Mechanism of Ce(IV) initiated graft copolymerization of acrylamide onto poly(vinyl alcohol)

Abstract

The mechanism of Cerium (IV) initiated graft copolymerization of acrylamide (AM) onto poly(vinyl alcohol) (PVAl) was investigated. Photolysis was found to significantly enhance the rate of graft copolymerization of AM onto PVAl. Under 'normal' conditions polymerization occurred almost exclusively at the 1,2-glycol units and under photolytic conditions polymerization occurred at the 1,3-glycol units. Molecular weight studies showed that fewer grafted chains of higher molecular weight were obtained under photolytic conditions compared to grafting under 'normal' conditions. Photolysis was ineffective in enhancing grafting onto an ethylene-PVAl copolymer (EVAl). This was because complex formation between Ce(IV) and the alcohol was inhibited by insolubility of the copolymer.The rate of Ce(IV) reduction during oxidation of PVAl in the absence of monomer was measured from the change in absorbance of the Ce(IV)/PVAl complex at 400 nm. ESR was used to measure the rate of radical formation when a model alcohol was oxidised by Ce(IV). Both rates showed a maximum when photolysed at about the absorption maximum of the Ce(IV)/alcohol complex. This showed that absorption of light by the Ce(IV)/alcohol stimulates oxidation of the alcohol which produces a free radical intermediate.The Ce(IV) photo-oxidation of small chain alcohols was studied by ESR spectroscopy as a model for PVAl. Primary and secondary radical intermediates were isolated by thermal trapping and the ESR spectra were assigned. The oxidation mechanism was found to be dependent on the structure of the alcohol.The effect of photolysis on the initiation of AM polymerization was also studied by ESR. In the absence of an alcohol, AM polymerization was only initiated by Ce(IV) when photolysed at wavelengths below 300 nm. In the presence of an alcohol polymerization occurred readily when photolysed at wavelengths above 340 nm. This confirmed that the radical intermediate, which is produced by Ce(IV) oxidation of the alcohol, initiates AM polymerization.PVAl, its precursor poly(vinyl acetate) (PVAc), EVAl and graft copolymers of AM onto PVAl and EVAl were characterised in detail by NMR spectroscopy. New assignments were made for the different stereochemical and compositional sequences in PVAc and EVAl using two dimensional NMR spectroscopy. Minor structures in PVAl were selectively modified by oxidation, reduction and hydrolysis. This enabled NMR assignments for the head-head sequence, branches, end groups and residual acetate groups to be made, confirmed or corrected.The mechanism of Ce(IV) oxidation of PVAl was also investigated by NMR spectroscopy. Oxidation of PVAl by sodium periodate was initially studied as a model for Ce(IV). The aldehyde end groups of the periodate oxidised PVAl formed a combination of cyclic and linear hemi-acetals and acetals. The NMR spectrum of PVAl after oxidation by Ce(IV) under, 'standard' conditions was similar to that obtained after periodate oxidation. This confirms that oxidation by Ce(IV) occurs preferentially at the 1,2-glycol units. The NMR spectrum of the PVAl after photolytic oxidation confirmed that photolysis stimulates oxidation of the 1,3- glycol units. The predominant mechanism involved carbon-carbon scission instead of carbon-hydrogen scission.