Study On Reversible Addition-Fragmentation Chain Transfer Polymerization In Emulsion Systems

The reversible addition-fragmentation chain transfer (RAFT) polymerizations ofmethyl methacrylate (MMA) were conducted in two heterogeneous systems, i.e.conventional emulsion and miniemulsion, with identical reaction condition. The mainobjective is to compare the living/controlled character in the both systems according as thenucleation mechanism, the latex stability, the particle sizes and particle size distributions oflatices, the molecular weight and PDI of PMMA, and the kinetics of the RAFTpolymerization in the two cases. The RAFT agent used in the both systems was2-cyanoprop-2-yl dithiobenzoate (CPDB). The effects of an oil-soluble initiator 2, 2(?)-azobisisobutyronitrile (AIBN) and a water-soluble initiator kalium persulfate (KPS) onthe RAFT/emulsion and RAFT/miniemulsion polymerizations were investigated.Methyl-β-cyclodextrin (Me-β-CD) was used as a solubilizer. The average molecularweights and molecular weight distributions (PDIs) were characterized by GPCmeasurement. The experimental results showed that the RAFT/miniemulsionpolymerization of MMA exhibited better living character than that of RAFT/emulsionpolymerization. The molecular weight distribution of PMMA in RAFT/miniemulsionpolymerization was decreased with the addition of Me-β-CD. However, Me-β-CD did notinfluence on the molecular weight distribution of PMMA prepared in RAFT/emulsionpolymerization.Poly(fluoroalkyl mathacrylate)-block-poly (butyl methacrylate) (PFAMA-b-PBMA)diblock copolymer latices were synthesized by two-step processe(?). In the first step, ahomopolymer PFAMA or PBMA end-capped with a dithiobenzoyl group was prepared inbulk via RAFT polymerization with CPDB as a RAFT agent. In the second step, the homopolymer chain-transfer agent (macro-CTA) was dissolved in the second monomer, mixed with a water phase containing a surfactant, and then ultrasonicated to fo(?) aminiemulsion. Subsequently, the RAFT-mediated miniemulsion polymerization of thesecond monomer (BMA or FAMA) was carried out in the presence of the first blockmacro-CTA. The influence of the polymerization sequence of the two kinds of monomerson the colloidal stability and molecular weight distribution was investigated. GPC analysesand particle size results indicated that using PFAMA macro-CTA as the first block wasbetter than using the PBMA RAFT agent with respect to the colloidal stability and thenarrow molecular weight distribution of the F-copolymers latices. The F-copolymers werecharacterized with ¹H NMR, ¹⁹F NMR and Fourier transform infrared spectroscopy.Comparing the contact angle of a water droplet on a thin film formed by fluorinatedcopolymer with that of PBMA, we found that for the diblock cop01ymers containingfluorinated block, the surface energy decreased greatly, and the hydrophobicity. increased.Amphiphilic rarndon copolymers of Poly(SMA-co-Mc80) (1: 9; 2: 8; 3: 7, molar ratio)with different composition were synthesized in anhydrous ethanol with dodecyl thiol as thechain transfer agent. Surface tension and conductivity were performed on the water solutionof the amphiphilic ramdon copolymers, and the results showed that they had obvioussurface activity. Althouth the composition of the three copolymers were different, thecritical micelle concentration were nearly the same, of about 3.4 g/L.RAFT/miniemulsion polymerization of MMA was performed, with the amphiphilicramdon copolymers as the polymeric surfactants. The efficiency of surfactant was evaluatedby measurement of the polymerization kinetics, the conversion of MMA, the amount of thecoagulation, the surface tensteon of the miniemulsion before and after polymerization, andthe particle size and particle size distribution of obtained lattice. The results showed that Poly(SMA₁-co-Mc80₉) and Poly(SMA₂-co-Mc80₈) copolymers could be used as efficientsurfactants to stabilize the latex in the RAFT/miniemulsion polymerization of MMA. Themolecular weight of PMMA increased linearly with conversion, and the PDIs were keptquite low. However, Poly(SMA₃-co-Mc80₇) copolymer was not suitable as the surfactant inthe RAFT/miniemulsion of MMA, because of its high hydrophobic property.