Poly(N-isopropylmethacrylamide) (PiPMA) has one more methyl group at each monomeric unit than poly(N-isopropylacrylamide) (PiPA). By use of laser light scattering (LLS) and ultrasensitive differential scanning calorimetry (US-DSC) we have investigated the association and dissociation of PiPMA chains in water. LLS studies reveal that PiPMA chains form larger aggregates at a temperature above its lower critical solution temperature (LCST) as the chain molar mass (M-w) decreases. In comparison with PiPA aggregates, PiPMA aggregates show. a larger ratio of average radius of gyration to average hydrodynamic radius (< R-g >/< R-h >), indicating that PiPMA aggregates are looser. US-DSC studies show PiPMA chains have smaller enthalpy change (Delta H) and entropy change (Delta S) than PiPA chains during the phase transition, indicating that PiPMA chains have smaller conformational change. Our experiments demonstrate that the additional methyl groups in PiPMA chains restrain the intrachain collapse and interchain association, leading the phase transition to occur at a higher temperature.