Small-angle neutron scattering and gel permeation chromatographic study on in-situ observation of N-isopropylacrylamide - Poly(ethylene glycol) diblock copolymer quasi-living polymerization

To synthesize an amphiphilic block copolymer of poly(N-isopropylacrylamide) -block-poly(ethylene glycol) (NE), a soap-free emulsion polymerization method was employed in an aqueous medium, where Poly(N-isopropylacrylamide) (PNIPA) was polymerized from the radically activated chain ends of poly(ethylene glycol) (PEG). As the polymerization proceeds, PNIPA block chains form micelle cores stabilized by PEG brush chains emanating therefrom. When this polymerization was carried out at temperatures equal to or higher than 34°C, narrowly-dispersed NE, which cannot be obtained by solution polymerization, was successfully obtained. Time-resolved gel permeation chromatography (GPC), enabling us to observe time-dependent monomer conversion and molecular weight of NE, revealed that radical polymerization proceeds living-likely. To elucidate origins of the living nature, we employed time-resolved ultra-small- and small-angle neutron scattering (USANS and SANS), which are advantageous to explore in-situ and at real time micelle formation during polymerization. SANS revealed that (i) from 25 to 60°C, polymerized NE forms micelle in the solution when M_n reaches to a critical molecular weight for micelle formation M_n,c, and (ii) as the temperature increases from 34°C, a water content in the micelle core becomes less because PNIPA block chains have a lower critical solution temperature around 34°C. These results obtained by GPC, USANS, and SANS strongly support our scenarios and low diffusivity of chain end radicals, compartmentalized in the micelle core, leads to the quasi-living behavior of the polymerization.