Controlled/living Radical Polymerization Research Articles

Controlled/Living Radical Polymerization in Dispersed Systems: An Update.

This review is an extensive update to the comprehensive review on controlled/living radical polymerization (CLRP) in dispersed systems published in 2008.

Preparation and characterization of poly(methyl methacrylate)/SiO2 organic–inorganic hybrid materials via RAFT-mediated miniemulsion Polymerization

Poly(methyl methacrylate)/SiO2 (PMMA/SiO2) organic–inorganic hybrid materials were prepared by traditional free radical polymerization and RAFT polymerization in miniemulsion. Comparison of these two reactions kinetics results indicated the polymerization behavior of methyl methacrylate (MMA) with control of the RAFT mechanism showed controlled/living radical polymerization (CLRP) characteristics. The hybrid materials were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform-infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM). Incorporation of the RAFT agent and SiO2 nanoparticles can effectively improve the thermal properties of PMMA polymers. TEM results showed that part of the SiO2 nanoparticles adsorbed on the surface of the PMMA latex particles and formed hybrid materials with partially wrapped structure.

Targeting Copolymer Composition Distribution via Model-Based Monomer Feeding Policy in Semibatch RAFT Mini-Emulsion Copolymerization of Styrene and Butyl Acrylate

Copolymer composition distribution (CCD) is an important parameter of chain microstructure that has significant impact on the material properties of polymer products. Controlled/living radical polymerization (CLRP) has provided a great opportunity in producing polymers with predesigned CCDs through a model-based monomer feeding policy (MMFP) in homogeneous systems. In this work, the MMFP has been expanded to heterogeneous systems. Reversible addition–fragmentation transfer (RAFT) mini-emulsion polymerizations of styrene (St) and butyl acrylate (BA) were carried out with 3-benzyltrithiocarbonyl propionic acid (BCPA) used as a RAFT agent. A kinetic model was developed and correlated to the batch St/BA RAFT mini-emulsion experimental data for parameter estimation. The MMFP was then developed by combining the kinetic model with a semibatch reactor model and applied to the semibatch St/BA RAFT mini-emulsion copolymerization for the synthesis of targeted CCDs. The model agreed well with the polymerization kinetics. A series of St/BA copolymers with predesigned CCDs were successfully synthesized.

Effect of tamoxifen in RAFT miniemulsion polymerization during the synthesis of polymeric nanoparticles

Tamoxifen (TXF) is currently the only hormonal agent used for treatment of breast cancer. Although very effective, TXF presents low solubility in water, which affects its absorption and bioavailability. A common strategy to overcome this barrier is the formulation of a drug delivery system (DDS) in order to increase the drug stability and improve the treatment effectiveness. Reversible addition-fragmentation chain transfer (RAFT) polymerization is the most versatile method of controlled/living radical polymerization (CLRP), allowing for synthesis of well-defined polymers and being adapted to a wide range of polymerization systems. Miniemulsion polymerization is a dispersed system that is commonly used to prepare nanoparticles (NP) with 50 to 500 nm of diameter. The aim of this work was to evaluate the effect of the in situ incorporation of TXF during miniemulsion conventional and RAFT polymerizations, using methyl methacrylate (MMA) as monomer. Although the in situ addition of TXF promoted a slight reduction of the reaction rate, it did not affect the final particle size distribution of the latex or the molecular weight control exerted by the RAFT agent. The obtained results suggest that in situ incorporation of TXF during the synthesis of polymer NP via RAFT polymerization allows for production of a polymer DDS for different uses, such as the breast cancer treatment.

Synthesis of amphiphilic PVC-b-poly(hydroxypropyl acrylate) (PHPA)-b-PVC block copolymers with low PHPA contents and different molecular weights by (Single Electron Transfer)-(Degenerative Chain Transfer) living radical polymerization

The aim of this work was to synthesize new amphiphilic block copolymers, based on poly(vinyl chloride) (PVC) and containing poly(hydroxypropyl acrylate) (PHPA), by using the controlled/“living” radical polymerization (CLRP) method. Various block copolymers containing a small proportion of PHPA were prepared, each having a different molecular weight. The technique used was the same as that employed in the production of commercial PVC made by free-radical polymerization. The materials were characterized in terms of their molecular structure, morphology, particle size, and surface and thermal properties. The CLRP preparation of block copolymers that are based on PVC and have low contents of other monomer units opens the possibility of synthesizing new materials whose properties are close to those of PVC but have new properties that may considerably enhance their performance. The incorporation of small amounts of PHPA into PVC block copolymers provided greater surface hydrophilicity and improved thermal stability while maintaining relevant processing properties, such as particle size and average molecular weight, so that they close to those of conventional PVC homopolymers. J. VINYL ADDIT. TECHNOL., 19:157-167, 2013. © 2013 Society of Plastics Engineers

