Inefficient mixing is caused by the long pathways of the molecules by diffusion through the complete cross section of the reaction tube.
Handbook of Polymer Synthesis: Second Edition
In contrast to T-pieces, commercially available multilamination mixers present clearly improved mixing patterns. Micrometer thin fluid layers lamellea get split up prior to the mixing process. Such multilamination mixers are also available for high-pressure applications. Another type of mixing is represented by split-and-recombine mixers Figure 5 , which are especially beneficial for chemical reactions with low-viscosity materials and are thus less suitable for higher molecular weight polymers.
Within such micromixers the fluid lamellae are split and subsequently recombined.
The repetition of this process in each step n of the mixers creates 2 n stacked fluid lamellae. In addition, the process of this serial multilamination shows further dominant fluid dynamic effects, e. In summary, all mixing types show high potential for polymerization in continuous flow, depending on the degrees of polymerization desired.
In the following section the currently established procedures, recent studies, and future challenges in polymer science utilizing different kinds of mixing geometries are discussed.
Microfluidic devices permit precise online monitoring of different stages of a reaction. In , Penlidis et al. On the one hand, the authors describe the sensor development to follow relevant physical parameters pressure, flow, and temperature. On the other hand, they give a valuable overview of the different monitoring techniques for in-situ determinable polymer properties, such as molecular weight, chemical composition, particle size, and cross-linking density. In a very recent work in this important area, Kumacheva and co-workers published a seminal kinetic study of the polymerization of N -isopropylacrylamide using in-situ attenuated total reflection Fourier transform infrared spectroscopy ATR-FTIR under different reaction conditions, benefiting from the rapid on-chip characterization method.
A central issue in current polymer science is the synthesis of complex polymer architectures with controlled molecular weight and narrow molecular weight distribution MWD. In a continuous setup, precise control can be achieved by efficient mixing using microreaction technology.
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First reports on the continuous preparation of homopolymers were published already in the s by Szwarc as well as Schulz and co-workers. Suprisingly, the area remained almost dormant in academic research, until the late s, which are marked by the implementation of different polymerization techniques in microreactors.
This renaissance comprises the successful execution of anionic, cationic, free radical, and controlled radical homopolymerization of different monomers in continuous flow, which will be outlined in this section. In the following paragraphs, the achievements in ionic polymerization in the flow will be discussed. For this type of polymerization, the reaction kinetics is strongly influenced by the solvent and counterion.
In particular, a more polar solvent leads to accelerated reaction kinetics, and control of the highly exothermic reaction becomes demanding. Furthermore, the living character and the sensitivity of the carbanionic chain ends demand strictly anhydrous and sealed reaction conditions. Microreaction technology provides suitable reaction compartments with high mixing efficiency to control extreme reaction conditions. In addition, the sealed reaction environment in a microfluidic device that enables living polymerization without tedious purification steps is a key feature.
The first carbocationic polymerization was transferred to a microfluidic device by Nagaki et al. The polymerization was controlled by adjusting the flow rate ratio and was completed within 0. Furthermore, the living character was confirmed by end-capping the polymer with allyltrimethylsilane. Scheme 1. Anionic polymerization also represents a highly suitable polymerization technique for microfluidic processes, especially due to the well-contained reaction zone.
Reaction time and experimental effort can be reduced significantly compared to established techniques and reactors. A significant decrease of the PDI value was obtained when utilizing a silicon-based microreactor fabricated by a dry etching procedure. For instance, the preparation of poly glutamine revealed a PDI of 1. This underlines the excellent control over the polymerization with microfluidic devices. Employing a decreased flow rate leads to higher residence times and a gradual increase of molecular weight cf. With respect to biomedical applications, these remarkable results offer a facile and novel access to poly amino acid s.
In our group, the anionic polymerization of poly styrene PS was transferred to a microstructured reactor. The monomer and initiator sec -butyllithium solutions were combined in the micromixer to initiate the anionic polymerization at room temperature in a polar solvent. Compared to batch reactions, the reaction time could be reduced from hours to several seconds, giving facile access to large amounts of well-defined polymers in a short period of time.
Furthermore, the adjustment of flow rate ratios afforded different molecular weights in a single experiment without interrupting the continuous flow process. It has to be mentioned that for these reactions the complete microreactor interior has to be made of stainless steel because the living carbanions react with most common materials, e. This work was extended to the preparation of specifically terminated polymers by investigating the end-capping process of living PS with different reagents in a microfluidic setup.
For this purpose a second mixer T-junction was added to achieve direct termination of PS in a continuous process Figure 7. In particular, living PS was end-capped with a variety of tailored glycidyl ethers e.
