Structure of the lowest triplet states of poly-α-methylstyryl sodium.: Ab initio calculations

Ab initio optimisation of a poly-alpha -methylstyryl sodium (PMSNa) fragment consisting of two cis units yields a triplet state energy which is close to the ground state energy. A new mechanism is proposed for depolymerization of "living" polymers, which implies that nit elementary step involves excitation to the low-lying triplet state with a charge transfer and with further bond cleavage. In the reaction structure, electronic excitation occurs with a minor (similar to0.5 Angstrom) displacement of the Na+ cation between the last and the last but one monomer units. The reversible polymerization/depolymerization reaction of PMSNa in THF was studied experimentally. The experimental (5.6 kcal/mole) and calculated (7.3 kcal/mole) polymerization enthalpies are in reasonable agreement.