Great interest has been shown in the kinetics and mechanisms of free radical copolymerizations that produce highly alternating copolymers. In many of these systems the production of alternating copolymers seems to arise naturally, while in others it is necessary to add a metal halide or an organometal halide regulating agent to modify reactivities. Much of the interest in alternating radical copolymerizations centers around the precise mechanism of alternation and in particular whether or not any of the alternation arises from the involvement of monomer-monomer donor-acceptor complexes (sometimes called charge-transfer complexes).