System control, microreactors

Recently, it has been demonstrated that good control of molecular weight and molecular weight distribution can be attained by using microreactor systems without stabilizing the carbocationic intermediates. The concept of this new technology (flow-microreactor-system-controlled polymerization) is described in the following... 

An example of microreactor systems for block copolymerization is shown in Fig. 7. The first monomer IBVE is mixed with TfOH in the first micromixer (Ml). Introduction of the second monomer (NB VE or EVE) at the second micromixer M2 results in the formation of the polymer of higher molecular weight with narrow molecular weight distribution [128]. Block copolymerization can be carried out with any combination and with either order of monomer addition, as shown in Table 3, demonstrating that the present method serves as a flexible method for the synthesis of block copolymers. Therefore, flow-microreactor-system-controlled polymerization can serve as a powerful method for synthesis of structurally well-defined polymers and copolymers in industry. 

It is easily anticipated that flow microreactors can enjoy industrial applications by virtue of inherent advantages based on their microstructure and flow nature. Significant progress in flow-microreactor-system-controlled polymerization to obtain structurally well-defined polymers has already been made to meet the demands of the chemical industry. The lack of need for cryogenic conditions for anionic polymerizations may enable commercial production. Some pilot plants have already been built and tested to examine the feasibility and durability of polymerization in flow microreactors. For example, a microchemical pilot plant... 

Similarly, MF synthesis offers crucial benefits for living anionic polymerization [39b,46]. For example, for polymerization of alkyl methacrylates conducted in a flow microreactor system, control over molecular weight distribution was achieved under easily accessible conditions at temperatures of —28 °C (methyl methacrylate, Mw/Mn = 1.16), 0°C (butyl methacrylate, M /M = 1.24), and 24°C (t-butyl methacrylate, M /M = 1.12) [39b]. The molecular weight of the polymer increased with increasing monomer to initiator ratio. Furthermore, a subsequent reaction of a reactive polymer chain with alkyl methacrylate yielded a block copolymer with a narrow molecular weight distribution. 

Lee, D.S. and Gloyna, E.E, Supercritical water oxidation microreactor system, paper presented at Water Poll. Control Fed. Specialty Conf. New Orleans, LA, April 17-19, 1988. 

In this chapter the use of electric fields as a means to control, activate, or modify chemistry in microreactor systems, and in some cases also in downstream work-up microsystems, was discussed. Particular attention was paid to microplasmas, in its many different configurations, because this is an upcoming field which allows us to perform chemistry at low... 

In the second chapter, Anil Agiral and Han J.G.E. Gardeniers take us to a fascinating world wherein "chemistry and electricity meet in narrow alleys." They claim that microreactor systems with integrated electrodes provide excellent platforms to investigate and exploit electrical principles as a means to control, activate, or modify chemical reactions, or even preparative separations. Their example of microplasmas shows that the chemistry can take place at moderate temperatures where the reacting species still have a high reactivity. Several electrical concepts are presented and novel principles to control adsorption and desorption, as well as the activity and orientation of adsorbed molecules are described. The relevance of these principles for the development of new reactor concepts and new chemistry is discussed. 

More recently, microreactor technology has entered the field of biocatalysis enzymes are used for synthesis rather than for diagnostics. The concept behind the use of biocatalytic microreactor systems is in fact twofold. First, a miniaturized reactor allows an efficient use of small amounts of enzyme, when enzyme kinetics determination is involved. Second, the classical advantages of microreactors in synthesis, namely, better control over heat- and mass-transfer... 

This microreactor system thus allows fast mixing, good temperature control and changing process parameters at short residence times - the overall time to pass all... 

