The effects of hydrophobic chain end groups on the cloud points of thermo-sensitive water-soluble polystyrenics were investigated. Well-defined poly (4-vinylbenzyl methoxytris(oxyethylene) ether) (PTEGSt) and poly(alpha-hydro-omega-(4-vinylbenzyl)tetrakis(oxyethylene)) (PHTrEGSt) were prepared by nitroxide-mediated radical polymerization using alpha-hydrido alkoxyamine initiators including two monomer-based initiators. The polymers were reduced with (n-BU)(3)SnH to replace the alkoxyamine end group with hydrogen. In the studied molecular weight range (M-n,M-GPC = 3000 to 28,000 g/mol), we found that the hydrophobic end groups decreased the cloud point by 1-20 degrees C depending on the molecular weight and the largest depression was observed at the lowest molar mass. The cloud points of PTEGSt and PHTrEGSt with two hydrophobic end groups, phenylethyl and alkoxyamine, exhibited a monotonic increase with the increase of molecular weight. For polymers with only one hydrophobic end group, either phenylethyl or alkoxyamine, the cloud point initially increased with the increase of molecular weight but leveled off/ decreased slightly with further increasing molar mass. For polymers with essentially no end groups, the cloud point decreased with the increase of chain length, which represents the "true" molecular weight dependence of the cloud point. The observed molecular weight dependences of the cloud points of polystyrenics with hydrophobic end group(s) are believed to result from the combined end group effect and "true" molecular weight effect. (C) 2007 Wiley Periodicals, Inc.