Masatoshi Tosaka

Ortho-nitrobenzyl (oNB) group is among the most widely used photodegradable units, serving in diverse fields, including its use as a protective group in organic synthesis, as a stimulus in photo-responsive and photo-degradable polymer materials, and in caged molecules in biology. Despite its extensive application, however, little is known about the fate of the photodegraded fragments of the oNB group and their potential impact on the system in which they are used. Here, we isolated ortho-nitrosobenzaldehyde (2), a proposed primary photoadduct of the oNB group, and investigated its stability and reactivity. Compound 2 rapidly decomposed under both thermal and photochemical conditions, forming oligomers bearing azo and amide linkages. The structure of a trimer containing both linkages was unambiguously determined by single-crystal X-ray diffraction. We further found that dienes effectively trapped the nitroso group in 2 by the hetero Diels-Alder reaction, thereby minimizing undesired oligomerization. This trapping strategy, in combination with a microflow reaction system, significantly enhances the photodegradation efficiency of oNB-containing polymers. These findings shed new light on the photochemistry of oNB groups and provide a rational basis for the design and implementation of oNB groups in advanced functional systems.

Dendritic hyperbranched poly(methyl acrylate)s (HBPMAs) with controlled molecular weight and low dispersity (Đ ≤ 2) were synthesized by radical copolymerization of MA and a branch-inducing monomer, evolmer 2b, in the presence of organotellurium chain transfer agent (CTA) 1, by targeting theoretical generation (Gn) of 3-5 and branch molecular weight (MBL) of 0.25Me - 2.0Me, with Me (= 11,000) being the entanglement molecular weight of PMA. The control of the self-similar branch layer structure in HBPMAs thus synthesized was examined by measuring their linear viscoelastic moduli (G* = G' + iG"). For all Gns having MBL = 0.25 Me, the storage modulus G' was lower than the loss modulus G" at all ω examined. This unentangled feature was observed even when the portion of the chain spanning the two outmost ends of the branches had the molecular weight of 2.0Me, possibly because of a lack of deep penetration among those Gn chains. In contrast, for MBL ≥ Me, G' was higher than G" in a given range of ω, and this range became wider and the relaxation mode distribution therein became broader on an increase of MBL, which is characteristic of entangled branch polymers. For the entangled HBPMA chains having the same MBL but different Gn, hierarchically self-similar relaxation based on the branch retraction mechanism was confirmed for the G' and G" data: The data for higher Gn agreed with the data for lower Gn shifted according to this self-similarity. For the unentangled BPMA chains of different Gn, the G' and G" data obeyed the Rouse-type relationship based on the self-similarity but without the hierarchy. These results support good control of the dendritic and branch structure of the HBPMAs synthesized with the current method.