Vitamin D Analogues - A patent covering the photochemical conversion of the diene (112) into the triene (113) within what is called a microreactor has been published. The microreactor system is a zeolite with the appropriate size of the cavity to provide stereochemical control of the reaction. The use of the 2,7-dimethyl-3,6-diazacyclohepta-1,6-diene tetrafluoroborate/biphenyl filter solution has allowed the double wavelength irradiation (290-300 nm and X. > 330 nm) of procalcitriol as a route to la,25-dihydroxycholecalciferol. A study of the control that can be exercised upon the reaction by changes in temperature was carried out. The results of a study of the influence of intensity on the picosecond laser irradiation of provitamin D have been published. Other research has been aimed at the examination of the photochemical behaviour of previtamin 03. ... 

Both oxyhydrochlorinatlon of methane and chloromethane oligomerization studies were conducted in essentially the same microreactor system. All the reactants were introduced at pressures slightly above one atmosphere from gas cylinders, and flow rates were controlled by a Brooks four-channel mass-flow controller. The feed stream for the oxyhydrochlorinatlon reaction was sampled before and after an experimental run, while the product stream was continuously sampled on-line during the run to obtain a mass balance. A quadrupole mass spectrometer was used to analyze the feed and product... 

While microreactors tend to offer advantages for reaction control, which facilitates careful study, not all reactions are equally easy to perform in the systems. General purpose equipment exists to study gas phase reactions as well as gas phase heterogeneous catalyst reactions. However, while liquid phase reactions are also easily studied, specialized microreactor systems are usually built to study liquid phase heterogeneous catalyst reactions. Consequently, these systems are much more specialized and are often tuned to the specific chemistry rmder study. 

To develop an EOF based system. Watts et al. have conducted an extensive study on peptide synthesis, where they prepared a library of peptide derivatives within a computer-controlled microreactor system operating under EOF [25-28]. The authors demonstrated that dipeptides could be prepared from pre-activated carboxylic acids. They optimized the reaction using the pentafluorophenyl (PFP) ester of Fmoc- 3-alanine 4 with amine 5 to give dipeptide 6 quantitatively in 20 min (Scheme 14.2). This represented a significant increase in yield compared with the traditional batch synthesis, where only a 50% yield was obtained in 24 h. 

The experiments were carried out in an all stainless steel microreactor system with four gas lines which was operated at pressures up to 100 bar. The gases were supplied by Linde with the following purities He 99.9999 %, N2 99.9999 %, H2 99.9999 %, the mixture of 25% N2 in H2 used as synthesis feed gas 99.9996 %. The feed gas was further purified by means of a purification unit described elsewhere [4]. The flows were regulated by electronic mass flow controllers. The reactor consisted of a glass-lined U-tube similar to the one described in ref. [13]. It was not possible to detect the desorption of N2, H2 or NH3 from the empty tube within the limits of detection. The U-tube was placed in a copper block to ensure isothermal operation. Gas analysis was performed using a mass spectrometer (Balzers GAM 445) which was calibrated for He, H2, N2 and NH3 by using a reference gas mixture. The calibration for H2O was carried out using a He stream saturated with H2O at room temperature. 

Similarly, radiolytically produced radical cations can be stabilized in zeohtes and related materials. This possibility was exploited by spectroscopists to study the EPR of radical cations and some neutral radicals even before the development of inert matrices such as rare gases and freons for radical cation stabilization. Recently, work in our laboratory has developed the use of inert zeolites as microreactors to control radical cation reactions and to study radiation chemistry in heterogeneous systems. In the case of active catalysts, radiolysis can potentially produce radical cations of products as weU as starting material. Thus, like the spontaneous oxidation process described above, radiolysis combined with EPR permits a method of post-reaction analysis of products by in situ spectroscopy. 

Microreactors have also been used for ionic polymerization or polycondensation processes. Nagaki et al. [136] have synthesized polystyrene-poly(alkyl methacrylate) block copolymers by butyllithium initiated anionic polymerization in an integrated flow microreactor system. A high level of control of molecular weight was achieved at temperatures between -28 and +24 °C due to fast mixing, fast heat transfer, and residence time control. Santos and Metzger... 

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