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

Several macro-photoinitiators of different composition and molecular weight s were prepared by the copolymerization of a photosensitive monomer, S-meth acryloyl O-ethyl xanthate (MAX), using MMA or styrene as the comonomers. Th ese 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 copol ymers 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 limiti ng value for free radical induced polymerization reactions, except in the c ase 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 polydisper sities has been explained on the basis of the distinctly different reactivi ties of the free radicals generated by the macroinitiator. The macroradical s 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-photo initiators is a simple and easy method for the synthesis of narrow disperse d graft copolymers, which are less contaminated with homopolymers.