Thermoresponsive polymers exhibiting upper critical solution temperature (UCST)-type behavior under physiologically relevant conditions have potential as biomaterials. The phase separation temperature (T-p) of the UCST-type polymers can be increased by copolymerization with hydrophobic comonomers. Quantitative index parameters that could be used to rationally tune the T-p are lacking, however. We have reported that ureido copolymers such as poly(allylamine-co-allylurea) (AM-PU) and poly(L-ornithine-co-citrulline) exhibit UCST-type solution behavior under physiologically relevant conditions. In this study, AM-PU was hydrophobized by acylation. T-p of AMPU can be regulated in a wide temperature range from about 20 to 80 degrees C or even higher by 20 mol % acylation with acetyl, propionyl, isobutanoyl, or pivaloyl groups, implying considerable impacts of hydrophobic groups on UCST phase separation. We observed a liner relationship between T-p and the hydrophobic parameter, log P, of the acyl groups. Furthermore, the acylation significantly reduced the influence of serum components on T-p by eliminating interactions of copolymers with serum components such as proteins and lipids. Acylation also abolished pH dependence of T-p which had been observed for unmodified AM-PU. Owing to the simple relationship between log P and T-p and the inertness of the acylated copolymer to serum components and pH changes, it is possible to rationally design copolymers exhibiting UCST-type behavior at a desired temperature under biological conditions.