Mechanically and Thermally Robust Ordered Nanoporous Monoliths Using Norbornene-Functional Block Polymers

Next-Generation Ultrafiltration Membranes Enabled by Block Polymers

Reliable and equitable access to safe drinking water is a major and growing challenge worldwide. Membrane separations represent one of the most promising strategies for the energy-efficient purification of potential water sources. In particular, porous membranes are used for the ultrafiltration (UF) of water to remove contaminants with nanometric sizes. However, despite exhibiting excellent water permeability and solution processability, existing UF membranes contain a broad distribution of pore sizes that limit their size selectivity. To maximize the potential utility of UF membranes and allow for precise separations, improvements in the size selectivity of these systems must be achieved. Block polymers represent a potentially transformative solution, as these materials self-assemble into well-defined domains of uniform size. Several different strategies have been reported for integrating block polymers into UF membranes, and each strategy has its own set of materials and processing considerations to ensure that uniform and continuous pores are generated. This Review aims to summarize and critically analyze the chemistries, processing techniques, and properties required for the most common methods for producing porous membranes from block polymers, with a particular focus on the fundamental mechanisms underlying block polymer self-assembly and pore formation. Critical structure-property-performance metrics will be analyzed for block polymer UF membranes to understand how these membranes compare to commercial UF membranes and to identify key research areas for continued improvements. This Review is intended to inform readers of the capabilities and current challenges of block polymer UF membranes, while stimulating critical thought on strategies to advance these technologies.

Block copolymers for designing nanostructured porous coatings

Highly ordered porous coatings find applications in many fields, such as nanotechnology, microfluidics and nanofluidics, membrane separation, and sensing. In recent years, there has been great interest regarding the synthesis of isoporous and well-ordered (in)organic coatings for the production of highly selective functional membranes. Among the different strategies that have been proposed to date for preparing these porous thin coatings, one simple route involves the use of self-assembled amphiphilic block copolymers either as the porogen (acting as sacrificial templating agents for the production of inorganic architectures) or as a source of the porogen (by self-assembly for the production of polymeric substrates). Therefore, an extended discussion around the exploitation of block copolymers is proposed here in this review, using polystyrene-block-polyethylene oxide (PS-b-PEO) as the model substrate, and critical points are highlighted.

Synthesis of ordered, functional, robust nanoporous membranes from liquid crystalline brush-like triblock copolymers

The fabrication of mechanically robust nanoporous polymeric membrane materials with aligned, accessible functionalized nanopores is extremely important for many applications.

Photoinitiated Polymerization-Induced Microphase Separation for the Preparation of Nanoporous Polymer Films

We report on the use of photoinitiated reversible addition-fragmentation chain transfer (RAFT) polymerization for the facile fabrication of cross-linked nanoporous polymer films with three-dimensionally (3D) continuous pore structure. The photoinitiated polymerization of isobornyl acrylate (IBA) in the presence of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (CTA) and 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator proceeded in a controlled manner, yet more rapidly compared to thermally initiated polymerization. When polylactide-macroCTA (PLA-CTA) was used, PLA-b-PIBA with high molar mass was obtained after several minutes of irradiation at room temperature. We confirmed that microphase separation occurs in the PLA-b-PIBA and that nanoporous PIBA can be derived from the PLA-b-PIBA precursor by selective PLA etching. To fabricate the cross-linked nanoporous polymer, IBA was copolymerized with ethylene glycol diacrylate (EGDA) in the presence of PLA-CTA to produce a cross-linked block polymer precursor consisting of bicontinuous PLA and P(IBA-co-EGDA) microdomains, via polymerization-induced microphase separation. We demonstrated that nanoporous P(IBA-co-EGDA) monoliths and films with 3D continuous pores can be readily obtained via this approach.

Molecular Design of Liquid Crystalline Brush-Like Block Copolymers for Magnetic Field Directed Self-Assembly: A Platform for Functional Materials

We report on the development of a liquid crystalline block copolymer with brush-type architecture as a platform for creating functional materials by magnetic-field-directed self-assembly. Ring-opening metathesis of n-alkyloxy cyanobiphenyl and polylactide (PLA) functionalized norbornene monomers provides efficient polymerization yielding low polydispersity block copolymers. The mesogenic species, spacer length, monomer functionality, brush-chain length, and overall molecular weight were chosen and optimized to produce hexagonally packed cylindrical PLA domains which self-assemble and align parallel to an applied magnetic field. The PLA domains can be selectively removed by hydrolytic degradation resulting in the production of nanoporous films. The polymers described here provide a versatile platform for scalable fabrication of aligned nanoporous materials and other functional materials based on such templates.

