Azobenzene polymers monomers

Alanine-derived optically active A-propargylamide 22 and azobenzene-containing monomer 25 afford a co-polymer forming a helix. The azobenzene moiety isomerizes from trans-ioxm to cis-ioxm upon UV light irradiation, accompanying transition from helix to random coil. Then upon irradiation of visible light, the m-azobenezene moiety re-isomerizes into trans, while the polymer main chain keeps a random structure. This is presumably due to large steric repulsion around the azobenzene moiety to disturb recovery of a helical structure. 

It is of obvious interest to explore the use of other polymerization techniques that, being more tolerant to the experimental conditions and monomers, can produce amphiphilie azobenzene BCPs with no need for post reactions. Notably, Su et al. have reeently reported the synthesis of such an amphiphilic diblock copolymer with PAA as the hydrophilic block using reversible addition-fragmentation transfer (RAFT) polymerization (structure d in Fig. 6.2) (Su et al., 2007). Using RAFT, they prepared PAA capped with dithiobenzoate and used it as the macro-RAFT transfer agent to polymerize the hydrophobic azobenzene polymer successfully. It ean be expected that more amphiphilic azobenzene BCPs will be synthesized using the eontrolled radical polymerization techniques (ATRP and RAFT) because of their simplicity, versatility, and efficiency. 

Zhao and coworkers [234, 235] reported that an azobenzene polymer network can also optically align ferroelectric liquid crystals. This was done by dissolving two chiral dimethacrylate and one chiral diacrylate monomers containing azobenzene groups in a commercial ferroelectric liquid crystal host. The monomers were illustrated as follows ... 

In addition, Zhao and coworkers [236] reported photo-induced alignment of ferroelectric hquid crystals using azobenzene polymer networks of polyethers and polyepoxides. Bulk alignment was achieved by polymerizing several divinyl ethers and diepoxide monomers bearing an azobenzene moiety. Here too, thermal polymerizations were conducted in solution within the ferroelectric hquid crystals, while exposing the reaction mixture to linearly polarized irradiation. The monomers can be shown as follows ... 

Scheme 16 shows the synthesis of azobenzene-derivatized monomers (55a-c) by reaction of complex 53 with the alcohol-functionalized chromophores (54a-c). The resulting monomers were isolated in 87-90% yields. Reaction of monomers 53 or 55a-c with dinucleophtlic reagents provides a route to polymers fimctionahzed with carboxylic acid or azobenzene groups. 

Polymers containing chromophores such as acridine and azobenzene have also been prepared by palladium-catalyzed amination according to two approaches [222-224]. The first approach involved the polymerization of monomers containing the chromophore. For example, 4-aminoazobenzene was condensed with 1,3-dibromobenzene (Eq. (39)) or 4,4 -dibromobiphenyl ether to form polymeric materials with Mw values of 9.0 x 103 and 19 x 103, respectively. Alternatively, polymers prepared by polymerization of 4-bromostyrene or copolymerization of styrene and 4-bromostyrene were coupled with N-phenyl-4-amino azobenzene. Substitution of the aryl bromides by the amino azobenzene unit was essentially quantitative when using P(tBu)3 as the ligand. 

The H and 13C NMR spectra of azo compounds incorporated into monomers and polymers have been measured.67-73 Chiral methacrylic polymers containing azobenzene chromophore,67,68 epoxy-based polymers functionalized with tricyanivinylphenylazo chromophores,69 4-vinylazobenzene and homo- and copolymers,70 microstructure of trans-4-acrylooyloxyazo-benzene/methyl methacrylate copolymers,71 optically active poly[(S)-4-(2-methacryloyloxypropanoyloxy)azobenzene]72 and polyetherurethane pendant with azo dye by TV-substitution73 have been studied. 

Block copolymers of butadiene and styrene are therefore readily synthesized an-ionically, with either of the two monomers polymerized first. Precursor copolymers of poly(styrene-h/oc -butadiene) have been used to prepare well-defined liquid crystalline block copolymers by the same polymer analogous reaction described in Sec. 2.5 of this chapter (Scheme 21) [201-203]. Following anionic polymerization by sequential monomer addition, the polymer analogous reactions of the cholesterol (PS-PBCh) [201] and azobenzene (PS-PBAz) [203] derivatives were essentially quantitative, while that of the phenyl benzoate (PS -PBBz) block went to up to 94% conversion [202]. The polydispersities of the liquid crystalline copolymers (pdi= 1.13-1.23) were nearly as narrow as those of their precursor copolymers (pdi = 1.08 -1.21, M =8.09-9.2xl0 ) [201, 203]. 

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