Stable,High‐Molecular‐Weight Poly(phthalaldehyde)

Low ceiling temperature, thermodynamically unstable polymers have been troublesome to synthesize and keep stable during storage. In this study, stable poly(phthalaldehyde) has been synthesized with BF₃‐OEt₂ catalyst. The role of BF₃ in the polymerization is described. The interaction of BF₃ with the monomer is described and used to maximize the yield and molecular weight of poly(phthalaldehyde). Various Lewis acids were used to investigate the effect of catalyst acidity on poly(phthalaldehyde) chain growth. In situ nuclear magnetic resonance was used to identify possible interactions formed between BF₃ and phthalaldehyde monomer and polymer. The molecular weight of the polymer tracks with polymerization yield. The ambient temperature stability of poly(phthalaldehyde) was investigated and the storage life of the polymer has been improved. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 55, 1166–1172     

Photodegradable transient bilayered poly(phthalaldehyde) with improved shelf life

Metastable poly(phthalaldehyde) (PPHA) can be triggered to depolymerize under visible light by incorporation of photosensitive compounds, such as a photoacid generator (PAG), which can generate a strong acid in situ. However, photosensitive compounds can be thermally unstable and have limited shelf life, causing inadvertent device triggering. It can also be difficult to fabricate components that are photosensitive because special lighting conditions are needed. In this paper, nonphotosensitive PPHA films were formed and made photosensitive at the point of use. This improved the material shelf life and manufacturability by adding a second, PAG‐containing layer to the original nonphotosensitive layer at an optimal point before use. The catalytic photoacid was generated rapidly by exposure of the PAG‐containing layer to radiation. Depolymerization of PPHA via the acid catalyst was followed by diffusion of the acid into the nonphotosensitive layer causing it to depolymerize. Diffusion of the photoacid into the nonphotosensitive medium was quantified at various temperatures. Photoacid diffusion in a liquid, moving‐front caused depolymerization of the nonphotosensitive PPHA layer. The fabricated bilayer structure allowed for better stability of the structural material using PPHA while still achieving transience.     

Time‐delayed photo‐induced depolymerization of poly(phthalaldehyde) self‐immolative polymer via in situ formation of weak conjugate acid

Poly(phthalaldehyde) (PPHA) can be used as a structural material in transient devices and photo‐catalytically depolymerized at the end of device life by the use of a photo‐acid generator (PAG). However, device degradation requires the presence of a radiation source at the end of device mission. It has been found that the onset of PPHA depolymerization after PAG photo‐exposure can be delayed by incorporation of a particular weak bases in the PPHA/PAG mixture. This method of delayed PPHA depolymerization allows for PAG activation prior to or during device deployment when the device is under full user control. The basicity of specific lactams and amides was found to slow the PPHA depolymerization, giving the transient device a longer but finite mission lifetime. The weak base reacts with the photo‐generated strong acid to form a weak conjugate acid, which reacts more slowly with PPHA to extend the onset of PPHA depolymerization. The addition of a molar excess of specific lactams or amides, with respect to PAG, maintains PPHA stability and mechanical properties for more than 80 minutes after photo‐exposure at room temperature. The amide or lactam mediated acid activation of PPHA follows first‐order kinetics. The time delay of PPHA depolymerization can allow for prelaunch photo‐exposure and eliminates the need for postmission photo‐exposure where reliable light‐sources may not be available.     

A Rare Low‐Spin CoIV Bis(β‐silyldiamide) with High Thermal Stability: Steric Enforcement of a Doublet Configuration

Attempted preparation of a chelated Co^II β‐silylamide resulted in the unprecedented disproportionation to Co⁰ and a spirocyclic cobalt(IV) bis(β‐silyldiamide): [Co[(N^tBu)₂SiMe₂]₂] ( 1 ). Compound 1 exhibited a room‐temperature magnetic moment of 1.8 B.M. and a solid‐state axial EPR spectrum diagnostic of a rare S= configuration for tetrahedral Co^IV. Ab initio semicanonical coupled‐cluster calculations (DLPNO‐CCSD(T)) revealed the doublet state was clearly preferred (?27 kcal mol^?1) over higher spin configurations only for the bulky tert‐butyl‐substituted analogue. Unlike other Co^IV complexes, 1 had remarkable thermal stability, and was demonstrated to form a stable self‐limiting monolayer in preliminary atomic layer deposition (ALD) surface saturation experiments. The ease of synthesis and high stability make 1 an attractive starting point to investigate otherwise inaccessible Co^IV intermediates and for synthesizing new materials.