活性自由基聚合研究的新动态──可逆加成-断裂链转移自由基聚合

介绍了基于可逆链转移思想的“活性”／控制自由基聚合。着重介绍澳大利亚Ｒｉｚｚａｒｄｏ研究小组的最新研究成果：以双硫酯类化合物为链转移剂的可逆加成－断裂链转移自由基聚合。分析其活性特征,并阐明其反应机理     

阻聚剂对自由基聚合的活性化影响

研究了阻聚剂对自由基聚合活性化的影响,用1H NM R表征了聚合物的结构。发现在甲基丙烯酸甲酯(MM A)自由基聚合体系中加入少量的加成型阻聚剂2,4,6-三硝基苯酚(TNP)或1,4-苯醌(BQ)时,体系转化率降低,引发效率降低,呈现出一定的缓聚性质;同时得到的聚合物分子量变小,分子量分布变窄,具有一定的“活性”/可控的特征。链转移型阻聚剂如对羟基苯甲醚(4-M P)和对苯二酚(HQ)则不能起到类似的效果。     

Probing the reaction kinetics of vinyl acetate free radical polymerization via living free radical polymerization (MADIX)

Living free radical polymerization technology (macromolecular design via the interchange of xanthates (MADIX)) was applied to give accesses to chain length and conversion dependent termination rate coefficients of vinyl acetate (VAc) at 80 °C using the MADIX agent 2-ethoxythiocarbonylsulfanyl-propionic acid methyl ester (EPAME). The kinetic data were verified and probed by simulations using the PREDICI^® modelling package. The reversible addition-fragmentation transfer (RAFT) chain length dependent termination (CLD-T) methodology can be applied using a monomer reaction order of unity, since VAc displays significantly lower monomer reaction orders than those observed in acrylate systems (ω(VAc, 80 °C)=1.17±0.05). The observed monomer reaction order for VAc is assigned to chain length dependent termination and a low presence of transfer reactions. The α value for the chain length regime of log(i)=1.25−3.25 (in the often employed expression ) reads 0.09±0.05 at low monomer to polymer conversion (10%) and increases significantly towards larger conversions (α=0.55±0.05 at 80%). Concomitantly with a lesser amount of midchain radicals, the chain length dependence of k_t is significantly less pronounced in the VAc system than in the corresponding acrylate systems under identical reaction conditions. The RAFT(MADIX)-CLD-T technique also allows for mapping of k_t as a function of conversion at constant chain lengths. Similar to observations made earlier with methyl acrylate, the decrease of k_t with conversion is more pronounced at increased chain lengths, with a strong decrease in k_t exceeding two logarithmic units from 10 to 80% conversion at chain lengths exceeding 1800.     

Application of living free radical polymerization for nucleic acid delivery

Therapeutic gene delivery can alter protein function either through the replacement of nonfunctional genes to restore cellular health or through RNA interference (RNAi) to mask mutated and harmful genes. Researchers have investigated a range of nucleic acid-based therapeutics as potential treatments for hereditary, acquired, and infectious diseases. Candidate drugs include plasmids that induce gene expression and small, interfering RNAs (siRNAs) that silence target genes. Because of their self-assembly with nucleic acids into virus-sized nanoparticles and high transfection efficiency in vitro, cationic polymers have been extensively studied for nucleic acid delivery applications, but toxicity and particle stability have limited the clinical applications of these systems. The advent of living free radical polymerization has improved the quality, control, and reproducibility of these synthesized materials. This process yields well-defined, narrowly disperse materials with designed architectures and molecular weights. As a result, researchers can study the effects of polymer architecture and molecular weight on transfection efficiency and cytotoxicity, which will improve the design of next-generation vectors. In this Account, we review findings from structure-function studies that have elucidated key design motifs necessary for the development of effective nucleic acid vectors. Researchers have used robust methods such as atom transfer radical polymerization (ATRP), reverse addition-fragmentation chain transfer polymerization (RAFT), and ring-opening metastasis polymerization (ROMP) to engineer materials that enhance extracellular stability and cellular specificity and decrease toxicity. In addition, we discuss polymers that are biodegradable, form supramolecular structures, target specific cells, or facilitate endosomal release. Finally, we describe promising materials with a range of in vivo applications from pulmonary gene delivery to DNA vaccines.     

