The effect of polymer architecture on molecular exchange in block copolymer micelles has been investigated using time-resolved small-angle neutron scattering (TR-SANS). Narrowly dispersed symmetric PEP−PS−PEP and PS−PEP−PS triblock copolymers were synthesized, where PS and PEP refer to poly(styrene) and poly(ethylene-alt-propylene), respectively. Spherical micelles of the triblocks in squalane, a selective solvent for the PEP blocks, were prepared using a cosolvent method. PEP−PS− PEP forms "hairy" micelles with the PS blocks looped in the cores, while PS−PEP−PS forms "flower-like" micelles with most of the PEP blocks looped in the corona. The micelle structure was characterized by small-angle X-ray scattering, providing in particular the core radius as a function of temperature. TR-SANS experiments were conducted on solutions containing 1 and 6 vol % PEP−PS− PEP, and 0.25 and 0.5 vol % PS−PEP−PEP, using matched pairs of deuterium-labeled (dPS) and normal (hPS) specimens and a mixture of normal and perdeuterated squalane contrast-matched to uniformly mixed hPS/dPS micelle cores. Blends of micelles with initially pure hPS and dPS cores produce scattering intensity that decays with the redistribution of block copolymer chains as a function of time, providing direct access to the rate of molecular exchange. Remarkably, the two triblock architectures display exchange rates that differ by approximately 9 orders of magnitude, even though the solvophobic PS blocks are of comparable size. This discovery is considered in the context of a model that successfully explained the exchange dynamics in PS−PEP diblock copolymer micelles.

Shell cross-linked micelles (SCMs) containing Co(III)-salen cores were prepared from amphiphilic poly(2-oxazoline) triblock copolymers. The catalytic activity of these nanoreactors for the hydrolytic kinetic resolution of various terminal epoxides was investigated. The SCM catalysts showed high catalytic efficiency and, more significantly, substrate selectivity based on the hydrophobic nature of the epoxide. Moreover, because of the nanoscale particle size and the high stability, the catalyst could be recovered easily by ultrafiltration and reused with high activity for eight cycles.

In Nature, incompatible catalytic transformations are being carried out simultaneously through compartmentalization that allows for the combination of incompatible catalysts in tandem reactions. Herein, we take the compartmentalization concept to the synthetic realm and present an approach that allows two incompatible transition metal catalyzed transformations to proceed in one pot in tandem. The key is the site isolation of both catalysts through compartmentalization using a core-shell micellar support in an aqueous environment. The support is based on amphiphilic triblock copolymers of poly(2-oxazoline)s with orthogonal functional groups on the side chain that can be used to cross-link covalently the micelle and to conjugate two metal catalysts in different domains of the micelle. The micelle core and shell provide different microenvironments for the transformations: Co-catalyzed hydration of an alkyne proceeds in the hydrophobic core, while the Rh-catalyzed asymmetric transfer hydrogenation of the intermediate ketone into a chiral alcohol occurs in the hydrophilic shell.

The mechanism of chain exchange in block polymer micelles at equilibrium is investigated using time-resolved small-angle neutron scattering (TR-SANS). The binary micelles are formed from blends of two poly(styrene-b-ethylene-alt-propylene) (PS-PEP) copolymers with different PS core block lengths, only one of which is contrast-matched with the solvent, squalane, so that the monitored scattering intensity only reflects the other species. Micelles prepared with an excess of deuterated PS chains (of the visible species) and those with the equivalent protonated PS chains are blended (“postmixed”) at room temperature, where the exchange of chains is suppressed. At several elevated temperatures these samples were monitored by TR-SANS, in which mixing of isotope-labeled visible species gives systematic reduction of scattering signals with time and provides a quantitative way to characterize the micelle exchange kinetics. Within experimental error, the results for each labeled chain (i.e., longer or shorter) in the binary micelles are identical to those recently reported for the same labeled chains in the corresponding single block copolymer component micelles, thus proving that chain exchange in these micelles involves independent chain motion. This reinforces the important conclusions that the single-chain exchange mechanism dominates in the studied micelle solutions and that micelle fusion or fission events are rare.

