Impact of Backbone Composition on Homopolymer Dynamics and Brush Block Copolymer Self-Assembly.

Four series of brush block copolymers (BBCP), with near identical side chain compositions but varying backbone structures, were synthesized to investigate the effect of backbone structure on the process of thermal BBCP self-assembly to photonic crystals (PCs). Each of the self-assembled PC films were examined by reflection measurements, small angle X-ray scattering measurements, and scanning electron microscopy to compare the resulting properties of the polymeric photonic crystal and the nanostructured morphology impacted by the backbone structure. It was found that the composition of the brush backbone within a BBCP has a dramatic effect on the ability of the BBCP to self-assemble into ordered nanostructures and on the local ordering of the nanostructure morphology accessed with higher molecular weight (MW) BBCPs (> 1,500 kg/mol). BBCPs with a norbornene imide-based backbone were able to thermally self-assemble to longer wavelength reflecting PCs and had higher fidelity ordering of lamellar nanostructures with higher MW polymers. By analyzing the melt rheological responses of the backbone compositions, both as linear polymers and homobrush polymers, it was concluded that the inherent fragility of the backbone promotes enhanced local ordering in the lamellar nanostructure morphology as well as access to larger domain sizes.

Solvent Effects on the Intramolecular Charge Transfer Character of N,N-Diaryl Dihydrophenazine Catalysts for Organocatalyzed Atom Transfer Radical Polymerization.

The nature of intramolecular charge transfer of N,N-diaryl dihydrophenazine photocatalysts (PCs) in different solvents is explored in context of their performance in organocatalyzed atom transfer radical polymerization (O-ATRP). PCs having a computationally predicted lowest energy excited state exhibiting charge transfer (CT) character can operate a highly controlled O-ATRP in a wide range of solvent polarities, from non-polar hexanes to highly polar N,N-dimethylacetamide. For PCs having a computationally predicted lowest energy excited state not possessing CT character, their ability to operate a controlled O-ATRP is decreased. This study confirms the importance of CT character in the excited state for N,N-diaryl dihydrophenazine PCs, and a deeper understanding of the activity of CT PCs has enabled the synthesis of polymers of low dispersity ( < 1.10) in a controlled fashion.

Strongly Reducing, Visible-Light Organic Photoredox Catalysts as Sustainable Alternatives to Precious Metals.

Photoredox catalysis is a versatile approach for the construction of challenging covalent bonds under mild reaction conditions, commonly using photoredox catalysts (PCs) derived from precious metals. As such, there is need to develop organic analogues as sustainable replacements. Although several organic PCs have been introduced, there remains a lack of strongly reducing, visible-light organic PCs. Herein, we establish the critical photophysical and electrochemical characteristics of both a dihydrophenazine and a phenoxazine system that enables their success as strongly reducing, visible-light PCs for trifluoromethylation reactions and dual photoredox/nickel-catalyzed C-N and C-S cross-coupling reactions, both of which have been historically exclusive to precious metal PCs.

Impact of Light Intensity on Control in Photoinduced Organocatalyzed Atom Transfer Radical Polymerization.

Organic photoredox catalysts have been shown to operate organocatalyzed atom transfer radical polymerizations (O-ATRP) using visible light as the driving force. In this work, the effect of light intensity from white LEDs was evaluated as an influential factor in control over the polymerization and the production of well-defined polymers. We posit the irradiation conditions control the concentrations of various catalyst states necessary to mediate a controlled radical polymerization. Systematic dimming of white LEDs allowed for consideration of the role of light intensity on the polymerization performance. The general effects of decreased irradiation intensity in photoinduced O-ATRP were investigated through comparing two different organic photoredox catalysts: perylene and an 3,7-di(4-biphenyl) 1-naphthalene-10-phenoxazine. Previous computational efforts have investigated catalyst photophysical and electrochemical characteristics, but the broad and complex effects of varied irradiation intensity as an experimental variable on the mechanism of O-ATRP have not been explored. This work revealed that perylene requires more stringent irradiation conditions to achieve controlled polymer molecular weight growth and produce polymers with dispersities <1.50. In contrast, the 3,7-di(4-biphenyl) 1-naphthalene-10-phenoxazine is more robust, achieving linear polymer molecular weight growth under relative irradiation intensity as low as 25%, to produce polymers with dispersities <1.50. This finding is significant, as the discovery of highly robust catalysts is necessary to allow for the adoption of successful O-ATRP in a wide scope of conditions, including those which necessitate low light intensity irradiation.

