Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
A few redox systems containing thiourea as reductant have been found to be quite effective in initiating vinyl polymerization in aqueous media and polymers obtained in the process have all been found to contain amino endgroups to various extents by the application of dye techniques. Quite a few oxidants have so far been utilized for this purpose; among them are ferric chloride (Fe3+), ethylene dibiguanide complex salts of tripositive silver (Ag3+), hydrogen peroxide (H2O2), persulfate (S2O82-), bromate (BrO3-) + hydrochloric acid (HCl). In case of oxidants Fe3+ and Ag3+, amino endgroups are mainly incorporated in polymers; but in case of oxidants H2O2 and S2O82-, fragments of oxidants are also incorporated as hydroxyl and sulfate endgroups. BrO3-, however, forms a very efficient redox-initiating system with thiourea, as is evidenced by its capability of polymerizing even hydroquinone-stabilized water-soluble vinyl monomers at very low temperature (∼0°C.) and at a quite rapid rate. Besides amino endgroups, sulfonate endgroups have also been detected in polymers in this case, and the relative extents of these two types of endgroups depend generally on the acid concentration of the system. Evidences so far collected indicate the generation of S—C(=NH)NH2 radicals in the system by oxidation of isothiourea, HS—C(=NH)NH2, and these are incorporated in polymers as endgroups. Sulfonate endgroups may be generated by oxidation of these amino-bearing endgroups. Suitable initiation mechanisms have been suggested in each case.
Type of Medium: