. As a result of this, dialysis progress is easily surveyed. In Fig. 4, the dialysis kinetics can be seen for the different macroRAFT species under investigation (we synthesized polymers of different degrees of polymerization, DP, to test for chain length influences of the dialysis). A waved clearance pattern was observed in all cases. This phenomenon is caused by the flow rate changing upon closing the dialysis loop. Since we performed dialysis immediately after synthesis, no flow stabilization time was added; otherwise the wave pattern would not be observed. An inline NMR spectrometer was placed in front of the dialysis block. This means that data are collected first before any dialysis has taken place, allowing the progress of the dialysis to be estimated correctly. If it were placed after the dialysis block, the wavy pattern would already be less pronounced. The dialysis loop is 5 mL in size, out of which the dialysis block takes a volume of 3 mL. As can be seen, all macroRAFT species show a similar clearance kinetics profile. The results are well reproducible, allowing for a deeper analysis of the clearance rates.
Fig. 4 Clearance kinetics by dialysis of PBA and PMMA monitored inline via low field NMR spectroscopy. |
(1) |
(2) |
The diffusion velocity coefficient is described using the coefficient k, whereas F describes an external flow replacing solvent in the outer volume. In our system, the outer purification volume is continuously refreshed. The monomer concentration in the purification volume is, therefore, always practically zero. This means that every time the sample volume passes, the only limiting factor in the rate of dialysis is the concentration of the monomer in the sample volume. The equation can therefore be largely simplified to:
(3) |
Results for the dialysis velocity coefficient k are given in Table 1. The difference in monomer concentrations with every pass through the NMR spectrometer is monitored. The volume within the dialysis block is 3 mL, with a flow rate of 0.2 mL min−1, which means that the effective time spent in dialysis per loop is only 900 seconds. The dialysis velocity coefficient k can be calculated from the initial concentration and the clearance of the monomer per loop. It was found that k is on average 4.1 × 10−4 s−1, irrespective of the monomer to be removed. As expected from eqn (3), the dialysis velocity is also independent of the concentration of the remaining monomer. Overall, when compared to conventional batch dialysis, the k coefficients in flow are 4–5 times higher, showing the high efficiency of the flow method.
Sample | k (s−1) | c (polymer) (mol L−1) | c initial (monomer) (mol L−1) |
---|---|---|---|
pBA–DP60 | 4.22 × 10−4 | 3.48 | 0.52 |
pBA–DP80 | 4.11 × 10−4 | 3.38 | 0.72 |
pBA–DP100 | 3.89 × 10−4 | 3.6 | 0.4 |
pMMA–DP40 | 4.00 × 10−4 | 2.67 | 0.33 |
pMMA–DP45 | 4.22 × 10−4 | 2.7 | 0.3 |
pMMA–DP65 | 4.33 × 10−4 | 2.67 | 0.33 |
The observed larger k is due to the design of the dialysis block. The very shallow flow channels increase the surface-volume ratio across the membrane, which increases the chance of the monomer being in contact with the membrane and therefore the clearance rate of the dialysis. Moreover, the zigzag structure of the channels creates turbulence within the solvents which increases the mixing of monomers in both phases. If a monomer crosses the membrane, it is immediately removed in the flow with the solvent phase always maintaining the highest concentration gradient. These adaptations mean that the purification process can be completed in significantly less time, making multi-reaction processes possible in hours. It should be thereby noted that no difference is seen if the purification solvent is pumped in crossflow, or in parallel to the polymer solution, underpinning that the concentration gradient is indeed always at its maximum.
With knowledge of the dialysis velocity coefficient, the dialysis becomes predictable and the inline NMR that we employed is in principle not required to monitor the progress of monomer removal. This is much more difficult to achieve for traditional batch reactions due to variation in setups and less control over concentration gradients. To exemplify the high predictability, we compared the clearance of the monomer as obtained per pass through the NMR with predictions made with the average k determined above. Fig. 5 nicely shows the close match that is obtained between experiment (markers) and prediction (full line) as an example for the clearance of the monomer from the pBA–DP60 sample. All clearance curves look fairly similar to each other as can be expected from the close match of the k values as indicated in Table 1.
Fig. 5 Comparison of the predicted clearance of the monomer viaeqn (3) for pBA–DP60 and the measured progress of clearance via inline NMR. |
Overall, one can see that the purification of the monomer is finished within six loops. With a flow rate of 0.2 mL min−1, every loop takes 25 minutes. Purification is hence finished after 2.5 hours. It should be noted that our previous experiments had already demonstrated that lowering the flow rate allowed for faster clearance of contaminants per loop, yet at the same time left the overall dialysis time unchanged as loops took longer. While 2.5 h may seem long at first glance, this is significantly shorter than any other method where purification often takes a day or longer. It is possible to increase the volume of the dialysis loop if larger quantities of purified polymers are needed, but this will decrease the ratio of contact time with the membrane over the total time of the dialysis and therefore make total purification slower. However, it is possible without problem to place several dialysis blocks in series, as is for example also performed for other liquid–liquid extractions in continuous flow. Hypothetically, if we had used 5 additional dialysis units in our setup, we could have purified all monomers from the first block in a single pass, allowing for completely continuous operation, while bringing the dialysis time even more down since dead volume from passing the NMR could be reduced. Furthermore, even though we used inline NMR to monitor the monomer removal for this work, the steady clearance kinetics that we observed allow dialysis to be performed without any online monitoring reliably.
Sample | Conversion (%) | M n (g mol−1) | Đ |
---|---|---|---|
pBA–DP60–pPEGMEA–DP25 | 16 | 8800 | 1.48 |
pBA–DP60–pPEGMEA–DP45 | 17 | 10560 | 1.56 |
pBA–DP60–pPEGMEA–DP65 | 16 | 11880 | 1.64 |
pMMA–DP25–pPEGMEA–DP25 | 78 | 9750 | 1.27 |
pMMA–DP25–pPEGMEA–DP45 | 71 | 14000 | 1.45 |
pMMA–DP25–pPEGMEA–DP65 | 65 | 18560 | 1.76 |
The control over the polymerization of the block copolymers means that there is control over the composition of the BCPs and in consequence also the size of the nanoaggregates. This complete design offers the opportunity to not only prepare nanoaggregates on a large scale in a rapid fashion but also gives a platform for screening different block lengths and studying their nanostructure-counterparts. Table 3 provides an overview of the DLS results. As can be seen, an impressive variability is observed in the particle sizes from 34 nm to 188 nm, even if PDIs are in the intermediate regime. This result underpins the general idea that flow dialysis can bridge the gap in BCP flow synthesis, finally reaching the goal of producing micelles in series starting directly from the monomer. As such, the presented setup and its attachment for micelle formation provide the foundation for much more detailed studies into effects of BCP nature, composition and length on nanoaggregate formation.
Sample | pPEGMEA25 | pPEGMEA45 | pPEGMEA65 | |||
---|---|---|---|---|---|---|
Size/nm | PDI | Size/nm | PDI | Size/nm | PDI | |
pBA–DP60 | 34 | 0.261 | 70 | 0.202 | 136 | 0.261 |
pBA–DP80 | 49 | 0.193 | 78 | 0.278 | 144 | 0.273 |
pBA–DP100 | 90 | 0.21 | 154 | 0.263 | 188 | 0.237 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc01819a |
This journal is © The Royal Society of Chemistry 2023 |