In an effort to create an in situ physically and chemically cross-linked hydrogel for in vivo applications, N-isopropylacrylamide (NIPAAm) was copolymerized with poly(ethylene glycol)-monoacrylate (PEG-monoacrylate) and then the hydroxyl terminus of the PEG was further modified with acryloyl chloride to form poly(NI-PAAm-co-PEG) with acrylate terminated pendant groups. In addition to physically gelling with temperature changes, when mixed with a multi-thiol compound such as pentaerythritol tetrakis 3-mercaptopropionate (QT) in phosphate buffer saline solution of pH 7.4, this polymer formed a chemical gel via a Michael-type addition reaction. The chemical gelation time of the polymer was affected by mixing time; swelling of the copolymer solutions was temperature dependant. Because of its unique gelation properties, this material may be better suited for long-term functional replacement applications than other thermo-sensitive physical gels. Also, the PEG content of this material may render it more biocompatible than similar HEMA-based precursors in previous simultaneous chemically and physically gelling materials. With its improved mechanical strength and biocompatibility, this material could potentially be applied as a thermally gelling injectable biomaterial for aneurysm or arteriovenous malformation (AVM) occlusion. (C) 2007 Wiley Periodicals, Inc.