UCST or LCST? Composition-Dependent Thermoresponsive Behavior of Poly(<i>N</i>‑acryloylglycinamide-<i>co</i>-diacetone acrylamide)

Copolymerization has been widely used to tune the thermoresponsive behavior of water-soluble polymers. However, the observation of both upper and lower critical solution temperature (UCST and LCST) from the same type of copolymer comprising only one monomer whose homopolymer is thermosensitive and the other monomer whose homopolymer is nonthermosensitive has not been reported. In this work, well-defined thermoresponsive copolymers with tunable compositions have been synthesized by copolymerization of <i>N</i>-acryloylglycinamide (NAGA) and diacetone acrylamide (DAAM) via reversible addition–fragmentation chain transfer (RAFT) polymerization. The thermal transitions of these copolymers are investigated using a combination of turbidimetry, dynamic light scattering (DLS), proton nuclear magnetic resonance (¹H NMR), and Fourier transform infrared (FTIR) spectroscopy. The solubility of these copolymers shows a distinct dependence on the composition. While copolymers with up to 30 mol % NAGA are essentially insoluble, copolymers with 35–55 mol % NAGA or 90–100 mol % NAGA have either LCST- or UCST-type transitions respectively, and soluble copolymers are obtained with 60–85 mol % NAGA. The LCST- and UCST-type transitions are tunable with respect to composition, degree of polymerization, polymer concentration, isotope effect and the presence of electrolyte. Insights from variable-temperature ¹H NMR and FTIR spectroscopies reveal the key role of hydrogen-bonding between the NAGA and DAAM units in determining the thermal transitions

Aqueous Polymerization-Induced Self-Assembly for the Synthesis of Ketone-Functionalized Nano-Objects with Low Polydispersity

Efficient synthesis of functionalized, uniform polymer nano-objects in water with controlled morphologies in one step and at high concentrations is extremely attractive, from perspectives of both materials applications and industrial scale-up. Herein, we report a novel formulation for aqueous reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization based on polymerization-induced self-assembly (PISA) to synthesize ketone-functionalized nanospheres and vesicles. Significantly, the core-forming block was composed entirely of a ketone-containing polymer from a commodity monomer diacetone acrylamide (DAAM), resulting in a high density of ketone functionality in the nano-objects. Producing uniform vesicles represents another challenge both in PISA and in the traditional self-assembly process. Aiming at producing uniform nano-objects, especially vesicles, in such a highly efficient aqueous PISA process, we devised strategies to allow sufficient time for the in situ generated polymers to relax and reorganize into vesicles with a remarkably low polydispersity. Specifically, both reducing the radical initiator concentration and lowering the polymerization temperature were shown to be effective for improving the uniformity of vesicles. Such an efficient, aqueous PISA to produce functionalized and uniform nano-objects with controlled morphologies at solid contents up to 20% represents important progress in the field

Exploring the Volume Phase Transition Behavior of POEGA- and PNIPAM-Based Core–Shell Nanogels from Infrared-Spectral Insights

The volume phase transition behavior of well-defined thermally responsive poly­(2-methoxyethyl acrylate-<i>co</i>-poly­(ethylene glycol) methyl ether acrylate)/poly­(<i><i>N,N</i></i>′-dimethylacrylamide) (P­(MEA-<i>co</i>-PEGA)/PDMA) and poly­(<i>N</i>-isopropylacrylamide)/poly­(<i><i>N,N</i></i>′-dimethylacrylamide) (PNIPAM/PDMA) core–shell nanogels, synthesized via reversible addition–fragmentation chain transfer (RAFT) mediated aqueous dispersion polymerization, is studied and compared by applying FTIR measurements in combination with two-dimensional correlation spectroscopy (2Dcos). Analysis through spectral insights clearly illustrates that the continuous dehydration of the CO groups in the P­(MEA-<i>co</i>-PEGA)/PDMA nanogel core predominates the linear volume phase transition while the hydrogen bonding transformation in the PNIPAM/PDMA nanogel core leads to the abrupt decrease in nanogel size on heating. Additionally, considering the core and the shell separately, the data shows that, for both nanogels, the inner core contributes much more to the volume phase transition and the outer shell only undergoes slight dehydration following the core on heating

