Design, synthesis, and aqueous aggregation behavior of nonionic single and multiple thermoresponsive polymers.

Nonionic water-soluble poly(acrylamide)s and poly(acrylate)s were synthesized by RAFT and ATRP methods. Similar to the synthesized poly(N-isopropylacrylamide) and poly(N-acryloylpyrrolidine), aqueous solutions of statistical acrylate copolymers bearing two different oligo(ethylene oxide) side chains showed a sharp clouding transition upon heating beyond characteristic temperatures. The temperature of the cloud point can be easily fine tuned by the copolymer composition. As for poly(N-isopropylacrylamide) and poly(N-acryloylpyrrolidine), the cloud-point temperatures of these statistical copolymers are rather insensitive to changes in the molar mass or the NaCl content of the solutions. Also, ternary triblock copolymers containing one permanently hydrophilic block and two different thermoresponsive blocks were synthesized, varying the block sequence systematically. Their aggregation in aqueous solution was followed by turbidimetry and dynamic light scattering. Depending on the heating process and the triblock sequence, micellar aggregates of 40 to 600 nm size were found. The thermally induced aggregation behavior depends sensitively on the block sequence but is also subject to major kinetic effects. For certain block sequences, a thermally induced two-step association is observed when heating beyond the first and second cloud points of the thermoresponsive blocks. However, the thermal-transition temperatures of the block polymers can differ from the thermal-transition temperatures of the individual homopolymers. This may be caused by end-group effects but also by mutual interactions of the different blocks in solution, as physical mixtures of the homopolymers exhibit deviations from a purely additive thermal behavior.

One-pot synthesis of pegylated ultrasmall iron-oxide nanoparticles and their in vivo evaluation as magnetic resonance imaging contrast agents.

A well-defined copolymer poly(oligo(ethylene glycol) methacrylate-co-methacrylic acid) P(OEGMA-co-MAA) was studied as a novel water-soluble biocompatible coating for superparamagnetic iron oxide nanoparticles. This copolymer was prepared via a two-step procedure: a well-defined precursor poly(oligo(ethylene glycol) methacrylate-co-tert-butyl methacrylate), P(OEGMA-co-tBMA) (M(n) = 17300 g mol(-1); M(w)/M(n) = 1.22), was first synthesized by atom-transfer radical polymerization in the presence of the catalyst system copper(I) chloride/2,2'-bipyridyl and subsequently selectively hydrolyzed in acidic conditions. The resulting P(OEGMA-co-MAA) was directly utilized as a polymeric stabilizer in the nanoparticle synthesis. Four batches of ultrasmall PEGylated magnetite nanoparticles (i.e., with an average diameter below 30 nm) were prepared via aqueous coprecipitation of iron salts in the presence of variable amounts of P(OEGMA-co-MAA). The diameter of the nanoparticles could be easily tuned in the range 10-25 nm by varying the initial copolymer concentration. Moreover, the formed PEGylated ferrofluids exhibited a long-term colloidal stability in physiological buffer and could therefore be studied in vivo by magnetic resonance (MR) imaging. Intravenous injection into rats showed no detectable signal in the liver within the first 2 h. Maximum liver accumulation was found after 6 h, suggesting a prolongated circulation of the nanoparticles in the bloodstream as compared to conventional MR imaging contrast agents.

Point by point comparison of two thermosensitive polymers exhibiting a similar LCST: is the age of poly(NIPAM) over?

The present Communication compares the thermosensitivity in dilute aqueous solutions of well-defined copolymers composed of 95% of 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and 5% of oligo(ethylene glycol) methacrylate (OEGMA, Mn = 475 g.mol-1) and poly(N-isopropylacrylamide) (PNIPAM) samples having similar degrees of polymerization and chain-ends. The thermoresponsive behavior of P(MEO2MA-co-OEGMA) was found to be overall comparable, and in some cases, superior to PNIPAM. Hence, P(MEO2MA-co-OEGMA) copolymers can be considered as ideal structures, which combine both the properties of poly(ethylene glycol) and PNIPAM in a single macromolecule.