Interface polymerization in a polymer micelle: An NMR study of the radical polymerization of methyl methacrylate at the core-shell interface of polystyrene-block-poly(methacrylic acid) micelles

摘要

Systems containing poly(styrene)-block-poly(methacrylic acid) micelles (1.67 g/L), methyl methacrylate (1.88-3.76 g/L), and initiator (polymerization) or inhibitor (pure solubilization) in D2O buffer solution were studied by H-1 NMR. The proton NMR signals of MMA dissolved in the micellar system have the same chemical shifts as those in the corresponding D2O buffer solution but exhibit a characteristically broadened and asymmetric shape. Separate signals with different chemical shifts and signal shapes were observed for MMA absorbed into the poly(styrene) core. From the shape? analysis of the signals of the water-dissolved MMA in the micellar system, it can be deduced that the monomer is almost exclusively accumulated near the core-shell interface. Its radial distribution can be approximated by a Gaussian function with the maximum at the core radius R and half-width b of 1.65R and 1.93R for the MMA concentrations 2 and 4 g/L, respectively. An increase in temperature from 295 to 330 K leads to a faster self-diffusion of MMA but not to an appreciable broadening of the distribution. When initiated with ammonium peroxysulfate (330 K) or its mixture with potassium disulfite (295 K), MMA at the interface undergoes polymerization. At elevated temperatures such as 330 K, a part of MMA diffuses into the outer layers of the polystyrene core and does not polymerize there, in particular at higher MMA concentrations, being shielded by the peel of PMMA from the initiator radicals. AL 295 K, the diffusion is very slow so that no detectable amount of MMA avoids polymerization. The observed polymerization kinetics as well as the MMA signal shape evolution indicate that (i) during polymerization, the PMMA formed diffuses into the vicinity of the core-shell interface and (ii) termination is markedly suppressed in analogy with a gel effect. Mathematical models of the NMR signal shape as well as of diffusion-affected polymerization kinetics under general radial monomer distribution in a micelle are presented and the numerical computer simulations are compared with the experimental data. The results of this study agree with the separately published SANS results in the conclusion that micelles with cores sheeted with multiple layers of different polymers can be prepared by the relatively simple technique presented. [References: 19]