Exploiting the tetrazine-norbornene reaction for single polymer chain collapse

Single chain polymer nanoparticles (SCNPs) have been formed using polystyrenes decorated with pendent norbornenes and a bifunctional tetrazine crosslinker. Characterisation by size exclusion chromatography and (1)H NMR gives evidence for the formation of SCNPs by the tetrazine-norbornene reaction, whilst light scattering, neutron scattering, transmission electron microscopy and atomic force microscopy show that discrete well-defined nanoparticles are formed and their size in solution calculated.

One-Step Synthesis of Cross-Linked Block Polymer Precursor to a Nanoporous Thermoset

Using a simultaneous block polymerization/in situ cross-linking from a heterofunctional initiator approach, we produced a nanostructured and cross-linked block polymer in a single step from a ternary mixture of monomers and used it as a precursor for a cross-linked nanoporous material. Using 2-(benzylsulfanylthiocarbonylsulfanyl)ethanol as a heterofunctional initiator, simultaneous ring-opening transesterification polymerization of d,l-lactide in the presence of tin 2-ethylhexanoate as a catalyst and reversible addition-fragmentation chain transfer polymerization of styrene at 120 °C produced a polylactide-b-polystyrene (PLA-b-PS) block polymer. Incorporation of divinylbenzene in the polymerization mixture allowed in situ cross-linking during the simultaneous block polymerization to result in the cross-linked block polymer precursor in one step. This material was converted into cross-linked nanoporous polymer by etching PLA in a basic solution.

Functionalized nanoporous polyethylene derived from miscible block polymer blends

Functionalized nanoporous polyethylene (PE) was prepared through controlled introduction of thermo-responsive poly[2-(2-methoxyethoxy)ethyl methacrylate] (PMe(OE)(2)MA), or poly{2-[2-(2-methoxyethoxy)ethoxy]ethyl methacrylate} (PMe(OE)(3)MA) onto the pore walls. The compatibility of polylactide (PLA) and PMe(OE)(x)MA (x = 2, 3) was investigated by blending the corresponding homopolymers. The blends showed only one glass transition when the molar masses of both components were relatively low, whereas two glass transitions were observed in case of higher molar mass samples. PMe(OE)(x)MA-b-PE-b-PMe(OE)(x)MA (x = 2, 3) block polymers were synthesized by a combination of ring-opening metathesis polymerization, atom transfer radical polymerization, and hydrogenation. Those block polymer blends formed a disordered bicontinuous structure consisting of a mixed PLA/PMe(OE)(x)MA domain and a semicrystalline PE domain. The PLA component was selectively removed from those blends by mild base treatment. The resulting nanoporous polyethylene showed an improved water uptake as a result of the hydrophilic PMe(OE)(x)MA on the pore walls.

A "Mix-and-Click" Approach to Double Core-Shell Micelle Functionalization

A micellar scaffold formed by self-assembly of a reversible addition-fragmentation chain transfer (RAFT)-synthesized amphiphilic diblock copolymer has been prepared to contain two orthogonal click-compatible functionalities in the core and shell. These functionalities (norbornenes in the core and terminal alkynes in the shell) have been used as handles to modify the micellar assembly in the core using tetrazine-norbornene chemistry or the shell using the copper-catalyzed azide-alkyne reaction. Additionally, both core and shell modifications were carried out in a tandem, one-pot process using the orthogonal chemistries mentioned above. In all cases the reactions were found to be highly efficient, requiring little excess of the modifying small molecule and very simple to perform under ambient conditions.

Living Radical Polymerization by the RAFT Process – A Third Update

This paper provides a third update to the review of reversible deactivation radical polymerization (RDRP) achieved with thiocarbonylthio compounds (ZC(=S)SR) by a mechanism of reversible addition-fragmentation chain transfer (RAFT) that was published in June 2005 (Aust. J. Chem. 2005, 58, 379). The first update was published in November 2006 (Aust. J. Chem. 2006, 59, 669) and the second in December 2009 (Aust. J. Chem. 2009, 62, 1402). This review cites over 700 publications that appeared during the period mid 2009 to early 2012 covering various aspects of RAFT polymerization which include reagent synthesis and properties, kinetics and mechanism of polymerization, novel polymer syntheses, and a diverse range of applications. This period has witnessed further significant developments, particularly in the areas of novel RAFT agents, techniques for end-group transformation, the production of micro/nanoparticles and modified surfaces, and biopolymer conjugates both for therapeutic and diagnostic applications.