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。     

Templateless Synthesis of Polyacrylamide‐Based Nanogels via RAFT Dispersion Polymerization

This paper reports on the synthesis of well‐defined polyacrylamide‐based nanogels via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization, highlighting a templateless route for the efficient synthesis of nanogels based on water‐soluble polymers. RAFT dispersion polymerization of acrylamide in co‐nonsolvents of water–tert‐butanol mixtures by chain extension from poly(dimethylacrylamide) shows well‐controlled polymerization process, uniform nanogel size, and excellent colloidal stability. The versatility of this approach is further demonstrated by introducing a hydrophobic co‐monomer (butyl acrylate) without disturbing the dispersion polymerization process.

     

Photoenzymatic RAFT Emulsion Polymerization with Oxygen Tolerance

Photoenzymatic reversible addition-fragmenatation chain transfer(RAFT) emulsion polymerization, surfactant-free or ab initio, of various monomers is reported with oxygen tolerance. In surfactant-free emulsion polymerizatoin, poly(N,N-dimethylacrylamide)s were used as stabilizer blocks for emulsion polymerization of methyl acrylate, n-butyl acrylate and styrene, producing well-defined amphiphilic block copolymers, including those with an ultrahigh molecular weight, at quantitative conversions. Th...     

Batch Emulsion Polymerization of Styrene Stabilized by a Hydrophilic Macro‐RAFT Agent

Summary: A well‐defined homopolymer of 2‐(diethylamino)ethyl methacrylate has been synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization using (4‐cyanopentanoic acid)‐4‐dithiobenzoate as a chain transfer agent. The corresponding protonated homopolymer with a very reactive dithiobenzoate end group has been used as a water‐soluble macromolecular chain transfer agent in the batch emulsion polymerization of styrene without any surfactant. The reaction leads to a stable latex, as a result of the in‐situ formation of an amphiphilic block copolymer stabilizer, via transfer reaction to the dithioester functions during the nucleation step. The work does not intend to apply controlled free‐radical polymerization in an aqueous dispersed system but takes advantage of the RAFT technique to create a well‐defined polyelectrolyte, with a high chain‐end reactivity.

Schematic of the formation of the stabilized latex by the in situ formation of an amphiphilic block copolymer stabilizer.     

RAFT Miniemulsion Polymerization Kinetics, 1 – Polymerization Rate

The polymerization kinetics of a RAFT‐mediated radical polymerization inside submicron particles (30 < D_p < 300 nm) is considered. When the time fraction of active radical period, ϕ_A, is larger than ca. 1%, the polymerization rate increases with reducing particle size, as for the cases of conventional emulsion polymerization. The rate retardation by the addition of RAFT agent occurs with or without intermediate termination in zero‐one systems. For the particles with D_p < 100 nm, the statistical variation of monomer concentration among particles may not be neglected. It was found that this monomer‐concentration‐variation (MCV) effect may slow down the polymerization rate. An analytical expression describing the MCV effect is proposed, which is valid for both RAFT and conventional miniemulsion polymerizations.

     

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     

Controlled Bimodal Molecular‐Weight‐Distribution Polymers: Facile Synthesis by RAFT Polymerization

The RAFT agents RAFT‐1 and RAFT‐2 were used for RAFT polymerization to synthesize well‐defined bimodal molecular‐weight‐distribution (MWD) polymers. The system showed excellent controllability and “living” characteristics toward both the higher‐ and lower‐molecular‐weight fractions. It is important that bimodal higher‐molecular‐weight (HMW) polymers and block copolymers with both well‐controlled molecular weight (MW) and MWD could be prepared easily due to the “living” features of RAFT polymerization. The strategy realized a mixture of higher/lower‐molecular‐weight polymers at the molecular level but also preserved the features of living radical polymerization (LRP) of the RAFT polymerization.     

Amine‐Reactive Polymers Synthesized by RAFT Polymerization Using an Azlactone Functional Trithiocarbonate RAFT Agent

A new azlactone‐derived trithiocarbonate is prepared and used as a chain‐transfer agent to mediate the reversible addition‐fragmentation chain transfer (RAFT) polymerization of styrene, ethyl acrylate, and N‐isopropyl acrylamide. Well‐defined polymers with controlled molecular weights (M _n = 1000–7000 g mol⁻¹) and narrow molecular weight distributions (PDI = 1.05–1.10) are thus obtained that retain the azlactone functionality at the chain end. The ability of the resulting end‐functionalized polymers to react quantitatively at room temperature with a stoichiometric amount of amino groups with retention of the thiocarbonylthio moiety is ascertained by using 4‐fluorobenzylamine and allylamine.     

Synthesis of Hyperbranched Polyethylene Amphiphiles by Chain Walking Polymerization in Tandem with RAFT Polymerization and Supramolecular Self‐Assembly Vesicles

A novel polymerization methodology for efficient synthesis of hyperbranched polyethylene amphiphiles by chain walking polymerization (CWP) followed by RAFT polymerization has been developed. Hyperbranched polyethylene with hydroxyl ends (HBPE‐OHs) is first synthesized via chain walking copolymerization of ethylene with 2‐hydroxyethyl acrylate with Pd‐α‐diimine catalyst. The hydroxyl groups of hyperbranched polyethylene are then converted into thiocarbonyl thio moieties by an esterification reaction with trithiocarbonate 3‐benzylsulfanylthiocarbonyl sulfanylpropionic acid (BSPA). The hyperbranched polyethylene with thiocarbonyl thio moiety ends (HBPE‐BSPAs) is used as a macro‐RAFT agent for the synthesis of hyperbranched polyethylene amphiphiles, HBPE‐PDMAEMAs, by RAFT polymerization of N,N‐dimethylaminoethyl methacrylate (DMAEMA). The resultant HBPE‐PDMAEMAs can self‐assemble to form supramolecular polymer vesicles in aqueous solution. A preliminary investigation on thermo‐ and pH‐responsive behaviors of the polymer is also reported.     

