This is an extension of our previous work on mechanistic studies of the degradation of the main chain during the chemical modification of poly(oxy-1-chloromethylethylene) (PECH) with phenolate to similar studies on the modification of poly(oxy-1-chloromethylethylene-co-oxyethylene) (PECR-PEO) with the same reagent. This polymer has been chemically modified with phenolate in the presence of a quaternary ammonium salt, i.e. tetrabutylammonium bromide (TBAB). Several reaction conditions were tested to provide the highest modification degrees with the minimum main-chain cleavage. The substitution of chlorine by phenolate was found to be accompanied by elimination reactions that led to vinylether moieties. The C-13 NMR technique was used to identify the different terminal groups arising from this cleavage. The nature of those groups confirmed that cleavage was caused by nucleophilic attack on the units neighbouring vinylic ones, mainly on the V (V) under bar dyads. The results are consistent with those obtained from the modification reaction of poly(oxy-1-chloromethylethylene) (PECH) with phenolate.