TEMPO‐Mediated Emulsion Polymerization

While miniemulsion polymerization has proven to be well‐suited for conducting living/controlled radical polymerizations, emulsion polymerizations have proven to be far more challenging. Ab initio emulsion polymerizations, in which monomer droplets are present during polymerization, have thus far not been successful with TEMPO‐mediated polymerizations, as a result of colloidal instability and coagulum formation. By selectively inhibiting polymerization in the monomer droplets, it is demonstrated that droplet polymerization is responsible for the formation of large (>1 µm) particles that can lead to coagulum formation. Furthermore, we show that coagulum‐free latexes can be produced using a TEMPO‐mediated ab initio emulsion polymerization by suppressing droplet polymerization.

     

Frozen‐In Magnetic Order in Uniaxial Magnetic Gels: Preparation and Physical Properties

We describe the preparation and characterization of uniaxial magnetic gels. Fibril formation of the embedded magnetic particles generates easily detectable magnetic and optical anisotropies. A finite magnetization is frozen‐in and leads to a ferromagnetic‐like response in small homogeneous external magnetic fields. We present, for the first time, frequency dependent measurements of the shear modulus G′. Despite their optical and magnetic anisotropy, the gels are mechanically isotropic.

The time‐dependent G′ for a ferrogel in the parallel orientation in a homogeneous field of 200 mT.     

Compartmentalization in TEMPO‐Mediated Radical Polymerization in Dispersed Systems: Effects of Macroinitiator Concentration

The influence of the initial macroinitiator concentration ([PT]₀) on compartmentalization effects (segregation effects and confined space effects) in 2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO)‐mediated radical polymerization of styrene in a dispersed system at 125 °C has been investigated by simulations employing modified Smith‐Ewart equations. The modeling approach accounts for compartmentalization of both propagating radicals and nitroxide, as well as the generation of radicals by thermal initiation of styrene. The manifestation of compartmentalization effects occurs at significantly greater particle diameters (d) for low [PT]₀; at [PT]₀ = 0.002 M , the polymerization rate, control and livingness are affected by compartmentalization for d < 120 nm, whereas the system behaves as in the corresponding bulk system for d > 45 nm at [PT]₀ = 0.2 M . The results are discussed with regards to the specific effects of compartmentalization on deactivation and bimolecular termination.

     

Nitroxide‐Mediated Radical Polymerization in Miniemulsion at Stationary State: Rationale for Independence of Polymerization Rate on Nitroxide Partitioning Using Oil‐Phase Initiation

Summary: Simulations based on the kinetics and mechanism of nitroxide‐mediated free radical polymerization (NMP) have been carried out in order to understand the hitherto largely unexplained effects (or lack thereof) of nitroxide partitioning in aqueous miniemulsion NMP. The focus has been on the miniemulsion NMP of styrene mediated by TEMPO and 4‐hydroxy‐TEMPO, two nitroxides with very similar activation‐deactivation equilibria, but very different organic phase‐aqueous phase partition coefficients. The general conclusion is that the organic phase propagating radical and nitroxide concentrations are unaffected by the partition coefficient in the stationary state, but the rate of polymerization and the extent of bimolecular termination increase with increasing nitroxide water solubility in the pre‐stationary state region. Specific NMP systems are, therefore, affected differently by nitroxide partitioning depending on whether polymerization predominantly occurs in the stationary state or not, which in turn is governed mainly by the activation‐deactivation equilibrium constant and the rate of thermal initiation.

Simulated organic‐phase propagating radical concentrations in the presence of thermal initiation for TEMPO‐mediated miniemulsion free radical polymerization of styrene for different nitroxide partitioning coefficients at 125 °C.     

Interfacial Mass Transfer in Nitroxide‐Mediated Miniemulsion Polymerization

A mathematical model has been developed to describe the interfacial mass transfer of TEMPO in a nitroxide‐mediated miniemulsion polymerization (NMMP) system in the absence of chemical reactions. The model is used to examine how the diffusivity of TEMPO in the aqueous and organic droplet phases, the average droplet diameter and the nitroxide partition coefficient influences the time required for the nitroxide to reach phase equilibrium under non‐steady state conditions. Our model predicts that phase equilibrium is achieved quickly (< 1 × 10⁻⁴ s) in NMMP systems under typical polymerization conditions and even at high monomer conversions when there is significant resistance to molecular diffusion. The characteristic time for reversible radical deactivation by TEMPO was found to be more than ten times greater than the predicted equilibration times, indicating that phase equilibrium will be achieved before TEMPO has an opportunity to react with active polymer radicals. However, significantly longer equilibration times are predicted, when average droplet diameters are as large as those typically found in emulsion and suspension polymerization systems, indicating that the aqueous and organic phase concentrations of nitroxide may not always be at phase equilibrium during polymerization in these systems.

Influence of droplet phase TEMPO diffusivity, D_TEMPO,drop, on the predicted organic phase concentration of TEMPO.     

