The C-13 DEFT NMR spectra of semi-alternating copolymers of citraconic (alpha-methylmaleic) anhydride (CA) and styrene (ST) prepared in a very polar solvent, N,N-dimethyformamide (DMF), with AIBN show that more than 50 % of the linkage configuration at the cyclic CA units is in cis configuration. Since the cis linkage configuration may result when the electron donor-acceptor complex formed between the comonomers is involved in the propagation, it is considered that, in very polar DMF, a participation of the complex is the driving force for the copolymerization.