Tailor-made polyfluoroacrylate and its block copolymer by RAFT polymerization in miniemulsion; improved hydrophobicity in the core–shell block copolymer

Controlled/living radical polymerization (CRP) of a fluoroacrylate was successfully carried out in miniemulsion by Reversible Addition Fragmentation chain Transfer (RAFT) process. In this case, 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA) was polymerized using 2-cyanopropyl dodecyl trithiocarbonate (CPDTC) as RAFT agent, Triton X-405 and sodium dodecyl sulfonate (SDS) as surfactant, and potassium persulphate (KPS) or 2,2′-azobis isobutyronitrile (AIBN) as initiator. Being compatible with hydrophobic fluoroacrylate, this RAFT agent offered very high conversion and good control over the molecular weight of the polymer. The miniemulsion was stable without any costabilizer. The long chain dodecyl group (–C12H25) (Z-group in the RAFT agent) had beneficial effect in stabilizing the miniemulsion. When 2-cyano 2-propyl benzodithioate (CPBD) (Z=–C6H5) was used as RAFT agent, the conversion was less and particle size distribution was very broad. Block copolymerization with butyl acrylate (BA) using PHFBA as macro-RAFT agent showed core–shell morphology with the aggregation of PHFBA segment in the shell. GPC as well as DSC analysis confirmed the formation of block copolymer. The core–shell morphology was confirmed by TEM analysis. The block copolymers (PHFBA-b-PBA) showed significantly higher water contact angle (WCA) showing much better hydrophobicity compared to PHFBA alone.

Functionalization of Graphene Oxide for the Production of Novel Graphene-Based Polymeric and Colloidal Materials

Graphene oxide (GO) has been long-considered the most convenient route towards the large scale production of graphene. Additionally, the functional groups present within GO permit both covalent and non-covalent chemical functionalization, in particular with polymeric materials. The functionalization of GO therefore enables the development of graphene-based composite materials which possess the properties of both the matrix and the remarkable electrical, thermal and mechanical properties of both GO and graphene. In this review, we discuss the functionalization of GO in two broad settings: macromolecular functionalization of GO, and the use of GO in dispersed/colloidal systems. We review numerous methods for the functionalization of GO with initiators or chain transfer agents to permit controlled/living radical polymerization (CLRP) to take place from the surface of GO; the use of GO as a polymerization initiator, as well as non-covalent polymeric modification is discussed. For applications of GO in dispersed systems, we discuss the incorporation of GO into miniemulsions, emulsions and other dispersed phase polymerization systems, as well as techniques such as layer-by-layer assembly and colloidal templating. The use of GO as a ‘colloidal surfactant’ is also reviewed. These functionalization methods are discussed within the framework of creating materials with enhanced properties for specific applications including electrodes, capsules, polymer particles and composite films.

Preparation and Characterization of PMMA/MMT Organic-Inorganic Hybrid Materials via RAFT Polymerization

In this article, the poly(methyl methacrylate)/montmorillonite (PMMA/MMT) organic-inorganic hybrid materials were prepared by conventional free radical polymerization and reversible addition-fragmentation chain transfer (RAFT) polymerization, respectively. The kinetics comparison of these two polymerizations was studied. The PMMA/MMT hybrid materials were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). According to these results, we knew that the polymerization behavior of MMA showed controlled/living radical polymerization (CLRP) characteristics under the control of RAFT agent. The incorporation of RAFT agent and MMT nanoparticles improved the thermal properties of polymers, and the thermal stability of polymers increased with increasing content of MMT nanoparticles. The structures and morphologies of PMMA/MMT hybrid materials were characterized by FT-IR, XRD and TEM. These results showed that the MMA monomer can be initiated and propagated in the clay layers of MMT via the control of RAFT agent, and then the exfoliated structure was obtained for the hybrid materials.

