Metal-free Atom Transfer Radical Polymerization Research Articles

Examining derivatives of quinacridone, diketopyrrolopyrrole and indigo as the visible-light organic photocatalysts for metal-free atom transfer radical polymerization

Conventional atom transfer radical polymerization (ATRP) involving metal/ligand complexes as the catalysts has been widely used for synthesis of myriads of polymers with controllable molecular weights, monomer sequences and chain end groups, but suffers from the metal-contamination in the final polymer products. This issue has been recently mediated to some extent by using organic photocatalysts, which not only enable controllable polymerization upon light irradiation, but also offers spatiotemporal control of the polymerizations. To date the majority of reported organic photocatalysts for ATRP mainly absorb light in the UV region. Here we examined three organic chromophores, quinacridone, diketopyrrolopyrrole and indigo, as visible-light-absorbing organic photocatalysts for controllable polymerization of a series of methacrylates monomers via a scheme of light-mediated ATRP. Among the three types of organic chromophores, N,N-bis(tert-butyloxycarbonyl) quinacridone shows the highest fluorescence quantum yield and the best electrochemical and optical stability, all of which contribute to its outstanding performance in controllable polymerization of methacrylates under visible LED light irradiation.

Facile Fabrication of Size-Tunable Core/Shell Ferroelectric/Polymeric Nanoparticles with Tailorable Dielectric Properties via Organocatalyzed Atom Transfer Radical Polymerization Driven by Visible Light

An unconventional but facile approach to prepare size-tunable core/shell ferroelectric/polymeric nanoparticles with uniform distribution is achieved by metal-free atom transfer radical polymerization (ATRP) driven by visible light under ambient temperature based on novel hyperbranched aromatic polyamides (HBPA) as a functional matrix. Cubic BaTiO3/HBPA nanocomposites can be prepared by in-situ polycondensation process with precursors (barium hydroxide (Ba(OH)2) and titanium(IV) tetraisopropoxide (TTIP)) of ferroelectric BaTiO3 nanocrystals, because precursors can be selectively loaded into the domain containing the benzimidazole rings. At 1200 °C, the aromatic polyamide coating of cubic BaTiO3 nanoparticles are carbonized to form carbon layer in the inert environment, which prevents regular nanoparticles from gathering. In addition, cubic BaTiO3 nanoparticles are simultaneously transformed into tetragonal BaTiO3 nanocrystals after high temperature calcination (1200 °C). The outer carbon shell of tetragonal BaTiO3 nanoparticles is removed via 500 °C calcination in air. Bi-functional ligand can modify the surface of tetragonal BaTiO3 nanoparticles. PMMA polymeric chains are growing from the initiating sites of ferroelectric BaTiO3 nanocrystal surface via the metal-free ATRP technique to obtain core/shell ferroelectric BaTiO3/PMMA hybrid nanoparticles. Changing the molar ratio between benzimidazole ring units and precursors can tune the size of ferroelectric BaTiO3 nanoparticles in the process of polycondensation, and the thickness of polymeric shell can be tailored by changing the white LED irradiation time in the organocatalyzed ATRP process. The dielectric properties of core/shell BaTiO3/PMMA hybrid nanoparticles can be also tuned by adjusting the dimension of BaTiO3 core and the molecular weight of PMMA shell.

BINOLs as visible light photocatalysts for metal-free atom transfer radical polymerization

1,1′-Bisnaphthol (BINOL) has been successfully identified as a new photocatalyst framework for organocatalyzed atom transfer radical polymerization (ATRP).

Evolution and Future Directions of Metal-Free Atom Transfer Radical Polymerization

The increasing impact of atom transfer radical polymerization (ATRP) in fields beyond traditional polymer science has necessitated the development of alternative strategies for controlling polymer growth. Driven by applications that are sensitive to metal ion contamination, “greener” methodologies are emerging as a powerful alternative to conventional ATRP. Organic catalysis represents a major evolution of ATRP with metal-free systems holding significant potential as user-friendly methods for utility in biological and microelectronic applications. In addition, shifting from a combination of thermal activation/metal ions/ligands to simpler organic catalysis/light activation increases compatibility with functional monomers and allows the development of novel surface patterning strategies. Herein, we highlight key discoveries and recent developments in metal-free ATRP, while providing a perspective for future opportunities in this emerging area.

