Synthesis of star polymers by a new "Core-First" method: Sequential polymerization of cross-linker and monomer

Star polymers containing a highly cross-linked core and many arms were synthesized by sequential polymerization of cross-linker and monomer. This novel method, termed as "star from in situ generated core", belongs to the category of "core-first" method and presents an alternative strategy for star synthesis when compared to the traditional "arm-first" method, in which monomer is polymerized first before cross-linker. To illustrate this new concept for the synthesis of star polymers by using controlled radical polymerization techniques, atom transfer radical polymerization was applied for homopolymerization of ethylene glycol diacrylate (EGDA) to generate a multifunctional cross-linked core (nanogel). At high cross-linker conversion, monovinyl monomers were added into the system and polymerized from the polyEGDA nanogel macroinitiator (MI) to form the star arms. By use of different acrylate monomers, including methyl acrylate, n-butyl acrylate, and tert-butyl acrylate, star polymers with different arm chemical compositions were successfully synthesized. Several parameters affected the star structures, such as the initial concentration of cross-linker, the molar ratio of cross-linker to initiator, and the addition moment of monovinyl monomer. The star polymers preserved the initiating sites at the chain ends, and they were further used as star MIs for arm extension by polymerization of a second monovinyl monomer to form a star block copolymer.