Quantum-chemical study of degradation of molecular analogs of butadiene-styrene copolymer fragments during stretching by end atoms

A comparative study of changes in the electronic structure during stretching of molecular fragments by end atoms is performed by using semiempirical quantum chemical method PM3 with geometry optimization for cis- and trans-hexa-3-ene, 3-methyl-1-pentene and sec-butylbenzene, which model butadiene-styrene copolymer units. In contrast to molecules modeling butadiene-1,4 units, the degradation of molecules modeling butadiene-1,2 and styrene units is found to proceed in two stages. At the first stage, when a maximum in the corresponding stress-strain curves is attained, the molecules with side vinyl and phenyl groups do not break down but experience intramolecular structural transition that is accompanied by the isomerization and displacement of double bonds. At the second stage, the molecules degrade.