Donor‐acceptor complexes in copolymerization. VI. A proposed mechanism for the homopolymerization of comonomer complexes to alternating copolymers

After a brief review of the characteristics of the copolymerization of donor and acceptor monomers in the presence of metal halides, a mechanism is proposed for the formation of alternating copolymers. The charge‐transfer donor‐acceptor complex resulting from the interaction of a strong electron‐acceptor monomer and a strong electron‐donor monomer undergoes spontaneous or radical initiated homopolymerization to yield an equimolar alternating copolymer, irrespective of the initial monomer ratio. The formation of the [D A] complex is enhanced by complexation of the acceptor monomer with a metal halide or organometal halide. The homopolymerization of the (D A… [MX] occurs at a faster rate than conventional radical copolymerization. The spontaneous polymerization occurs when the equilibrium concentration of the complex is sufficiently high. The latter depends upon the electron‐donating strength of the donor monomer and the electron‐accepting strength of the acceptor monomer and the activator. Complex formation is favored at lower temperatures while polymerization may require thermal or catalytic activation. The complexes may be aligned in an ordered array or matrix, and polymerization proceeds through a rapid initial stage in which the molecular weight increases with conversion, followed by a slower stage in which the molecular weight remains constant at the highest level attained in the first stage, indicative of the first‐stage polymer acting as a template for the second‐stage polymerization. The proposed polymerization mechanism involves initiation by a head‐to‐head reaction of two complexes during which hydrogen abstraction or transfer occurs through a six‐membered cyclic transition state to generate a polymer chain with an attached terminal complex. The propagation reaction is head‐to‐tail and involves the addition of complexes to the chain end through hydrogen abstraction via a six‐membered cyclic transition state to regenerate the complex chain end.