A General Method for the Synthesis and Isolation of Well-Defined Core Cross-Linked Multistar Assemblies: A Route toward Enhanced pH-Responsive Polymers

Abstract

A general method for the synthesis of well-defined multistar assemblies (in which the stars contain cross-linked cores) involving the atom transfer radical polymerization (ATRP) of macroinitiators (MIs) and cross-linker with the use of telechelic macroinitiators (TMIs) as the connecting blocks is described. Isolation of each type of multistar assembly with a high degree of specificity was achieved via a simple fractional precipitation process. Selective degradation of the PMMA-based and PtBMA-based multistar assemblies into their individual star units via cleavage of the disulfide group within the connecting blocks confirmed the synthesis of well-defined, low-polydispersity PMMA-based tristar and dumbbell polymers and PtBMA-based tetrastar, tristar, and dumbbell polymers. Hydrolysis of the PtBMA-based assemblies afforded amphiphilic poly(methacrylic acid) (PMAA) multistar assemblies and dumbbell polymers with enhanced pH responsiveness over their single star analogues, with changes in dh of 42% and 30%, respectively (this is in comparison to 22% for discrete PMAA CCS polymers). Furthermore, the first reported native-state images of these PMAA-based multistar assemblies and CCS polymers in solution obtained using cryogenic transmission electron microscopy (cryo-TEM) are presented, revealing the different conformations of the various multistar assemblies.