Synthesis, Folding, and Association of Long Multiblock (PEO₂₃-b-PNIPAM₁₂₄)₇₅₀ Chains in Aqueous Solutions

Abstract

A thermally sensitive ultralong multiblock copolymer, [poly(ethylene oxide) 23 -b-poly(N-isopropylacrylamide) 124 ] 750 (M w ) 1.78 × 10 7 g/mol and M z /M w ) 1.49), was prepared using the oxidative coupling of two mercapto groups at the two ends of triblock PNIPAM 62 -b-PEO 23 -b-PNIPAM 62 (M n,PEO ) 1.0 × 10 3 g/mol and M n,PNIPAM ) 1.4 × 10 4 g/mol) copolymer chains. The folding of individual multiblock copolymer chains in an extremely dilute solution (10 -6 g/mL) was studied by laser light scattering (LLS). Moreover, the association of multiblock and triblock copolymer chains in relatively concentrated aqueous solutions (10 -3 g/mL) was also comparatively studied by a combination of LLS, fluorescence spectrometry, and microcalorimetry. We found that in the single-chain folding process the average radius of gyration (〈R g 〉) remains a constant in one heatingand-cooling cycle, but the average hydrodynamic radius (〈R h 〉) decreases as the solution temperature increases. Our result reveals that the single-chain folding undergoes two stages at ∼32 and ∼40 °C, presumably due to the successive contraction of thermally sensitive PNIPAM segments in the middle around each hydrophobic S-S coupling point and near the hydrophilic PEO block. Each PNIPAM block collapses into a small globule (bead) stabilized by the two attached PEO blocks on the chain backbone, a string-bead conformation, which makes the chain thicker and more extended. The association of multiblock chains also undergoes similar two stages to form stable mesoglobules during the heating. In contrast, the triblock chains associate at ∼30 °C to form polymeric micelles.

Oxidatively Responsive Chain Extension to Entangle Engineered Protein Hydrogels

“…Recently, responsive disulfide formation has been used to stabilize self-assembled nanofibers from peptide amphiphiles 44 and to chemically crosslink coiled-coil domains in artificially engineered protein hydrogels. 45 Although high molecular weight synthetic copolymers have been prepared by oxidation of thiols on the polymer chain ends, 46 to the best of our knowledge, this strategy has not been applied to synthesize high molecular weight protein gels.…”

Synthesis of poly(N-isopropylacrylamide)-b-poly(2-vinylpyridine) block copolymers via RAFT polymerization and micellization behavior in aqueous solution

“…R g / R h ranges between 0.83 and 1.22, which is much higher than that of the typical spherical micelle (∼0.775) [31] (Table S1 in supplementary material). It is probably due to the formation of loosely packed P2VP-core spherical micelles [32,33].…”

Smart Nanohybrids of RAFT Polymers and Inorganic Particles

“…[151] For the system pNIPAm/water/methanol, it is for example speculated that water pentamers are formed with methanol by hydrogen bonding, [216] but most of the formed structures have only been predicted theoretically, since they are very hard to detect experimentally with currently used methods. [214,217] These structures are then assumed to be poorer solvents for the polymer than the two pure solvents. [216,[218][219][220] Put simply, there are just less water molecules available to hydrate the polymer.…”

“…[213] The reaction is illustrated in Scheme 5.6. On a side note, these polymers could theoretically be employed as macro-RAFT agents in a second radical polymerization with another monomer [214] to yield multisegmented copolymers, [215][216][217][218] polymers with alternating homogeneous segments composed of the two monomers. Such macromolecules, which are otherwise difficult to synthesize, [219] reveal a unique selfassembly behavior, [220][221][222] such as microphase separation, [223][224][225] when the individual segments are sufficiently long [226,227] and have distinctly different properties.…”