Polymers with Norbornadiene Moieties

One approach to harvesting light energy is to utilize pendant groups that reversibly absorb light energy, rearrange, and then release this absorbed energy as heatina rearrangement back to the original structure. To that end, research is going on in various laboratories to develop systematically derivatized polymer arrays than can collect and convert light energy. Among these, photo-rearrangements from norbornadiene to quadricyclane and back are of considerable interest, because photo-energy can be stored as strain energy (about 96 kJ/mol) in a quadricyclane molecule and later recovered [184]  

This photo-isomerization reaction is referred to as a valence isomerization. It is a reaction in which electron reshuffling occurs and the nuclei move to make or break new p and s bonds. A number of polymers were, therefore, prepared with the norbornandiene moieties either in the backbone or as pendant groups. Among them are polyesters that were synthesized with donor-acceptor norbornadiene residues in the main chain [184] by polyaddition of 5-(4-methoxyphenyl)-l,4,6,7,7-pentamethyl-2,5 norbornadiene-2,3-dicarboxylic acid or 5,6-bis(4-methoxyphenyl)-7,7- dimethyl-2,5-norbornadiene-2, 3-dicarboxylic acid, with bis(epoxide)s. This preparation of and the accompanying photo-rearrangements can be illustrated as follows:

The photo-rearrangements of the norbornadiene residues in the resulting polyesters were observed to proceed smoothly to the quadricyclane groups. Also, it was found that the norbornadiene residues in these polyesters show resistance to fatigue in repeated cycles of the interconversions [184]. The goal to utilize photochemical valence isomerization between norbornadiene and quadricyclane for solar energy collection and storage was reported by others [185]. Nagai et al. synthesized five different polymers with trifluoromethyl-substituted norbornadiene moieties in the side chains and in the main chain [185]:

All of the above polymers exhibit large absorption bands in the visible region of the spectra and the norbornadiene moieties in these polymers isomerize very rapidly. In addition, the norbornadiene moieties also exhibit efficient fatigue resistance [185]. Kawatsuki et al. [186] synthesized styrene polymers with pendant norbornadiene groups attached via amide linkages:

where R is a methoxy or a ring-substituted aniline group attached through the nitrogen or at another position. These pendant groups also undergo reversible conversions into quadricycline units in polymer films when irradiated by ultraviolet light of two different wavelengths. The materials exhibit high photosensitivity as well as a large red-shift in the absorption spectrum upon irradiation. Sampei et al. [187] reported that polyaddition of 2,5-norbornadiene-2,3-dicarboxylic acid diglycidyl ester to adipoyl chloride gave a polyester containing norbornadiene residues in the polymer backbone and in the pendant groups. When a photochemical rearrangement of norbornadiene residues took place in polymer films, the rate of the photochemical reactions in the polymer backbones was higher than that in the side chains [187]. Kawashima et al. [188] reported preparation of donor-acceptor type norbornadiene carboxylic acids compounds with carbamoyl groups, such as dipropylcarbamoyl, methylphenylcarbamoyl, propylcarbamoyl, and phenyl-carbamoyl. Benzyl esters were also prepared. Addition of these groups to polystyrenes formed polymers with pendant donor-acceptor type norbornadiene. Some were formed with 100% substitution. It was found that the polymers containing phenylcarbamoyl groups exhibit especially high photo-reactivity. In addition, the rate of the photochemical reaction in films of these norbornadiene polymers increases efficiently by an addition of 4,40-bis(diethyl-amino) benzo phenone photosensitizer. As a result, all the norbornadiene groups of the polymers isomerize to the quadricycline groups in as little as 20 s. The stored thermal energy of the irradiated polymers was found to be 32–52 kJ/mol.

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مواضيع ذات صلة

Changes in Viscosity and Solubility of Polymeric Solutions

Photo-Isomerization of the Azo Group

Photo-Isomerization of the Olefinic Group

Copolymers of Methacrylates with Cholesteric Monomers

Photo-Responsive Polymers

Polymers Designed to Cross-link Upon Irradiation with Laser Beams

Polymers with Pendant Azide Groups

Polymers with Pendant Abietate and Dibenzazepine Groups

Polymers with Pendant Maleimide Groups

Polymers with Pendant Furan Groups

Polymers with Functional Groups Similar to Cinnamates

Polymers with Functional Chalcone Groups