We report the development of a new gelation mechanism and a new family of polymers that self-assembles to form gels in a thermoreversible fashion. The polymers are block or star copolymers with a central hydrophilic poly(ethylene glycol) (PEG) segment (A) and temperature responsive poly(N-isopropylacrylamide) (PNIPAAm) terminal segments (B). Copolymers of various architectures, AB, A(B)(2), A(B)(4), and A(B)(8), were synthesized to investigate the structures and properties relationship. At 5 degreesC, the viscosities of 20 wt % solutions were between 700 and 950 cP, and they could be easily injected through a 25 G needle. Upon warming to body temperature, A(B)(2), A(B)(4), and A(B)(8) formed a strong associative network gel with aggregates of PNIPAAm segments acting as physical cross-links, whereas AB formed a weaker gel by micellar packing and entanglement. The values of elastic modulus, loss tangent, and yield strength were 1000-2500 Pa, 0.24-0.62, and 200-860 Pa, respectively. The gelation kinetic was fast; a typical gelation time for a solution of 5 mt in volume was less than a minute. No significant syneresis was observed after 2 months at 37 degreesC. DSC results indicated that the thermal behavior of material was completely reversible even after 30 heat-and-cool cycles. These materials are promising candidates for in-situ gelation applications such as injectable drug delivery, tissue engineering scaffolds, and anatomical barriers.