Phoenix Flow Reactor™ Application Note
Figure 1. Schematic diagram illustrating the grafting polymerization of particles (silica) packed in a column via SI-RAFT.
INTRODUCTION
Polymer grafted inorganic particles are attractive building blocks for numerous chemical and material applications. Surface initiated controlled radical polymerization (SI-RAFT) is one of the most feasible methods to fabricate these materials. However, conventional in-batch approaches still suffer from several disadvantages, such as time-consuming purification
processes, inefficient grafting, and possible gelation problems.1
A facile method was demonstrated to synthesize homopolymers and block copolymer grafted inorganic particles using continuous flow chemistry in an environmentally friendly aqueous media using the Phoenix Flow ReactorTM. 2 Immobilization of the initiator (or chain transfer) agent and subsequent polymerization can be accomplished sequentially in this continuous flow system, avoiding multi-step purification processes in-between. The chain length (MW) of the grafted polymers is tunable by adjusting the flow rate or monomer concentration, and the narrower molar mass dispersity of the grafted polymers reveals the uniform polymer chains on the particles.
State of the art:
A facile, controlled and scalable method to fabricate polymer grafted particles (PGP) in a continuous flow reactor system at a relatively high temperature in water/ethanol was reported.
Utilization of surface initiated reversible addition fragmentation and chain transfer (SI-RAFT) polymerization allowed grafted polymers with relatively low molar mass dispersity, and the length of the grafted polymers was adjustable by changing the reaction time or modifying the monomer concentration. Moreover, comparing with the conventional “grafting from” reaction in a batch reactor, the in-solution “gelation” problem is avoided owing to the flow nature of the continuous system. (Figure 1.).
EXPERIMENTAL
The synthesis of the initiator: butyl (1-oxo-1-((3-(trimethoxysilyl)propyl)amino)prop-an-2-yl) carbonotrithioate (BTPAPC) is reported in the publication. The grafting of the BTPAPC initiators to the Silica microparticles (SiMP) was done in situ via flow reactor procedures. The synthesis and grafting of PPEGMEMA grafted SiMPs (PPEGMEMA-g-SiMPs) was done via flow reactor procedures and is reported in the publication. Finally, the synthesis and grafting of the block copolymer: PPEGMEMA-b-PNIPAM grafted SiMPs (PPEGMEMAb-PNIPAM-g-SiMPs) via flow reactor was also reported in the publication. Further characterization of the products was done by Size Exclusion Chromatography (SEC), NMR, IR, and thermogravimentric analysis (TGA).
Chemicals and Reagents: (3-Aminopropyl)trimethoxysilane (APSi, 97%), N-(3-Dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC, commercial grade), Poly(ethylene
glycol) methyl ether methacrylate (PEGMEMA, Mn=300), N-Isopropylacrylamide (NIPAM, 97%), 4,4’-Azobis(4-cyano-valeric acid) (ACVA, >=98%), and hydrofluoric acid (HF, 48%) were purchased from Sigma-Aldrich and used as received. N-Hydroxysuccinimide (NHS, 97%) was purchased from Alfa Aesar. Unless otherwise specified, the purity and chemical structures of the chemical reagents were verified by 1H NMR. Hydrochloric acid (HCl) solution (37%) was purchased from British Drug Houses (BDH). Anhydrous ethanol (EtOH) was purchased from Millipore. Deionized (DI) water was produced using a Milli-Q water system. Silica micro-particles were purchased from BDH (BDH9000, 40-63 µm diameter, 60 Å pore size). The carboxylic acid terminated RAFT-CTA, i.e., 2-(((butylthio)carbonothioyl)thio)propanoic acid (BCPA), was synthesized according to the previous literature.
CONCLUSION
In conclusion, a facile, controlled and scalable method to fabricate PGPs in the Phoenix Flow Reactor system at relatively high temperature in water/ethanol was demonstrated.
