Poly(oxy-1-chloromethylethylene-co-oxyethylene) copolymer (PECH-PEO) has been modified chemically by phenolate, producing dehydrochlorination as a side reaction, because of its basic character. The comonomer composition of the resulting poly(oxy-1-chloromethylethylene-co-oxyethylene-co-oxy-1-methyleneethylene-co-oxy-1-phenoxy-methylethylene) can be quantitatively determined by C-13 NMR spectroscopy. A series of copolymeric models were synthesized to make the complete assignment of all the signals appearing in the spectrum of the tetrapolymer. The various C-13 NMR signals in the spectra were assigned to dyads, triads or tetrads of all the possible comonomer sequences and in some cases to the tacticity.