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Emulsion polymerization is a unique technique to produce polymer particles on the nanoscale that are dispersed in water. It is possible to produce polymer nanoparticles withparticle sizes ranging from 15 nm to 2 ím, blends and composites consisting of a broad range of polymers, and a wide variety of particle morphologies. The synthetic versatility of this technique allows polymer materials to be made for a wide range of industrial applications, ranging from high strength coatings to biomedical devices (e.g., diagnostic kits). “Living” radical emulsion polymerization using the reversible addition-fragmentation chain transfer (RAFT) process5-8 has not only provided a synthetic tool to produce polymer with controlled Mn and PDI9-17 but allowed the production of complex architectures and particle morphologies with different mechanical properties.19 However, the most successful synthesis of RAFT-mediated ab initio emulsion polymerization (where the surfactant above its critical micelle concentration (cmc), water, monomer, initiator and RAFT agent are added to a reaction vessel and polymerized) is through the use of low reactive RAFT agents. The use of high reactive RAFT agents (e.g., cumyl dithiobenzoate) gave poor control of the molecular weight distribution and the resulting latex was invariably unstable leading to a red layer consisting of monomer, RAFT agent (red in color) and possibly some oligomeric species. This was overcome by using miniemulsion polymerizations,in which monomer droplets were stabilized using both a surfactant and organic hydrophobe (e.g., hexadecane). Other methods included the use of polymeric surfactant grown in situ. 和 Surfactant-free, batch emulsion polymerization of styrene was carried out in the presence of sodium acrylate as a comonomer and dibenzyltrithiocarbonate (DBTTC) as a reversible addition-fragmentation chain transfer (RAFT) agent. Very stable latex was recovered with narrow particle size distribution. Because of the low water-solubility of DBTTC, diffusion of the RAFT agent from the monomer droplets toward the polymer particles was slow, which did not allow a linear increase of molar mass with monomer conversion. However, Mneventually reached the expected value and the final polymer chains could be extended, when the latex was used as a seed for a second polymerization step. To overcome the slow diffusion of the RAFT agent, a new process was proposed, based on a spontaneous phase inversion mechanism. The method relies upon a first bulk copolymerization of styrene and acrylic acid, followed by neutralization of the carboxylic acid moieties by the addition of a sodium hydroxide solution under gentle stirring, leading to spontaneous phase inversion when a sufficient amount of water has been added. The method was applied to generate stable polymer nuclei, which were further used as a seed for chain extension upon monomer addition. 和 Amphiphilic, RAFT-capped, (acrylic acid)x(styrene)y diblock copolymers (x= 10, y ) 10, 5, 0) were synthesized and used as stabilizers in emulsion polymerization. Above the critical micelle concentration (cmc) of the diblocks and under appropriate reaction conditions micelles of the more hydrophobic diblocks were sufficiently nonlabile to be nucleated and act as seed particles for latex particle formation. The key parameters which allow control over the system are diblock hydrophobicity and initiator concentration. A homogeneous nucleation mechanism is most likely to operate below the cmc of the diblocks. 和 Reversible addition fragmentation chain transfer (RAFT)-mediated polymerization was successfully applied for the synthesis of poly(4-vinylpyridine) (P4VP) polymers of predetermined molar mass and of low polydispersity index. These RAFT end-functionalized polymers were then used as macro-RAFT agents and further chain extended with an azeotropic mixture of styrene (STY) and acrylonitrile (AN) (63 mol % STY). Initially, these chain extension experiments were carried out in solution. In that case, the formation of the P4VP-b-SAN block copolymers clearly demonstrated the large fraction of chain end functionality in these RAFT-functional P4VP polymers. Proof of the formation of low molar mass P4VP-b-SAN block copolymer was obtained by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis. Gradient polymer elution chromatographic (GPEC) analysis confirmed successful formation of P4VP-b-SAN block copolymers. Block copolymer synthesis in emulsion was also investigated. The polymerization mediated by a RAFT-functional P4VP, macro-RAFT agent, was carried out as a semicontinuous process. The complete transformation of the P4VP starting block into P4VP-b-SAN block copolymer points to an efficient control of the polymerization in emulsion. This procedure leads to the formation of a colloidally stable latex. The results of GPEC analysis confirmed the successful block copolymer latex formation. |
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2楼2010-05-18 15:07:21
