Christopher P. Kabb

Thermally-labile segmented hyperbranched copolymers: using reversible-covalent chemistry to investigate the mechanism of self-condensing vinyl copolymerization

A thermally-reversible inimer was used to confirm the controlled growth of individual branches during self-condensing vinyl atom transfer radical polymerization (ATRP). Segmented hyperbranched polymers were synthesized by ATRP of methyl methacrylate (MMA) and a novel inimer that contained a thermally labile Diels–Alder linkage between its initiating and polymerizable moieties. Three distinct feed ratios of MMA to inimer (15 : 1, 30 : 1, and 60 : 1) yielded hyperbranched polymers with variable degrees of branching and molecular weights in the range of 120 000 to 515 000 g mol−1. The resulting hyperbranched polymers contained thermally-reversible branch points that were cleaved quantitatively on heating to yield linear polymers with molecular weights that were similar to the theoretical values that would be expected based on controlled chain growth of individual branches during self-condensing vinyl polymerization (SCVP). The cleaved linear polymers contained pendant furan and terminal maleimide functionalities that allowed reassembly at 50 °C to form “healed” hyperbranched polymers. The healing efficiency was determined by 1H NMR spectroscopy, and the molecular weights of the repaired hyperbranched polymers were characterized by gel permeation chromatography. A segmented hyperbranched polymer was employed as a multifunctional macroinitiator to prepare an amphiphilic “hyper-star” via chain extension with poly(ethylene glycol) methyl ether methacrylate. Assembly of these “hyper-stars” into well-defined micelles (∼23 nm) in neutral water was confirmed by transmission electron microscopy and dynamic light scattering.

Self-assembled micro-organogels for 3D printing silicone structures

High-precision 3D printing of liquid silicone is achieved using a new oil-based microgel as a support medium. The widespread prevalence of commercial products made from microgels illustrates the immense practical value of harnessing the jamming transition; there are countless ways to use soft, solid materials that fluidize and become solid again with small variations in applied stress. The traditional routes of microgel synthesis produce materials that predominantly swell in aqueous solvents or, less often, in aggressive organic solvents, constraining ways that these exceptionally useful materials can be used. For example, aqueous microgels have been used as the foundation of three-dimensional (3D) bioprinting applications, yet the incompatibility of available microgels with nonpolar liquids, such as oils, limits their use in 3D printing with oil-based materials, such as silicone. We present a method to make micro-organogels swollen in mineral oil, using block copolymer self-assembly. The rheological properties of this micro-organogel material can be tuned, leveraging the jamming transition to facilitate its use in 3D printing of silicone structures. We find that the minimum printed feature size can be controlled by the yield stress of the micro-organogel medium, enabling the fabrication of numerous complex silicone structures, including branched perfusable networks and functional fluid pumps.

Poly(N-(2-hydroxypropyl)methacrylamide)–valproic acid conjugates as block copolymer nanocarriers

We report nanoassemblies based on block copolymers of N-(2-hydroxypropyl) methacrylamide (HPMA) in which drug cleavage enhances the biological compatibility of the original polymer carrier by regeneration of HPMA units. Drug release via ester hydrolysis suggests this approach offers potential for stimuli-responsive drug delivery under acidic conditions.

Photoreversible Covalent Hydrogels for Soft-Matter Additive Manufacturing

Reversible covalent chemistry provides access to robust materials with the ability to be degraded and reformed upon exposure to an appropriate stimulus. Photoresponsive units are attractive for this purpose, as the spatial and temporal application of light is easily controlled. Coumarin derivatives undergo a [2 + 2] cycloaddition upon exposure to long-wave UV irradiation (365 nm), and this process can be reversed using short-wave UV light (254 nm). Therefore, polymers cross-linked by coumarin groups are excellent candidates as reversible covalent gels. In this work, copolymerization of coumarin-containing monomers with the hydrophilic comonomer N, N-dimethylacrylamide yielded water-soluble, linear polymers that could be cured with long-wave UV light into free-standing hydrogels, even in the absence of a photoinitiator. Importantly, the gels were reverted back to soluble copolymers upon short-wave UV irradiation. This process could be cycled, allowing for recycling and remolding of the hydrogel into additional shapes. Further, this hydrogel can be imprinted with patterns through a mask-based, post-gelation photoetching method. Traditional limitations of this technique, such as the requirement for uniform etching in one direction, have been overcome by combining these materials with a soft-matter additive manufacturing methodology. In a representative application of this approach, we printed solid structures in which the interior coumarin-cross-linked gel is surrounded by a nondegradable gel. Upon exposure to short-wave UV irradiation, the coumarin-cross-linked gel was reverted to soluble prepolymers that were washed away to yield hollow hydrogel objects.