Ambient temperature rapid SARA ATRP of acrylates and methacrylates in alcohol–water solutions mediated by a mixed sulfite/Cu(ii)Br2 catalytic system

The new generation of catalytic systems for Controlled/“Living” Radical Polymerization (CLRP) of vinyl monomers should be non-toxic, inexpensive and provide fast polymerizations in environmentally friendly media. Herein, we report the successful ambient temperature ATRP of several vinyl monomers (MA, n-BA, MMA and DMAEMA) catalyzed by inorganic sulfites (Na2S2O4 and Na2S2O5) and small amounts of a Cu(II)Br2/Me6TREN system in alcohol–water mixtures. The controlled character of ATRP of acrylates and methacrylates was confirmed by the linear increase of molecular weights with monomer conversion, narrow molecular weight distributions (Mw/Mn ∼ 1.05) and by reinitiation experiments (copolymerization and chain extension). 1H NMR and MALDI-TOF analyses confirmed the molecular structure and chain-end functionality of the obtained polymers. ATRP of MA using this novel catalytic system in alcohol–water mixtures with multifunctional Br-based initiators provides 4 and 6 arm star polyacrylates in a controlled manner without any observable gel formation. The data presented open up the possibility of using fast ATRP catalyzed by inorganic sulfites (approved by FDA as food and beverage additives) in solvents that are inexpensive, eco-friendly and widely used in chemical industrial processes.

Optimization of brush-like cationic copolymers for nonviral gene delivery.

Polyethylenimine (PEI) is one of the most broadly used polycations for gene delivery due to its high transfection efficiency and commercial availability but materials are cytotoxic and often polydisperse. The goal of current work is to develop an alternative family of polycations based on controlled living radical polymerization (CLRP) and to optimize the polymer structure for efficient gene delivery. In this study, well-defined poly(glycidyl methacrylate)(P(GMA)) homopolymers were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization followed by decoration using three different types of oligoamines, i.e., tetraethylenepentamine (TEPA), pentaethylenehexamine (PEHA), and tris(2-aminoethyl)amine (TREN), respectively, to generate various P(GMA-oligoamine) homopolycations. The effect of P(GMA) backbone length and structure of oligoamine on gene transfer efficiency was then determined. The optimal polymer, P(GMA-TEPA)(50), provided comparable transfection efficiency but lower cytotoxicity than PEI. P(GMA-TEPA)(50) was then used as the cationic block in diblock copolymers containing hydrophilic N-(2-hydroxypropyl) methacrylamide (HPMA) and oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA). Polyplexes of block copolymers were stable against aggregation in physiological salt condition and in Opti-MEM due to the shielding effect of P(HPMA) and P(OEGMA). However, the presence of the HPMA/OEGMA block significantly decreased the transfection efficacy of P(GMA-TEPA)(50) homopolycation. To compensate for reduced cell uptake caused by the hydrophilic shell of polyplex, the integrin-binding peptide, RGD, was conjugated to the hydrophilic chain end of P(OEGMA)(15)-b-P(GMA-TEPA)(50) copolymer by Michael-type addition reaction. At low polymer to DNA ratios, the RGD-functionalized polymer showed increased gene delivery efficiency to HeLa cells compared to analogous polymers lacking RGD.

Synthesis of densely grafted copolymers with tert-butyl methacrylate/2-(dimethylamino ethyl) methacrylate side chains as precursors for brush polyelectrolytes and polyampholytes

Methacrylate-based densely grafted copolymers were synthesized by atom transfer radical polymerization (ATRP) and activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) techniques. The linear poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) PBIEM prepared by ATRP served as a macroinitiator backbone. The “grafting from” strategy was used to initiate polymerization of tert-butyl methacrylate (tBuMA) from PBIEM under ATRP and/or AGET ATRP conditions yielding densely grafted copolymers PBIEM-graft-P(tBuMA). The low polydispersity indices (PDI) of the synthesized brushes evidenced by SEC analysis were consistent with a controlled/living radical polymerization (CLRP) mechanism. The chlorine-terminated PBIEM-graft-P(tBuMA)-Cl macroinitiators were subsequently employed for chain extension with 2-(dimethylamino ethyl) methacrylate (DMAEMA) yielding densely grafted copolymers with diblock copolymer side chains PBIEM-graft-P(tBuMA)-block-PDMAEMA. Further, PBIEM macroinitiator was used to initiate the copolymerization of a binary mixture of tBuMA and DMAEMA through both ATRP and AGET ATRP initiating systems, yielding densely grafted copolymers with statistical distribution of the side chains. The reactivity ratios for random graft copolymerization of tBuMA and DMAEMA from PBIEM backbone established by three different methods (Finemann-Ross, Kelen-Tüdös and Error-in-Variable) did not substantially differ from literature values for conventional free-radical copolymerization of the same monomers. Polyampholyte brushes with PMAA-stat-PDMAEMA side chains were eventually synthesized by hydrolysis of the shielding tert-butyl groups.