Intramolecular Charge Transfer and Ion Pairing in N,N-Diaryl Dihydrophenazine Photoredox Catalysts for Efficient Organocatalyzed Atom Transfer Radical Polymerization.

Photoexcited intramolecular charge transfer (CT) states in N,N-diaryl dihydrophenazine photoredox catalysts are accessed through catalyst design and investigated through combined experimental studies and density functional theory (DFT) calculations. These CT states are reminiscent of the metal to ligand charge transfer (MLCT) states of ruthenium and iridium polypyridyl complexes. For cases where the polar CT state is the lowest energy excited state, we observe its population through significant solvatochromic shifts in emission wavelength across the visible spectrum by varying solvent polarity. We propose the importance of accessing CT states for photoredox catalysis of atom transfer radical polymerization lies in their ability to minimize fluorescence while enhancing electron transfer rates between the photoexcited photoredox catalyst and the substrate. Additionally, solvent polarity influences the deactivation pathway, greatly affecting the strength of ion pairing between the oxidized photocatalyst and the bromide anion and thus the ability to realize a controlled radical polymerization. Greater understanding of these photoredox catalysts with respect to CT and ion pairing enables their application toward the polymerization of methyl methacrylate for the synthesis of polymers with precisely tunable molecular weights and dispersities typically lower than 1.10.

Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst.

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The procedure is performed under an inert atmosphere using air- and water-exclusion techniques. The outcome of the polymerization is affected by the ratios of monomer, initiator, and catalyst used as well as the reaction concentration, solvent, and nature of the light source. Temporal control over the polymerization can be exercised by turning the visible light source off and on. Low dispersities of the resultant polymers as well as the ability to chain-extend to form block copolymers suggest control over the polymerization, while chain end-group analysis provides evidence supporting an atom-transfer radical polymerization mechanism.

Organocatalyzed atom transfer radical polymerization driven by visible light.

Atom transfer radical polymerization (ATRP) has become one of the most implemented methods for polymer synthesis, owing to impressive control over polymer composition and associated properties. However, contamination of the polymer by the metal catalyst remains a major limitation. Organic ATRP photoredox catalysts have been sought to address this difficult challenge but have not achieved the precision performance of metal catalysts. Here, we introduce diaryl dihydrophenazines, identified through computationally directed discovery, as a class of strongly reducing photoredox catalysts. These catalysts achieve high initiator efficiencies through activation by visible light to synthesize polymers with tunable molecular weights and low dispersities.

Adhesional small bowel obstruction after colorectal surgery.

BACKGROUND: Acute clinical indicators of complications in colorectal surgery can be readily attained but it is much harder to gather indicators of long-term outcomes such as small bowel obstruction (SBO). However, with improved in-hospital data collection and coding such information is becoming available. The aim of the present study was to examine our own hospital database for SBO rates post-colorectal surgery. METHODS: The database was searched and then the relevant medical records were reviewed of all patients admitted to Flinders Medical Centre (Bedford Park, Australia) between July 1999 and November 2002 with a diagnosis of SBO following a colorectal procedure during this same time period. RESULTS: There were 21 patients that accounted for 28 readmissions from a total colorectal procedure group of 583 patients. The arbitrary subgroups were: 13/325 (4%) for colonic resections; 7/186 (3.7%) for rectal resections; 0/12 (0%) for laparotomies/other procedures; and 1/39 (2.6%) for stoma formation/reversals. The overall SBO rate requiring readmission was therefore 3.6%/pt in the 3 years. There was a large variation in the first readmission interval, 38% occurred within 3 months, 43% between 3 and 12 months, and 19% after 1 year. At the first readmission 38% of patients had operative treatment. The mean length of stay was 6.12 days for non-operative vs 21.62 days for operative treatment. CONCLUSION: The reported rate of SBO of 3.6% (at 3 years time interval) is in accordance with other studies. With 38% of patients being treated operatively at first admission there is good acceptance for conservative management in non-strangulated SBO. The prolonged hospital stay for patients needing surgery warrants further investigation.