Multicompartment Vesicles: A Key Intermediate Structure in Polymerization-Induced Self-Assembly of Graft Copolymers

The morphological evolution of graft copolymer-based nano-objects was monitored by light scattering and electron microscopy during their formulation in water by polymerization induced self-assembly using a photo-mediated reversible addition–fragmentation chain transfer mechanism. The copolymer models used were composed of a dextran backbone bearing poly(2-hydroxypropyl methacrylate) grafts of two degrees of polymerization (X). At a full monomer conversion, unilamellar vesicles (ULVs) and large compound nano-objects (LCNs) were formed when targeting X= 100 and 500, respectively. For X= 100, some spherical, worm-like, then jellyfish-like structures were progressively observed before the ULVs formation. For X= 500, electron cryotomography revealed an unprecedented intermediate morphology formed from the onset of self-assembly called a multicompartment vesicle (MCV) that fused to form LCN. The formation of MCV was attributed to a local phase separation between dextran and the residual 2-hydroxypropyl methacrylate inducing the appearance of multiple hydrophilic cores.