Rapid-flow and spin-trapping ESR study on carbonyl-conjugated radicals of modified poly(vinyl alcohol) bearing 1,2-propanediol pendant moiety as intermediates of graft copolymerization

Based on the results of rapid-flow electron spin resonance (RF-ESR) and spin-trapping (ST-ESR) measurements, carbonyl-conjugated radicals derived from the vicinal diol moiety (2, i.e., the head-to-head structure) of poly(vinyl alcohol) (PVA) have been proposed to be the most probable intermediate species involved in the initial stages of the graft reaction of PVA with methyl methacrylate (MMA). A modified poly(vinyl alcohol) (PPVA) bearing a 1,2-propanediol pendant moiety (3) with a molar ratio of 8% (based on the monomer unit) was prepared to clarify the role of the carbonyl-conjugated radicals of PPVA in the graft reaction with MMA. The RF-ESR spectra observed for the mixtures composed of PPVA, hydrogen peroxide (HPO) or ammonium persulfate (APS), and Ti-2(SO4)(3) (pH ca. 2) revealed the formation of four radical species (A, B-1, B-2, and C) derived from PPVA. By comparison of the RF-ESR spectra observed for PPVA and PVA, species A (g = 2.0041) was confirmed to be the pendant derived radical of PPVA, and species B-1 and B-2 (g = 2.0023) and C (g = 2.0032) were assigned to be the PVA main-chain derived radicals. Species B-1 and B-2 were deduced to be a pair of carbonyl-conjugated radicals (4, 5, R-1-CH2-CO center dot-CH-CH2-R-2) derived from the vicinal diol (2), and species C was assigned to be the secondary alcohol (1) derived radicals (6, R-1-(CH2COH)-C-center dot-CH2R2). In terms of g value and proton hyperfine coupling constants (hfcc), the molecular structure of species A was ascribed to be the carbonyl-conjugated radical of the pendant moiety (7, (CH2CO)-C-center dot-(CHRR2)-R-1). The results of ST-ESR measurements using the sodium salt of 3,5-dibromo-4-nitroso-benzene-sulfonate (DBNBS) show that a pair of pendant radicals (7) and (8, O=CH center dot-CH-(CHRR2)-R-1) was formed in the graft polymerization reaction solutions containing APS or HPO as an oxidant (80 degrees C, pH ca. 2). The grafted copolymers of PPVA and PVA (PPVA-g-MMA and PVA-g-MMA) were separated from similar graft reaction solutions composed of APS and MMA after heating at 70 degrees C for 1 h (pH ca. 2), and the grafting efficiencies of PPVA ((PPVA)g(e)%) and PVA ((PVA)g(e)%) were, respectively, estimated to be 4% and 2%. The molecular and electronic structure of the carbonyl-conjugated radicals derived from the main-chain (4, 5) and from the pendant moiety (7, 8) of PPVA will be discussed in relation to the reaction mechanisms of the initial reaction stage of the graft copolymerization with MMA.