Spontaneous Polymerization Mechanism of Electron-Accepting Substituted Quinodimethane with p-Methoxystyrene

Abstract

Spontaneous reactions of 1-(2,2-dimethyl-1,3-dioxane-4,6-dione-5-ylidene)-4-(dicyanomethylene)-2,5-cyclohexadiene (QM 1) with p-methoxystyrene (MeOSt) were investigated in chloroform at room temperature at different monomer feed ratios. Reaction products obtained as hexane-insoluble and hexanesoluble fractions were characterized by elemental analysis and IR, 1 H NMR, and 13 C NMR spectroscopies. The hexane-insoluble products were to be copolymers of 7-(4-methoxyphenyl)-8,8-dicyanoquinodimethane (QM 2) with small amounts (3-6 mol %) of the QM 1, and the reaction product in the hexane-soluble fractions was to be the one-to-one adduct (Cycloadduct) of methylene Meldrum's acid (MM) with MeOSt. It was found that in the spontaneous reaction of the QM 1 with MeOSt a side reaction involving a metathesis process took place to form new two products, QM 2 and MM, and the former polymerized and the latter underwent the cycloaddition reaction with MeOSt. The formation of the Cycloadduct was also confirmed by a model reaction using 1-[(6-hydroxy-2,2-dimethyl-4-oxo-4H-1,3-dioxin-5-yl)-methyl]pyridinium hydroxide as a precursor of the MM and MeOSt. To identify the reactive intermediate, the spontaneous reactions of the QM 1 with MeOSt were carried out in the presence of 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) and methanol. No products trapped by TEMPO were obtained, but in the case of methanol the one-to-one-to-one adduct (Adduct) of the QM 1, MeOSt, and methanol was isolated. It was concluded, therefore, that the spontaneous reaction of the QM 1 with MeOSt might proceed via a zwitterionic tetramethylene intermediate.

Spontaneous reactions of electron‐accepting substituted quinodimethane with substituted styrenes: Inductive and steric effects on the formation of a zwitterionic intermediate

“…The same spontaneous reactions in the presence of TEMPO as a radical inhibitor also gave terpolymers, but their molecular weights were much lower than those obtained in the absence of TEMPO. These results for MeSt, HSt, AcOSt, and ClSt are similar to those for MeOSt reported previously6 and indicate that in these spontaneous reactions a zwitterionic tetramethylene intermediate is formed as a reactive intermediate and the terpolymers are formed via a radical mechanism. It is thought, therefore, that these spontaneous reactions might proceed via the mechanism shown in Scheme , where a cyclobutane is formed via a gauche‐form zwitterionic tetramethylene intermediate and then cleaves via a metathesis process to generate the trisubstituted quinodimethane (QM2) and MM.…”

“…This concept has been applied to other spontaneous systems such as Lewis acid promoted free‐radical polymerization, photochemical charge‐transfer polymerization, and copolymerizations of p ‐quinodimethanes. Recently, we found that an electron‐accepting unsymmetrically substituted quinodimethane compound, 1‐(2,2‐dimethyl‐1,3‐dioxane‐4,6‐dione‐5‐ylidene)‐4‐(dicyanomethylene)‐2,5‐cyclohexadiene (QM1), reacts spontaneously with p ‐methoxystyrene (MeOSt) to give a copolymer as well as a cycloadduct, and we proposed a reaction mechanism involving a gauche‐form zwitterionic tetramethylene intermediate followed by cyclobutane formation (Scheme ) 6…”

A paradigm for the mechanisms and products of spontaneous polymerizations

“…With less hindered and less electron‐rich olefins such as p ‐methoxystyrene, vinyl ethers, and NVCz, no polycombination occurs, and the reactions with QDM1 follow a more complicated pathway described here for p ‐methoxystyrene 105. In this case, cyclobutane adducts can form from the P8 zwitterionic intermediate.…”

Spontaneous polymerization mechanism of electron‐accepting substituted quinodimethane with vinyl ether and cyclic ketene acetal

“…This strongly indicates that the zwitterionic intermediate was formed in the spontaneous reaction of QM‐1 with BVE. Previously, we proposed a reaction mechanism7 for the spontaneous reaction of QM‐1 with MeOSt, in which the cyclobutane ring formed through a zwitterionic intermediate cleaved via a metathesis process to form QM‐2 and MM. It is, therefore, thought that in the spontaneous reaction of QM‐1 with BVE, a similar mechanism is operative.…”

“…In addition, we have reported that the polymerization modes (cationic or radical) of TCNQ with vinyl ethers might change according to the electron‐donating property of the vinyl ethers 3, 5, 6. Recently, we investigated the spontaneous reaction of an unsymmetrical substituted quinodimethane, 1‐(2,2‐dimethyl‐1,3‐dioxane‐4,6‐dione‐5‐ylidene)‐4‐(dicyanomethylene)‐2,5‐cyclohexadiene (QM‐1), with p ‐methoxystyrene (MeOSt), and we proposed a reaction mechanism in which the cyclobutane is formed via a zwitterionic intermediate and then cleaves via a metathesis process to generate the trisubstituted quinodimethane, 7‐(4‐methoxyphenyl)‐8,8‐dicyanoquinodimethane, and methylene Meldrum's acid (MM), the former of which forms a copolymer with QM‐1 and the latter of which forms a one‐to‐one cycloadduct with MeOSt 7. More recently, we found that the spontaneous reaction of weaker electron‐accepting 7,7‐dicyanobenzoquinone methide (CQM) with MeOSt proceeds via a zwitterionic phenylene oxytrimethylene intermediate and that the alternating copolymer is formed by polycombination 8.…”