A thermal responsive block copolymer made up of ethylene oxide (EO) and propylene oxide (PO) blocks was simulated with optimized atomistic potentials and enhanced sampling methods over a range of temperatures. The results for the L42 pluronic polymer (EO)(4)(PO)(22)(EO)(4), which is known to undergo a transition in this temperature range, and the similarly sized (EO)(30) polymer, which does not, are compared. The thermal responsive L42 polymers in a dilute solution tend to aggregate, and this tendency gets stronger as temperature increases. The poly(ethylene oxide) polymer shows no such tendency. The aggregation is stabilized by the hydrophobic contact of the propylene oxide methyl groups, which outweighs a small loss in hydrogen bonds between the ether oxygens and water.