Atom transfer radical polymerization: Fundamentals, challenges, and application

Various aspects of atom transfer radical polymerization (ATRP) were investigated. In an attempt to find a novel catalyst system for ATRP, we screened the activities of various metal complexes using a combinatorial approach. Several new catalyst systems including FeCl2/bam(TMS) were found to be active ATRP catalysts in the polymerization of styrene and MMA. In order to make this combinatorial screening a viable method of quickly discovering usable systems, we tried to find a fast and reliable method to evaluate the catalysts. A parameter estimation method based on nonlinear regression was developed to evaluate various catalyst systems by determining kinetic parameters of polymerization. From our model system considering small molecular atom transfer addition reaction, we found that equilibrium constant of atom transfer reaction could be successfully estimated. A new model dealing polymerization itself was also developed, and we could demonstrate that each values of activation and deactivation reaction rate constant can be estimated unambiguously. On screening the catalyst systems for ATRP, we found some titanium complexes gave a control in the polymerization of styrene without the aid of Group I–III cocatalysts. A series of experiments to elucidate the mechanism of polymerization all support that radical mechanism is involved in the polymerization using bis-(cyclopentadienyl)titanium dichloride. A possibility of ATRP mechanism was checked by isolating intermediate species. It is found that the polymerization is not followed the pure ATRP pathway, but is comprised of various competing reactions. Several strategies has been developed to prepare polymers having higher order structure including branched, hyperbranched, star, and dendrigrafts. The combination of nitroxide mediated SFRP and ATRP techniques successfully provided relatively simple routes to from branched and hyperbranched polymers in controlled structures. To overcome this limitation of backbone polymer prepared by SFRP, a new strategy using protection-deprotection chemistry was employed. Among the various protected monomers tested, we could prepare branched polystyrene having controlled structure using VBt-BOC and 4-methyl styrene. As an example of diversity of this strategy, we also could prepare the branched acrylate polymer having controlled structure

Colorless and Transparent Copolyimides and Their Nanocomposites: Thermo-Optical Properties, Morphologies, and Gas Permeabilities

A series of linear aromatic copolyimides (Co-PIs) were synthesized by reacting 4,4′-biphthalic anhydride (BPA) with various molar contents of 2,2′-bis(trifluoromethyl)benzidine (TFB) and p-xylylenediamine (p-XDA) in N,N′-dimethylacetamide (DMAc). Co-PI films were fabricated by solution casting and thermal imidization with poly(amic acid) (PAA) on glass plates. The thermo-optical properties and gas permeabilities of Co-PI films composed of various molar ratios of p-XDA (0.2–1.0 relative to BPA) were investigated. Thermal properties were observed to deteriorate with increasing p-XDA concentration. However, oxygen-transmission rates (O2TRs) and optical transparencies improved with increasing p-XDA concentration. Co-PI hybrids with a 1:0.2:0.8 molar ratio of BPA:TFB:p-XDA and organically modified hectorite (STN) were prepared by the in situ intercalation method. The morphologies and the thermo-optical and gas permeation properties of the hybrids were examined as functions of STN loading (5–50 wt %). XRD and TEM revealed substantial increases in clay particle agglomeration in the Co-PI hybrid films as the clay loading was increased from 5 to 50 wt %. The coefficient of thermal expansion (CTE) and the O2TR of a Co-PI hybrid film were observed to improve with increasing STN concentration; however, its optical transparency decreased gradually with increasing STN concentration

Dual Roles of a Xanthate as a Radical Source and Chain Transfer Agent in the Photoinitiated RAFT Polymerization of Vinyl Acetate

A high level of control over the photoinitiated RAFT polymerization of vinyl acetate (VAc) was achieved using a specifically designed xanthate, <i>S</i>-2-cyano-2-propyl-<i>O</i>-ethyl xanthate (CPEC), which acted as a radical source and a chain transfer agent simultaneously. Unlike other RAFT processes, the present system did not use any additional radical initiator, while achieving greater control over the polymerization than the photoiniferter process. The molecular weight of the resulting polymer could be modulated by changing the initial [VAc]₀/[CPEC]₀ ratio, but the control over the polymerization was lost with a very low initial [VAc]₀/[CPEC]₀ ratio. The intensity of UV irradiation affected the polymerization by reducing the induction period and increasing the rate of polymerization but did not affect the molecular weight of the resulting polymer

