1 3
Topics in Current Chemistry (2018) 376:44
Since the increased radiation homogeneity of the entire reaction medium can be
easily obtained in continuous-flow reactors with small characteristic dimensions, the
combination of photochemistry and flow chemistry is highly reasonable [33–39]. As
compared with conventional batch photoreactors, photochemical transformations in
continuous-flow microreactors can significantly shorten the reaction/residence time
and achieve a higher reaction selectivity with lower loading of photocatalysts. It
was reported that there was about 150 times photon flux density in continuous-flow
microreactors when compared to batch reactors, which may be one of the main reasons for the much faster reaction rate obtained in the continuous-flow microreactors
[103, 162, 163]. Therefore, the microreactor technology has been recognized as a
practical and robust way to produce small molecules as well as polymers through
photochemical reactions [164, 165]. Moreover, the selection and purification of photocatalysts deserve careful considerations, which are important parameters for process design. For certain processes in industry, the residual catalyst content typically
has stringent standards [166].
4.8.1 Photo‑induced Radical Polymerization
Wenn and coworkers reported a photo-induced copper-mediated radical polymerization of methyl acrylate (MA) in continuous-flow microreactors including tubular
photoreactor and a glass-chip photomicroreactor [167]. The reaction solvent consisted of dimethylsulfoxide and CuBr 2 , and Me 6 TREN were used as the catalyst/
ligand pair. The control of the polymerization kinetics was very desirable in flow,
the resulting polymer had a low PDI value, and the average molecular weight was
confirmed to be in the linear evolution during the polymerization. Moreover, the
glass-chip photomicroreactor could accelerate the photo-induced polymerization of
methyl acrylate compared to the conventional tank reactor, achieving nearly 90%
monomer conversion with 20 min, and poly(methyl acrylate) with average molecular
weight of 9800 g mol
−1
was successfully synthesized with the use of microreactor
technology. In addition to the homopolymer poly(methyl acrylate), the block copolymer poly(MA)-block-poly(nBA) was also successfully synthesized in flow by these
authors, and the obtained block copolymer had similar low dispersibility [167].
Chuang et al. reported the synthesis of mixed acrylate/methacrylate (methyl
acrylate, MA and methyl methacrylate, MMA) block copolymers by copper-mediated free-radical photopolymerization in a microreactor with the use of a 100-W
high-pressure mercury vapor short arc lamp (320–500 nm) as the light source [168].
Proper selection of ligand and polymerization sequence was performed in order to
get optimized conditions. Compared with the batch photoreactor, the tubular photomicroreactor provided a significant acceleration of polymerization, concomitant
reduction in product dispersity and largely simplified block copolymer synthesis
conditions. Rubens et al. prepared the diblock and triblock polymethacrylate copolymers in a single or coupled microreactor through the photo-induced polymerization
using an initiation-transfer-terminator (iniferter) with the blue light illumination, and
the PDI value of synthesized polymers indicated in a narrow molecular weight distribution (PDI = 1.2–1.3) [169].
179
Reprinted from the journal
Topics in Current Chemistry (2018) 376:44
Since the increased radiation homogeneity of the entire reaction medium can be
easily obtained in continuous-flow reactors with small characteristic dimensions, the
combination of photochemistry and flow chemistry is highly reasonable [33–39]. As
compared with conventional batch photoreactors, photochemical transformations in
continuous-flow microreactors can significantly shorten the reaction/residence time
and achieve a higher reaction selectivity with lower loading of photocatalysts. It
was reported that there was about 150 times photon flux density in continuous-flow
microreactors when compared to batch reactors, which may be one of the main reasons for the much faster reaction rate obtained in the continuous-flow microreactors
[103, 162, 163]. Therefore, the microreactor technology has been recognized as a
practical and robust way to produce small molecules as well as polymers through
photochemical reactions [164, 165]. Moreover, the selection and purification of photocatalysts deserve careful considerations, which are important parameters for process design. For certain processes in industry, the residual catalyst content typically
has stringent standards [166].
4.8.1 Photo‑induced Radical Polymerization
Wenn and coworkers reported a photo-induced copper-mediated radical polymerization of methyl acrylate (MA) in continuous-flow microreactors including tubular
photoreactor and a glass-chip photomicroreactor [167]. The reaction solvent consisted of dimethylsulfoxide and CuBr 2 , and Me 6 TREN were used as the catalyst/
ligand pair. The control of the polymerization kinetics was very desirable in flow,
the resulting polymer had a low PDI value, and the average molecular weight was
confirmed to be in the linear evolution during the polymerization. Moreover, the
glass-chip photomicroreactor could accelerate the photo-induced polymerization of
methyl acrylate compared to the conventional tank reactor, achieving nearly 90%
monomer conversion with 20 min, and poly(methyl acrylate) with average molecular
weight of 9800 g mol
−1
was successfully synthesized with the use of microreactor
technology. In addition to the homopolymer poly(methyl acrylate), the block copolymer poly(MA)-block-poly(nBA) was also successfully synthesized in flow by these
authors, and the obtained block copolymer had similar low dispersibility [167].
Chuang et al. reported the synthesis of mixed acrylate/methacrylate (methyl
acrylate, MA and methyl methacrylate, MMA) block copolymers by copper-mediated free-radical photopolymerization in a microreactor with the use of a 100-W
high-pressure mercury vapor short arc lamp (320–500 nm) as the light source [168].
Proper selection of ligand and polymerization sequence was performed in order to
get optimized conditions. Compared with the batch photoreactor, the tubular photomicroreactor provided a significant acceleration of polymerization, concomitant
reduction in product dispersity and largely simplified block copolymer synthesis
conditions. Rubens et al. prepared the diblock and triblock polymethacrylate copolymers in a single or coupled microreactor through the photo-induced polymerization
using an initiation-transfer-terminator (iniferter) with the blue light illumination, and
the PDI value of synthesized polymers indicated in a narrow molecular weight distribution (PDI = 1.2–1.3) [169].
179
Reprinted from the journal