Yuting Li

RAFT synthesis of sterically stabilized methacrylic nanolatexes and vesicles by aqueous dispersion polymerization.

Emulsion polymerization is widely used for waterborne coatings. Reducing the latex particle size is known to promote coalescence and hence enhance film formation. However, the synthesis of smaller latexes usually requires additional surfactant, which can compromise the quality of waterborne coatings (e.g. poor adhesion and reduced film quality due to migration of excess surfactant). In principle, reactive surfactants offer a potentially decisive advantage over conventional surfactants in emulsion polymerization because they bind irreversibly to the latex and hence cannot migrate during film formation; this allows defect-free coatings to be produced with reduced moisture sensitivity. Over the last two decades, controlled/living radical polymerization techniques have become powerful tools in polymer synthesis. There are many examples of latex syntheses based on these approaches. For example, nitroxide-mediated living radical polymerization has been used by Charleux, El-Aasser, Okubo, and Georges to mediate the miniemulsion polymerization of n-butyl acrylate and styrene. ATRP has been optimized by Matyjaszewski and Okubo for the (mini)emulsion polymerization of (meth)acrylic and styrene monomers. Reversible addition– fragmentation chain transfer (RAFT) polymerization has been extensively exploited in the context of both emulsion and miniemulsion polymerization by Hawkett, Charleux, El-Aasser, Cunningham, and Zhu. There are also a number of RAFT syntheses conducted under nonaqueous dispersion polymerization conditions. However, as far as we are aware, there are only three examples of the application of controlled/living radical polymerization techniques for latex syntheses by aqueous dispersion polymerization. In each case, a relatively expensive speciality monomer was utilized for the latex core, namely N-isopropylacrylamide or N,N’-diethylacrylamide. This relative lack of research is perhaps surprising, because aqueous dispersion polymerization is conceptually much simpler than aqueous emulsion polymerization since the initial reaction solution is homogeneous in the former case. Presumably, the paucity of experimental data merely reflects the fact that there are relatively few vinyl monomers that are suitable for latex syntheses by aqueous dispersion polymerization. Recently, we reported the use of conventional (nonliving) free radical chemistry for the aqueous dispersion polymerization of a commodity methacrylic monomer, 2hydroxypropyl methacrylate (HPMA). The resulting PHPMA latexes were stabilized by poly(N-vinylpyrrolidone) and the mean particle diameter could be varied from approximately 100 to 1000 nm diameter, with good control over the particle size distribution being achieved in most cases. Herein we explore the RAFT synthesis of sterically stabilized PHPMA nanolatexes of 20 to 100 nm diameter by surfactant-free aqueous dispersion polymerization using a poly(glycerol monomethacrylate)-based chain transfer agent (CTA) as the reactive steric stabilizer. Thus both the latex cores and the steric stabilizer chains of the resulting nanolatexes are highly hydroxylated for this prototype formulation. Moreover, varying the length of the targeted PHPMA chains allows the final size of the sterically stabilized nanolatex particles to be controlled quite precisely (see Scheme 1, Table 1, and the Supporting Information).

Substrate-directed formation of calcium carbonate fibres

This article describes the formation of fibres of calcium carbonate on a range of substrates in the presence of a polyacid diblock copolymer, and it is demonstrated that the morphologies and structures of the fibres can be controlled by judicious selection of both the substrate and the reaction conditions. While fibres precipitated on single crystals of calcite and aragonite proved to be single crystals of calcite, those formed on glass and mica were amorphous and frequently displayed remarkable helical morphologies. Investigation of the fibre growth mechanism(s) suggests that all formed via the self-assembly of polymer-stabilised precursor units. Particles forming the calcite single-crystal fibres were elongated calcite crystallites, and fibre formation was interpreted in terms of an oriented assembly process, where anisotropy in the precursor unit morphology and surface distribution of copolymer chains promoted one-dimensional assembly. In contrast, the particles that formed amorphous fibres were spherical and amorphous, with fibre formation being attributed to polarisation of these copolymer-rich particles. This work therefore demonstrates that directional aggregation processes can be applied to both amorphous and crystalline units, opening up the possibility of using block copolymers to control the morphologies of a very wide range of materials.

Shell Cross-Linked Micelles as Cationic Templates for the Preparation of Silica-Coated Nanoparticles: Strategies for Controlling the Mean Particle Diameter

The mean diameter of poly[2-(dimethylamino)ethyl methacrylate]-block-poly[2-(diisopropylamino)ethyl methacrylate] (PDMA-PDPA) diblock copolymer micelles can be easily adjusted from 27-155 nm (as measured by DLS) by either selective quaternisation of the PDMA block or by adding PDPA homopolymer prior to micellisation; these self-assembled nanostructures can be shell crosslinked with 1,2-bis-(2-iodoethoxy)ethane and subsequently used as templates for the preparation of silica-coated nanoparticles and, ultimately, hollow silica nanoparticles.

Use of quaternised methacrylate polymers and copolymers as catalysts and structure directors for the formation of silica from silicic acid

A range of quaternised tertiary amine methacrylate-based homopolymers and copolymers were synthesised as mimics of the biopolymers implicated in biosilica formation. These synthetic polymers were evaluated for their ability to catalyse and direct the structure of silica formed by condensation of silicic acid in aqueous solution and at neutral pH. Homo- and co-polymers of differing degrees of quaternisation were studied, while some of the homopolymers also differed in their chain length. All polymers acted as catalysts for the condensation reaction, but at different rates according to their architecture and degree of quaternisation. The resulting silica–polymer hybrids were characterised fully, as were pure silicas obtained by calcination of the hybrids. Some crystallites were present in the hybrids and differences in crystal structure were observed in the calcined silicas, depending on the structure of the polymer, indicating that the polymers exert a structure-directing effect during initial silica formation. The work provides some new insights into structural factors affecting silica growth catalysed by synthetic cationic polymers.

Biomimetic thermo-responsive star diblock gelators.

We report the synthesis of novel biomimetic gelators with star diblock copolymer architectures by sequential monomer addition via alcoholic ATRP at 20 degrees C; free-standing gels can be formed from 5% aqueous copolymer solutions at 37 degrees C.