One-Enzyme Triple Catalysis: Employing the Promiscuity of Horseradish Peroxidase for Synthesis and Functionalization of Well-Defined Polymers

We demonstrate a new concept in polymer chemistry that the promiscuity of enzymes, as represented by horseradish peroxidase, can be employed for RAFT polymerization and thiol–ene and Diels–Alder reactions to synthesize well-defined functional polymers, via three different catalytic reactions mediated by one single enzyme

Single Monomer for Multiple Tasks: Polymerization Induced Self-Assembly, Functionalization and Cross-Linking, and Nanoparticle Loading

Efficient preparation of multifunctional nano-objects with controlled morphologies in one step at high concentrations is synthetically challenging, yet is highly desirable, in a broad range of materials applications. Herein, we address this synthetic hurdle by introducing a single commodity monomer 2-(acetoacetoxy)­ethyl methacrylate (AEMA) to realize multiple functions. Facile preparation of both nanospheres and vesicles via polymerization induced self-assembly at concentrations of 20–30% provided defined polymeric nanomaterials with reactive handles inherent to the AEMA units. High-yielding keto-alkoxylamine chemistry was utilized to decorate and cross-link the nano-objects. Nanoparticle loading into the designated location within both nano-objects was exemplified with in situ formation of silver nanoparticles. The concept of using a single monomer capable of both morphology control and multifunctionalization is expected to offer significant opportunities in functional nanomaterials

Photocontrolled RAFT Polymerization Mediated by a Supramolecular Catalyst

A photocontrolled reversible addition–fragmentation chain transfer (RAFT) polymerization mediated by a supramolecular photoredox catalyst is reported. Cucurbit[7]­uril (CB[7]) was used to form a host–guest complex with Zn­(II) meso-tetra­(4-naphthalylmethylpyridyl) porphyrin (ZnTPOR) to prevent aggregation of ZnTPOR, which in combination with a chain transfer agent (CTA) initiated efficient and controlled RAFT polymerization in water under visible light. RAFT polymerization was significantly affected by the subtle interplay of host–guest, electrostatic, and steric interactions among CB[7], ZnTPOR, and CTA. Polymerization rate was remarkably improved using CB[7]@ZnTPOR in comparison with that using ZnTPOR. The use of supramolecular interactions to modulate photocontrolled RAFT polymerization provides new opportunities to manipulate controlled radical polymerizations

Temperature-Induced Morphological Transitions of Poly(dimethylacrylamide)–Poly(diacetone acrylamide) Block Copolymer Lamellae Synthesized via Aqueous Polymerization-Induced Self-Assembly

Aqueous dispersion polymerization of diacetone acrylamide (DAAM) by chain extension from a hydrophilic poly­(<i>N</i>,<i>N</i>-dimethyl­acrylamide) (PDMA₃₀) macromolecular chain transfer agent (macro-CTA) to produce PDMA₃₀–PDAAM_<i>x</i> block copolymer nano-objects was investigated in detail by systematically varying solids content and degree of polymerization of the core-forming PDAAM, leading to the formation of pure lamellae, mixed lamellae/vesicles, and pure vesicles as revealed by dynamic light scattering (DLS), transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). PDMA₃₀–PDAAM_<i>x</i> lamellae were found to span an unprecedented wide space in the morphology phase diagram. Moreover, in situ cross-linking of lamellae via statistical copolymerization of DAAM with an asymmetric cross-linker allyl acrylamide and the effect of cross-linking density on the colloidal and morphological stabilities were studied, representing the first report on in situ cross-linking of lamellae during polymerization-induced self-assembly (PISA). Finally, reversible, temperature-induced morphological transitions from lamellae to worms/spheres on cooling were investigated by DLS, TEM, ¹H NMR spectroscopy, and rheology. The kinetics of the temperature-dependent morphological transitions and the rheological properties could be tuned by the cross-linking density