Copper‐Mediated Controlled/“Living” Radical Polymerization in Polar Solvents: Insights into Some Relevant Mechanistic Aspects

The field of transition-metal-mediated controlled/"living" radical polymerization (CLRP) has become the subject of intense discussion regarding the mechanism of this widely-used and versatile process. Most mechanistic analyses (atom transfer radical polymerization (ATRP) vs. single-electron transfer living radical polymerization (SET-LRP)) have been based on model experiments, which cannot correctly mimic the true reaction conditions. We present, for the first time, a determination of the [Cu(I)Br]/[L] (L=nitrogen-based chelating ligand) ratio and the extent of Cu(I)Br/L disproportionation during CLRP of methyl acrylate (MA) in dimethylsulfoxide (DMSO) with Cu(0) wire as a transition-metal catalyst source. The results suggest that Cu(0) acts as a supplemental activator and reducing agent of Cu(II)Br(2)/L to Cu(I)Br/L. More importantly, the Cu(I)Br/L species seem to be responsible for the activation of SET-LRP.

Reversible Addition–Fragmentation Chain Transfer Polymerization of Vinyl Chloride

Controlled/“living” radical polymerization (CLRP) of vinyl chloride (VC) via the reversible addition–fragmentation chain transfer (RAFT) process is reported for the first time. The cyanomethyl methyl(phenyl)carbamodithioate (CMPCD) was found to be an efficient RAFT agent enabling the CLRP polymerization of VC monomer under certain experimental conditions. Two different radical initiators, having very distinct half-life times at room temperature, were employed in this study. The kinetic studies of RAFT polymerization of VC show a linear increase of the molecular weight with the monomer conversion and the lowest polydispersity (PDI) ever reported for poly(vinyl chloride) (PVC) synthesized with CLRP method (PDI ∼ 1.4). The resulting PVC was fully characterized using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), 1H nuclear magnetic resonance spectroscopy (1H NMR), and gel permeation chromatography (GPC) techniques. The 1H NMR and MALDI-TOF-MS analysis of PVC prepared via RAFT polymerization method have shown the absence of structural defects and the presence of chain-end functional groups. The “livingness” of the PVC was also confirmed by a successful reinitiation experiment. The suitability of the RAFT agent was also confirmed via high-level ab initio molecular orbital calculations.

3D Single Cyclized Polymer Chain Structure from Controlled Polymerization of Multi-Vinyl Monomers: Beyond Flory–Stockmayer Theory

Controlled/living radical polymerization (CRP) is a widely used technique that allows the synthesis of defined polymer architectures through precise control of molecular weights and distributions. However, the architectures of polymers prepared by the CRP techniques are limited to linear, cross-linked, and branched/dendritic structures. Here, we report the preparation of a new 3D single cyclized polymer chain structure from an in situ deactivation enhanced atom transfer radical polymerization of multivinyl monomers (MVMs), which are conventionally used for the production of branched/cross-linked polymeric materials as defined by P. Flory and W. Stockmayer nearly 70 years ago. We provide new evidence to demonstrate that it is possible to kinetically control both the macromolecular architecture and the critical gelling point in the polymerization of MVMs, suggesting the classical Flory-Stockmayer mean field theory should be supplemented with a new kinetic theory based on the space and instantaneous growth boundary concept.

How Fast Can a CRP Be Conducted with Preserved Chain End Functionality?

The preservation of chain end functionality (CEF) is perhaps the most important criterion of the “livingness” for any controlled/living radical polymerization (CRP) system. CEF depends on several parameters but most importantly on concentration of propagating radicals, i.e., rate of polymerization. There are claims in the literature about ultrafast CRP leading to ultrahigh molecular weight with perfectly preserved CEF. However, faster CRP always results in lower CEF. Thus, how fast can a CRP be conducted with preserved CEF? In this paper we evaluate, by kinetic analysis and computation, the dependence of CEF on the rate of polymerization, monomer structure (rate constants of propagation and termination), targeted degree of polymerization, initial monomer concentration, and monomer conversion. Some subtle differences in the definition of CEF are discussed for different types of atom transfer radical polymerization (ATRP) techniques. CEF for polymer chains with and without an α-alkyl group from a (macro)ini...

Engineering soft nanostructured functional materials via orthogonal chemistry

Nanostructured amphiphilic block copolymers, graft copolymers, polymeric thermally responsive molecular brushes and polymer stars are only few examples of macromolecular architectures accessible either via controlled/living radical polymerization (CLRP) techniques or the combination of CLRP mechanisms with efficient post-polymerization routes including click chemistry. Precise control over the composition, molecular weight and functionalities is a prerequisite for soft polymeric materials to selforganize into ordered morphologies. This contribution describes novel orthogonal chemical routes for the synthesis of macromolecular architectures and selfassembly of functional soft polymeric materials. Emerging potential applications of well-defined block and graft copolymers are outlined as well.

