Here we report the preparation of PEG-based thermoresponsive hyperbranched polymers via a facile in situ reversible addition-fragmentation chain transfer (RAFT) copolymerization using bis(thiobenzoyl) disulphide to form 2-cyanoprop-2-yl dithiobenzoate in situ. This novel one-pot in situ RAFT approach was studied firstly using methyl methacrylate (MMA) monomer, then was used to prepare thermoresponsive hyperbranched polymers by copolymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMEMA, M-n = 475), poly(propylene glycol) methacrylate (PPGMA, M-n = 375) and up to 30 % of ethylene glycol dimethacrylate (EGDMA) as the branching agent. The resultant PEGMEMA-PPGMA-EGDMA copolymers from in situ RAFT were characterized by Gel Permeation Chromatography (GPC) and H-1-NMR analysis. The results confirmed the copolymers with multiple methacrylate groups and hyperbranched structure as well as RAFT functional residues. These water-soluble copolymers with tailored compositions demonstrated tuneable lower critical solution temperature (LCST) from 22 degrees C to 32 degrees C. The phase transition temperature can be further altered by post functionalization via aminolysis of RAFT agent residues in polymer chains. Moreover, it was demonstrated by rheological studies and particle size measurements that these copolymers can form either micro- or macro photocrosslinked gels at suitable concentrations due to the presence of multiple methacrylate groups. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3751-3761