The formation of core cross-linked star polymer and nanogel assemblies facilitated by the formation of dynamic covalent imine bonds

Abstract

Reversible addition-fragmentation chain transfer (RAFT) polymerization has been utilised to prepare diblock copolymer chains possessing ‘inert’ blocks of polystyrene and ‘reactive’ blocks displaying aldehyde or amino functional groups. These polymer chains were shown to cross-link through the formation of imine bonds in organic solvents to form kinetically stable polymer assemblies possessing core-cross-linked star (CCS) polymers architectures which display a size-dependency upon the concentration at which the cross-linking reactions are performed. Evidence was found for the formation of a 2-armed species as a side-product in the formation of CCS polymers. RAFT polymerization was utilised to prepare a series of methacrylate copolymers possessing only ‘reactive’ blocks displaying a significantly lower density of aldehyde and amino functional groups than the diblock copolymers utilized in the formation of CCS polymers. These copolymers were shown to cross-link through the formation of imine bonds in organic solvents to form polymer assemblies possessing nanogel architectures. The lower density of functional groups was required to promote the formation of discrete nanogel assemblies. This cross-linking also displayed a size-dependency upon the concentration at which the cross-linking reactions are performed. Macroscopic gelation of these polymer chains was observed at higher concentrations of the copolymer chains. Both the CCS polymers and nanogels underwent structural reconfiguration to linear polymer chains through component exchange facilitated by trans-imination with a small molecule amine. We speculate that the formation of both the CCS polymers and nanogels is under kinetic control on account of the lack of reversibility of imine bonds in organic solvents.