Photomediated atom transfer radical polymerization of MMA under long-wavelength light irradiation

In this study, a novel photocatalyst, pentarylenebis(dicarboximide) dye: (1,6,13,18-tetra(4-(2,3,3-trimethylbut-2-yl)phenoxy)-N,N’-(2,6-diisopropylphenyl)-pentarylene-3,4,15,16-tetracarboxidiimide) (TTPDPT), was first used in metal-free photoinduced atom transfer radical polymerization (ATRP) of methyl methacrylates (MMA). The initiator was methyl α-bromoisobutyrate (MBI) and the light source was mild near-infrared (NIR) light irradiation (λmax ≈ 870 nm). The TTPDPT-mediated ATRP relies on in situ photoreduction of a MBI through an electron transfer process to generate the desired alkyl radical, which could induce polymerization of the monomer. The photoinduced metal-free ATRP of MMA shows typical characteristics of controlled free radical polymerization, showing the linear evolution of number-average molecular weight (Mn,GPC) with monomer conversion, where polymers with predetermined degree of polymerization have well-controlled molecular weights and narrow molecular weight distribution (Mw/Mn). The photoinduced metal-free ATRP of MMA can be carried out with just ppm level of TTPDPT. The polymerization initiation and propagation can be operated by the aid of pulsed light sequences while NIR light source was used to promote carbon–carbon bond formation and to produce poly(methyl methacrylate) (PMMA) with Mw/Mn as low as 1.5. The synthesized PMMA was characterized by 1H nuclear magnetic resonance (1H NMR). The resultant PMMA contained a bromide end group that can be employed to reinitiate styrene polymerization to produce block copolymers through chain extension experiments.

Simultaneous Preparation of Multiple Polymer Brushes under Ambient Conditions using Microliter Volumes

The fabrication of well-defined, multifunctional polymer brushes under ambient conditions is described. This facile method uses light-mediated, metal-free atom-transfer radical polymerization (ATRP) to grow polymer brushes with only microliter volumes required. Key to the success of this strategy is the dual action of N-phenylphenothiazine (PTH) as both an oxygen scavenger and polymerization catalyst. Use of simple glass cover slips results in a high degree of spatial and temporal control and allows for multiple polymer brushes to be grown simultaneously. The preparation of arbitrary 3D patterns and functional/emissive polymer brushes demonstrates the practicality and versatility of this novel strategy.

Aqueous Metal-Free Atom Transfer Radical Polymerization: Experiments and Model-Based Approach for Mechanistic Understanding

Metal-free atom transfer radical polymerization (ATRP) was successfully achieved in aqueous media for the first time. Polymerization of poly(ethylene oxide) methyl ether acrylate (PEGA480) was well controlled (Đ < 1.40) under visible light irradiation using tetrabromofluorescein (Eosin Y) as catalyst and pentamethyldiethylenetriamine (PMDETA) as electron donor. A validated kinetic model was developed to investigate the process of photoredox catalytic cycle via reductive quenching pathway. Experimental and simulation results showed that electron donor not only had an important influence on the ATRP activation, but also participated in the ATRP deactivation. Furthermore, the effects of water content, catalyst concentration, and degree of polymerization on the polymerization were studied thoroughly by a series of experiments. Good controllability of the polymerization regulated by light on and off confirmed the high degree of temporal control. The livingness of the chains was proved by a successful chain ext...

Synthesis of poly(3-hexylthiophene) based rod–coil conjugated block copolymers via photoinduced metal-free atom transfer radical polymerization

Photo-mediated metal-free ATRP for the synthesis of P3HT-based rod–coil copolymers.

Photo-induced metal-free ATRP of MMA with 2,7-bi-(N-penothiazinyl)fluorenone as photocatalyst

ABSTRACTIn this study, 2,7-bi-(N-penothiazinyl)fluorenone was employed as photocatalyst (PC), ethyl α-bromophenylacetate (EBP) as atom transfer radical polymerization (ATRP) initiator, and photo-induced metal-free ATRP of methyl methacrylate (MMA) was performed at 25°C under blue light irradiation. PMMAs with well-defined architectures and precisely controlled chain lengths were synthsized. The kinetics results confirmed that molecular weights increased linearly with monomer consumption. The molecular weight distributions (Mw/Mn) of the resultant PMMA were narrow. The polymerization was activated and deactivated by periodic light control process. 1H nuclear magnetic resonance spectrometer (NMR) and gel permeation chromatography (GPC) were used to characterize the obtained PMMAs. The living characters of the polymerization system were further confirmed by chain extension of from the PMMA-Br macroinitiator.