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This continuous end-functionalization provides a valuable method for rapid and cost-efficient polymer screening with respect to novel end-functionalized polymers that can be utilized for the rapid generation of complex macromolecular architectures. Recently, in analogy to the continuous homopolymerization of PS, other research groups extended this concept utilizing different mixing geometries or styrene derivatives. Free radical polymerizations can also be carried out in microfluidic devices. An important benefit of the transfer to continuous flow is the suppression of the Trommsdorff effect by the superior heat and mass transfer.
Detailed studies of five different monomers in a free radical polymerization were accomplished by Yoshida et al. The authors obtained a significant improvement of the MWD for the polymerization of benzyl methacrylate, methyl methacrylate MMA , and butyl methacrylate compared to the corresponding batch reactions.
This effect was less pronounced for the free radical polymerization of vinyl benzoate and styrene due to the decreased polymerization rate, leading to a reduced benefit from the efficient heat dissipation.
In addition, the successful implementation of the radical MMA polymerization to a larger scale was accomplished by Iwasaki et al. Moreover, industrial application of microflow polymerizations with continuously operating pumps has been described in several patents. Since the s, a variety of novel techniques for the controlled synthesis of polymers by radical polymerization have been developed.
Well-defined polymers with large monomer variation can be obtained in a facile manner. CRP can be subdivided in three major techniques viz. The challenge to transfer the major CRP techniques to flow systems has been taken up by several research groups in recent years. First reports on the transfer of controlled radical polymerization into continuous flow reaction systems were published by Zhu and co-workers.
The column exhibited good catalyst retention and a long-term catalytic reactivity, representing a promising development for commercial ATRP applications, since removal of copper subsequent to polymer synthesis is not necessary. The system was compared to the batch process, and monomer conversion was comparable to bulk reaction kinetics.
In a more tubular dimension, a rather long continuous reactor m length, i. Narrow molecular weight distributions and a slightly higher conversion were obtained compared to analogous batch experiments. The fundamental concept was the aminolysis followed by thiol—ene reaction of various termination reagents.
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Quantitative conversion was realized within 20 min and proven by soft ionization mass spectrometry. Reconsidering microdimensional devices, also other CRP techniques, e. Rosenfeld et al. In comparison to lab-scale batch reactors the exothermic polymerization of n -butyl acrylate can benefit from the superior heat release and provide a narrower MWD, whereas styrene shows no difference to the batch processes. This novel GPC characterization method allows the automatic sampling, dilution, injection, and analysis every 12 min of the crude co polymer samples, which are recovered from the reactor outlet.
Similar results were found for microwave-assisted RAFT polymerization, but the continuous process offers a pronounced scalable and cost-efficient procedure. The comprehensive investigation considered polymerization under a large variety of conditions. Controlled radical polymerization generally exhibits significantly slower polymerization kinetics compared to ionic or free radical polymerizations.
Therefore, CRPs do not directly benefit from the microflow technology with its superior heat and mass transfer. Furthermore, additional requirements have to be fulfilled; e. Even a small amount of precipitated material is problematic and can lead to problems for the long-term stability of the microfluidic device. Thus, from a scientific and industrial point of view it is advantageous to create a method for facile screening of the correlation between polymer architecture, composition, and reaction parameters. In summary, to date a variety of monomers has been polymerized homogeneously in various microfluidic devices with different polymerization techniques, such as anionic, cationic, free radical, and controlled radical polymerization cf.
Although the basic feasibility of the synthesis of these homopolymers has been established in microfluidic setups, future challenges are the implementation of additional monomers for the preparation of block copolymers and other, nonconventional architectures using continuous flow strategies. An impressive technical implementation of microflow technology was presented by the ultrafast anionic co polymerization of methacrylate monomers leading to mainly syndiotactic polymers. Table 1.
Polymer Synthesis and Characterization
The implementation of homopolymerizations in microfluidic systems represents an important foundation for the preparation of complex polymer architectures in microfluidic devices. The synthesis of linear block copolymers represents a key target, and in particular living polymerization techniques provide an ideal platform for the synthesis of multiblock copolymers, since they can be prepared in a one-pot reaction by sequential addition of the desired monomers. By connecting several mixers in a microfluidic device, sequential addition of monomers can be achieved, enabling the preparation of block copolymers with different block length ratio and degree of polymerization within short time.
As an alternative access, diblock copolymers can be obtained by semicontinuous polymerizations. Here, a conventionally synthesized macroinitiator can be introduced in the microfluidic devices to initiate the second polymerization, or the macroinitiator can be synthesized in continuous flow and utilized in a batch reactor for a subsequent block polymerization. In several reports the semi continuous flow synthesis of block copolymers has been described.