Photochromic block copolymers

The present report describes the observation and measurement of the photo-optical and photo-mechanical responses on polyurethane-acrylate block copolymers in which a photochrome (spiropyrane or w-spiropyrane) has been incorporated in the form of a simple solid solution or chemically bonded. 

Photochromic diarylethenes can be incorporated into polymers by following free-radical polymerization [75], polycondensation [76], Fridel-Crafts alkylation [77], or oxidative polymerization [78], The side-chain and main-chain homopolymers, copolymers, block copolymers, and jr-conjugated polymers synthesized by the above-mentioned methods have shown good photochromic properties in films and solution phase owing to the higher content of diarylethene chromophores in the polymer system compared to that in simple guest-host systems [79-81], Selected examples are illustrated in Figure 9.8. 

Hawker et al. [95] and Fukuda et al. [96] both reported on the copolymerization of St with various monomers in 1996. Hawker reported copolymerizations with nBA, MMA, and p-chloromethylstyrene (CMSt), while Fukuda focused on several acrylates, 9-vinylcarbazole, and acrylonitrile (AN), and succeeded in preparing block-random copolymers of St with AN (details below). Neither group found the polymerizations to be well-controlled when low concentrations of St were present in the comonomer feed. However, since then, NMP has been used extensively to prepare copolymers. Pozzo et aL copolymerized St with 4-vinyl pyridine (VP), initiated by benzoyl peroxide (BPO) and using TEMPO as the radical mediator [97]. After purification, the copolymer was reacted with spiro [fluorenecyclopropene] to prepare photochromic copolymers with controlled molecular weights (Scheme 6). 

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