Synthesis of Hyperbranched Polymers by Photoinduced Metal-Free ATRP

A novel synthetic strategy for the syntheses of hyperbranched homo and block copolymers was developed. By applying the photoinduced metal-free atom transfer radical polymerization (ATRP) using perylene as photocatalyst, monomer/inimer pairs were transformed into hyperbranched polymer structures under visible light irradiation. Using the halide functional groups in thus-formed structures as initiating sites, further polymerizations were also performed to construct the secondary segments successfully. The strategy described here enables the synthesis of such complex polymeric structures by a simple procedure, under mild and metal-free conditions. All polymers were characterized by spectroscopic and chromatographic techniques.

A novel self-catalyzed photoATRP strategy for preparation of fluorescent hydroxyapatite nanoparticles and their biological imaging

Hydroxyapatite (HAp), as an important biomaterial for the regeneration and reconstruction of bone tissue, has attracted more and more attention of researchers and scientists due to its unique structure and compositions. However, the preparation of fluorescent HAp with controllable morphology has achieved only limited success. In this work, we reported a novel strategy to construct the water dispersible fluorescent HAp nanorods via the combination of ligand exchange and metal-free atom transfer radical polymerization (ATRP). The Br-containing fluorescent HAp nanorods with controllable size and morphology were first prepared through hydrothermal treatment. A multifunctional organic molecule (named as PTH-Br) with aggregation-induced emission feature was immobilized on the surface of hydrophobic HAp nanorods through ligand exchange reaction. The PTH-Br could be used as the initiator and catalyst for surface-initiated metal-free ATRP using poly(ethylene glycol) methacrylate as monomer to obtain hydrophilic fluorescent HAp polymer nanoparticles. This strategy successfully endowed HAp nanorods excellent fluorescence properties and favorable water dispersibility but well preserved their regular morphology. Biological assays demonstrated that the HAp-PTH-poly(PEGMA) nanoparticles exhibited good biocompatibility and efficient cell uptake performance. Taken together, we have developed a rather facile strategy based on the surface ligand exchange reaction and metal-free photoATRP to fabricate fluorescent HAp with controllable size and morphology, high water dispersibility and biological properties. These HAp-PTH-poly(PEGMA) nanoparticles should be novel and promising candidates for biomedical applications.

Amphiphilic diblock and crosslinked copolymers synthesized via metal‐free atom transfer radical polymerization

AbstractThe use of metal‐free atom transfer radical polymerization (MF‐ATRP) was explored for the formation of diverse macromolecular structures to assess the versatility of this advanced polymerization process. In MF‐ATRP using an organic photocatalyst, 10‐phenylphenothiazine, the influences of various monomers, initiators and solvents were examined, showing that molecular weight and polydispersity could be tailored through appropriate selection of each component. Using this modern polymerization technology, metal‐free amphiphilic diblock and crosslinked copolymers were prepared successfully. Especially, demonstration of amphiphilic diblock copolymer synthesis provides a basis for further applications to biomedical materials. © 2017 Society of Chemical Industry

Photoinduced Metal-Free Atom Transfer Radical Polymerization Using Highly Conjugated Thienothiophene Derivatives

Photoinduced metal-free atom transfer radical polymerization (ATRP) activated by highly conjugated electron-rich thienothiophene derivatives, namely 4-[2-(4-diphenylaminophenyl)-thieno[3,2-b]thiophen-3-yl]benzonitrile (TT-TPA), 4-[2,5-bis(4-diphenylaminophenyl)-thieno[3,2-b]thiophen-3-yl]benzonitrile (TPA-TT-TPA), 4-(2-(4-(1,2,2-triphenylvinyl)phenyl) thieno[3,2-b]thiophen-3-yl)benzonitrile (TT-TPE) and 4-(2,5-bis(4-(1,2,2-triphenylvinyl)phenyl)thieno[3,2-b]thiophen-3- yl)benzonitrile (TPE-TT-TPE) is reported. Polymerization of methyl methacrylate (MMA) is efficiently activated and deactivated with light, forming polymers with controlled the molecular weight characteristics, dispersity, and chain end functionality. Polymerization studies and DFT calculations revealed that TT-TPA is the most efficient activator due to the favorable thermodynamic properties.

Phenyl Benzo[b]phenothiazine as a Visible Light Photoredox Catalyst for Metal-Free Atom Transfer Radical Polymerization.

This paper reports use of phenyl benzo[b]phenothiazine (Ph-benzoPTZ) as a visible light-induced metal-free atom transfer radical polymerization (ATRP) photoredox catalyst. Well-controlled polymerizations of various methacrylate monomers were conducted under a 392 nm visible light LED using Ph-benzoPTZ to activate different alkyl halides. The use of the photocatalyst enabled temporal control over the growth of polymer chains during intermittent on/off periods. The polymerization was initiated and progressed only under stimulation by light and completely stopped in the absence of light. Block copolymers were synthesized to demonstrate high retention of chain end fidelity in the polymers and livingness of the process.