Utilization of SI-RAFT polymerization allows to synthesize grafted polymers with relatively low molar mass dispersity, and the length of grafted polymers is adjustable by changing
the reaction time or modifying the monomer concentration. Moreover, compared with the conventional “graft from” reaction in a batch reactor, the in-solution “gelation” problem
is avoided owing to the flow nature of the continuous system. Fabrication of block copolymer grafted SiMPs was also demonstrated due to the living nature of the RAFT polymerization. Compared with the cumbersome and timeconsuming batch fabrication of PGPs and possible gelation problem, the continuous flow chemistry approach allows a facile fabrication of PGP in a continuous manner with a more environmentally friendly solvent. This should open the possibility for more process control in actual commercial applications.
ACKNOWLEDGEMENT
ThalesNano would like to thank the authors for their contribution
INSTRUMENTATION
Phoenix Flow ReactorTM is designed to perform reactions up to 450 °C. The pressure range can reach up to 200 bar applying a back pressure regulator.
The Phoenix Flow ReactorTM is equipped with a stainless steel column (99 mm length, 3.8 mm inner diameter), an HPLC pump, and a Back Pressure Regulator Module (controlling pressure of the whole system up to 200 bar), was supplied by ThalesNano Nanotechology Inc. Both ends of the column were covered with a stainless steel filter (10 micron, Valco Instruments) to prevent the particles from being flushed out.
Risk assessment and hazards:
Always use the system in a well ventilated fume hood to avoid inhalation of solvent vapors. Never open it at high pressure or temperature, the overheated or pressurized solvents can cause injuries. Avoid contact with the heated parts.
RESULTS AND DISCUSSION
The synthesis of the initiator (or chain transfer agent) (CTA) was based on a previously reported procedure. This CTA was then grafted in situ to the microparticles (SiMP) packed in the column. After removal of the excess initiators with flushing, the first homopolymer, PPEGMEMA-gSiMPs, was grafted via activation of the CTA with the introduction of the free radical initiator ACVA. Controlling the temperature and pressure allowed for the shorter half-life for activation. By controlling the flow rate, residence time (circulation time) or the monomer feed ratio, it is possible
to control the MW (Figure 2).
Figure 2. SEC Data of PEGMEMA Homopolymers grafted on the particle surace: A) Residence time control (longer time, higher MW), B) Monomer feed control (higher Monomer ratio, higher MW). Lower Retention volume = higher MW.
Secondly, after removing the excess monomers, the block copolymer PPEGMEMA-b-PNIPAM-g-SiMPs was prepared. Again, by controlling the flow rate, residence time (circulation time) or the monomer feed ratio, it is possible to control the MW of the second block. The successful grafting of block copolymer with low molar mass dispersity was confirmed by IR spectra, SEC curve and 1H NMR spectra. TGA measurements confirmed the change in weight composition upon grafting of the successive blocks of polymers. Because of the thermo-sensitivity of the outer PNIPAM block, the
surface of PPEGMEMA-b-PNIPAM-g-SiMPs became more hydrophobic when raising the temperature above the lower critical solution temperature (LCST). This was proven by doing contact angle–wetting experiments on the modified particles (Figure 3).
Figure 3. Contact angle measurement on the particles at various stages of the surface modification, A) unmodified particle, B) SiMP, C) PPEGMEMA, D) PPEGMEMA-b-PNIPAM at room temperature, E) PPEGMEMA-b-PNIPAM at higher temperature, and F) Optical Micrograph of modified particles.
REFERENCES
1) Y. Tsujii, M. Ejaz, K. Sato, A. Goto, T. Fukuda, Macromolecules 2001, 34, 8872.
2) Ye, P.; Cao, P.; Chen, Q.; Advincula, R. “Continuous Flow Fabrication of Block Copolymer–Grafted Silica Micro‐Particles in Environmentally Friendly Water/Ethanol Media” Macro. Mat. Eng. 2018 https://doi.org/10.1002/mame.201800451
