可逆加成-断裂链转移自由基聚合研究进展

RAFT(Reversible addition-fragmentation chain transfer,可逆加成-断裂链转移)自由基存在链增长自由基与链转移剂(RAFT试剂)之间的可逆蜕化转移,现已广泛应用于聚合物分子结构设计及众多功能高分子材料的合成,受到众多高分子研究者的关注,是一种发展较快的可控/活性聚合技术.本文在简要介绍了RAFT聚合发展历程基础上,综述了RAFT聚合反应机理,RAFT试剂的结构及其对聚合性能的影响,RAFT试剂与单体的匹配性,RAFT聚合实施方法等.同时也对RAFT聚合反应的发展进行了展望.     

RAFT聚合在聚合物分子结构设计中的应用

可逆加成-断裂链转移(reversible addition-fragmentation chain transfer,RAFT)聚合是一种新型的活性/可控自由基聚合方法,在制备窄分子量聚合物和设计聚合物分子结构方面具有独特的优势。本文首先介绍RAFT活性自由基聚合的机理、体系、特点及链转移(RAFT)试剂的选择,然后总结了近年来国内外利用RAFT聚合技术在设计无规和交替共聚物方面的应用,详细介绍了该方法在制备特殊结构共聚物,如嵌段、梯度、接枝、星形、树形和梳形结构聚合物的新应用,并对RAFT聚合技术在今后的研究重点和应用前景做了展望。     

可逆加成-断裂链转移(RAFT)分散聚合

RAFT分散聚合是在分散体系中实施RAFT聚合的一种非均相聚合方法。RAFT分散聚合的最大特点是它可以直接制备聚合物分子量可控、分子量分布窄的聚合物粒子。本文简要介绍了在小分子RAFT试剂和大分子RAFT试剂(Macro-RAFT)存在下,RAFT分散聚合的聚合动力学、聚合物的成核和粒子的增长。小分子RAFT试剂存在下的RAFT分散聚合是一个与普通的分散聚合类似,可以看作为非均相条件下的RAFT聚合,它可以制备微米尺度的聚合物粒子。Macro-RAFT存在下的RAFT分散聚合,是制备高浓度、纳米尺度的嵌段共聚物胶体的重要方法,它包含嵌段共聚物胶束化之前的均相聚合和嵌段共聚物胶束化后的非均相聚合两个阶段。     

RAFT乳液聚合

高分子材料性能追本朔源主要由分子链微结构决定。以RAFT聚合为代表的"活性"/可控自由基聚合结合了传统自由基聚合和活性阴离子聚合各自的优点,提供了一种有效调控聚合物分子链微结构的聚合方法。RAFT乳液聚合作为"活性"/可控自由基聚合中具有工业应用前景的聚合方法,在过去二十年受到了学术界的广泛关注。本文总结了RAFT乳液聚合乳液失稳机理、聚合动力学、链结构的可控性等方面的进展。在此基础上,介绍了通过RAFT乳液聚合这一可控制备聚合物新材料的平台制备得到的新型嵌段共聚物、梯度共聚物,并展望了RAFT乳液聚合在高分子合成材料领域的应用前景。     

水溶性RAFT链转移剂的制备及应用进展

可逆加成-断裂链转移(reversible addition-fragmentation chain transfer,RAFT)聚合是一种有效的可控/活性自由基聚合方法,在功能型高分子的制备中有广泛的应用,RAFT聚合的关键就在于选择合适的RAFT链转移剂。基于环保无害的要求,水溶性RAFT链转移剂的制备就至关重要。本文介绍了RAFT聚合的机理,综述了水溶性RAFT链转移剂的制备及应用进展,探讨出RAFT链转移剂水溶性的作用机理,一方面是极性基团的作用,另一方面是离子键氢键等的作用,这对水溶性RAFT链转移剂的制备有一定的启发。大分子RAFT链转移剂分子中常含有亲水基团和疏水基团,具有一定的分散作用,在水相条件下不仅可以通过扩链反应制备窄分子量分布的嵌段共聚物,还可以制备出微纳米凝胶。     

可控自由基聚合系列实验教学中的一些尝试与经验*

设计了基于可控自由基聚合的系列实验教学,包括:单体和引发剂精制、RAFT试剂合成、不同单体的RAFT/ATRP聚合、RAFT聚合制备嵌段聚合物、ATRP制备嵌段聚合物等。这些实验环环相扣,互相支撑,又有着明显的对比效果。这种尝试有效改进和扩充了常规高分子合成教学中的自由基聚合部分,有效激发了学生的主动性,提高了其分析问题、解决问题的能力。     

