Rate-Enhanced Nitroxide-Mediated Miniemulsion Polymerization

A novel approach is presented whereby nitroxide-mediated radical polymerization (NMP) is conducted in an aqueous heterogeneous system at an initial polymerization rate an order of magnitude greater than the corresponding bulk system, accompanied by an improvement in the level of control over the molecular weight distribution. NMP of styrene mediated by <i>N</i>-<i>tert</i>-butyl-<i>N</i>-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1) at 90 °C was performed in a miniemulsion with exceptionally small particles of number-average diameter ∼10 nm, generated by ultrasonication combined with in situ surfactant formation. The results are discussed in terms of the effects of compartmentalization, nitroxide partitioning (exit/entry), and a rate-enhancing effect of oleic acid. These findings illustrate that it is possible to significantly improve the performance of an NMP process by the exploitation of intrinsic effects of heterogeneous systems

Synergistic Effects of Compartmentalization and Nitroxide Exit/Entry in Nitroxide-Mediated Radical Polymerization in Dispersed Systems

Modeling and simulations of compartmentalization effects in tandem with nitroxide exit and entry have been performed for the nitroxide-mediated radical polymerization (NMP) of styrene in an aqueous dispersed system employing 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) at 125 °C. It is demonstrated that, even for a relatively water-insoluble nitroxide like TEMPO, exit and entry can strongly influence the polymerization kinetics in submicrometer-size droplets/particles. In such systems, the polymerization is expected to proceed at a markedly higher rate than the corresponding bulk system at the expense of control/livingness. Depending on the deactivator water solubility, these findings will apply qualitatively to all controlled/living radical polymerization systems governed by the persistent radical effect [e.g., NMP and atom transfer radical polymerization (ATRP)]

Preparation of Composite Materials by Using Graphene Oxide as a Surfactant in Ab Initio Emulsion Polymerization Systems

In this letter, we report a simple and unexpected method of producing polymer–graphene oxide (GO) composite materials via ab initio emulsion polymerization in water. On the basis of the recent reports concerning the surfactant-like behavior of GO for stabilizing oil-in-water emulsions, we prepared exfoliated GO sheets with lateral dimension approximately 200 nm for use as surfactant in the emulsion polymerization of styrene. We observed an expected “classic” surfactant behavior to produce stable nanoparticles at extremely low GO loadings (<0.1% w/w); however, at higher concentrations a highly aggregated, fibrous morphology was formed. This morphology is predominantly due to the electrolyte concentration (ionic strength) of the aqueous phase resulting in heterocoagulation of growing oligomers with dispersed GO sheets, which offers a convenient route toward preparing hybrid materials

Inverse Miniemulsion Periphery RAFT Polymerization: A Convenient Route to Hollow Polymeric Nanoparticles with an Aqueous Core

The recently developed [Chem. Commun. 2012, 48, 11103−11105] inverse miniemulsion periphery RAFT polymerization (IMEPP) approach to prepare hollow polymeric nanoparticles (∼200 nm) with an aqueous core has been explored in detail. The method is based on an amphiphilic macroRAFT agent acting as stabilizer of water droplets in an organic continuous phase while also mediating cross-linking chain growth in a controlled/living manner on the outer periphery of the droplets. The macroRAFT agent comprised a hydrophilic block of poly­(<i>N</i>-(2-hydroxypropyl)­methacrylamide) and a hydrophobic block of either polystyrene or poly­(methyl methacrylate), and the cross-linked shell was formed on polymerization of styrene/divinylbenzene or methyl methacrylate/ethylene glycol dimethacrylate, respectively. The effects of various reaction parameters on the resulting hollow nanoparticles have been systematically investigated, and it has been demonstrated that the shell thickness can be tuned based on initial stoichiometry and monomer conversion. This method is particularly relevant for encapsulation of proteinssuccessful incorporation of proteins (bovine serum albumin) into the miniemulsion did not negatively affect the droplet size and stability

Visible-Light-Regulated Controlled/Living Radical Polymerization in Miniemulsion

The implementation of photopolymerization processes in aqueous dispersed systems has the potential to afford greener approaches to the preparation of polymeric materials and has motivated researchers to perform photopolymerization in emulsion. However, these previous works have employed UV irradiation to induce photodegradation of a photoinitiator in addition to specialized equipment setups, thus limiting widespread use of these approaches. In this work, we aim to remedy these drawbacks via the implementation of photoredox catalysis in the regulation of a controlled/living radical polymerization under visible light. Utilizing the photoinduced electron transfer–reversible addition–fragmentation chain transfer (PET-RAFT) process, we report the miniemulsion polymerization of styrene mediated by a household grade blue LED (λ_max = 460 nm, 0.73 mW/cm²). The polymerization rate can be easily manipulated by light intensity and catalyst concentration. Finally, temporal control was demonstrated via ON/OFF experiments, which shows that no significant detriment is caused by prolonged interruptions to the light exposure

