Enhancement of Intramolecular Photocycloaddition of Bichromophoric Compounds via Inclusion in Low-Density Polyethylene Films

Abstract

The photocycloaddition of tetra- and penta(ethylene glycol) labeled at the chain terminals with 2-naphthoyl groups (N-Pn-N; n = 4 or 5) and tetra(ethylene glycol) terminated by 7-(4-methylcoumarinyl) groups (C-P4-C) has been investigated in hexane solutions and in low-density polyethylene (LDPE) films. At loading levels less than 1 × 10-2 mol/g-film, irradiation of the compounds in LDPE leads to formation of intramolecular photocyclomers to the exclusion of intermolecular products. Irradiations in hexane containing 1 × 10-3 M of a bichromophoric compound produce large amounts of oligomeric material. The difference between the results in solution and the film is explained in terms of the compartmentalization of the guest molecules in the LDPE amorphous regions. Cold-stretching the LDPE films significantly increases the relative efficiency of the intramolecular photocycloadditions due to reduction of the free volume at the reaction sites. This work reports a new approach to synthesize large-ring compounds in high chemical yields at relatively high substrate concentrations.