Alternation Polar Effects and Complex Participation

Two mechanisms have been proposed to explain the strong alternation tendency between electron-acceptor and electron-donor monomers. The polar effect mechanism (analogous to the polar effect in chain transfer—Sec. 3-6c-2) considers that interaction between an electron-acceptor radical and an electron-donor monomer or an electron-donor radical and 

The monomer complex participation (MCP) mechanism suggests that alternation results from homopolymerization of a 1 1 complex formed between donor and acceptor monomers [Cowie, 1989 Furukawa, 1986]  

Some pairs of very strongly electron-donor and electron-acceptor monomers, such as p-methoxystyrene and dimethyl cyanofumarate, undergo spontaneous alternating copolymerizations without any added free-radical initiator, although heat may be required [Hall and Padias, 1997, 2001]. Initiation involves reaction of the comonomer pair to form a diradical, 

Both the cross-propagation and complex mechanisms may be operative in alternating copolymerizations with the relative importance of each depending on the particular reaction system. The tendency toward alternation, with or without added Lewis, acid, is temperature-and concentration-dependent. Alternation decreases with increasing temperature and decreasing total monomer concentration since the extent of complex formation decreases. When the alternation tendency is less than absolute because of high reaction temperature, low monomer concentration, absence of a Lewis acid, or an imbalance in the coordinating abilities of the two monomers, copolymerization proceeds simultaneously by the two mechanisms. The quantitative aspects of this situation are considered in Sec. 6-5. 

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