Pushing the Limit of the RAFT Process: Multiblock Copolymers by One-Pot Rapid Multiple Chain Extensions at Full Monomer Conversion

We describe an optimized method to prepare multiblock copolymers. The approach is based on our previously reported use of reversible addition–fragmentation chain transfer (RAFT) polymerization, which here has been optimized into a fast, versatile, efficient, and scalable process. The one-pot, multistep sequential polymerization proceeds in water, to quantitative yields (>99%) for each monomer addition, thus circumventing requirements for intermediate purification, in 2 h of polymerization per block. The optimization of the process is initially demonstrated via the synthesis of a model decablock homopolymer (10 blocks) of 4-acryloylmorpholine with an average degree of polymerization of 10 for each block (<i><i>Đ</i></i> = 1.15 and livingness >93% for the final polymer). Both the potential and the limitations of this approach are illustrated by the synthesis of more complex high-order multiblock copolymers: a dodecablock copolymer (12 blocks with 4 different acrylamide monomers) with an average degree of polymerization of 10 for each block and two higher molecular weight pentablock copolymers (5 blocks with 3 different acrylamide monomers) with an average degree of polymerization of 100 per block

Exploitation of the Degenerative Transfer Mechanism in RAFT Polymerization for Synthesis of Polymer of High Livingness at Full Monomer Conversion

We report the synthesis by the reversible addition–fragmentation chain transfer process of well-defined decablock polymers with a final dispersity as low as 1.15 and a fraction of living chain as high as 97% after 10 successful block extensions, each taken to >99% monomer conversion. By using model decablock homopolymers of poly­(<i>N</i>,<i>N</i>-dimethylacrylamide) and poly­(4-acryloylmorpholine) of relatively low DP (10 units per block in average), we describe the theoretical and experimental considerations required to access high-order multiblock copolymers with excellent control over molecular weight distributions and high livingness

Synthesis of Complex Multiblock Copolymers via a Simple Iterative Cu(0)-Mediated Radical Polymerization Approach

Controlled/living radical polymerization is an efficient technique for the synthesis of well-defined polymeric architectures, including copolymers, block copolymers, stars, graft, and variations thereof. In this article, we report for the first time the synthesis of a decablock copolymer via a simple and efficient iterative Cu(0)-mediated radical polymerization technique. In this approach, purification is not required between the iterative chain extension steps, as each block formation is taken to full conversion. The final decablock copolymer can be obtained with a yield in mass of ∼90%. Using traditional controlled/living radical polymerization techniques, including reversible addition–fragmentation chain transfer (RAFT) polymerization, nitroxide-mediated polymerization (NMP), or atom transfer radical polymerization (ATRP), synthesis of decablock copolymer in such high yield is very difficult (and may be impossible) and requires numerous purification steps. The synthesis of the final complex copolymers required the concomitant synthesis of 16 architecturally discrete block copolymers

Visible Light-Mediated Polymerization-Induced Self-Assembly Using Continuous Flow Reactors

We present the synthesis of polymeric nanoparticles of targeted morphology in a continuous process via visible light-mediated aqueous RAFT polymerization-induced self-assembly (PISA). A trithiocarbonate-derived poly­(ethylene glycol) (PEG) macroRAFT was activated in the presence of hydroxypropyl methacrylate (HPMA) at 37 °C under blue light irradiation (460 nm), leading to the formation of PEG-<i>b</i>-P­(HPMA) nanoparticles. The method is attractive in its simplicityspheres, worms, and vesicles can easily be obtained in a continuous fashion with higher control in comparison to conventional batch procedures. This allows for more accurate production of particle morphologies and scalable synthesis of these nano-objects. The versatility of this process was demonstrated by the <i>in situ</i> encapsulation of an active compound

Successful Miniemulsion ATRP Using an Anionic Surfactant: Minimization of Deactivator Loss by Addition of a Halide Salt

To date, it has been generally assumed, based on early experimental work, that ATRP in aqueous dispersed systems is incompatible with anionic surfactants. In the present work, it is clarified that this incompatibility originates in the anionic surfactant (sodium dodecyl sulfate, SDS) displacing the halide ligand from the Cu^II bromide-based deactivator, converting it to a Cu^II complex, unable to deactivate radicals. This results in a very high polymerization rate as well as essentially no control over the molecular weight distribution. It is demonstrated how such loss of deactivator can be minimized by the addition of a source of halide ions, thus enabling one to conduct ATRP in aqueous dispersed systems using commonly available and inexpensive anionic surfactants such as SDS