Functional polyolefins responding to light stimuli

Functional polymers which can undergo light-induced shape changes can have interesting applications as non linear optical devices and reversible memory storage systems. In this contest, the functionalization of commercial polyolefins with chromophores having the properties to change their configuration upon irradiation can give the opportunity to obtain functional polymers which can modify the main chain orientation in response of light stimuli, through isomerization of side chain groups [1]. Recently, the functionalization of HDPE with 4-(phenylazo)benzoil-2,2,6,6-tetramethyl piperidine-1-oxyl (AzO-TEMPO) and 4-[4-(2-thienylazo)benzoyl]-2,2,6,6-tetramethylpiperidine-1-oxyl (TiO-TEMPO), which are TEMPO derivatives bearing azobenzene chromophore, has been obtained by free radical post reactor modification initiated by peroxide and carried out in the melt. Photo-physical properties of free TEMPO derivatives and of functionalized polymers have been studied by UV-VIS spectroscopy both in solution and on polymer film. In all cases a total or partial isomerization of the azo-type moiety has been observed upon irradiation with high pressure mercury lamp. Interestingly, comparison of all data allowed to state that photo-physical behavior of free TEMPO derivatives has been completely transferred to the polymer matrix which responds to light stimuli also as polymer film. Further analyses concerning the reversibility and the kinetic of the isomerization between trans and cis isomers are in progress. The conformational and photophysical properties in solution of TiO-TEMPO-Me has been studied through density functional theory (DFT) calculations and its time-dependent extension (TD-DFT) coupled to a polarizable continuum model (PCM) of the surrounding medium.