Rings of Hydrogel Fabricated by a Micro‐Transfer Technique

Microgel rings have been fabricated on a glass surface by a micro‐transfer technique. The polymer melt is transferred to the substrate surface in a confined space with a picoliter volume along the boundary of the polymeric stamp. The surface of the polymer features is smoothened by surface tension in an annealing cycle, which results in isotropic rings. After cross‐linking, the microgels respond swiftly when immersed in water. Asymmetric rings are also obtained by tilting the substrate. The microgels can detach from the substrate to form a suspension of the rings. Conjugated rings have also been fabricated by supplementing functional molecules into the poly(ethylene glycol) network.

     

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.

     

Anionic Polysaccharides as Templates for the Synthesis of Conducting Polyaniline and as Structural Matrix for Conducting Biocomposites

A green chemoenzymatic pathway for the synthesis of conducting polyaniline (PANI) composites is presented. Laccase‐catalyzed polymerization in combination with anionic polysaccharides is used to produce polysaccharide/PANI composites, which can be processed into flexible films or coated onto cellulose surfaces. Different polysaccharide templates are assessed, including κ‐carrageenan, native spruce O‐acetyl galactoglucomannan (GGM), and TEMPO‐oxidized cellulose and GGM. The resulted conducting biocomposites derived from natural materials provide a broad range of potential applications, such as in biosensors, electronic devices, and tissue engineering.

     

Synthesis of Well‐Defined Rod‐Coil Diblock Copolymer of Aromatic Polyether and Polyacrylonitrile by Chain‐Growth Condensation Polymerization and Atom Transfer Radical Polymerization

The synthesis of diblock copolymers of aromatic polyether and polyacrylonitrile (PAN) was conducted by chain‐growth condensation polymerization (CGCP) and atom transfer radical polymerization (ATRP) from an orthogonal initiator. When CGCP for aromatic polyether was carried out from a PAN macroinitiator obtained by ATRP with an orthogonal initiator, decomposition of the PAN backbone occurred. However, when ATRP of acrylonitrile was conducted from an aromatic polyether macroinitiator obtained by CGCP followed by introduction of an ATRP initiator unit, the polymerization proceeded in a well‐controlled manner to yield aromatic polyether‐block‐polyacrylonitrile (polyether‐b‐PAN) with low polydispersity. This block copolymer self‐assembled in N,N‐dimethylformamide to form bundle‐like or spherical aggregates, depending on the length of the PAN units in the block copolymer.

     

Thermoresponsive Block Copolymer–Protein Conjugates Prepared by Grafting‐from via RAFT Polymerization

Well‐defined “smart” block copolymer–protein conjugates were prepared by two consecutive “grafting‐from” reactions via reversible addition–fragmentation chain transfer (RAFT) polymerization. The initiating portion (R‐group) of the RAFT agent was anchored to a model protein such that the thiocarbonylthio moiety was readily accessible for chain transfer with propagating chains in solution. Well‐defined polymer‐protein conjugates of poly(N‐isopropylacrylamide) (PNIPAM) and bovine serum albumin (BSA) were prepared at room temperature in aqueous media. The retained trithiocarbonate moiety on the free end group of the immobilized polymer allowed the homopolymer conjugate to be extended by polymerization of N,N‐dimethylacrylamide. Polyacrylamide gel electrophoresis, size exclusion chromatography, and NMR spectroscopy confirmed the synthesis of the various conjugates and revealed that the polymerizations were well controlled. As expected, the resulting block copolymer–protein conjugates demonstrated thermoresponsive behavior due to the temperature‐sensitivity of the PNIPAM block, as evidenced by turbidity measurements and dynamic light scattering analysis.

     

Polymer Brushes by Nitroxide‐Mediated Polymerization

This review article describes the preparation of polymer brushes by nitroxide‐mediated radical polymerization using either the ‘grafting to’ or the ‘grafting from’ approach. The use of TEMPO as a classical initiator is intensively described. More sophisticated nitroxides are also included in the discussion. Brush formation on flat surfaces such as wafers and also on particles is reported. Finally, some applications of polymer brushes are presented.

     

Polymer Brushes with Phototriggered and Phototunable Swelling and pH Response

Photolabile polymer brushes with tailored length containing a photoremovable protecting group (NVOC) are prepared via the SI‐ATRP method. Upon light irradiation, the NVOC group is removed to generate controlled densities of free amine groups (PAMA) randomly distributed along the brush. The presence of the ionizable groups induces a photo‐triggered swelling response. The swelling degree can be tuned by the irradiation dose. A dual (light and pH), tunable response is demonstrated.