RAFT链转移剂的选用原则及通用型RAFT链转移剂

可逆加成-断裂链转移聚合(RAFT Polymerization)是目前最为常用的活性可控自由基聚合方法之一,因其产物分子量分布较窄、适用单体范围广、反应条件温和等优势得到了不同领域科学家的广泛应用。然而,科学家们在选择RAFT链转移剂(也称RAFT试剂)时,经常会忽略RAFT链转移剂与单体活性的匹配原则,导致在制备高活单体与低活单体的嵌段共聚物方面存在产物分子量分布宽、聚合速率慢,甚至反应无法成功进行的问题。基于此,本文首先综述聚合中RAFT链转移剂的选用原则,随后介绍近几年开发的一类同时适用于高/低活性单体聚合的通用型RAFT链转移剂(Universal/Switchable RAFT agent)的作用原理及适用条件,并着重探讨了基于通用型RAFT链转移剂制备高/低活性单体的嵌段共聚物的最新进展及应用。     

可逆加成-断裂链转移自由基聚合研究进展

综述了活性/可控自由基聚合中的可逆加成-断裂链转移(RAFT)自由基聚合研究进展;总结了RAFT试剂、RAFT聚合反应条件、RAFT聚合物及其结构形貌的最新研究进展;指出RAFT自由基聚合反应已被作为重要方法之一用于合成具有特定分子结构的聚合物.     

"活性"/可控自由基聚合新进展

概述了当前“活性”／可控自由基聚合(CPR)的三种主要方法，硝基氧调介聚合(NMP)、原子转移自由基聚合(ATRP)、可逆加成-断裂链转移聚合(RAFT)，特别是近年来的进展情况。     

RAFT自由基聚合技术在表面改性领域的研究进展

总结了近十年来可逆加成一断裂链转移（RAFT）自由基聚合技术在材料表面改性领域的研究进展。主要介绍了相关的四大类改性途径：（1）物理涂覆RAFT聚合物;（2）从表面接枝聚合（graftingfrom）,即在表面自由基和溶液中自由RAFT链转移剂存在下进行表面接枝聚合反应;（3）将聚合物接枝到表面（graftingto）...     

Reversible addition-fragmentation chain transfer polymerization in microemulsion

This tutorial review first details the uncontrolled microemulsion polymerization mechanism, and the RAFT polymerization mechanism to provide the necessary background for examining the RAFT microemulsion polymerization mechanism. The effect of the chain transfer agent per micelle ratio and the chain transfer agent aqueous solubility on the RAFT microemulsion polymerization kinetics, polymer molecular weight and polydispersity, and polymer nanoparticle size are discussed with a focus on oil-in-water microemulsions. Modeling of RAFT microemulsion polymerization kinetics and the resulting final polymer molecular weight are presented to assist with the analysis of observed experimental trends. Lastly, the current significance of RAFT microemulsion polymerization and the future directions are discussed.     

Reversible Addition-Fragmentation Chain Transfer Polymerization of Methyl Methacrylate in Microemulsion: The Influence of Reaction Conditions on Polymerization

The reversible addition-fragmentation chain transfer (RAFT) polymerization of methyl methacrylate (MMA) using cetyltrimethylammonium bromide (CTAB) as surfactant and a difunctional RAFT agent S,S′-bis (α, α′-dimethylacetic acid) trithiocarbonate (BDAT) as chain transfer were conducted in microemulsion. The influence of polymerization temperature and concentration of RAFT agent on the polymerization were investigated, respectively. The results showed that the molecular weight of products increased linearly with conversion, the polydispersity indexes remained low value, and the polymerization processes were totally under control with increasing concentration of RAFT agent, the polymerization behavior exhibited living polymerization characters. In addition, the influence of RAFT concentration on the particle size was investigated by TEM. The results indicated that the particles were highly monodispersed and the particle size increased with increasing concentration of RAFT agent.     

