RESUMO
As many physical properties of polymers scale with molecular weight, the ability to achieve polymers of nearly inaccessibly high molecular weight provides an opportunity to probe the upper size limit of macromolecular phenomena. Yet many of the most stimulating macromolecular designs remain out of reach of current ultrahigh molecular weight (UHMW) polymer synthetic approaches. Herein, we show that UHMW polymers of diverse composition can be achieved by irradiation of thiocarbonylthio photoiniferters with long-wave ultraviolet or visible light in concentrated organic solution. This facile photopolymerization strategy is general to acrylic-, acrylamido-, methacrylic-, and styrenic-based monomers, enabling the synthesis of well-defined macromolecules with molecular weights in excess of 106 g/mol. The high chain-end fidelity afforded by photoiniferter polymerization conditions facilitated the design of UHMW amphiphilic block copolymers, which were found to self-assemble into micellar morphologies up to 200 nm in diameter.
RESUMO
An initiator- and catalyst-free method for polymer end-group modification has been designed. Under long-wave ultraviolet irradiation, polymers with thiocarbonylthio end groups undergo photolytic cleavage to reveal an active macroradical capable of irreversible termination with a suitable hydrogen source. This straightforward method was successfully demonstrated by the removal of a range of end groups that commonly result from reversible addition-fragmentation chain transfer or photoiniferter polymerizations, including trithiocarbonate, dithiobenzoate, xanthate, and dithiocarbamate mediating agents. This strategy proved efficient for polymers derived from acrylamido, acrylic, methacrylic, styrenic, and vinylpyrrolidone monomers.
RESUMO
A biphasic one-pot polymerization method enables the preparation of block copolymers from monomers with similar and competitive reactivities without the addition of external materials. AB diblock copolymers were prepared by encapsulating a frozen solution of monomerâ B on the bottom of a reaction vessel, while the solution polymerization of monomerâ A was conducted in a liquid layer above. Physical separation between the solid and liquid phases permitted only homopolymerization of monomerâ A until heating above the melting point of the lower phase, which released monomerâ B, allowing the addition of the second block to occur. The triggered release of monomerâ B allowed for chain extension without additional deoxygenation steps or exogenous monomer addition. A method for the closed (i.e., without addition of external reagents) one-pot synthesis of block copolymers with conventional glassware using straightforward experimental techniques has thus been developed.
RESUMO
The surface-localized hyperthermia of gold nanoparticles under microwave irradiation was examined. Gold nanoparticles with a hydrodynamic diameter of â¼6 nm stabilized by polymeric "thermometers" were used to gather information on the extent of heating as well as its spatial confinements. Reversible addition-fragmentation chain transfer polymerization was employed to synthesize well-defined, functional polymers of predetermined molecular weights, allowing for estimation of the distance between the nanoparticle surface and the polymer chain end. The polymers were conjugated with a fluorescent dye separated by a thermally-labile azo linkage, and these polymeric ligands were bound to gold nanoparticles via gold-thiolate bonds. Conventional heating experiments elucidated the relationship between temperature and the extent of dye release from the gold nanoparticle using fluorescence spectroscopy. The local temperature increase experienced under microwave irradiation was calculated using the same methodology. This approach indicated the temperature near the surface of the nanoparticle was nearly 70 °C higher than the bulk solution temperature, but decreased rapidly with distance, with no noticeable temperature increase when the azo linkage was approximately 2 nm away.