A series of well-defined, semibranched hydrophilic-hydrophobic diblock and triblock copolymers have been synthesized in high yield under environmentally friendly conditions via atom transfer radical polymerization (ATRP). The hydrophilic block is based on methoxy-capped oligo(ethylene glycol) methacrylate (OEGMA), a branched analogue of PEO, and the hydrophobic component is a linear PPO block. In view of the commercial importance of linear, nonionic PEO-PPO type surfactants, we were interested in the aqueous solution properties of these related, semibranched POEGMA-PPO diblock and triblock copolymers. High conversions of POEGMA (> 95%) and good control (high initiator efficiencies and low polydispersities) were routinely achieved within 16 h at 20 degreesC. The polymerization is first order with respect to monomer up to 95% conversion, which implies that the polymer radical concentration remains constant for the duration of the polymerization. As the PPO block length was fixed at 33 units, the hydrophilic-hydrophobic balance of the block copolymers was adjusted by varying the Dp of the POEGMA block; this was achieved simply by changing the monomer-to-macroinitiator ratio from 14 to 97. With linear PEO-PPO block copolymers the limiting surface tension is governed by the PPO block length. With the semibranched POEGMA-PPO copolymers a similar trend was also evident, since, for the fixed Dp of 33 for the PPO block, the surface tension curves are almost independent of both the length of the POEGMA block and the diblock/triblock copolymer architecture. Combined H-1 NMR and DLS studies indicated that the POEGMA-PPO copolymers are thermoresponsive, forming large, highly hydrated micelles reversibly at 20-60 degreesC. This is understandable since coronal branching should reduce micellar packing efficiency, leading to bigger, looser micellar aggregates.