SynopsisAdvantages in using the pyrophosphate complex of trivalent manganese over the sulfate complex as initiator for graft copolymerization onto starch are discussed. The first successful attempts to graft copolymerize acrylonitrile, methyl methacrylate, and acrylamide to starch and starch derivatives are described using manganic pyrophosphate as initiator. Selective solvent extraction of the reaction products and very low conversions of monomer to homopolymer in absence of starch substrates provide evidence for high grafting efficiencies obtained with acrylonitrile and methyl methacrylate. With acrylamide as monomer, however, low grafting efficiencies and considerable amounts of homopolymer are obtained under the experimental conditions investigated. Reaction mechanisms responsible for initiation of graft copolymerization are discussed. These are (a) glycol cleavage in the anhydroglucose units by Mn3+ ions leading to formation of a radical, and (b) enolization and further oxidation of oxidized starch by Mn3+ ions also leading to radical species. Mechanisms are also proposed for homopolymerization through Mn"+ oxidation of enols which probably are formed by "vinylogous" addition of water molecules to acrylamide and methyl methacrylate.
SynopsisRecent development of synthesis and applications in grafting of acrylonitrile (AN) to gelatinized starch by Ce4+ initiation have been briefly reviewed. Using the Mn3+ method, the effects of starch pretreatment temperature on grafting parameters in grafting of AN onto starch have been studied. Total conversion of AN to PAN and % add-on were almost constant and showed only slight variation with increasing starch pretreatment temperature. Average molecular weight of grafts showed, however, an increase by a factor of nearly 10 (i.e., from 0.4 X 105 to 4 X lo5) after swelling and gelatinization of the starch, and the grafting frequency (anhydroglucose units per grafted chain) increased in proportion. In grafting of AN onto gelatinized starch, increasing Mn3+ concentration increased the conversion of monomer and % add-on, whereas the average molecular weight of grafts and the grafting frequency (AGU/chain) decreased. An increase in the amount of starch also increased the conversion of monomer and the grafting frequency (AGU/chain) but decreased the % add-on and the average molecular weight of the grafts. Selective solution of PAN homopolymer by dimethylformamide indicated that grafting efficiencies were high in all cases. The results are interpreted in terms of slow termination rates due to high viscosity of gelatinized starch.
SynopsisThe effect of reaction conditions on the composition of native potato starch-polyacrylonitrile graft copolymers initiated by manganic pyrophosphate onto starch slurries at 30°C has been examined.In general, when the Mn3+ ion concentration was increased from 0.15 X 10-3M to 3.0 X 10-3M (other conditions kept constant), an increase in conversion of monomer to polymer and % add-on was observed, whereas frequency of grafts (anhydroglucose units, AGU, per grafted chain) decreased. Also, the average molecular weights of grafts showed a decrease from 2.2 X lo5 to 1.5 X lo5. Increasing the concentration ratio of starch to monomer during polymerization by a factor of 3 produced an increase in the conversion of monomer to polymer, whereas an increase in frequency of grafts (AGU/chain) was obtained. Values of % add-on and average molecular weights of the grafts showed, however, a decreasing tendency. It was observed that grafting onto starch took place readily even at acid additions as low as 10 X lO-3M H&04 (pH N 1.8). Selective solvent extraction of homopolymer and extremely low conversions of monomer to polymer (0.1%1.5%) in duplicate runs without addition of starch indicated that grafting efficiencies were high in all cases. An attempt has been made to interpret the results in terms of variations in factors such as initial ratio of (Mn3+)/(AGU), termination rate of acrylonitrile chain radicals by oxidation by Mn3+ ions, oxidation rate of radicals formed on anhydroglucose units by Mn3+ ions, and physical factors such as diffusion rate of Mn3+ ions through the polyacrylonitrile-grafted starch granules for terminating the radicals.
SynopsisA study has been made of graft copolymerization of methyl methacrylate onto native potato starch in aqueous slurry at 30°C. As Mn3+ concentration was increased from 0.15 X 10-3M to 1.0 X 10-3M, conversion of monomer to polymer and add-on of polymer to starch increased and frequency of grafts (anhydroglucose units per grafted chain) decreased sharply. The average molecular weights of the PMMA grafts also decreased in this range. At Mn3+ concentrations from 1.0 X 10-3M to 3.0 X 10-3M, only minor changes in grafting parameters were observed. When the amount of starch charged per batch was increased threefold, the add-on decreased sharply, the molecular weight increased slightly, and the conversion of MMA monomer to polymer remained almost constant. The increase in frequency of grafts (AGU/chain) was almost directly proportional to the increase in the amount of starch charged. In all cases the average molecular weights of grafts were of the order of lo6 and the grafting efficiencies high, normally greater than 85%. These results were compared with those previously obtained for grafting of acrylonitrile onto starch. They were interpreted in terms of initial (Mn3+)/(ACU) ratio, total number of radicals initiating grafting, and compatibility of methyl methacrylate monomer with poly(methy1 methacrylate) chains.
A series of starch‐g‐polyacrylonitrile copolymers of starch granules and starch gel were synthesized with the Mn3+ initiating system. The samples were examined and photographed in a scanning electron microscope before and after homopolymer extraction in N, N‐dimethylformamide., The grafted starch was hydrolyzed in aqueous acid solution to remove the starch moiety. The graft copolymers were also saponified with aqueous sodium hydroxide to convert nitrile groups to carboxamide and sodium carboxylate groups. The micrographs show that polyacrylonitrile is grafted both on the surface and in the interior of the granule. At higher grafting ratios the products contain increasing amounts of grafts inside the granule, but the centre of the granule is always hollow. The saponified graft copolymer retains its granular structure, in spite of the swelling and the high temperature during the hydrolysis, although the surface of the granules becomes smoother.
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