The Application Of CaCO₃Particles In Aqueous RAFT Polymerization

Since initial introduced in1998, the reversible addition-fragmentation chaintransfer (RAFT) polymerization has proven to be perhaps the most versatile of thecontrolled/living radical polymerization methods. Up to now, RAFT polymerizationunder homogeneous condition (solution/bulk) has been extensively studied, andvarious polymers including homopolymer, and block polymer with narrow molecularweight distribution have been synthesized. However, the RAFT polymerization underheterogeneous conditions, especially the aqueous RAFT polymerization, is far frombeing perfectly accomplished. A new method to achieve well-controlled aqueousRAFT polymerization of hydrophobic monomer and hydrophilic monomer in thepresence of CaCO3particles were proposed.Polystyrene (PS) was synthesized by aqueous RAFT polymerization. Thesurfactant plays the crucial role to ensure stability of the emulsion. However, thesurfactant brings some side effects. In this paper, the RAFT agent ofS,S’-bis(,’-dimethyl-’’-acetic acid) trithiocarbonate (BDMAT) and St were addedinto water mediated with CaCO3particles, and it was found that they were adsorbedon the surface of CaCO3particles and well-controlled aqueous RAFT polymerizationwas performed. The weight ratio of CaCO3/St on the aqueous RAFT polymerizationwas investigated, and it was found that high CaCO3/St weight ratio led to fast aqueousRAFT polymerization. The aqueous RAFT polymerization kinetics was checked, andthe fast RAFT polymerization rate at low molar ratio of styrene/BDMAT/K2S2O8, thelinear ln([M]o/[M])-time plot, the linear increase in the number-average molecularweight with the monomer conversion, and the relatively low polydispersity index(PDI) values were demonstrated. The aqueous RAFT polymerization was comparedwith the general emulsion RAFT polymerization in the presence of the sodiumdodecyl sulfate (SDS) surfactant. It was found that the aqueous RAFT polymerizationseemed more valid than the emulsion RAFT polymerization, despite that aqueousRAFT polymerization ran slower than the general emulsion RAFT polymerization.Poly(N-isopropylacrylamide)(PNIPAM) was prepared by a new method.PNIPAM possesses a readily accessible lower critical solution temperature (LCST) inwater and the chains become hydrophobic above the LCST, so the polymerization ofPNIPAM demonstrates the need for organic solvents. In this paper, the aqueous RAFTpolymerization of N-isopropylacrylamide (NIPAM) was carried out successfully inthe presence of CaCO3particles. The weight ratio of CaCO3/NIPAM on the aqueous RAFT polymerization, the molar ratio of RAFT agent to initiator and thepolymerization kinetics were investigated. The linear ln([M]o/[M])-time plot, thelinear increase in the number-average molecular weight with monomer conversion,and the relatively low PDI values were demonstrated. The proposed method isbelieved to be a new strategy to achieve well-controlled aqueous RAFTpolymerization.PNIPAM-b-PS-b-PNIPAM was synthesized by aqueous RAFT polymerizationmediated with the macro-RAFT agent of PNIPAM-TTC, and the copolymer hadthermosensitive block and hydrophobic block. It was found that the polymerizationwas stable in the presence of CaCO3particles, and high CaCO3/St weight ratio led tofast aqueous RAFT polymerization. The aqueous RAFT polymerization kinetics waschecked, the linear ln([M]o/[M])-time plot, the linear increase in the number-averagemolecular weight with the monomer conversion, and the relatively low PDI values inthe presence of CaCO3particles were demonstrated.PDMA-b-PS-b-PDMA, which had hydrophilic block and hydrophobic block,was synthesized by aqueous RAFT polymerization mediated with the macro-RAFTagent of PDMA-TTC. It was found that high CaCO3/St weight ratio led to fastaqueous RAFT polymerization, however, the PDI of polymer had been little affected.The aqueous RAFT polymerization kinetics was checked, the linearln([M]o/[M])-time plot, the linear increase in the number-average molecular weightwith the monomer conversion were demonstrated. At the same time, in the presenceof CaCO3particles, the polymerization rate was fast and the PDI values were lowrelatively.