This article deals with the estimation of the reactivity of a novel methacrylic macromonomer with amphiphilic character derived from Triton X-100 (NIT) in radical copolymerisation reaction with acrylic acid (AA). Two approaches to estimate the reactivity ratio of the macromonomer are described. The first involves the use of the Jaacks equation valid for systems with a large excess of one of the comonomers and leads to r(AA) = 0.31. The second method uses a low molecular weight model monomer (MTm) that reproduces the chemical structure of the macromonomer. A non-linear fitting of the experimental data of the system MTm-AA to an integrated form of the copolymefisation equation which describes the terminal model postulated by Mayo and Lewis, gave reactivity ratio values of r(MTm) = 2.5 and r(AA) = 0.30. The results obtained from the two estimations suggest that methacrylic double bond reactivity is not affected by poly(oxyethylene oxide) chain length. In situ quantitative H-1 NMR analysis was used to monitor monomer consumption and therefore to follow the course of the copolymerisation reaction in both approaches. (c) 2005 Elsevier Ltd. All rights reserved.