Reversible side-group bonding

Reversible Side-Group Bonding for Self-Healing. 80... 

Meng, X., Natansohn, A., Rochon, P. (1997). Azo polymers for reversible optical storage. 13. Photoorientation of rigid side groups containing two azo bonds. Polymer 38, 2677-2682. 

Cases a to c are results from the X-ray experiments, while configuration d is the fully optimized structure. The central bond C2-C3 comes close to a triple bond for all three experimental structures although it appears that the values for the two polyTCDU cases are too short for a true triple bond. Notice the reversal of the bond order for C1-C2 and C4-C5 for the two polyTCDU cases. The reported 1.38 A bond distance in b for C1-C2 has been considered as the main argument for the existence of the butatrienic backbone. This result is not confirmed in the new structure determination (case c). As can be seen from Table I, the Enkelmann-Lando structure (case b) deviates the most from an idealized diacetylene configuration. In cases e, f, and g the distance of the methyl group to the backbone and the side group angle has been varied to simulate strain conditions. 

Depolymerization is the reverse of addition polymerization [reaction (23-50)]. It only occurs without side reactions when the bonds to the side groups are much more stable than the main-chain bonds. Depolymerization begins spontaneously only in living problems. In all other macromolecules, bonds in the main chain must first be broken homolytically in a start reaction. In these cases, therefore, depolymerization proceeds according to a free radical mechanism. 

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