The role of core block size dispersity on the rate of molecular exchange in spherical micelles formed from 1% by volume poly(styrene-b-ethylenepropylene) (PS-PEP) diblock copolymers in squalane (C 30 H 62 ) was investigated using timeresolved small-angle neutron scattering (TR-SANS). Separate copolymer solutions (total polymer 1% by volume) containing either deuterium labeled (dPS) or normal (hPS) poly(styrene) core blocks were prepared and mixed at room temperature, below the core glass transition temperature. Each preparation (dPS or hPS) contained equal volume fractions of M n = 26 and 42 kg/mol (h-equivalent) poly(styrene) blocks. Heating to temperatures between 87 and 146 °C resulted in block copolymer exchange as evidenced by a systematic reduction in the SANS intensity; C 30 H 62 and C 30 D 62 were blended so as to contrast match the fully exchanged cores. Following a protocol established in a previous report, the time-dependent intensity data were shifted with respect to time and temperature, leading to a master curve covering nearly 7 orders of magnitude in reduced time. These results are quantitatively accounted for by summing the weighted relaxation functions obtained from the individual components, consistent with a previously published model that accounts for the dramatic sensitivity of the molecular exchange dynamics to core block dispersity.

Shell cross-linked micelles (SCMs) containing acid sites in the shell and base sites in the core are prepared from amphiphilic poly(2-oxazoline) triblock copolymers. The materials are utilized as two-chamber nanoreactors for a prototypical acid− base bifunctional tandem deacetalization−nitroaldol reaction. The acid and base sites are localized in different regions of the micelle, allowing the two steps in the reaction sequence to largely proceed in separate compartments, akin to the compartmentalization that occurs in biological systems.

The effect of corona block length on micelle structure and molecular chain exchange kinetics has been investigated for a series of dilute poly­(styrene)-b-poly­(ethylene-alt-propylene) (SEP) diblock copolymer micelles with constant PS core block length (⟨N core⟩ ≈ 255) but different PEP corona block lengths (⟨N corona⟩ = 256–2080), using a combination of dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and time-resolved small-angle neutron scattering (TR-SANS). Smaller core radii and aggregation numbers, but significantly thicker corona layers (proportional to N corona 0.7), were observed with increasing corona block length. Furthermore, 2 orders of magnitude more rapid chain exchange was observed in SEP micelles with the longest corona block compared to the shortest. This effect is attributed to the entropic gain arising from the relief of corona chain stretching upon chain expulsion. We further extend a previous theoretical model by explicitly including a corona term associated with the entropy change in the chain exchange process, which successfully explains the influence of the corona blocks on chain exchange. Our results are in excellent agreement with simulation results of Li and Dormidontova but are apparently contradictory with Halperin and Alexander’s theory for hairy micelles and with experimental observations in two other systems that the exchange kinetics slow down with increasing corona block length. These discrepancies reveal unanticipated complexity regarding the role of the corona block in chain exchange.

The exchange of copolymer chains between 1 vol % PS–PEP (poly­(styrene-b-ethylene-alt-propylene)) diblock copolymer micelles in squalane (selective for PEP) is investigated using time-resolved small-angle neutron scattering (TR-SANS) as a function of added PEP homopolymer. The solvent squalane, C30H62, is substituted in part or completely with PEP homopolymers that are the same molecular weight as the corona blocks. Polymer solutions/mixtures (1 vol % PS–PEP, plus 2, 7, or 15 vol % PEP in squalane, and 1 vol % PS–PEP in PEP) were separately prepared using normal (h-PS) or deuterated equivalent (d-PS) PS–PEP diblock copolymers. The solvent was contrast matched to a 50/50 mixed h-/d-PS micelle core, so that the scattering intensity decays with the mixing of h- and d-PS–PEP chains undergoing exchange between micelles. The chain exchange rate can therefore be assessed quantitatively. As the concentration of added homopolymer in solution increases above the overlap concentration of PEP chains, the chain exchange rate drops significantly. The results are compared to an earlier study of chain exchange between PS–PEP micelles in a 15% solution in squalane, which was also found to be significantly slower than when the solution is dilute. The primary factor in this slowing down of chain exchange is an increased screening of excluded volume interactions among the corona blocks. The role of increasing micelle aggregation number with PEP concentration is found not to be the dominant effect up to 15% added PEP but may play an increasingly important role in the PEP melt matrix, where no chain exchange could be detected in these experiments.