RAFT Dispersion PISA with Poly(methyl methacrylate) as Stabilizer Block in Alcohol/Water: Unconventional PISA Morphology Transitions.

Reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization-induced self-assembly (PISA) was conducted in the presence of poly(methyl methacrylate) (PMMA) stabilizer in ethanol/water mixture (80/20 by volume). Two different systems were explored by utilizing (i) 2-ethylhexyl methacrylate (EHMA) and (ii) n-butyl methacrylate (BMA). The morphology transitions of these systems were investigated by varying the polymerization conditions, i.e., the presence of the solvophilic comonomer MMA, the solids content, and the target degree of polymerization (DP). As observed in conventional PISA, the presence of solvophilic comonomer, increase in solids content and target DP promoted the formation of high-order morphology. However, unusual morphology transitions were observed whereby the morphology transformed from high-order morphologies to a mixture of spherical nanoparticles, worms, and vesicles and finally to vesicles with increasing target DP. This unusual evolution may be attributed to the limited solubility of PMMA in the ethanol/water solvent mixture, whereby PMMA is soluble at the polymerization temperature but insoluble at lower temperatures.

Preparation of Block Copolymer Self-Assemblies via Pisa in a Non-Polar Medium Based on RCMP.

Polymerization-induced self-assembly (PISA) is conducted in a non-polar medium (n-dodecane) via reversible complexation-mediated polymerization (RCMP). Stearyl methacrylate (SMA) is used to synthesize a macroinitiator, and subsequent block polymerization of benzyl methacrylate (BzMA) from the macroinitiator in n-dodecane afforded a PSMA-PBzMA block copolymer, where PSMA is poly(stearyl methacrylate) and PBzMA is poly(benzyl methacrylate). Because PSMA is soluble but PBzMA is insoluble in n-dodecane, the block copolymer formed a self-assembly during the block polymerization (PISA). Spherical micelles, worms, and vesicles are obtained, depending on the degrees of polymerization of PSMA and PBzMA. "One-pot" PISA is also attained; namely, BzMA is directly added to the reaction mixture of the macroinitiator synthesis, and PISA is conducted in the same pot without purification of the macroinitiator. The spherical micelle and vesicle structures are also fixed using a crosslinkable monomer during PISA. RCMP-PISA is highly attractive as it is odorless and metal-free. The "one-pot" synthesis does not require the purification of the macroinitiator. RCMP-PISA can provide a practical approach to synthesize self-assemblies in non-polar media.

Reversible Complexation Mediated Living Radical Polymerization Using Tetraalkylammonium Chloride Catalysts.

This work reports the first use of organic chloride salts as catalysts for reversible complexation mediated living radical polymerization. Owing to the strong halogen-bond forming ability of Cl- , the studied four tetraalkylammonium chloride catalysts (R4 N+ Cl- ) successfully control the polymerizations of methyl methacrylate, yielding polymers with low dispersities up to high monomer conversion (>90%). Benzyldodecyldimethylammonium chloride is further exploited to other methacrylates and yields low-dispersity block copolymers. The advantages of the chloride salt catalysts are wide monomer scope, good livingness, accessibility to block copolymers, and good solubility in organic media. Because of the good solubility, the use of the chloride salt catalysts can prevent agglomeration of catalysts on reactor walls in organic media, which is an industrially attractive feature. Among halide anions, chloride anion is the most abundant and least expensive halide anion, and therefore, the use of the chloride salt catalysts may lower the cost of the polymerization.

Polymeric Nanofibers of Various Degrees of Cross-Linking as Fillers in Poly(styrene-stat-n-butyl acrylate) Nanocomposites: Overcoming the Trade-Off between Tensile Strength and Stretchability.

Synthesis of light polymer nanocomposites with high strength and toughness has been a significant interest for its potential applications in industry. Herein, the authors have synthesized polymerization-induced self-assembly (PISA) derived nanodimensional polymeric worm (fiber) reinforced polymer nanocomposites by a simple and environmentally friendly synthesis process without the addition of volatile organic compounds. PISA-derived worms with a core-forming block of low glass transition temperature (Tg  ≈ 27.1 °C) comprising poly(styrene-stat-n-butyl acrylate) have been employed as reinforcing filler. The influence of core-segment cross-linking on reinforcement efficiency has been explored by comparing noncross-linked worms, and worms cross-linked with a small amount of ethylene glycol diacrylate introduced at t = 0 h or t = 2 h of polymerization. Upon addition of 1 wt% of noncross-linked, t = 0 h cross-linked, and t = 2 h cross-linked worms, toughness of polymer nanocomposites can be enhanced by 62%, 114%, and 120%, respectively. The results suggest that the reinforcement efficiency of worms is significantly influenced by the cross-linking of core-segments regardless of cross-linking methods. This work broadens the understanding in application of PISA-derived worms as reinforcing filler by demonstrating the efficient reinforcement with low Tg worms.

Development of glassy step-growth thiol-vinyl sulfone polymer networks.

Thermomechanical properties of neat phosphine-catalyzed thiol-Michael networks fabricated in a controlled manner are reported, and a comparison between thiol-acrylate and thiol-vinyl sulfone step-growth networks is performed. When highly reactive vinyl sulfone monomers are used as Michael acceptors, glassy polymer networks are obtained with glass transition temperatures ranging from 30 to 80 °C. Also, the effect of side-chain functionality on the mechanical properties of thiol-vinyl sulfone networks is investigated. It is found that the inclusion of thiourethane functionalities, aryl structures, and most importantly the elimination of interchain ester linkages in the networks significantly elevate the network's glass transition temperature as compared with neat ester-based thiol-Michael networks.

