Intelligent build-up of complementarily reactive diblock copolymers via dynamic covalent exchange toward symmetrical and miktoarm star-like nanogels

The formation of symmetrical and asymmetrical (miktoarm) star-like nanogels by the crosslinking reaction of complementarity reactive diblock copolymers via a radical crossover reaction of alkoxyamine units was successfully accomplished. Four types of diblock copolymers with alkoxyamine units connected at different positions and different chemical compositions were prepared by random copolymerization of methyl methacrylate and methacrylic esters with alkoxyamine from poly(methyl methacrylate) or polystyrene prepolymers using the atom transfer radical polymerization method. Star-like nanogels were formed, by dynamic covalent exchange among alkoxyamine units when heating a mixture of two different types of diblock copolymers in anisole. It was revealed that one block of diblock copolymers complementarity reacted; the formation of star-like arms and nanogel parts was revealed by small-angle X-ray scattering (SAXS) measurements and scanning force microscopic (SFM) observations. Furthermore, miktoarm starlike nanogels with different molecular weights and chemically asymmetrical chains were also formed by using several types of diblock copolymers. At the same time, structural transformation from star-like nanogels to diblock copolymers was successfully accomplished by dynamic covalent exchange among alkoxyamine units in the star-like nanogels and excess added alkoxyamine compounds. The molecular weights and sizes of the diblock copolymers and star-like nanogels were evaluated by gel permeation chromatography-multiangle light scattering (GPC-MALS), quasi elastic light scattering (QELS), and SAXS measurements, and their molecular conformations were estimated from SAXS profiles and SFM images.