Studies on pyrolysis mechanisms of syndiotactic polystyrene using DFT method

Abstract

The pyrolysis reactions of styrene trimer as polystyrene (PS) model compound were researched through using density functional theory (DFT) method M06-2X with the 6-311G(d) basis set to clarify the evolution mechanisms of PS pyrolysis products. The kinetic and thermodynamic parameters in all reactions (including bond cleavage, β-scission, hydrogen transfer, radical addition and disproportionation) were calculated. The calculation results of bond dissociation energies (BDE) indicate that the BDE values of CC on the backbone are obviously lower than those of C-Caromatic of branched chain, and the structure of syndiotactic PS is more stable than that of isotactic and atactic PS. PS can be decomposed into a methylene-end radical and a benzyl-end radical through the main-chain homolytic reaction, and these radicals further decompose to generate styrene through the end-chain β-scission reaction, or generate α-methylstyrene through the mid-chain β-scission after 1,2-hydrogen transfer reaction, or generate dimer through the mid-chain β-scission after 1,3-hydrogen transfer reaction. The kinetic analysis indicates that styrene is the major pyrolysis product, and α-methylstyrene and dimer are the main competitive products.