Copolymerization of 1‐vinylindol with maleic anhydride

The formation of a charge‐transfer complex between 1‐vinylindole (1a) and maleic anhydride (MA) was detected by 1H NMR analysis, and the corresponding Keq was measured (Keq=0,251 mol in CDCl3 at 25°C). It could be shown that this complex can be involved both in the initiation and propagation steps of the 1a‐MA copolymerization by carrying out spontaneous copolymerization experiments and studying the influence of the feed composition on the conversion and [η] of the copolymer. The structure of the 1a‐MA copolymer, investigated by 13C NMR spectroscopy, was found to be complicated due to the formation of indoline rings along the chains by an attack of the propagating MA radical at position 2 in the indole ring. As a consequence an excess of MA, with respect to a 1:1 mole ratio of monomer feed, was usually found in the copolymer. The mechanism of the cyclization reaction was clarified by copolymerizing MA with 2‐methyl‐1‐vinylindole (1b), 3‐methyl‐1‐vinylindole (1c), or 2,3‐dimethyl‐1‐vinylindole (1d). An alternating copolymer with MA (mole ratio 1:1 of monomeric units), without irregularities, was obtained only by employing 1d as a comonomer.