Topics in Current Chemistry (2018) 376:44
1 3
concerns, as discussed in the front sections. As suggested by Wang et al., liquid–liquid slug flow or liquid–liquid heterogeneous operations could decrease the
flow resistance and avoid polymer particles adhering in continuous-flow reactors,
with the inert continuous phase generating a lubrication layer between the polymer
solution (dispersed phase) and the reactor wall. With the protection of n-octane films
around dispersed droplets, polyacrylamide with high molecular weights (> 1000 kg/
mol) was successfully synthesized in a tubular reactor with 38-min residence time.
In contrast, while performing the control experiment in a batch reactor, polymer
adhesion to the flask was observed, even though a large number of surfactants were
used, owing to poor heat exchange ability [157].
Some research groups have studied the channel clogging mechanisms in detail.
Tubular reactors are attractive for the synthesis of complex dispersed polymers by
photoinitiated miniemulsion polymerization. However, when a fine emulsion polymerization process with the rather high dispersion between the organic and aqueous
phases is carried out in a tubular reactor, the channel clogging might easily occur.
Asua et al. revealed that when the interfacial tension between the monomer and the
reactor wall ( MS ) was lower than that between the aqueous phase and the reactor
wall ( WS ), monomers could diffuse onto the reactor wall and the polymerization
took place there [158]. Substantial accumulation of polymers on the reactor wall
might cause channel clogging. For í µí»¾ MS
í µí»¾ WS < 1 , the production of polyurethane
with 45% solid content from prepolymer/acrylics system could be successfully conducted with the monomer conversion over 90% in a quartz microreactor.
Bistac et al. delved into the parameters causing channel plugging, as well as complex clogging mechanisms. Taking the BA photo radical miniemulsion polymerization in a helix quartz tubular microreactor as an example, they revealed that the
miniemulsion stability, the size of dispersed phase droplets, and the flow rate had
minimal impact, while the solids content, the surfactant concentration, and the characteristic diameter of the reactor were the key factors for channel clogging, which
should be carefully selected [159].
4.8 Photopolymerization in Microreactors
In many photopolymerization processes, the reaction rate will strongly depend on
the light intensity. However, the increase of light intensity has deleterious effects
on the molecular weight distribution when the monomer conversion reaches a high
level [9]. Low light intensity and long reaction time are usually required in order
to obtain polymers with narrow molecular weight distributions. Furthermore, it is
difficult to achieve uniform irradiation when large-scale reactors for the photopolymerization are required considering the scale-up process [160, 161]. Therefore, it is
very important to ensure uniform and sufficient irradiation in photopolymerization
processes. Compared with thermal polymerization, reactor design for photopolymerization is indeed more complex. Therefore, simple and scalable photopolymerization reactors should be developed to provide uniform irradiation for the synthesis of
polymers with narrow molecular weight distributions.
178
Reprinted from the journal
1 3
concerns, as discussed in the front sections. As suggested by Wang et al., liquid–liquid slug flow or liquid–liquid heterogeneous operations could decrease the
flow resistance and avoid polymer particles adhering in continuous-flow reactors,
with the inert continuous phase generating a lubrication layer between the polymer
solution (dispersed phase) and the reactor wall. With the protection of n-octane films
around dispersed droplets, polyacrylamide with high molecular weights (> 1000 kg/
mol) was successfully synthesized in a tubular reactor with 38-min residence time.
In contrast, while performing the control experiment in a batch reactor, polymer
adhesion to the flask was observed, even though a large number of surfactants were
used, owing to poor heat exchange ability [157].
Some research groups have studied the channel clogging mechanisms in detail.
Tubular reactors are attractive for the synthesis of complex dispersed polymers by
photoinitiated miniemulsion polymerization. However, when a fine emulsion polymerization process with the rather high dispersion between the organic and aqueous
phases is carried out in a tubular reactor, the channel clogging might easily occur.
Asua et al. revealed that when the interfacial tension between the monomer and the
reactor wall ( MS ) was lower than that between the aqueous phase and the reactor
wall ( WS ), monomers could diffuse onto the reactor wall and the polymerization
took place there [158]. Substantial accumulation of polymers on the reactor wall
might cause channel clogging. For í µí»¾ MS
í µí»¾ WS < 1 , the production of polyurethane
with 45% solid content from prepolymer/acrylics system could be successfully conducted with the monomer conversion over 90% in a quartz microreactor.
Bistac et al. delved into the parameters causing channel plugging, as well as complex clogging mechanisms. Taking the BA photo radical miniemulsion polymerization in a helix quartz tubular microreactor as an example, they revealed that the
miniemulsion stability, the size of dispersed phase droplets, and the flow rate had
minimal impact, while the solids content, the surfactant concentration, and the characteristic diameter of the reactor were the key factors for channel clogging, which
should be carefully selected [159].
4.8 Photopolymerization in Microreactors
In many photopolymerization processes, the reaction rate will strongly depend on
the light intensity. However, the increase of light intensity has deleterious effects
on the molecular weight distribution when the monomer conversion reaches a high
level [9]. Low light intensity and long reaction time are usually required in order
to obtain polymers with narrow molecular weight distributions. Furthermore, it is
difficult to achieve uniform irradiation when large-scale reactors for the photopolymerization are required considering the scale-up process [160, 161]. Therefore, it is
very important to ensure uniform and sufficient irradiation in photopolymerization
processes. Compared with thermal polymerization, reactor design for photopolymerization is indeed more complex. Therefore, simple and scalable photopolymerization reactors should be developed to provide uniform irradiation for the synthesis of
polymers with narrow molecular weight distributions.
178
Reprinted from the journal