The Combined Effects of Sr(II) and Poly(Acrylic Acid) on the Morphology of Calcite

Biomineralization of calcium carbonate has interesting characteristics of intricate morphology formation with controlled crystal polymorphs. In particular, modification of calcite morphology with diverse additives has been the focus of many biomimetic and bioinspired studies. The possible role of strontium ions in enhancing the morphology-modifying ability of macromolecules was investigated. In the present study, concentrations of strontium ions were comparable to that in seawater, and anionic poly(acrylic acid) and cationic poly(ethylene imine) were used as model macromolecules. When strontium ions were combined with anionic poly(acrylic acid), new types of calcite surfaces, most likely {hk0}, appeared to drastically change the morphology of the crystals, which was not observed with cationic poly(ethylene imine). This behavior of strontium ions was quite similar to that of magnesium ions, which is intriguing because both ions are available from seawater to be utilized during biomineralization

Doxorubicin-Loaded Alginate‑<i>g</i>‑Poly(<i>N</i>‑isopropylacrylamide) Micelles for Cancer Imaging and Therapy

Chemotherapy is a widely adopted method for the treatment of cancer. However, its use is often limited due to side effects produced by anti-cancer drugs. Therefore, various drug carriers, including polymeric micelles, have been investigated to find a method to overcome this limitation. In this study, alginate-based, self-assembled polymeric micelles were designed and prepared using alginate-<i>g</i>-poly­(<i>N</i>-isopropylacrylamide) (PNIPAAm). Amino-PNIPAAm was chemically introduced to the alginate backbone via carbodiimide chemistry. The resulting polymer was dissolved in distilled water at room temperature and formed self-assembled micelles at 37 °C. Characteristics of alginate-<i>g</i>-PNIPAAm micelles were dependent on the molecular weight of PNIPAAm, the degree of substitution, and the polymer concentration. Doxorubicin (DOX), a model anti-cancer drug, was efficiently encapsulated in alginate-<i>g</i>-PNIPAAm micelles, and sustained release of DOX from the micelles was achieved at 37 °C in vitro. These micelles accumulated at the tumor site of a tumor-bearing mouse model as a result of the enhanced permeability and retention effect. Interestingly, DOX-loaded alginate-<i>g</i>-PNIPAAm micelles showed excellent anti-cancer therapeutic efficacy in a mouse model without any significant side effects. This approach to designing and tailoring natural polymer-based systems to fabricate nanoparticles at human body temperature may provide a useful means for cancer imaging and therapy

Solvent-Free Processable and Photo-Patternable Hybrid Gate Dielectric for Flexible Top-Gate Organic Field-Effect Transistors

We report a novel solvent-free and direct photopatternable poly­[(mercaptopropyl)­methyl-siloxane] (PMMS) hybrid dielectric for flexible top-gate organic field-effect transistors (OFETs) utilizing a photoactivated thiol–ene reaction under UV irradiation of 254 nm to induce cross-linking, even in air and at low temperatures. In particular, a solvent-free PMMS-f dielectric film, for which an optimal cross-linking density is shown by a well-organized molar ratio between thiol and vinyl in the thiol–ene reaction, exhibited a high dielectric constant (5.4 @ 100 Hz) and a low leakage current (<1 nA mm^–2 @ 2 MV cm^–1). The excellent dielectric characteristics of the solvent-free PMMS-hybrid dielectrics, along with their other unique characteristics of a direct photopatternability for which UV-nanoimprint lithography is used and a high surface energy of 45.6 mJ m^–2, allowed the successful application of the dielectrics to flexible solvent-free top-gate OFETs with a high reliability against the radius of curvature (9.5, 7.0, and 5.5 mm) and repetitive bending cycles at the radius of curvature of 5.5 mm. This will eventually enable the proposed dielectric design to be used in a variety of applications such as flexible displays and soft organic sensors including chemical and tactile capability