Living anionic polymerization of methyl methacrylate controlled by metal-free phosphazene catalyst as observed by small-angle neutron scattering, gel-permeation chromatography and UV–visible spectroscopy

Abstract

Phosphazene (PZN) catalyst, PZN catalyst coexisting with a co-catalyst 1-hydroxycyclohexyl phenyl ketone (Irgacure 184; IRG) and polymethylmethacrylate (PMMA) (prepared by catalytic living anionic polymerization using the PZN catalyst and IRG) have been observed for the first time by small-angle neutron scattering (SANS) and UV–visible spectroscopy to elucidate the aggregation behavior of the PZN molecules themselves and the state of living chain ends in a living polymer solution. PZN catalyst in deuterated tetrahydrofuran (thf-d8) showed SANS curves fitted by a form factor for a sphere whose radius Rs is larger (1.4–1.6 nm) than a single PZN molecule (0.65 nm), indicating formation of PZN aggregates in thf-d8. In a nonpolar solvent, benzene-d6, Rs was even larger (3.1 nm), indicating formation of larger aggregates. By adding IRG to PZN solution, an excess scattering appeared in the SANS profile and a strong band emerged in the UV–visible spectrum. This result indicates strong interaction of IRG with PZN not only on a molecular scale but also on a mesoscopic scale. The SANS profile from the living polymer solution in thf-d8 was observed to be fitted by the sum of the profile for the aggregated PZN/IRG complex and that for Gaussian chains of PMMA. The molecular weight of the PMMA determined by SANS, 2100 g mol−1, was in agreement with that estimated from gel-permeation chromatography, indicating that the anionic living chain ends and their counter ions (PZN) are dissociated in thf-d8; thus, the chains are not associated into multiple-ion pairs.