Phase transition behavior of unimolecular dendritic three-layer nanostructures with dual thermoresponsive coronas is studied. Successive reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide (NIPAM) and 2-(dimethylamino)ethyl methacrylate (DMA) were conducted using fractionated fourth-generation hyperbranched polyester (Bolton H40) based macroRAFT agent. At lower temperatures (<20 degrees C), dendritic macromolecules H40-poly(N-isopropylacrylamide)-poly(2-(dimethylamino)ethyl methacrylate) (H40-PNIPAM-PDMA) exist as unimolcular core-shell-corona nanostructures with hydrophobic H40 as the core, swollen PNIPAM as the inner shell, and swollen PDMA as the corona. PNIPAM and PDMA homopolymers undergo phase transitions at their lower critical solution temperatures (LCST), which are found to be 32 degrees C for PNIPAM and 40-50 degrees C for PDMA, respectively. Upon continuously heating through the LCSTs of PNIPAM and PDMA, such dendritic unimolecular micelles exhibit two-stage thermally induced collapse. This process is reversible with a two-stage reswelling upon cooling. Laser light scattering, micro-differential scanning calorimetry, and excimer fluorescence measurements are used to investigate the double phase transitions.

This paper describes the double phase transition behavior of a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush at the surface of a hydrophobic core. Reversible addition-fragmentation transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) was conducted by using a hyperbranched polyester (Boltorn H40) based macroRAFT agent. The resultant multiarm star block copolymer (H40-PNIPAM) exists as unimolecular micelles with hydrophobic H40 as the core, densely grafted PNIPAM brush as the shell. A combination of laser light scattering (LLS) and microdifferential scanning calorimetry (micro-DSC) studies of H40-PNIPAM in aqueous solution reveals double phase transitions of the PNIPAM corona, which is in contrast to the fact that free PNIPAM homopolymer in aqueous solution exhibits a lower critical solution temperature (LCST) at approximately 32 degrees C. The first phase transition takes place in the broad temperature range 20-30 degrees C, which can be tentatively ascribed to the n-cluster-induced collapse of the inner region of the PNIPAM brush close to the H40 core; the second phase transition occurs above 30 degrees C, which can be ascribed to the outer region of PNIPAM brush. Employing the RAFT chain extension technique, the inner and outer part of PNIPAM brush were then selectively labeled with pyrene derivatives, respectively; temperature-dependent excimer fluorescence measurements further support the conclusion that the inner part of PNIPAM brush collapses first at lower temperatures, followed by the collapse of the outer part at higher temperatures.

A triblock copolymer, poly(ethylene glycol)-b-poly(glycerol monomethacrylate)-b-poly(2-(diethylamino)ethyl methacrylate) (PEG−PGMA−PDEA), was synthesized via atom transfer radical polymerization (ATRP) by successive polymerization of glycerol monomethacrylate (GMA) and 2-(diethylamino)ethyl methacrylate (DEA) using a PEG-based ATRP macroinitiator. Reacting the obtained triblock copolymer with varying amounts of cinnamoyl chloride in anhydrous pyridine yielded PEG−(PCGMA-co-PGMA)−PDEA triblock copolymer with photo-cross-linkable moieties, where PCGMA is poly(3-cinnamoyl glycerol monomethacrylate) and the mean degree of cinnamoylation ranges from 5 to 50 mol % relative to the PGMA block. All PEG−(PCGMA-co-PGMA)−PDEA triblock copolymers molecularly dissolve in aqueous media at acidic pH; upon addition of NaOH, micellization occurred above pH 7−8 to form three-layer “onionlike” micelles comprising PDEA cores, PCGMA-co-PGMA inner shells, and PEG outer coronas. The pH-induced micellization kinetics of PEG113−(CGMA0.5-co-GMA0.5)50−DEA65 triblock copolymers was investigated by stopped-flow light scattering upon a pH jump from 3 to 10, and compared to that of PEG113−PGMA50−PDEA65. Facile cross-linking of the PCGMA-co-PGMA inner shell was then conducted via UV irradiation. The PDEA cores of the resulting shell cross-linked (SCL) micelles exhibited reversible pH-responsive behavior. The extent of pH-induced swelling/shrinking and the colloidal stability of SCL micelles were mainly determined by the extent of cross-linking. The dissociation kinetics of the triblock copolymer micelles before and after shell cross-linking was also investigated employing the stopped-flow technique. It was found that SCL micelles prepared at higher degrees (>20 mol %) of cross-linking exhibited excellent colloidal stability to external pH changes.