Phosphonium Tetraphenylborate: A Photocatalyst for Visible-Light-Induced, Nucleophile-Initiated Thiol-Michael Addition Photopolymerization.

A photoinitiation system that utilizes phosphonium tetraphenylborate as the key component was developed for the visible light-triggered nucleophile-catalyzed thiol-Michael addition reaction. This highly reactive catalyst was composed of a photocaged phosphine (methyldiphenylphosphonium tetraphenylborate, MDPP·HBPh4), a photosensitizer (isopropylthioxanthone, ITX), and a radical scavenger (TEMPO). Unlike the prevailing photobase catalysts, this photoactivatable phosphine system triggers the thiol-Michael addition polymerization by a nucleophile-catalyzed mechanism and provides a controlled stoichiometric reaction between the thiol and the vinyl precursors. This approach enables the formation of homogeneous polymer networks upon low-energy visible light exposure and, thus, broadens its potential applications in bulk polymer materials synthesis and UV-sensitive bioscaffold formation.

Thermoreversible Folding as a Route to the Unique Shape-Memory Character in Ductile Polymer Networks.

Ductile, cross-linked films were folded as a means to program temporary shapes without the need for complex heating cycles or specialized equipment. Certain cross-linked polymer networks, formed here with the thiol-isocyanate reaction, possessed the ability to be pseudoplastically deformed below the glass transition, and the original shape was recovered during heating through the glass transition. To circumvent the large forces required to plastically deform a glassy polymer network, we have utilized folding, which localizes the deformation in small creases, and achieved large dimensional changes with simple programming procedures. In addition to dimension changes, three-dimensional objects such as swans and airplanes were developed to demonstrate applying origami principles to shape memory. We explored the fundamental mechanical properties that are required to fold polymer sheets and observed that a yield point that does not correspond to catastrophic failure is required. Unfolding occurred during heating through the glass transition, indicating the vitrification of the network that maintained the temporary, folded shape. Folding was demonstrated as a powerful tool to simply and effectively program ductile shape-memory polymers without the need for thermal cycling.

Synthesis of C2-chiral bifunctionalised spin labels and their application to troponin C.

An enantiomeric pair of C2-chiral bifunctionalised spin labels having a pyrrolidine nitroxide moiety, whose configurations were determined by X-ray crystal diffraction analysis, was prepared and applied to troponin C whose binding mode of double disulfide linkage was proved by EPR spectroscopy.

Ester-free Thiol-X Resins: New Materials with Enhanced Mechanical Behavior and Solvent Resistance.

A series of thiol-Michael and radical thiol-ene network polymers were successfully prepared from ester-free as well as ester-containing monomer formulations. Polymerization reaction rates, dynamic mechanical analysis, and solvent resistance experiments were performed and compared between compositions with varied ester loading. The incorporation of ester-free alkyl thiol, vinyl sulfone and allylic monomers significantly improved the mechanical properties when compared with commercial, mercaptopropionate-based thiol-ene or thiol-Michael networks. For polymers with no hydrolytically degradable esters, glass transition temperatures (Tg's) as high as 100 °C were achieved. Importantly, solvent resistance tests demonstrated enhanced stability of ester-free formulations over PETMP-based polymers, especially in concentrated basic solutions. Kinetic analysis showed that glassy step-growth polymers are readily formed at ambient conditions with conversions reaching 80% and higher.

Programmable mechanically assisted geometric deformations of glassy two-stage reactive polymeric materials.

Thiol-isocyanate-methacrylate two-stage reactive network polymers were developed and used for fabrication of well-defined surface patterns as well as functional geometric shapes to demonstrate a new methodology for processing of "smart materials". The dynamic stage I networks were synthesized in base-catalyzed thiol-isocyanate cross-linking reactions to yield tough, glassy materials at ambient conditions. Methacrylate-rich stage I networks, incorporating photoinitiator and photoabsorber, were irradiated with UV light to generate stage II networks with intricate property gradients. Upon directional straining and subsequent temperature-dependent stress relief of the predefined gradient regions, the desired surface or bulk geometric transformations were achieved. Depending on the gradient extent in conjunction with photoorthogonal initiators, the introduced deformations were shown to be easily erasable by heat or permanently fixable by bulk polymerization.

Visible-Light Initiated Thiol-Michael Addition Photopolymerization Reactions.

A visible-light base generating system was successfully employed in catalyzing the thiol-Michael addition reaction to yield cross-linked polymers from a stoichiometric mixture of model thiol and vinyl monomers. Implementation of the radical inhibitor TEMPO with a combination of a photosensitizer (isopropylthioxanthone, ITX) and a photobase generator (triazabicyclodecene tetraphenylborate, TBD·HBPh4) resulted in suppression of radical mediated side reactions and provided stoichiometric and complete conversion of both thiol and vinyl functional groups. The new initiating system acts as an efficient visible-light photobase generator that improves the orthogonality of the thiol-Michael addition with respect to off-stoichiometric radical thiol-vinyl addition/vinyl chain reactions. This approach opens up a variety of possibilities for base-catalyzed reactions in multiple applications such as coatings and biomaterials that require biocompatible, environmentally friendly, and low-energy visible-light initiation.