4-vinylazobenzene: Polymerizability and photochromic properties of its polymers

The polymerizability of 4-vinylazobenzene; (VAB), a vinyl monomer containing an azobenzene chromophore directly attached to the polymerizable double bond, was investigated in the presence of radical, anionic, cationic, and Ziegler-Natta type catalysts. The homo- and copolymerization of VAB with (-)-menthyl methacrylate, carried out in the presence of AIBN as a radical initiator, afforded a fairly high yield of photochromic polymeric materials. On the contrary, no high molecular weight product was formed in the presence of cationic, anionic, and anionic coordinate catalysts, whereas polymeric products were obtained by anionic copolymerization of VAB with styrene. Determination of C-13 relaxation times allowed us to evaluate the poly(VAB) molecular mobility, also, in comparison, that of 4-isopropylazobenzene, a low molecular weight model compound, and of other polymeric systems containing azobenzene chromophores spaced from the main chain. Thermally and photoinduced trans --> cis and cis --> trans isomerization rates were almost independent of molecular weight and chemical composition of the investigated samples, thus suggesting that azobenzene isomerization implies an in-plane inversion of the phenyl ring most distant from the polymer backbone. Asymmetric perturbation of the azobenzene chromophores, attributable to the presence of short ordered segments along the polymer chain of MnMA/VAB copolymers, was evidenced by circular dichroism. The limited dependence of the intensity of dichroic bands on copolymer composition was interpreted in terms of distribution and mobility of VAB units.