Synthesis of Low-Polydispersity Miktoarm Star Copolymers via a Simple “Arm-First” Method: Macromonomers as Arm Precursors

Abstract

Miktoarm star copolymers with variable arm compositions, high star yield (>85%), and low polydispersity (Mw/Mn < 1.2) were successfully synthesized by a simple “arm-first” method, one-pot cross-linking a mixture of linear macromonomers (MMs) and/or macroinitiators (MIs), via the atom transfer radical polymerization (ATRP) technique. Characterization of the star product by liquid adsorption chromatography proved that the star polymers obtained by cross-linking different MMs and MIs were miktoarm stars containing two arm species in one molecule. The incorporation of MM into miktoarm star molecule increased the star molecular weight and the average number of arms but did not change the number of initiating sites in the star core because the initiating sites independently originated from the incorporated MIs or low molar mass ATRP initiators. By using lower amount of initiator, the molar ratio of initiating sites to incorporated arms in the miktoarm star was decreased, which effectively reduced the extent of star−star coupling reactions and resulted in the formation of miktoarm star copolymers with low polydispersity. The molar ratios of arms in the miktoarm star copolymers were in agreement with the composition of initial MMs and/or MIs mixture. During the cross-linking reaction of a mixture of two MMs, addition of extra cross-linker and low molar mass initiator increased the molecular weight and yield of the miktoarm star molecules while preserving their low polydispersity.