Controlled/living radical polymerization of styrene catalyzed by cobaltocene

Controlled/living radical polymerization of styrene has been achieved by atom transfer radical polymerization (ATRP) catalyzed by cobaltocene (PDI = 1.27-1.41). The effects of the initiators, temperatures and solvents were studied. The end group of PS-Br was characterized by ¹H NMR. Block copolymerization proved that the polymer end is still living and the PMMA-b-PSt block copolymer was synthesized.     

Emulsifier-free, organotellurium-mediated living radical emulsion polymerization of Styrene: Initial stage of polymerization

Behavior of the particle formation based on self-assembling in emulsifier-free, organotellurium-mediated living radical emulsion polymerization (emulsion TERP) of styrene was studied from the molecular weight distributions (MWDs) of polystyrene (PS) formed in an initial stage of the polymerization at different temperatures from 50 °C to 70 °C. As the polymerization temperature was decreased, the larger number of poly(methacrylic acid) (PMAA; degree of polymerization, 30)-methyltellanyl (TeMe) (PMAA₃₀-TeMe) participated in the polymerization, resulting in amphiphilic PMAA₃₀-b-PS-TeMe oligomers. Almost all control agents were consumed and a self-assembly nucleation occurred in the initial stage of the polymerization at 50 °C, which lead to depress of particle formation of a homogeneous nucleation. The consumption rate of PMAA₃₀-TeMe affected directly the particle formation. From these results, it is concluded that it is important for the emulsion TERP of styrene with excellent control/livingness that the self-assembly nucleation proceeds without the homogeneous nucleation in the initial stage of the polymerization.     

Solvent effect on TEMPO-mediated living free radical dispersion polymerization of styrene

Living free radical dispersion polymerization of styrene in the presence of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 2,2-azobisisobutyronitrile (AIBN) was conducted in various glycol media having different solubility parameters. Genuine heterogeneous polymerization nature was observed from the initial stage of the reaction. The solubility parameter of the media was found to significantly influence the living characteristics of the polymerization including the polymerization kinetics and the molecular weight evolution at a fixed concentration of TEMPO. A grafting of stabilizer molecules onto monomer and a possible partitioning of TEMPO between the continuous medium and particle phase were postulated for the deviation of the experimentally obtained molecular weight from the calculated value. For a poor medium for styrene, such as diethylene glycol, the living nature was achieved by increasing the amount of TEMPO. The polystyrene (PS) microsphere was well obtained in all tested glycol media and the average size was increased with enhanced affinity of the media to styrene and with decreasing concentration of TEMPO.     

用聚合转化法合成聚异丁烯-聚丙烯酸叔丁酯嵌段聚合物

使用BCl3/CH2Cl2/DTBP/IB反应体系实现了异丁烯的正离子聚合,得到末端为硼氧烷基的聚异丁烯,其与苯基溴化镁发生金属转移反应,生成末端为二苯基硼烷的聚异丁烯,其在常温下与氧气发生氧化反应,生成碳氧自由基,从而引发丙烯酸叔丁酯的自由基聚合,得到异丁烯和丙烯酸叔丁酯的嵌段聚合物.     

Reversible chain transfer catalyzed polymerization (RTCP): A new class of living radical polymerization

This article introduces the new family of living radical polymerizations with germanium (Ge), tin (Sn), phosphorus (P), and nitrogen (N) catalysts which we recently developed. The polymerizations are based on a new reversible activation mechanism, Reversible chain Transfer (RT) catalysis. Low-polydispersity polymers are obtained in the homopolymerizations and random and block copolymerizations of styrene, methyl methacrylate, and functional methacrylates. The background, performance, and kinetic features of the polymerizations are described. Attractive features of the catalysts include their high reactivity, low toxicity (Ge, P, and N), low cost (P and N), and ease of handling (robustness).     

