Construction Of Highly-efficient And Ultra-fast RAFT Polymerization System Based On Continuous Tubular Reactors

Tubular reactor has a unique position in industrial production because of its structural characteristics, including continuous operation, no quench time and improvements in chemical reaction control. As one of the “living” radical polymerization methods, Reversible Addition-Fragmentation Chain Transfer(RAFT) polymerization has gained a significant meaning in the field of polymer chemistry since it was developed in 1998. It has become a powerful synthesis strategy in polymer materials with controlled molecular weights and structures due to its strong ability of molecular design, simple composition and extensive applicability. But, there still have some problems when “living” radical polymerization conducted in a scale-up tank reactor. Multi-operations restrict the process of industrialization; the ununiformity, backmixing of raw materials and the heating effect of reactors made chemical reactions out of control, producing undesirable polymers. However, by combing the advantages of “continuousness” and “livingness”, we built RAFT systems in tubular reactors, making the continuous operation of highly-efficient and ultra-fast RAFT polymerization possible, and the obtained polymers showed typical features of “livingness”. Some foundational work has been done to facilitate industrialization of “living” radical polymerizations.System I: Surfactant-free emulsion polymerization in a continuous tubular reactor. We constructed a surfactant-free emulsion RAFT polymerization within a tubular reactor of 90 oC, using water(H2O) and N,N-dimethyl formamide(DMF) as the co-solvent, which allowed the mixture, including methyl methacrylate(MMA), 2,2′-azobisisobutyronitrile(AIBN), and 4-cyano-4-(thiobenzoylthio) pentanoic acid(CPADB) with additive sodium hydroxide(NaOH), to form a homogeneous phase. Utilizing ionized R groups attached in the polymer chain ends to stablize the latex particles, we got a surfactant-free emulsion from the homogeneous raw materials directly, without any additional process of preemulsification and prepared hydrophilic macro-RAFT agent in advance. Moreover, when the reaction system was applied to a tubular reactor, the polymerization rate increased slightly, even if in the presence of a limited amount of dissolved oxygen. In addition, the resulting latexes showed good stability and the polymerization demonstrated typical “living” features of RAFT polymerization.System II: Consecutive synthesis of hydrophilic block copolymer in a tubular reactor. Herein, to avoid the use of organic solvent, water was set as mobile phase(solvent). In virtue of the convenience of tubular reactors, a green and highly-efficient system was established to continuously synthesize double hydrophilic block copolymers without any extra process of handling the intermediate macro-RAFT agent. In this system, we used 3-sulfopropyl methacrylate potassium salt(SPMA) and polyethylene glycol methyl ether methacrylate(PEGMA, Mn = 500 g mol-1) as monomers, CPADB as the RAFT agent and 2,2′-azobis-[2-(2-imidazolin-2-yl)propane] dihydrochloride(AIBI) as the water soluble initiator. Through the tubular reactor at 70 oC, the conversion of SPMA reached 99% in 40 min, and so did PEGMA in 60 min. Therefore, ultra-fast polymerization rate, ultra-high monomer utilization and consecutive productions endowed the polymerization system with a great industrial prospect. Typical “living” features of the polymerization were demonstrated and tri-block copolymer PSPMA-b-PPEGMA-b-PNaSS was obtained by chain extension reaction. This polymerization system also showed the applicability to different repeat unit ratios of PSPMA-b-PPEGMA.