Emily A. Hoff

Dynamic-covalent nanostructures prepared by Diels–Alder reactions of styrene-maleic anhydride-derived copolymers obtained by one-step cascade block copolymerization

Macromolecular star formation by Diels–Alder chemistry resulted in dynamic nanomaterials capable of reversibly demonstrating the properties of both linear and highly branched macromolecules. Well-defined block copolymers of maleic anhydride (MAn) and styrene [poly(styrene-alt-MAn)-b-polystyrene (P(S-alt-MAn)-b-PS)] were prepared via a one-pot cascade approach by reversible addition–fragmentation chain transfer (RAFT) polymerization. Subsequent ring opening of the anhydride groups in the P(S-alt-MAn) segments by amidation with furfurylamine led to the formation of block copolymers with pendant furan functionality. Diels–Alder reactions of the furan-functional block copolymer with a bismaleimide crosslinker resulted in core-crosslinked stars by an arm-first approach. Star-like structures were also prepared by first allowing the furan-functional block copolymers to pre-assemble into polymeric micelles in a solvent selective for the polystyrene block. Subsequent addition of a bismaleimide and heating to allow the Diels–Alder reaction resulted in core-crosslinked micelles with similar structures to the polymeric stars prepared by the arm-first approach. Regardless of the synthetic approach employed, the thermoreversibility of the Diels–Alder linkages within the cores rendered the stars/crosslinked micelles dynamic-covalent, as demonstrated by their ability to reversibly dissociate back to the individual arms on heating.

Stimuli-Responsive Peptide-Based ABA-Triblock Copolymers: Unique Morphology Transitions With pH

We report the synthesis and solution characterization of poly(L-lysine)-b-poly(propylene oxide)-b-poly(L-lysine) (KPK) triblock copolymers with high lysine weight fractions (>75 wt%). In contrast to PK diblock copolymers in this composition range, KPK triblock copolymers exhibit morphology transitions as a function of pH. Using a combination of light-scattering and microscopy techniques, we demonstrate spherical micelle-vesicle and spherical micelle-disk micelle transitions for different K fractions. We interpret these morphology changes in terms of the energy penalty associated with folding the core P block to form a spherical micelle in relation to the interfacial curvature associated with different charged states of the K block.

Rapid Synthesis of Polymer Brush Surfaces via Microwave-Assisted Surface-Initiated Radical Polymerization

Microwave-assisted surface-initiated radical polymerization (μW-SIP) is demonstrated for the rapid synthesis of polymer brush surfaces on two-dimensional substrates. μW-SIP is carried out at constant temperature and microwave power allowing comparison with conventional SIP carried out in an oil bath at the same effective solution temperature. We show μW-SIP enables significant enhancements (up to 39-fold increase) in brush thickness at reduced reaction times for a range of monomer types (i.e. acrylamides, acrylates, methacrylates, and styrene). The effects of reaction time, monomer concentration, and microwave power on film thickness are explored.

A robust and high-throughput measurement platform for monomer reactivity ratios from surface-initiated polymerization

This article describes a robust approach to measure monomer reactivity ratios from surface-initiated copolymerization, by measuring composition of statistical copolymer brush surfaces using X-ray photoelectron spectroscopy. Statistical copolymer brushes were prepared from various monomer feeds by surface-initiated radical copolymerization at room temperature under ultraviolet (UV) irradiation. The copolymer brush composition data were fit to the terminal copolymerization kinetic model resulting in point estimates for the monomer reactivity ratios that are in good agreement with values measured under bulk reaction conditions. Additionally, a high-throughput approach was demonstrated to measure reactivity ratios using a single substrate exhibiting a gradient in copolymer brush composition. This high-throughput approach significantly reduces the time and effort required to generate reliable and reproducible point estimates of reactivity ratios, and these values are in good agreement with values obtained from both the discrete sample surface measurements and classical bulk analytical methods.

Aqueous RAFT at pH zero: enabling controlled polymerization of unprotected acyl hydrazide methacrylamides

We report aqueous RAFT polymerization at pH = 0 mediated by a novel imidazolium-containing chain transfer agent. In 1 M HCl, unprecedented controlled polymerization and chain-extension of unprotected acyl hydrazide methacrylamides is achieved enabling the synthesis of well-defined acyl hydrazide functionalized polymer scaffolds of interest for dynamic covalent and bioconjugation strategies.