The spontaneous polymerization of methyl methacrylate, 7. External heavy atom effect on the initiation

Abstract

A dimeric biradical 'Mi plays a central role in the initiation of the spontaneous polymerization of methyl methacrylate (MI). Besides the initiation step by chain transfer reaction, a change of the multiplicity of ' M; seems to be of importance. Evidence is given by heavy atoms in the solvent or in an inert gas atmosphere, leading to an acceleration of the overall polymerization rate. The observed effect is related to the atomic number of the respective heavy atom. Measurements of the temperature dependence of the effect for carbon tetrachloride show thatas expectedno activation energy is needed for this reaction step. The results are summarized in a reaction scheme for the initiation mechanism of the spontaneous polymerization of methyl methacrylate. C-CHZ-CH CH COOCH, COOCH, 2-I COOCH, COOCH, (HI) (HHI) Part 6: cf.@. Pressent address: Kalle, Niederlassung der Hoechst AG., D-6200 Wiesbaden-Biebrich. b, Part of the dissertation of J. L., Mainz 1982.0025-1 16X/84/$03.00 *) Thiophenol is an exception, the transfer effect of which is strong as compared to the heavy **) The authors want to express their thanks to the Rohm company for the generosity in atom effect to be expected (cf. discussion in6)).supplying unstabilized monomer of extremely high purity.

The Low Energy of Concert in Many Symmetry‐Allowed Cycloadditions Supports a Stepwise‐Diradical Mechanism

“…Nevertheless, to eliminate altogether this unlikely loophole, an example of external HAE, which cannot possibly alter the mechanism, was pursued. A sound precedent for external HAE on the cycloaddition timescale exists , in which halogenated solvents or xenon gas extend the lifetime of reversibly formed 1,4‐diradicals by temporarily converting them into triplets. Each radical site, regardless of spin, can initiate a polymer chain, but cleavage of a triplet diradical back to reactants is impossible.…”

Experimental study of the spontaneous thermal homopolymerization of methyl andn‐butyl acrylate

“…In nonstyrenic monomers, such as methyl methacrylate (MMA), Diels–Alder adducts lack the tendency to undergo hydrogen abstraction to generate monoradicals. Stickler, Meyerhoff, and coworkers18–25 studied the spontaneous thermal polymerization of MMA at high temperatures (100–130°C) and reported low‐conversion (≤ ∼ 5%), high‐average‐molecular‐weight polymers and the formation of a significant amount of dimers, trimers, and oligomers after 75 h of batch operation. The presence of diradical species and the initiation mechanism were identified from the predominant formation of a linear unsaturated dimer, dimethyl 1‐hexene 2,5 carboxylate 19–21…”

“…Theoretical considerations lead to the conclusion that sustained polymer chain growth is not possible from diradical species 23. The role of the solvent as a chain‐transfer species was studied in the spontaneous thermal polymerization of MMA23–25 and pentafluorostyrene 26. Low rates of polymerization of MMA were found in nonpolar solvents,23 and high rates of polymerization were found in halogenated solvents and polar, protic solvents, such as thiophenol.…”

“…Low rates of polymerization of MMA were found in nonpolar solvents,23 and high rates of polymerization were found in halogenated solvents and polar, protic solvents, such as thiophenol. Halogenated solvents and thiophenol were reported to increase the rate of polymerization because of the rapid chain transfer and crossing of diradical species from an unstable singlet to a stable triplet state via collisions with the heavy halogen or sulfur atom 23–25. However, the heavy atom effect in thiophenol was found to be less evident in comparison to halogenated solvents.…”

“…However, the heavy atom effect in thiophenol was found to be less evident in comparison to halogenated solvents. The triplet diradical species were postulated to generate initiating species for the polymerization of MMA 24. High rates of thermal spontaneous polymerization of alkyl acrylates accompanied with significant chain‐transfer reactions, such as back‐biting and β scission, in solvents of varying polarity, such as xylene, benzene, isopropyl alcohol, ethyl acetate, and methyl amyl ketone, have been reported 10.…”