Modeling Controlled/Living Radical Polymerization Kinetics: Bulk and Miniemulsion

AbstractA first‐principle model for the controlled/living radical polymerization (CLRP) is discussed. The polymerization rate of CLRP is conveniently represented by a simple relationship, Rp ∝ [Intermediate]/[Trapping Agent], which highlights the important characteristics of various types of CLRPs. In stable free radical polymerization and atom‐transfer radical polymerization, the relationship, [Trap] ≪ [Interm] holds, and the polymerization rate is controlled by [Trap]. When the polymerization is conducted in nanosized particles, even a single trapping agent in a particle may lead to a larger [Trap] than for bulk polymerization. This single‐molecule‐concentration (SMC) effect theory leads to determine the particle diameter, Dp,SMC below which Rp starts to decrease significantly compared with the corresponding bulk polymerization, and $R_{{\rm p}} \,{\propto} \,D_{{\rm p}}^{3} $ for Dp < Dp,SMC. For the particle sizes somewhat larger than Dp,SMC, the statistical variation in the number of trapping agents can make Rp larger. A simple equation to estimate the Dp,Fluct‐value, below which the acceleration due to the fluctuation effect is predicted to occur, is presented. In conjunction with the SMC effect, an acceleration window, in which Rp is larger than for bulk polymerization, may be observed for Dp,SMC < Dp < Dp,Fluct. On the other hand, many reversible‐addition‐fragmentation chain transfer polymerizations conform to the condition [Interm] ≪ [Trap], and Rp is controlled by [Interm]. If [Interm] in a particle under the zero‐one condition is larger than for bulk polymerization, Rp can be increased significantly by reducing the particle size due to the zero‐one intermediate molecule (ZIM) effect. The ZIM effect theory leads to determine the particle diameter, Dp,ZIM below which Rp increases significantly compared with the bulk polymerization, and $R_{{\rm p}} \,{\propto} \,D_{{\rm p}}^{{-} 3} $ for Dp < Dp,ZIM. magnified image

Mechanism study and molecular design in controlled/“living” radical polymerization

This tutorial review summarizes recent progress in the research field of controlled/“living” radical polymerization (CLRP) from Soochow University. The present paper gives a broad overview of the mechanism study and molecular design in CLRP. The mechanism study in CLRP aided by microwave, initiated by γ-radiation at low temperature, mediated by iron, in reversible addition-fragmentation chain transfer (RAFT) polymerization and the mechanism transfer between different CLRP processes are reviewed and summarized. The molecular design in CLRP, especially in RAFT polymerization for mechanism study, and in achieving tailor-made functional polymers is studied and discussed in the later part.

Fundamentals of Atom Transfer Radical Polymerization

Today’s market increasingly demands sophisticated materials for advanced technologies and high-value applications, such as nanocomposites, optoelectronic, or biomedical materials. Therefore, the demand for well-defined polymers with very specific molecular architecture and properties increases. Until recently, these kinds of polymers could only be prepared via living ionic polymerization, a process that requires very stringent experimental conditions. Recently, atom transfer radical polymerization (ATRP), a controlled/living radical polymerization (CRP) process, has emerged as a viable alternative. ATRP allows the synthesis of polymeric structures that are well-defined in terms of composition and molecular architecture. By virtue of its simplicity, versatility and scope to make polymers with site-specific functionality and novel architecture, ATRP has become the most extensively studied CRP technique.

Synthesis of hyperbranched polymers via a facile self-condensing vinyl polymerization system – Glycidyl methacrylate/Cp 2TiCl 2/Zn

A facile self-condensing vinyl polymerization (SCVP) system, the combination of glycidyl methacrylate, Cp 2TiCl 2 and Zn, has been firstly used to prepare novel hyperbranched polymers, consisting of vinyl polymers as the backbone, and cyclic ester polymers (poly(ɛ-caprolactone) or poly( l-lactide)) as the side chains. The polymerizations are initiated by the epoxide radical ring-opening catalyzed by Cp 2Ti(III)Cl which is generated in situ via the reaction of Cp 2TiCl 2 with Zn. The key to success is that the polymerizations can proceed concurrently via two dissimilar chemistries possessing the opposite active initiating species, including ring-opening polymerization (ROP) and controlled/living radical polymerization (CRP). We have demonstrated that this facile one-step polymerization technique can be applied successfully to prepare highly branched polymers with a multiplicity of end reactive functionalities including Ti alkoxide, hydroxyl and vinyl functional groups.