Photo‐Cross‐Linking of Polymethacrylates with Stilbene Chromophores in the Side Chains

Abstract

Methacrylates (¼ 2-methylpropenoates) 5 with (E)-stilbene (¼ (E)-1,2-diphenylethene) building blocks on tethers of variable length were prepared (Scheme 2) and polymerized (i.e., 5 ! 6; Scheme 3) in the presence of AIBN (¼ 2,2'-azobis(2-methylpropanenitrile). 4-[(E)-2-Phenylethenyl]phenyl acetate (7) as model compound established the cyclodimerization as a single irreversible photoreaction. i.e., (7 ! 8 -11; Scheme 4) in the absence of oxygen. The solution photolysis of the polymers 6 provided a similar result, whereby [2p þ 2p] cycloadditions of stilbene units of neighboring tethers predominated. On the contrary, the desired photo-cross-linking of chaines occurred in the irradiation of polymer films.

Photo-Crosslinking of Copolymeric Methacrylates Bearing Stilbene Chromophores

“…Recently we reported on polymethacrylates with (E)-stilbene chromophores in the side chains [9]. Similar results were obtained for stilbenoid dendrimers some years ago [10].…”

“…Although the number of (E)-stilbene units in the copolymer chains was much lower than in the homopolymers [9], the irradiation in solution (CH 2 Cl 2 or C 6 H 6 , λ ≥ 300 nm) led predominantly to intrachain reactions to give 7a, b (Scheme 3). The corresponding [2π + 2π] photocycloaddition worked very well for 3a-6 and 3b-1, 2.…”

“…The signal c at 4.35 ppm grows during the irradiation. It results from head-to-head and head-to-tail stilbene dimers as in the case of the homopolymers [9]. Both regioisomers have predominantly a stereochemical arrangement in which each proton of the four-membered ring has only one cis-standing phenyl neighbor.…”

Deformable and conformal silk hydrogel inverse opal

“…The rate of photo-cross-linking is increased by the aromaticity of the side chain that silk possesses in abundance. Consequently, stilbene can be a good cross-linker for silk fibroin (24,26). We found that a solution containing 0.02 M of stilbene led to the most rapid rate of cross-linking.…”

“…UVC light induces [2π + 2π] cycloadditions between the stilbene units, whereas the radicals generated in the silk tethers undergo dityrosine bonding (for details, see Supporting Information and Fig. S1) (24,25). The rate of photo-cross-linking is increased by the aromaticity of the side chain that silk possesses in abundance.…”