Backbiting and β-scission reactions in free-radical polymerization of methyl acrylate

Abstract

Multiple mechanisms of backbiting and b-scission reactions in free-radical polymerization of methyl acrylate are modeled using different levels of theory, and the rigid-rotor harmonicoscillator (RRHO) and hindered-rotor (HR) approximations. We identify the most cost-effective computational method(s) for studying the reactions and assess the effects of different factors (e.g., functional type and chain length) on thermodynamic quantities, and then identify the most likely mechanisms with first-principles thermodynamic calculations and simulations of nuclear magnetic resonance (NMR) spectra. To this end, the composite method G4(MP2)-6X is used to calculate the energy barrier of a representative backbiting reaction. This calculated barrier is then compared with values obtained using density functional theory (DFT) (B3LYP, M06-2X, and PBE0) and a wavefunction-based quantum chemistry method (MP2) to establish the benchmark method. Our study reveals that the barriers predicted using B3LYP, M06-2X, and G4(MP2)-6X are comparable. The entropies calculated using the RRHO and HR approximations are also comparable. DFT calculations indicate that the 1:5 backbiting mechanism with a six-membered ring transition state and 1:7 backbiting with an eight-membered ring transition state are energetically more favored than 1:3 backbiting and 1:9 backbiting mechanisms. The thermodynamic favorability of 1:5 versus 1:7 backbiting depends on the live polymer chain length. The activation energies and rate constants of the left and right b-scission reactions are nearly equal. The calculated and experimental 13 C and 1 H NMR chemical shifts of polymer chains affected by backbiting and b-scission reactions agree with each other, which provides further evidence in favor of the proposed mechanisms.The G4(MP2)-6X calculations are performed with Gaussian 09 [61] to take advantage of the parallel algorithm of open-shell

On the Use of Quantum Chemistry for the Determination of Propagation, Copolymerization, and Secondary Reaction Kinetics in Free Radical Polymerization

“…Intermolecular hydrogen abstractions like chain transfer to monomer and chain transfer to polymer were studied in various systems [106,[137][138][139][140][141]. Intramolecular hydrogen abstractions were investigated as well, focusing on the backbiting reactions that occur when a chain-end radical (CER) abstracts a hydrogen from its own backbone, forming a mid-chain radical (MCR) [70,106,[141][142][143][144]. Specific attention was paid to the 1:5 backbiting due to its energetically favorable transition state configuration involving a six-atom ring [145].…”

“…In particular, the rate coefficients of propagation reaction of MCRs were estimated for various systems [70,104,142,144]. Furthermore, both the two symmetric breaks of the carbon-carbon bonds that can occur when a MCR undergoes a β-scission reaction were studied [70,106,143,144,151,152]. Finally, reactions relevant in high temperature processes like thermal self-initiation were approached by quantum chemical investigations [151,153,154].…”

On the Thermal Self-Initiation Reaction of n-Butyl Acrylate in Free-Radical Polymerization

“…A singlet diradical has a pair of electrons, one spin-up and one spin-down (+1/2 and −1/2), in one orbital with the second, equal energy orbital, empty [38]. The singlet diradical then goes through a spin crossover reaction and changes into a triplet diradical (*MM* T ):…”

“…A triplet diradical has two "spin-up" electrons in adjacent, degenerate (equal energy) orbitals (two electrons of the same spin, +1/2 and +1/2) [38]. Finally, a third monomer molecule abstracts a hydrogen atom from the triplet diradical and forms two monoradicals, one with two monomer units (MM*) and the other with one monomer unit (M*):…”

A Quantum Mechanical Study on the Propagation Kinetics ofN-methylacrylamide: Comparison WithN,N-Dimethylacrylamide in Free Radical Polymerization

“…Various groups have contributed to this area by doing broad type of calculations so far. For example, chain‐length and penultimate effects, backbiting and chain transfer mechanisms and initiation kinetics were discussed by several groups. Solvent and substituent effects on the propagation kinetics of water‐soluble monomers, acrylic acid and methacrylic acid,, acrylamide and methacrylamide, acrylonitrile and vinylchloride in their free radical polymerization were studied with quantum mechanical calculations.…”