Narrowly Distributed Dendronized Polymethacrylates by Reversible Addition‐Fragmentation Chain Transfer (RAFT) Polymerization

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.

     

Light‐Regulated Polymerization under Near‐Infrared/Far‐Red Irradiation Catalyzed by Bacteriochlorophyll a

Photoregulated polymerizations are typically conducted using high‐energy (UV and blue) light, which may lead to undesired side reactions. Furthermore, as the penetration of visible light is rather limited, the range of applications with such wavelengths is likewise limited. We herein report the first living radical polymerization that can be activated and deactivated by irradiation with near‐infrared (NIR) and far‐red light. Bacteriochlorophyll a (Bachl a) was employed as a photoredox catalyst for photoinduced electron transfer/reversible addition–fragmentation chain transfer (PET‐RAFT) polymerization. Well‐defined polymers were thus synthesized within a few hours under NIR (λ=850 nm) and far‐red (λ=780 nm) irradiation with excellent control over the molecular weight (M_n/M_w<1.25). Taking advantage of the good penetration of NIR light, we showed that the polymerization also proceeded smoothly when a translucent barrier was placed between light source and reaction vessel.     

SET‐RAFT Polymerization of Progargyl Methacrylate and a One‐Pot/One‐Step Preparation of Side‐chain Functionalized Polymers via Combination of SET‐RAFT and Click Chemistry

A clickable alkyne monomer, PgMA, was successfully polymerized in a well‐controlled manner via single electron transfer initiation and propagation through the radical addition fragmentation chain transfer (SET‐RAFT) method. The living nature of the polymerization was confirmed by the first‐order kinetic plots, the linear relationships between molecular weights and the monomer conversions while keeping relatively narrow (≤1.55), and the successful chain‐extension with MMA. The better controllability of SET‐RAFT than other CRP methods is attributed to the less competitive termination in view of the presence of the CTA as well as the Cu(II) that is generated in situ. Moreover, a one‐pot/one‐step technique combining SET‐RAFT and “click chemistry” methods has been successfully employed to prepare the side‐chain functionalized polymers.

     

A Critical Survey of Dithiocarbamate Reversible Addition‐Fragmentation Chain Transfer (RAFT) Agents in Radical Polymerization

This article provides a critical review of the properties, synthesis, and applications of dithiocarbamates Z′Z″NC(=S)SR as mediators in reversible addition‐fragmentation chain transfer (RAFT) polymerization. These are among the most versatile RAFT agents. Through choice of substituents on nitrogen (Z′, Z″), the polymerization of most monomer types can be controlled to provide living characteristics (i.e., low dispersities, high end‐group fidelity, and access to complex architectures). These include the more activated monomers (MAMs; e.g., styrenes and acrylates) and the less activated monomers (LAMs; e.g., vinyl esters and vinylamides). Dithiocarbamates with balanced activity (e.g., 1H‐pyrazole‐1‐carbodithioates) or switchable RAFT agents [e.g., a N‐methyl‐N‐(4‐pyridinyl)dithiocarbamate] allow control MAMs and LAMs with a single RAFT agent and provide a pathway to low‐dispersity poly(MAM)‐block‐poly(LAM). © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 216–227     

Thermoprecipitation of Glutathione S‐Transferase by Glutathione–Poly(N‐isopropylacrylamide) Prepared by RAFT Polymerization

Herein, we report an effective and rapid method to purify glutathione S‐transferase (GST) using glutathione (GSH)‐modified poly(N‐isopropylacrylamide) (pNIPAAm) and mild, thermal conditions. A chain transfer agent modified with pyridyl disulfide was employed in the reversible addition–fragmentation chain transfer (RAFT) polymerization of NIPAAm. The resulting polymer had a narrow molecular weight distribution (polydispersity index = 1.21). Conjugation of GSH to the pyridyl disulfide–pNIPAAm reached 95% within 30 min as determined by UV–Vis monitoring of the release of pyridine‐2‐thione. GST was successfully thermoprecipitated upon heating the GSH–pNIPAAm above the lower critical solution temperature (LCST). The pull down assay was repeated with bovine serum albumin (BSA) and T4 lysozyme (T4L), which demonstrated the specificity of the polymer for GST. Due to its simplicity and high efficiency, this method holds great potential for large‐scale purification of GST‐tagged proteins.

     

Cationic RAFT Polymerization Using ppm Concentrations of Organic Acid

A metal‐free, cationic, reversible addition–fragmentation chain‐transfer (RAFT) polymerization was proposed and realized. A series of thiocarbonylthio compounds were used in the presence of a small amount of triflic acid for isobutyl vinyl ether to give polymers with controlled molecular weight of up to 1×10⁵ and narrow molecular‐weight distributions (M_w/M_n<1.1). This “living” or controlled cationic polymerization is applicable to various electron‐rich monomers including vinyl ethers, p‐methoxystyrene, and even p‐hydroxystyrene that possesses an unprotected phenol group. A transformation from cationic to radical RAFT polymerization enables the synthesis of block copolymers between cationically and radically polymerizable monomers, such as vinyl ether and vinyl acetate or methyl acrylate.