Investigation of the radical copolymerization and terpolymerization of maleic anhydride and norbornenes by an in situ ¹H NMR analysis of kinetics and by the mercury method: Evidence for the lack of charge-transfer-complex propagation

Abstract

The radical copolymerization of electron-deficient maleic anhydride (MA) and electron-rich norbornene (NB) derivatives with 2,2′-azobis(isobutyronitrile) (AIBN) in dioxane-d8 has been monitored in situ by 1H NMR spectroscopy with free induction decays recorded every 30 min at 60, 70, or 84 °C. The ratios of the monomer pairs were varied in some cases. The NB derivatives employed in this study included bicyclo[2.2.1]hept-2-ene (NB), t-butyl 5-norbornene-2-carboxylate, methyl 5-norbornene-2-methyl-2-carboxylate, and ethyl tetracyclo[4.4.0.12,5.17,10]dodec-3-ene-8-carboxylate. Decomposition of AIBN, consumption of the monomers, feed ratios, endo/exo ratios, copolymer compositions, and copolymer yields were studied as a function of polymerization time. Furthermore, a homopolymerizable third monomer (t-butyl methacrylate, methacrylic acid, t-butyl acrylate, or acrylic acid) was added to the NB/MA 1/1 system, revealing that the methacrylic monomer polymerizes rapidly in the early stage and that the ratio of MA to NB in the terpolymer strongly deviates from 1/1. In contrast, however, the acrylic monomers are more uniformly incorporated into the polymer. Nevertheless, these studies indicate that MA and NB do not always behave as a pair in radical polymerization and disproves the commonly believed charge-transfer mechanism. Electron-deficient fumaronitrile was also included in the kinetics study. To further understand the copolymerization mechanism, MA and NB were competitively reacted with a cyclohexyl radical generated by the treatment of cyclohexylmercuric chloride with sodium borohydride (mercury method). A gas chromatographic analysis of the reaction mixtures has revealed that a cyclohexyl radical reacts with MA almost exclusively in competition and that the cyclohexyl adduct of MA essentially accounts for all the products in a mass balance experiment, eliminating a possibility of the formation of an adduct involving the MA-NB charge-transfer complex. Thus, the participation of a charge-transfer complex in the copolymerization of MA and NB cannot be important.