RAFT种子乳液聚合反应初期的Monte Carlo模拟

自由基聚合是制备聚合物材料最为重要的技术 .但由于自由基极易进行双基终止 ,一般很难对其结构进行精确的控制 ,所得产物分子量宽 ,组成分布不易控制 ,很难制备嵌段共聚物 . 2 0世纪 90年代出现的活性自由基聚合技术 (RAFT)克服了上述缺点 ,成为高分子化学研究的热点[1] .RAFT聚合以其适用单体广、聚合条件温和以及活性高而成为最具前途的活性自由基聚合技术之一 .迄今为止 ,RAFT的研究大多集中在溶液和本体等均相聚合体系 [2～ 5] .乳液聚合有聚合速率快、环境友好、体系粘度低等优点 ,是活性自由基聚合工业化首选工艺 ,因而近年来活…     

Orthogonal Cationic and Radical RAFT Polymerizations to Prepare Bottlebrush Polymers

An orthogonal combination of cationic and radical RAFT polymerizations is used to synthesize bottlebrush polymers using two distinct RAFT agents. Selective consumption of the first RAFT agent is used to control the cationic RAFT polymerization of a vinyl ether monomer bearing a secondary dormant RAFT agent, which subsequently allows side‐chain polymers to be grafted from the pendant RAFT agent by a radical‐mediated RAFT polymerization of a different monomer, thus completing the synthesis of bottlebrush polymers. The high efficiency and selectivity of the cationic and radical RAFT polymerizations allow both polymerizations to be conducted in one‐pot tandem without intermediate purification.     

Initialization of RAFT Agents with Different Leaving Groups – Determination of the Transfer Coefficients

The leaving group of the initial RAFT agent is one key factor during the initialization period of the RAFT polymerization. Benzyl-, Phenylethyl- and Cumyl Dithiobenzoate RAFT mediated bulk polymerization of styrene at 120 °C was investigated by direct measurement of the concentration of the RAFT agent and determination of the molecular weight in the initial phase of the RAFT polymerization using Size Exclusion Chromatography (SEC). The initialization period can be described by means of two transfer coefficients that are determined by the stability of the free-radical leaving group. By means of a Monte Carlo simulation the RAFT initialization process was characterized and the transfer coefficients from the experimental data were determined.     

Reversible Addition Fragmentation Chain Transfer Mediated Dispersion Polymerization of Styrene

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.     

Polymerization of styrene in alcohol/water mediated by a macro‐RAFT agent of poly(N‐isopropylacrylamide) trithiocarbonate: From homogeneous to heterogeneous RAFT polymerization

The reversible addition fragmentation chain transfer (RAFT) polymerization of styrene in alcohol/water mixture mediated with the poly(N‐isopropylacrylamide) trithiocarbonate macro‐RAFT agent (PNIPAM‐TTC) is studied and compared with the general RAFT dispersion polymerization in the presence of a small molecular RAFT agent. Both the homogeneous/quasi‐homogeneous polymerization before particle nucleation and the heterogeneous polymerization after particle nucleation are involved in the PNIPAM‐TTC‐mediated RAFT polymerization, and the two‐stage increase in the molecular weight (M_n) and nanoparticle size of the synthesized block copolymer is found. In the initial homogeneous/quasi‐homogeneous polymerization, the M_n and nanoparticle size slowly increase with monomer conversion, whereas the M_n and particle size quickly increase in the subsequent heterogeneous RAFT polymerization, which is much different from those in the general RAFT dispersion polymerization. Besides, the PNIPAM‐TTC‐mediated RAFT polymerization runs much faster than the general RAFT dispersion polymerization. This study is anticipated to be helpful to understand the polymer chain extension through RAFT polymerization under dispersion conditions. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

RAFT-mediated emulsion polymerization of vinyl acetate: a challenge towards producing high molecular weight poly(vinyl acetate)

The preparation of poly(vinyl acetate) with well-controlled structure has received a great deal of interest in recent years because of a large number of developments in living radical polymerization techniques. Among these techniques, the use of reversible addition–fragmentation chain transfer (RAFT)-mediated polymerization has been employed for the controlled polymerization of vinyl acetate due to the high susceptibility of this monomer towards chain transfer reactions. Here, a novel water-soluble N,N-dialkyl dithiocarbamate RAFT agent has been prepared and employed in the emulsion polymerization of vinyl acetate. The kinetic results reveal that the polymerization nucleation mechanism changes from homogeneous to micellar and RAFT-generated radicals can change the kinetic behavior from conventional emulsion polymerization to living radical polymerization. At higher concentrations of the modified RAFT agent, as a result of an aqueous phase reaction between RAFT and sulfate radicals, relatively more hydrophobic radicals are generated, which favors entry and propagation into micelles swollen with monomer. This observation was determined from the investigation of the polymerization rate and measurements of the average particle diameter and the number of particles per liter of the aqueous phase. Molecular weight analysis also demonstrated the participation of the RAFT agent in the polymerization in such a way as to restrict chain transfer reactions. This was determined by examining the evolution of polymer chain length and attaining higher molecular weights, even up to 50?% greater than the samples obtained from the conventional emulsion polymerization of vinyl acetate in the absence of the synthesized modified RAFT agent.