We describe the first account of the synthesis and intriguing micellization properties of nonlinear double hydrophilic block copolymers (DHBCs) of the A 2 BA 2 and A 4 BA 4 type. Atom transfer radical polymerization (ATRP) macroinitiators with two and four initiating sites at both ends of poly(propylene oxide) (PPO) chain were synthesized via reacting 2-hydroxyethyl acrylate and glycidol with commercially available diamine-terminated PPO, respectively, followed by esterification with excess 2-bromoisobutyryl bromide or 2-bromopropionyl bromide. Well-defined nonlinear DHBCs, (PDEA) 2 PPO(PDEA) 2 (H-shaped) and (PDEA) 4 PPO(PDEA) 4 (star-b-linear-bstar), were then prepared by polymerizing 2-(diethylamino)ethyl methacrylate (DEA) via ATRP in 2-propanol at 40 °C using the prepared macroinitiators, where PDEA was poly(2-(diethylamino)ethyl methacrylate). The structures of the resulting nonlinear shaped copolymers were characterized by 1 H NMR and gel permeation chromatography (GPC). The pH-and thermoresponsive micellization behavior of (PDEA 10 ) 2 PPO(PDEA 10 ) 2 and (PDEA 11 ) 4 PPO-(PDEA 11 ) 4 was then investigated by a combination of dynamic laser light scattering (LLS) and fluorescence techniques. Compared to the linear counterpart, the nonlinear block copolymers exhibited complex and interesting micellization properties.

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Antimicrobial peptides (AMPs) are considered as potential antibiotic substitutes because of their potent activities. Previous studies mainly focused on the effects of peptide charges and secondary structures, but the self-assembly of AMPs was neglected. As more and more researchers notice the roles of peptide self-assembly in AMPs, it has been considered as another important property. In this review, we will discuss the influences of peptide self-assembly on the activity and mode of action, and some specific features it introduces to the AMPs, such as particular responsiveness, improved cell selectivity and stability and sustained release. In addition, some methods to design self-assembling AMPs are primarily discussed. With further understanding about the self-assembling regularity, design of particular self-assembling AMPs will be very helpful for their applications, especially in the fields of drug delivery and biomedical engineering.

Thermoresponsive polymer-encapsulated silica hybrid nanoparticles were fabricated via self-assembling of block copolymer in aqueous solution into micelles and subsequent sol-gel process inside the micellar core. Poly(N-isopropylacrylamide)-b-poly(γ-methacryloxypropyltrimethoxysilane) (PNIPAMb-PMPS) was prepared by successive reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide (NIPAM) and γ-methacryloxypropyltrimethoxysilane (MPS) in 1,4-dioxane. In aqueous solution, amphiphilic PNIPAM-b-PMPS self-assembles into micelles with PMPS core and PNIPAM shell. Base-catalyzed sol-gel process inside PMPS core results in PNIPAM-encapsulated silica hybrid core-shell nanoparticles. Transmission electron microscopy (TEM), dynamic light scattering (DLS), and static light scattering (SLS) studies reveal monodisperse hybrid nanoparticles with densely grafted PNIPAM brush at the surface of silica core. Grafted PNIPAM brush shows thermoresponsive two-stage collapse upon heating. Because of the high conversion of RAFT polymerization of MPS, we also show that a one-pot synthesis of thermoresponsive hybrid nanoparticles is feasible. This is the first report of stimuli-responsive hybrid core-shell nanoparticles via the block copolymer self-assembling approach.

Using a narrowly distributed poly(N-isoporpylacrylamide) (PNIPAM) chain with a degree of polymerization (N) of 3100, randomly labeled with pyrene, we have, for the first time, observed the two-stage kinetics of the coil-to-globule transition. Two characteristic relaxation times, tau(fast) for the crumpling of a random coil (approximately 12 ms) and tau(slow) for the collapsing of the crumpled chain to a compact globule (approximately 270 ms), were measured. To our knowledge, this is the first experimental evidence supporting the two-stage collapsing kinetics of single synthetic polymer chain previously proposed by de Gennes, Dawson, and Grosberg.