Comonomer donor-acceptor complexes

This paper summarizes chemical grafting techniques explored in this laboratory that have potential biomedical application. These reactions, initiated by ceric ions, persulfate-bisulfite redox systems, or the presence of comonomers forming donor-acceptor complexes, were carried out in an aqueous environment under conditions which, with suitable modifications, might be tolerated in vivo. Grafting onto tissue surfaces by means of ionizing radiation will not be discussed since techniques for avoiding undesirable side reactions have not yet been developed. 

First, from e.g. strong donor — acceptor complex between comonomers or even a betaine structure. This intermediate undergoes faster ion-coupling than the corresponding ion-molecule reaction. 

In this paper, we report efforts to find donor/acceptor systems, comprised of at least one multifunctional monomer, capable of sustaining rapid free-radical polymerization without the need for external photoinitiators. Although we will include in this report comonomer systems which form ground state CT complexes, we stress that the primary mechanism for generating free-radical in each case may not be via excitation of ground state CT complexes. 

The formation of alternating copolymer is attributed to the homopolymerization of a comonomer charge-transfer complex. The latter is formed spontaneously, subject to equilibrium considerations, with the interaction of a strong donor monomer and a strong acceptor monomer. 

The most widely accepted explanation for these surprising results is the formation of an charge-transfer complex between the electron acceptor MAH and an electron donor comonomer that exclusively or predominantly participates in the chain growth process [973,974]. 

While attempting to establish the existence and role of the CTC in alternating copolymerizations, many authors have determined the equilibrium constant K) for a large number of MA-comonomer pairs (table in appendix to this chapter). Other charge-transfer complexes of MA have also been extensively investigated,by the use of ultravioletand spectroscopic methods (table in appendix to this chapter). Identification of the CTC is centered on the concentration and temperature dependence of the absorption in the visible and ultraviolet spectra. In NMR, the acceptor or donor protons undergo a chemical shift when the CTC is formed and this variation is used to determine the K values. Calorimetric methods have also been used to determine equilibrium constants. 

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