Facile synthesis of size-tunable superparamagnetic/polymeric core/shell nanoparticles by metal-free atom transfer radical polymerization at ambient temperature

Size-tunable superparamagnetic/polymeric core/shell nanoparticles with uniform distribution was fabricated based on metal-free atom transfer radical polymerization at ambient temperature.

Conventional Type II photoinitiators as activators for photoinduced metal-free atom transfer radical polymerization

A novel methodology for photoinduced metal-free Atom Transfer Radical Polymerization (ATRP) by using conventional Type II photoinitiators such as benzophenone, thioxanthone, isopropyl thioxanthone and camphorquinone as sensitizers is presented.

Block copolymer synthesis in one shot: concurrent metal-free ATRP and ROP processes under sunlight

A completely metal-free strategy was developed by combining Atom Transfer Radical Polymerization (ATRP) and Ring Opening Polymerization (ROP) for the syntheses of block copolymers.

Surface polyPEGylation of Eu3+ doped luminescent hydroxyapatite nanorods through the combination of ligand exchange and metal free surface initiated atom transfer radical polymerization

The Eu3+ doped luminescent hydroxyapatite (HAp) nanorods with uniform size and morphology can be synthesized by hydrothermal route. However, these HAp nanorods are coated by hydrophobic oleylamine, which makes them difficult to be dispersed in aqueous solution and impede their biomedical applications. In this work, Eu3+ doped luminescent polymers functionalized HAp nanorods were prepared through the combination of ligand exchange reaction and metal free surface initiated atom transfer radical polymerization (ATRP) method. In this procedure, the amino group functionalized HAp nanorods were first prepared by ligand exchange reaction using adenosine monophosphate (AMP) as ligand. Then the Br-containing initiators (HAp-Br) were introduced onto the surface of HAp-AMP nanorods through the amidation reaction. Finally, polymers functionalized HAp nanorods were prepared by metal free ATRP method using poly(ethylene glycol) methacrylate (PEGMA) as monomer and 10-phenylphenothiazine (PTH) as organic photocatalyst. The properties of these obtained HAp nanocomposites (HAP-polyPEGMA nanorods) were characterized by means of transmission electron microscopy, Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis in detail. The cell imaging of these HAP-polyPEGMA nanorods was examined using laser scanning confocal microscope to evaluate their biomedical applications. We demonstrated for the first time that hydrophobic luminescent HAp nanorods can be functionalized with polyPEGMA through the combination of ligand exchange reaction and metal free surface initiated ATRP. As compared with the traditional ATRP, the metal free ATRP can overcome the toxic and fluorescence quenching effects of metal catalysts such as copper ions. More importantly, the strategy described in this work should also be utilized for fabrications of many other luminescent polymer nanocomposites due to its good monomer adoptability.

Metal-Free Atom Transfer Radical Polymerization of Methyl Methacrylate with ppm Level of Organic Photocatalyst.

It is well known that the recently developed photoinduced metal-free atom transfer radical polymerization (ATRP) has been considered as a promising methodology to completely eliminate transition metal residue in polymers. However, a serious problem needs to be improved, namely, large amount of organic photocatalysts should be used to keep the controllability over molecular weights and molecular weight distributions. In this work, a novel photocatalyst 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) with strong excited state reduction potential is successfully used to mediate a metal-free ATRP of methyl methacrylate just with parts per million (ppm) level usage under irradiation of blue light emitting diode at room temperature, using ethyl α-bromophenyl-acetate as a typical initiator with high initiator efficiency. The polymerization kinetic study, multiple controlled "on-off" light switching cycle regulation, and chain extension experiment confirm the "living"/controlled features of this promising photoinduced metal-free ATRP system with good molecular weight control in the presence of ppm level photocatalyst 4CzIPN.

Photoinduced Metal-Free Atom Transfer Radical Polymerization of Biomass-Based Monomers

Well-defined polymers derived from biomass feedstock (e.g., soybean oil, rosin acid, and furfural) were successfully prepared by metal-free atom transfer radical polymerization (ATRP). In the presence of photoredox catalysts and UV irradiation, three biomass-based methacrylate monomers were efficiently polymerized with good control over molecular weight and dispersity. NMR and MALDI-TOF MS confirmed high fidelity of chain end groups originated from initiators. Furthermore, block copolymers from these monomers were also achieved through chain extension by metal-free ATRP.