Aqueous dispersions of polyurethane polyacrylic acid multiblock copolymers through living radical polymerization

Polyurethane macroiniferter (PUMI) including tetraphenylethane was synthesized and used to prepare polyurethane–polyacrylic acid multiblock copolymers. Film‐forming aqueous dispersions without any added external emulsifiers were prepared from polyurethane–polyacrylic acid multiblock copolymers. The effect of varying PUMI content, polymerization time, and percent ionization on the properties of multiblock copolymeric dispersions were studied in detail. Interfacial tension of the dispersions and critical surface tension measurements of the films formed thereof have shown that the polymers exhibit a hydrophilic character in the dispersed phase and a hydrophobic character in the solid phase. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1109–1115, 2003     

Photochemically initiated free radical promoted living cationic polymerization of isobutyl vinyl ether

A new photoinitiating system for living cationic polymerization of vinyl ethers such as isobutyl vinyl ether (IBVE) has been reported. The photoinitiating system comprises free radical photoinitiators such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA), benzophenone or thioxanthone, together with an onium salt, such as diphenyliodonium chloride and zinc bromide. In the first step, photochemically generated free radicals are oxidized to the corresponding carbocations which subsequently react with vinyl ether monomer to yield an adduct. In the presence of zinc salt, this adduct initiates living cationic polymerization of IBVE.     

Synthesis of polymeric nanofiber and polymeric nanosphere by living free radical polymerization

Living free radical polymerizations are employed to synthesize the polystyrene copolymer. The applications of those copolymer in nanomaterials are elucidated in this research. It involves the copolymerization of styrene with 2‐hydroxyethyl methacrylate. The copolymers are reacted with cinnamoyl chloride, and then irradiated with UV light. The polymeric nanofibers are formed by solvent splitting. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3550–3558, 2006     

Molecularly imprinted polymer nanomaterials and nanocomposites by controlled/living radical polymerization

Since the pioneering work of Wulff and Mosbach more than 30 years ago, molecular imprinting of synthetic polymers has emerged as a robust and convenient way for synthesizing polymeric receptor materials bearing specific recognition sites for target molecules. The resulting materials, molecularly imprinted polymers (MIPs), are therefore commonly referred to as ‘plastic antibodies’. They are obtained by polymerizing a scaffold around a target, or a derivate thereof, which acts as a molecular template. MIPs have been successfully applied in many areas including affinity separation, immunoassays, chemical sensing, solid-phase extraction, drug delivery, cell and tissue imaging, direct synthesis and catalysis. In terms of affinity and selectivity, MIPs are on a par with biological receptors like antibodies, and this is accompanied by a superior chemical and physical stability, compatibility with organic media, reusability, easy engineering and low cost. These advantages represent the main reasons for the wide interest raised around molecularly imprinted materials. Mainly produced by free radical polymerization (FRP) of vinyl monomers, MIPs have also taken advantage of the introduction of controlled/living radical polymerization (CRP) techniques, which have literally transformed polymer chemistry over the last decade. This review describes the advantages arising from the use of CRP in synthesizing MIPs, both in terms of sheer binding properties as well as for their remarkable potential for post-polymerization functionalization, for the synthesis of MIP nanomaterials and for the integration of MIPs into composites and hybrid materials. The benefits of using CRP are critically assessed with respect to the still largely applied FRP and guidelines are provided for choosing the most convenient technique to fit a specific targeted application of MIPs.     

Lithographic results of electron beam photoresists prepared by living free radical polymerization

Summary A new family of random copolymers composed of chloromethylstyrene and a silicon based styrenic monomer was prepared using living radical polymerization. The lithographic efficiency of the resulting electron beam resists was examined. A pronounced improvement on the lithographic resolution and image quality of resists with a narrow molecular mass distribution was observed and is described. Received: 20 April 1999/Revised version: 11 June 1999/Accepted: 17 June 1999     

