Minimization of homopolymer formation and control of dispersity in free radical induced graft polymerization using xanthate derived macro-photoinitiators

Abstract

Several macro-photoinitiators of different composition and molecular weights were prepared by the copolymerization of a photosensitive monomer, S-methacryloyl O-ethyl xanthate (MAX), using MMA or styrene as the comonomers. These macro-photoinitiators were used for the controlled grafting of monomers such as MMA and styrene under various experimental conditions using 350 nm irradiation. The graft yields and the molecular weights of the graft copolymers increased with increase in irradiation time. Similarly, the molecular weights of the graft copolymers increased with increase in mole percentage of the xanthate chromophores in the macroinitiators. The polydispersities of the graft copolymers remained below 1.5, which is the theoretical limiting value for free radical induced polymerization reactions, except in the case of graft polymers obtained after prolonged irradiation. Noticeably, the formation of homopolymers could be suppressed, which is a great advantage of the present approach. The control over homopolymerization and polydispersities has been explained on the basis of the distinctly different reactivities of the free radicals generated by the macroinitiator. The macroradicals generated are mainly responsible for initiating the grafting of monomers whereas the less reactive thiocarbonylthiyl radicals are primarily involved in the termination of the graft chains, thereby preventing the homopolymer formation to a large extent. Thus, the use of xanthate-derived macro-photoinitiators is a simple and easy method for the synthesis of narrow dispersed graft copolymers, which are less contaminated with homopolymers.