In Situ Cross-Linking of Vesicles in Polymerization-Induced Self-Assembly

In situ cross-linking of nano-objects with controllable morphologies in polymerization-induced self-assembly (PISA) has been a challenge because cross-linking lowers chain mobility and hence inhibits morphology transition. Herein, we propose a novel strategy that allows in situ cross-linking of vesicles in PISA in an aqueous dispersion polymerization formulation. This is realized by utilizing an asymmetric cross-linker bearing two vinyl groups of differing reactivities such that cross-linking is delayed to the late stage of polymerization when morphology transition has completed. Cross-linked vesicles with varying degrees (1–5 mol %) of cross-links were prepared, and their resistance to solvent dissolution and surfactant disruption was investigated. It was found that vesicles with ≥2 mol % cross-links were able to retain their structural integrity and colloidal stability when dispersed in DMF or in the presence of 1% of an anionic surfactant sodium dodecyl sulfate

Polymerization-Induced Cooperative Assembly of Block Copolymer and Homopolymer via RAFT Dispersion Polymerization

Polymerization-induced cooperative assembly (PICA) is developed to promote morphological transitions at high solids via RAFT dispersion polymerization, using both a macromolecular chain transfer agent (macro-CTA) and a small molecule chain transfer agent (CTA) to generate nano-objects consisting of well-defined block copolymer and homopolymer. PICA is demonstrated to promote morphological transitions under various conditions. Elemental mapping provides unambiguous evidence for the uniform distribution of the homopolymer within the core of the nano-objects. It is proposed that the growing homopolymer first reaches its solubility limit and forms aggregates, which induce the adsorption of the growing block copolymer. This effective and robust PICA approach significantly expands the capability to promote morphological transitions in RAFT dispersion polymerization and will facilitate the efficient synthesis of various higher-order morphologies at high solids

In Situ Cross-Linking as a Platform for the Synthesis of Triblock Copolymer Vesicles with Diverse Surface Chemistry and Enhanced Stability via RAFT Dispersion Polymerization

An intrinsic dilemma exists for block copolymer vesiclesimproving the colloidal stability of vesicles using long/charged stabilizing blocks lowers the propensity of morphological transition to vesicles. Moreover, maintaining the vesicular morphology requires effective structure stabilization via cross-linking. We report a strategy to circumvent this problem and simultaneously improve the colloidal and structural stability of vesicles synthesized via polymerization-induced self-assembly (PISA) using dispersion polymerization. More specifically, in situ cross-linked poly­(<i>N</i>,<i>N</i>-dimethyl­acrylamide)-<i>b</i>-poly­(diacetone acrylamide-<i>co</i>-allylacrylamide) diblock copolymer vesicles are first synthesized via aqueous dispersion polymerization, which then serve as a robust platform to initiate the growth of a third hydrophilic block of either neutral poly­(<i>N</i>,<i>N</i>-dimethyl­acrylamide), anionic poly­(2-acrylamido-2-methyl-1-propane­sulfonic acid sodium salt), or cationic poly­(3-acrylamido­propyl trimethyl­ammonium chloride) with retained vesicular morphology. The formed cross-linked triblock copolymer vesicles have advantages of diverse surface chemistry and arbitrary stabilizing block length. As a control experiment, synthesis from linear diblock copolymer vesicles provides a mixture of triblock copolymer vesicles and spheres. The successful synthesis of triblock copolymer vesicles with a binary mixture of two hydrophilic stabilizing blocks is supported by dynamic light scattering (DLS), transmission electron microscopy (TEM), electrophoresis, and X-ray photoelectron spectroscopy (XPS). Both linear and cross-linked triblock copolymer vesicles are subjected to solvent dissolution, freeze-drying, and surfactant challenge studies, which collectively demonstrate that cross-linked triblock copolymers can maintain their vesicular structure and show excellent colloidal and structural stability, as indicated by DLS, TEM, and transmittance measurements