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

Abstract

The theoretical enthalpies of propagation reactions at 0 K without zeropoint vibrational energy corrections according to terminal and penultimate models of the radical copolymerization of styrene with acrylonitrile are reported from molecular orbital calculations at the following levels of theory and basis sets: HF/6-31G(d); B3-LYP/6-31G(d); B3-LYP/6-311G(d,p) and B3-LYP/6-311ϩG(3df)//6-311G(d,p). Both the enthalpic terminal and penultimate unit effects, determined according to the theoretical thermochemistry, depend on the level of theory and basis set used for the molecular orbital calculations. The best performing B3LYP/6-311ϩG(3df)//B3LYP/6-311G(d,p) procedure gives theoretical enthalpies for the addition of styrene and acrylonitrile to CH 3 • that differ from experimental values by 0.6 and 1.6 kcal mol Ϫ1 , respectively. An analysis of the results obtained here leads to the conclusion that at least for the styrene-acrylonitrile monomer system, that is, a monomer system known as one of the few systems that do not conform to terminal model composition and microstructure equations, the enthalpic terminal unit effects seem to depend on the penultimate units of the growing radical. This finding, together with the outcome from our previous work on the dependence of the penultimate effects on the terminal units in a growing macroradical, indicates the inseparability of the enthalpic terminal (implicit) and explicit penultimate unit effects on the radical copolymerization.

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

“…Because of the size of the studied systems, all calculations were performed at a relatively low theory level, namely, the B3-LYP/6-31G(d). Nevertheless, as it was shown previously, 26 dependencies of the reaction enthalpies on the theory level and basis set form almost parallel curves the differences between enthalpic effects for elementary propagation reactions within the same kinetic model and can be satisfactory shown by performing calculations at an even relatively lower level of theory.…”

“…Δ H and (Δ H were calculated as the difference between the enthalpy of addition of a given monomer to growing radical with A (S–A system) or M (S–M system) and S as the terminal and penultimate units, respectively. Results calculated at the B3‐LYP/6‐31G(d) level of theory without ZPE energies for reactions in the gas phase at 0 K (kilocalories per mole) (data for S–A monomer system published previously26). Terminal model: (▾) CH 3 ‐S · + S → CH 3 ‐S‐S · and CH 3 ‐A · + S → CH 3 ‐A‐S ·; CH 3 ‐S · + S → CH 3 ‐S‐S · and CH 3 ‐M · + S → CH 3 ‐M‐S · (▴) CH 3 ‐S · + A → CH 3 ‐S‐A · and CH 3 ‐A · + A → CH 3 ‐A‐A ·; CH 3 ‐S · + M → CH 3 ‐S‐M · and CH 3 ‐M · + M → CH 3 ‐M‐M ·.…”

“…This value seems to were calculated as the difference between the enthalpy of addition of a given monomer to growing radical with A (S-A system) or M (S-M system) and S as the terminal and penultimate units, respectively. Results calculated at the B3-LYP/ 6-31G(d) level of theory without ZPE energies for reactions in the gas phase at 0 K (kilocalories per mole) (data for S-A monomer system published previously 26 ). be quite substantial for the final composition of the polymer considering that the estimated enthalpy of hompolymerization of liquid A to crystalline polymer and photopolymerization of M in bulk is reported to be 18.3 32 and 13.1 kcal/mol, 34 respectively.…”

Transition structures and reaction barriers in the styrene–acrylonitrile copolymerization system according to quantum mechanical calculations

“…The influence of the basis set on thermochemistry was investigated for the B3LYP method for which the impact of two basis sets—a medium size basis set 6–31G(d) and a large 6–311+G(3df, 2p) one—on the calculated enthalpies were compared. The comparison was performed to verify previous conclusions from thermochemical studies performed on the same monomer system, namely, that the dependencies of enthalpic effects on the basis sets form parallel curves within the same copolymerization Mayo–Lewis kinetic model and, therefore, they can be compared, at least qualitatively, within the set of elementary propagation reactions for the given comonomer system 27. Optionally, theoretical calculations with the Gaussian deliver data on electronic structures and vibrational frequencies for the optimized molecular structures that, after scaling,28 are usually used29 to obtain thermochemical and thermodynamic quantities, including the enthalpy of reaction and change of entropy.…”