Polymerization of 1‐vinylindole and its methyl derivatives. Structure of the polymers

The polymerization of 1‐vinylindole (1a) induced by free radical and cationic initiators is studied and the structure of the resulting polymers is elucidated by 13C NMR spectroscopy. Also 2‐methyl‐1‐vinylindole (1b), 3‐methyl‐1‐vinylindole (1c), and 2,3‐dimethyl‐1‐vinylindole (1d) are polymerized in order to understand the polymerization mechanism of this class of monomers. Radical polymerization affords a linear polyvinylic structure in the case of 1b and 1d, and cyclic indoline type structures in the case of 1a and 1c. Cationic polymerization gives linear structures for poly(1c) and poly(1d), but cyclic structures for 1a. Evidence is given that cyclization involves position 2 of the indole ring in the radical polymerization, and position 3 in the cationic polymerization. Five and six membered rings are formed, respectively. The indole ring was found to be unreactive, under the adopted conditions, in the linear radical propagation reaction.