     

Color Indicator for Supramolecular Polymer Chemistry: Phenolphthalein‐Containing Thermo‐ and pH‐Sensitive N‐(Isopropyl)acrylamide Copolymers and β‐Cyclodextrin Complexation

The copolymerization parameters of N‐(isopropyl)acrylamide ( 1 ) and N‐(2‐hydroxy‐5‐(1‐(4‐hydroxyphenyl)‐3‐oxo‐1,3‐dihydroisobenzofuran‐1‐yl)benzyl)acrylamide ( 2 ) are determined. For both monomers, the homoaddition proceeds slightly faster than the heteroaddition step; however, the polymer formation occurs in a statistic fashion. Copolymers of different compositions are prepared and the cloud points are determined. Thereby, a significant influence of the concentration of monomer 2 and the pH value is found. For the first time, the complexation of polymer attached phenolphthalein by β‐cyclodextrins is shown. Furthermore, it is possible to achieve a decomplexation by the addition of suitable guest molecules. Both procedures can be followed with the naked eye.

     

Synthesis and Self‐Assembly of Amphiphilic Diblock Copolymers using a Fluorescently Labeled N‐Alkoxyamine Initiator

Summary: An initiator for nitroxide mediated ‘living’ free radical polymerization was prepared with a fluorescent tag attached to the initiating alkyl radical terminus. This was used to synthesize amphiphilic poly(acrylic acid)‐block‐polystyrene diblock copolymers, which self assembled in a tetrahydrofuran/buffer solution to form structures that are visible by fluorescence.

     

Nitroxide‐Mediated Controlled/Living Free Radical Copolymerization of Styrene and Divinylbenzene in Aqueous Miniemulsion

Summary: The nitroxide‐mediated controlled/living free radical copolymerization of styrene and divinylbenzene using a polystyrene‐TEMPO macroinitiator in aqueous miniemulsion and in bulk have been investigated. The crosslink densities were estimated based on the content of pendant vinyl groups as determined by ¹H NMR. Considerably lower crosslink densities were revealed in the miniemulsion than in the corresponding bulk system. The rate of polymerization in the miniemulsion increased with decreasing particle size, and was significantly higher than in bulk.

Crosslink density for the TEMPO‐mediated free radical copolymerization of S(1) and DVB(2) (f = 0.99, f = 0.01) at 125 °C in bulk (□) and in miniemulsions with d_n = 585 nm (○) and 53.3 nm (•).     

1,1‐Diphenyl Ethylene‐Mediated Radical Polymerisation: A General Non‐Metal‐Based Technique For The Synthesis Of Precise Core Cross‐Linked Star Polymers

This communication details the successful synthesis of low polydispersity core cross‐linked star (CCS) polymers via DPE‐mediated polymerisation. We demonstrate the ability to produce poly(methyl methacrylate) and poly(acrylonitrile) CCS polymers that are currently inaccessible via the two most common non‐metal‐based controlled radical polymerisation techniques (NMP and RAFT polymerisations).

     

Synthesis of Nanosized (<20 nm) Polymer Particles by Radical Polymerization in Miniemulsion Employing in situ Surfactant Formation

A novel method for synthesis of ultrafine polymeric nanoparticles of diameters less than 20 nm has been developed. The method is based on miniemulsion polymerization exploiting combination of the in situ surfactant generation approach (whereby the surfactant is formed at the oil–water interface by reaction between an organic acid and a base) and ultrasonication. Conventional radical polymerization and nitroxide‐mediated radical polymerization of styrene have been conducted in miniemulsion using oleic acid/potassium hydroxide, demonstrating that particles with diameters less than 20 nm can be obtained by this approach at surfactant contents much lower than traditionally required in microemulsion polymerizations.

     

α,ω‐Nitroxide‐Capped Polystyrenes and Poly(butyl acrylate)s as Macroinitiators for the Free‐Radical Polymerization of Methyl Methacrylate

The use of a bisaminooxy compound as initiator for nitroxide‐mediated radical polymerization (NMRP) of styrene or n‐butyl acrylate allows the synthesis of α,ω‐nitroxide‐capped polymers. At high temperatures and with the addition of acetic anhydride, it was found that these polymers could be applied as macroinitiators in the free‐radical polymerization of methyl methacrylate. This enables the synthesis of block copolymers with only minor contents of homopolymer.

The structure of bis‐TIPNO, the bisaminooxy compound used as an initiator for the nitroxide‐mediated radical polymerization of styrene or n‐butyl acrylate.     

Synthesis and Characterization of Amphiphilic Biodegradable Copolymer,Poly(aspartic acid‐co‐lactic acid)

An amphiphilic biodegradable polymer, poly(aspartic acid‐co‐lactic acid) (PAL), was synthesized by simply heating a mixture of aspartic acid (Asp) and L ‐lactide without additional catalysts or solvents. The unique branched architecture comprising succinimide units and lactic acid units was confirmed by IR and NMR spectroscopy. A copolymer of sodium aspartate and lactic acid (PALNa) was prepared by reacting PAL with an aqueous sodium hydroxide solution. The PAL was soluble in many organic solvents, while the PALNa was soluble in methanol and water. The hydrolytic degradation behavior of PAL varied with the copolymer composition. A higher Asp content resulted in a faster molecular weight decrease, and introducing glycolic acid units accelerated the degradation rate.

Microphotograph of microsphere of PAL‐1/5.