1 3
Topics in Current Chemistry (2018) 376:44
inherently low viscosity could be prepared in the bulk up to full conversion within
20 min under UV irradiation and solvent-free conditions, which was very untypical
for flow radical polymerization. The average molecular weight of the obtained polymer materials was from 5000 g mol
−1
up to 100,000 g mol
−1
, strongly depending
on the monomer–thiol ratio. As claimed, the scalable continuous-flow photoreactor realized 24-h continuous synthesis of 300 g of branched poly(n-butyl acrylate)
s, which allowed the polymerization to occur under homogenous irradiation and
prevented local hot spots, thus increasing the monomer conversion and improving
control over reaction conditions (i.e., temperature and viscosity). Furthermore, the
robustness of this photoreactor system was demonstrated by using different photoinitiators, thiols, and tube diameters.
Gong et al. reported the development of the metal-free photocontrolled radical
polymerization of semifluorinated (meth) acrylates using a new visible-light-absorbing organocatalyst in flow for the production of a variety of semifluorinated polymers with narrow molecular weight distributions [174]. Compared with the batch
processing, the reaction time could be reduced to one-fourth and the block copolymer was produced with excellent properties (Mn = 10,800 g mol
−1
and PDI = 1.07)
in the photomicroreactor system. As demonstrated, this continuous-flow technique
in combination with photochemistry allowed for the controllable synthesis of tailormade semifluorinated polymers for advanced material engineering.
4.8.2 PhotoRAFT Processes
Thermal initiators have commonly been used for reversible addition–fragmentation chain-transfer (RAFT) polymerization. However, traditionally induced RAFT
polymerization is less efficient, leading to recent developments such as photoelectron transfer RAFT (PET-RAFT), which was demonstrated to be an efficient technology for a variety of complex synthetic targets [175]. Recently, Gardiner et al.
reported the RAFT polymerization of (meth) acrylates or acrylamides using conventional photoinitiators for the RAFT polymerization in a microreactor under the
irradiation of a 150-W mercury lamp or a range of LED arrays [176]. They used
millimeter-scale fluoropolymer capillaries with excellent light transmission properties to synthesize multigrams/kgs of RAFT polymers. The effects of different wavelengths and photoinitiators on the polymerization performance and the product quality were investigated. It was found that the photoinitiated polymerization achieved
good conversion and low polydispersity (< 1.3) using wavelengths between 310 and
380 nm. A careful interaction between the range of wavelengths used and residences
time should be checked since broader wavelength ranges resulted in faster conversion but an increase in polydispersity due to scission and termination side reactions
caused by further activation of the RAFT end groups. Compared to standard thermal
RAFT usually requiring high temperatures or long reaction times, this work demonstrated very fast polymerization with nearly full conversion after only 5 min at
30 °C with the use of high initiator and RAFT agent loadings. Apparently, photoinitiated RAFT polymerization in flow will find a wide application potential in polymer research and industry.
181
Reprinted from the journal
Topics in Current Chemistry (2018) 376:44
inherently low viscosity could be prepared in the bulk up to full conversion within
20 min under UV irradiation and solvent-free conditions, which was very untypical
for flow radical polymerization. The average molecular weight of the obtained polymer materials was from 5000 g mol
−1
up to 100,000 g mol
−1
, strongly depending
on the monomer–thiol ratio. As claimed, the scalable continuous-flow photoreactor realized 24-h continuous synthesis of 300 g of branched poly(n-butyl acrylate)
s, which allowed the polymerization to occur under homogenous irradiation and
prevented local hot spots, thus increasing the monomer conversion and improving
control over reaction conditions (i.e., temperature and viscosity). Furthermore, the
robustness of this photoreactor system was demonstrated by using different photoinitiators, thiols, and tube diameters.
Gong et al. reported the development of the metal-free photocontrolled radical
polymerization of semifluorinated (meth) acrylates using a new visible-light-absorbing organocatalyst in flow for the production of a variety of semifluorinated polymers with narrow molecular weight distributions [174]. Compared with the batch
processing, the reaction time could be reduced to one-fourth and the block copolymer was produced with excellent properties (Mn = 10,800 g mol
−1
and PDI = 1.07)
in the photomicroreactor system. As demonstrated, this continuous-flow technique
in combination with photochemistry allowed for the controllable synthesis of tailormade semifluorinated polymers for advanced material engineering.
4.8.2 PhotoRAFT Processes
Thermal initiators have commonly been used for reversible addition–fragmentation chain-transfer (RAFT) polymerization. However, traditionally induced RAFT
polymerization is less efficient, leading to recent developments such as photoelectron transfer RAFT (PET-RAFT), which was demonstrated to be an efficient technology for a variety of complex synthetic targets [175]. Recently, Gardiner et al.
reported the RAFT polymerization of (meth) acrylates or acrylamides using conventional photoinitiators for the RAFT polymerization in a microreactor under the
irradiation of a 150-W mercury lamp or a range of LED arrays [176]. They used
millimeter-scale fluoropolymer capillaries with excellent light transmission properties to synthesize multigrams/kgs of RAFT polymers. The effects of different wavelengths and photoinitiators on the polymerization performance and the product quality were investigated. It was found that the photoinitiated polymerization achieved
good conversion and low polydispersity (< 1.3) using wavelengths between 310 and
380 nm. A careful interaction between the range of wavelengths used and residences
time should be checked since broader wavelength ranges resulted in faster conversion but an increase in polydispersity due to scission and termination side reactions
caused by further activation of the RAFT end groups. Compared to standard thermal
RAFT usually requiring high temperatures or long reaction times, this work demonstrated very fast polymerization with nearly full conversion after only 5 min at
30 °C with the use of high initiator and RAFT agent loadings. Apparently, photoinitiated RAFT polymerization in flow will find a wide application potential in polymer research and industry.
181
Reprinted from the journal