Penultimate unit effects in the free‐radical copolymerization of styrene with acrylonitrile according to theoretical thermochemistry

Abstract

Penultimate unit effects in the free‐radical copolymerization of styrene with acrylonitrile were investigated by the consideration of the theoretical thermochemistry of three subsequent propagation steps in the copolymerization process at 298 K and the electronic properties of the relevant reactants. The total energies, zero‐point energies incorporating a 0.96 scale factor, and thermal enthalpy corrections for all optimized structures were computed with the B3LYP density functional theory and the 6‐311G(d,p) basis set. The penultimate unit effect on the enthalpy of reaction for elementary propagation reactions ranged from −1.2 to 2.7 kcal/mol. The enthalpies of elementary copolymerization propagation reactions showed that penultimate unit effects depended not only on the γ substituent itself but also on the terminal unit of the growing radical and the monomer being attached. The exothermicity of the addition of radicals, varying in the penultimate unit for a given monomer, was lower for more polar radicals and smaller Mulliken charges at the radical atom, except for radicals with a CN substituent placed in the γ position when they reacted with acrylonitrile. For the latter system, the repulsive interactions between the CN substituent and nitrile group of the monomer being added contributed to the reaction enthalpy. Almost no penultimate unit effect was detected upon spin distribution at the radical atom, and this probably indicated the absence of the independent implicit penultimate model. The results obtained strengthen the concept of the inseparability of implicit and explicit penultimate unit effects in radical copolymerization. However, it appears that for the styrene–acrylonitrile copolymerization system, the explicit penultimate model prevails over the implicit penultimate model. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3592–3603, 2002

Theoretical study of the free‐radical copolymerization of styrene with methyl methacrylate: Comparative study to the styrene–acrylonitrile monomer system

“…However, a comparison between dependencies of the penultimate unit effects on the total spin density at the radical atom for the S-M and S-A systems did not reveal any significant difference between the two systems. Results for the S-M system reproduced the former relation 24 for the S-A system, showing that there is almost no incidence of the penultimate monomer unit on the total spin density at the radical atom ( Fig. 6).…”

“…However, both dependencies, that is, the enthalpy of the addition reaction versus polarity of the radical and Mulliken charges at radical atoms, are regular in contrast to the equivalent dependencies for the S-A monomer system. 24 Here again, the kinetics of the elementary propagation reaction CH 3 -A-A ⅐ ϩ A 3 CH 3 -A-A-A ⅐ seemed to play a key role in differentiation between the penultimate effects in the two systems of monomers.…”

Further evidence for the inseparability of the theoretical enthalpic terminal and penultimate unit effects in the radical copolymerization of styrene with acrylonitrile

“…Recently, we applied the B3‐LYP density functional theory (DFT)14, 15 to study the free‐radical copolymerization of the styrene–acrylonitrile monomer system. The experimental results for the composition16–18 and kinetics19–21 proved that this system could display a significant penultimate effect in a copolymerization reaction. Therefore, for the monomer system, the theoretical enthalpies of reactions for three subsequent propagation steps in the radical copolymerization at 298 K21 were evaluated.…”

“…The experimental results for the composition16–18 and kinetics19–21 proved that this system could display a significant penultimate effect in a copolymerization reaction. Therefore, for the monomer system, the theoretical enthalpies of reactions for three subsequent propagation steps in the radical copolymerization at 298 K21 were evaluated. This allowed us to estimate the theoretical enthalpic terminal (Δ H ; implicit penultimate) and penultimate (Δ H ; explicit penultimate) unit effects, which could be defined as the differences between the enthalpies of addition of a given monomer and those of growing radicals differing in their terminal and penultimate units, respectively.…”

A QM Approach to the Calculation of Reactivity Ratios in Free‐Radical Copolymerization

“…In fact, the rapid and continuing increase in computer power has made it feasible to use ab initio QM to determine different properties, such as molecular geometries (i.e., bond lengths, bond angles, and torsional angles), energetic profiles of reactions, vibrational frequencies of molecular species, transition state structures, and reaction frequency factors, with good accuracy. Only a few pioneering works that use this technique to study free radical copolymerization reactions have been proposed in the literature 56, 62–67…”