Preparation of initiator and cross-linker-free poly(N-isopropylacrylamide) nanogels by photopolymerization

Poly(N-isopropylacrylamide) or PNIPAm nanogels with diameter of 50–200 nm were prepared from N-isopropylacrylamide monomer by photopolymerization in the absence of initiator, cross-linker and surfactant. Morphology transition of the nanogels from branch to compact, global one was tuned with NIPAm(N-isopropylacrylamide) concentration in reaction. Reaction mechanism of the nanogels formation was proposed. The yield of prepared nanogels can increased from ca. 20 up to 86% when solution pH varied from neutral to 2. ESR signals confirmed that the existence of H+ in reaction could accelerate the polymerization of NIPAm.