A new approach to controlled/living radical polymerization by DPE method

Controlled free radical polymerizations of methyl methacrylate and styrene in bulk by 1,1-diphenylethene (DPE) were demonstrated in a two-step process, preheating treatment of initiators followed by a living polymerization of monomers. Over the course of polymerization, continuous growing of polymers with unimodal molecular weight distribution and a relatively small polydispersity index (around 1.5 even in the range of Mn ∼ 10⁵ g/mol) on GPC diagrams was observed. In our previous study, the DPE controlled radical polymerization with constant molecular weight throughout the polymerization was caused by the intrinsically low reactivation rate constant (k₂) of DPE capped dormant chains. To raise the reaction temperature in order to increase k₂, a continuous molecular weight growing but broader or bimodal molecular weight distribution was obtained if the living polymerization was conducted in a one-step process. In this work, a two-step polymerization process was proposed. In the first step, the initiator 2,2′-azobisisobutyronitrile (AIBN), control agent DPE, and small amount of monomer were mixed and heated for a specific time period. Then a living polymerization of monomers was conducted in the second step of polymerization. This two-step new approach had minimized the imperfections of the DPE system; thus the polymerization showed better living characters and revealed its enhanced control abilities.     

A convenient synthesis of functionalized alkoxyamines as initiators for living free radical polymerization

Summary 2,2,6,6-Tetramethylpiperidine- 1-oxyl (TEMPO) was reacted with ethylbenzene (la), 1-bromo-4-ethylbenzene (lb), and 4-ethylphenyl acetate (lc), respectively, using tert-BuOOWCo(OAc)·4H₂O in acetonitrile at room temperature. The reactions produced the respective TEMPO-adducts (2a, 2b, and 2c) in the yields of 37, 44, and 45 %, which were based on TEMPO. Similarly, TEMPO was reacted with 4-ethylphenyl 2,3,6,2',3',4',6'-hepta-O-acetyl-β-D-cellobioside (1d) to afford the glycoconjugated TEMPO-adduct (2d) in 45 5% yield, which was based on 1d. These results indicated that the reaction has the potential to become an easy and also safe strategy, which provided various functionalized alkoxylamines. Received 20 November 2002/Revised Version: 5 December 2002/ Accepted: 7 December 2002 Correspondence to Naoya Sugimoto     

Nitroxide mediated living radical polymerization of styrene in miniemulsion—modelling persulfate-initiated systems

Recently we have constructed a mechanistic model describing the nitroxide mediated miniemulsion polymerization (NMMP) of styrene at 135°C, using alkoxyamine initiators to control polymer growth (Nitroxide-Mediated Polymerization of Styrene in Miniemulsion. Modeling Studies of Alkoxyamine-Initiated Systems, 2001b). The model has since been expanded to describe styrene NMMP at 135°C using TEMPO and the free radical initiator, potassium persulfate (KPS). The model includes mechanisms describing reactions in the aqueous and organic phases, particle nucleation, the entry and exit of oligomeric radicals, and the partitioning of nitroxide and styrene between the aqueous and organic phases. Predicted monomer conversions, number average molecular weights and polydispersities were in agreement with experimentally measured values. Model simulations revealed that for systems employing high ratios of TEMPO:KPS, the consumption of TEMPO by polymer radicals derived from KPS decomposition and styrene thermal initiation (using the accepted literature kinetic rates) is not sufficient to lower TEMPO concentrations to levels where polymer growth can occur. By accounting for the consumption of TEMPO by acid-catalyzed disproportionation, TEMPO concentrations are significantly reduced, allowing for accurate model predictions of monomer conversion, number average molecular weight and polydispersity at every experimental condition considered.     

Controlled/living radical polymerization in aqueous media: homogeneous and heterogeneous systems

Controlled/living radical polymerizations carried out in the presence of water have been examined. These aqueous systems include both the homogeneous solutions and the various heterogeneous media, namely dispersion, suspension, emulsion and miniemulsion. Among them, the most common methods allowing control of the radical polymerization, such as nitroxide-mediated polymerization, atom transfer radical polymerization and reversible transfer, are presented in detail.     

活性自由基聚合制备结构明晰的苯乙烯和丙烯酸八氟戊酯接枝共聚物?貂

通过苯乙烯与对氯甲基苯乙烯进行氮氧稳定自由基共聚合,合成了苯乙烯和对氯甲基苯乙烯的无规共聚物,以上矣物为丙烯酸八氟戊酯原子转移自由基聚合的大分子引发剂,成功地合成了结构明晰的以聚苯乙烯为主链,聚丙烯酸八氟戊酯为侧链的接枝共聚物。