MOLECULAR-ORBITAL STUDY ON THE MECHANISM OF RADICAL POLYMERIZATION OFSPIRO-ORTHOCARBONATES BEARING EXO-METHYLENE GROUPS

Molecular orbital studies were carried out to compare the easiness of ring-opening in the radical polymerization of spiro-orthocarbonates be aring exo-methylene groups at alpha-position of the ether oxygen, -ben zodioxepine-4'-methylene-3,2'-[1,3]-dioxolane] (1), and at beta-positi on of the ether oxygen, ,4-benzodioxepine-5'-methylene-3,2'-[1,3]-diox ane] (2). The formation energy suggests that 1 would show a degree of ring-opening larger than 2, contrary to the experimental result. There fore, a reverse relation in activation energy was suggested, i.e., the kinetic factor surpasses the thermodynamic factor in the ring-opening reaction of 1 and 2. Although the calculation of the activation energ y of the radical ring-opening reaction was not successful, the result extracted from the perturbation energy calculations of vinyl polymeriz ations of 1 and 2 agree well with their radical vinyl polymerizability . Namely, 1 was confirmed to have a larger vinyl polymerizability than 2, which agrees well with the smaller ring-opening polymerizability o f 1 compared with 2. The real HOMO and LUMO (highest occupied and lowe st unoccupied molecular orbitals) of 1, 2 and their intermediates for the radical addition were confirmed not to correspond to the apparent HOMO and LUMO from the detailed analysis of their coefficients of atom ic orbitals (AOs). The frontier electron density of 1 and 2 agrees wel l with the fact that the beta-carbon of their exo-methylene group has a higher reactivity toward radical species than the alpha-carbon.