Polystyrene microspheres have been synthesized by the reversible addition-fragmentation chain transfer (RAFT) mediated dispersion polymerization in an alcoholic media in the presence of poly(N-vinylpyrrolidone) as stabilizer and 2,2′-azobisisobutyronitrile as a conventional radical initiator. In order to obtain monodisperse polystyrene particles with controlled architecture, the post–addition of RAFT agent was employed to replace the weak point from the pre-addition of RAFT. The feature of preaddition and postaddition of RAFT agent was studied on the polymerization kinetics, particle size and its distribution and on the particle stability. The living polymerization behavior as well as the particle stability was observed only in the postaddition of RAFT. The effects of different concentration on the postaddition of RAFT agent were investigated in terms of molecular weight, molecular weight distribution, particle size and its distribution. The final polydispersity index (PDI) value, particle size and the stability of the dispersion system were found to be greatly influenced by the RAFT agent. This result showed that the postaddition of RAFT agent in the dispersion polymerization not only controls the molecular weight and PDI but also produces stable monodisperse polymer particles. 相似文献
Abstract The reversible addition fragmentation chain transfer (RAFT) bulk polymerization of isobutyl methacrylate (i‐BMA) has been studied using 2‐cyanoprop‐2‐yl dithionaphthalate (CPDN) as RAFT agent in the presence of 2,2′‐azobisisobutyronitrile (AIBN). The results of polymerizations of i‐BMA show that i‐BMA can polymerize in a controlled way by RAFT polymerization using CPDN as RAFT agent; i.e., the polymerization rate is first order with respect to monomer concentration, molecular weight increases linearly with monomer conversion, and polydispersities are relatively low (PDI?<?1.2). The structure of the polymer was characterized by 1H‐NMR. A chain‐extension experiment of the resulting polymer was successfully carried out. The influences of [i‐BMA]0/[CPDN]0/[AIBN]0 molar ratio and reaction temperature were investigated. 相似文献
Various versatile chain transfer agents (CTAs) have been synthesized for reversible addition fragmentation chain transfer (RAFT) polymerzation. Such CTAs have been used to modify hydroxyl containing materials and produce well-controlled molecular architectures such as amphiphilic copolymer from poly (ethylene glycol), AB block copolymer consisting of a biodegradable segment, poly (l-lactic acid) (PLLA) and grafted copolymers of poly (styrene), poly (methyl methacrylate) and poly (methyl acrylate) from cellulose. 相似文献
运用RAFT活性自由基聚合方法探索了具有一定立构规整性的聚丙烯腈的合成。合成得到RAFT聚合的链转移剂MESA,并以1 H NMR进行了表征;以MESA作为链转移剂、碳酸乙烯酯为溶剂,在单体浓度为0.80 M、60℃、原料配比[AN]0/[MESA]0/[AIBN]0为2500∶5∶1的聚合条件下,成功合成出较高分子量(Mn=5.60×104g/mol)、窄分子量分布(PDI=1.15)的聚丙烯腈;进一步在各单体浓度的RAFT聚合中,加入单体摩尔量3%的AlCl3,得到聚丙烯腈数均分子量为6.1×104~6.5×104g/mol,全同立构组成为mm=32.1%~32.6%,聚合产物分子量分布宽度介于1.31~1.38之间,从而实现了在RAFT活性聚合体系中通过Lewis酸的作用合成得到具有一定立构规整性的聚丙烯腈。 相似文献
A novel process to produce homo‐ and copolymers by RAFT polymerization in emulsion is presented. It is known that RAFT‐controlled radical polymerization can be conducted in emulsion polymerization without disturbing the radical segregation characteristic of this process, thus leading to polymerization rates identical to those encountered in the corresponding nonliving systems. However, RAFT agents are often characterized by very low water solubility and, therefore, they diffuse very slowly from the monomer droplets, where they are initially solubilized, to the reaction loci, i.e., the polymer particles. Accordingly, when used in emulsion polymerization, they are practically excluded from the reaction. In this work, we show that cyclodextrins, well‐known for their ability to form water‐soluble complexes with hydrophobic molecules, facilitate the transport across the H2O phase of the RAFT agent to the polymer particles. Accordingly, chains grow through the entire process in a controlled way. This leads to the production of low‐polydispersity polymers with well‐defined structure and end functionalities as well as to the possibility of synthesizing block copolymers by a radical mechanism. 相似文献
Reversible addition–fragmentation chain transfer (RAFT) chemistry can be effectively employed to construct macromolecular architectures of varying topologies. The present article explores the principle design routes to star, block, and comb polymers in the context of theoretical design criteria for the so‐called Z‐ and R‐group approaches. The specific advantages and disadvantages of each approach are underpinned by selected examples generated in the CAMD laboratories. In particular, we demonstrate how the modeling of full molecular weight distributions can be employed to guide the synthetic effort. We further explore the theory and practice of generating amphiphilic block copolymer structures and their self‐assembly. In addition, the article foreshadows how modern synthetic techniques that combine RAFT chemistry with highly orthogonal click chemistry can be employed as a powerful tool that furthers the enhancement of macromolecular design possibilities to generate block (star) copolymers of monomers with extremely disparate reactivities. Finally, the ability of RAFT chemistry to modify the surface of well‐defined nano‐ and microspheres as devices in biomedical application is detailed.
Summary: RAFT is applied to the dendronized macromonomers of the first and second generation, 1 and 2 , respectively. Good results are obtained in the presence of AIBN as radical initiator, with compound 6 as mediator and at mediator to monomer ratios of 2:200 for monomer 1 ( = 320 000, PDI = 1.24) and monomer 2 ( = 178 000, PDI = 1.20). The common characteristics of a controlled polymerization are reasonably met. The more sterically demanding G2 monomer 2 requires higher polymerization temperatures.
The present paper reports the first example of a controlled radical polymerization of ethylene using reversible addition–fragmentation chain transfer (RAFT) in the presence of xanthates (Alkyl‐OC(?S)S‐R) as controlling agents under relative mild conditions (70 °C, <200 bars). The specific reactivity of the produced alkyl‐type propagating radicals induces a side fragmentation reaction of the stabilizing O‐alkyl Z group of the controlling agents. This fragmentation, rarely observed in RAFT, was proven by NMR analyses. In addition, semicrystalline copolymers of ethylene and vinyl acetate were also prepared with a similar level of control. 相似文献
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