We establish the relationship between pendant group chemical identity and the ability of a specific radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), to transport charge in the solid state. Radical polymers (i.e., macromolecules with aliphatic carbon backbones and pendant groups containing stable radical moieties) have attracted much attention in organic electronic applications due to straightforward synthetic methods, easily tunable electronic properties, and relatively high-performance with respect to charge transport. Because charge transport can occur only through the pendant group of these completely amorphous radical polymers, controlling the precise chemical nature of these functional groups is of key import. Specifically, we have determined that the deprotection step, which converts the pendant group functionality through a simple oxidation reaction, can lead to four distinct chemical functionalities along the radical polymer, as monitored by a range of complementary spectroscopic techniques. Of these four functionalities, only two (i.e., the stable free radical and the corresponding oxoammonium cation) are able to contribute positively to the charge transport ability of the macromolecule. As such, manipulating the reaction conditions for this deprotection step, and monitoring closely the resultant chemical functionalities, is critical in tuning the electrical properties of radical polymers. However, if these parameters are controlled well, we are able to generate transparent, conducting thin films of pristine (i.e., not doped) nonconjugated radical polymers with electrical conductivities as high as (1.5 ± 0.3) × 10–5 S cm–1.

Radical polymers (i.e., macromolecules composed of a nonconjugated polymer backbone and with stable radical sites present on the side chains of the repeat units) can transport charge in the solid state through oxidation−reduction (redox) reactions that occur between the electronically localized open-shell pendant groups. As such, pristine (i.e., not doped) thin films of these functional macromolecules have electrical conductivity values on the same order of magnitude as some common electronically active conjugated polymers. However, unlike the heavily evaluated regime of conjugated polymer semiconductors, the impact of molecular dopants on the optical, electrochemical, and solid-state electronic properties of radical polymers has not been established. Here, we combine a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), with a small molecule redox-active salt, 4-acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate (TEMPOnium), in order to elucidate the effect of molecular doping on this emerging class of functional macromolecular thin films. Note that the TEMPOnium salt was specifically selected because the cation in the salt has a very similar molecular architecture to that of an oxidized repeat unit of the PTMA polymer. Importantly, we demonstrate that the addition of the TEMPOnium salt simultaneously alters the electrochemical environment of the thin film without quenching the number of openshell sites present in the PTMA-based composite thin film. This environmental alteration changes the chemical signature of the PTMA thin films in a manner that modifies the electrical conductivity of the radical polymer-based composites. By decoupling the ionic and electronic contributions of the observed current passed through the PTMA-based thin films, we are able to establish how the presence of the redox-active TEMPOnium salts affects both the transient and steady-state transport abilities of doped radical polymer thin films. Additionally, at an optimal loading (i.e., doping density) of the redox-active salt, the electrical conductivity of PTMA increased by a factor of 5 relative to that of pristine PTMA. Therefore, these data establish an underlying mechanism of doping in electronically active radical polymers, and they provide a template by which to guide the design of nextgeneration radical polymer composites.

Radical polymers are an emerging class of electronically active macromolecules; however, the fundamental mechanism by which charge is transferred in these polymers has yet to be established in full. To address this issue, well‐defined norbornene‐based nitroxide radical polymers were synthesized using the controlled ring‐opening metathesis polymerization technique. These polymers were blended in solution with a quenched, electrically insulating hydroxylamine derivative to dilute the radical content of the system. Electron paramagnetic resonance spectroscopy data were used to characterize the radical content as well as to reveal that hydrogen atom transfer occurred between the open‐shell and closed‐shell polynorbornene derivatives when they were blended in solution. Using these platform macromolecules, we demonstrate that the systematic manipulation of the radical content in open‐shell macromolecules leads to exponential changes in the macroscopic electrical conductivity. When coupled with the fact that these materials show a clear temperature‐independent charge transport behavior, a picture emerges that charge transfer in radical polymers is dictated by a tunneling mechanism between localized donor and acceptor sites within the redox‐active thin films. These results constitute the first experimental insight into the mechanism of solid‐state electrical conduction in radical polymers, and this provides a design paradigm for open‐shell macromolecular charge transport. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1516–1525

Congenital aortic valve disease is a life-long condition that can require multiple interventions. It is one of the most common causes of congenital heart defect, with bicuspid aortic valve present in at least 1−2% of the general population. Surgical management of congenital aortic valve disease consists of either valve repair or replacement. While aortic valve replacement using the Ross procedure can be considered the gold standard management in the pediatric population, advancements in aortic valve repair techniques have proved its usefulness as an initial management approach as it prevents prosthesis-related complications and patient-prostheses mismatch while the patient grows. Overall, all techniques have their benefits and limitations in terms of growth potential, durability of repair, freedom from reoperation and anticoagulation, infection risk, and mortality. Each patient will require an individualized judiciously selected management plan to minimize the number of interventions over their lifetime. The aim of this review is to discuss the merits and drawbacks of the major techniques currently used in both aortic valve repair and replacement.