Reversible addition-fragmentation transfer polymerization in nanosized micelles formed in situ

The reversible addition-fragmentation transfer (RAFT) polymerizations of 4-vinylpyridine (4VP) in tetrahydrofuran (THF) and in cyclohexane with RAFT agent, dithiobenzoate-terminated polystyrene (PS-SC(S)Ph), involve one polymerization rate (R-p = 0.083 mol L-1 h(-1)) and two stages of polymerization (Rp = 0.164 and 0.0024 mol L-1 h(-1) before and after 5 h), respectively. The polymerization of 4VP and divinylbenzene (less than 10% relative to 4VP) in THF led to gelation, but the polymerization in cyclohexane displayed clear solution consistently throughout polymerization, and kinetic studies showed sudden decrease of polymerization rate and sharp increase of molecular weight at around 5 h. Polymerization was followed by gel permeation chromatography (GPC) and the combination of GPC and multiangle laser light scattering (MALLS). The results show the formation of micelles with PS as shell and poly(PVP-co-DVB) as core by microphase separation; the micelles' size increased fast around 5 h polymerization, and then the micelles grew slowly with progress of polymerization. A series of experiments were made to look for reasons for the decrease of polymerization rate at around 5 h of polymerization, and the possible reasons are the restriction of diffusion and higher concentration of dithiobenzoate groups in the cores of micelles. The effects of molecular weight of RAFT agent and the content of DVB in the mixture of 4VP and DVB on the polymerization and the formation of micelles were also investigated. H-1 NMR, dynamic and static light scattering (DLS, SLS), transmission electron micrograph (TEM), and atomic force microscopy (AFM) were used to characterize the micelles, and the micelles with less than 50 nm in diameter and narrow size distribution were obtained. Thus, an efficient synthetic method of stable micelles was developed in comparison with the self-assembling of block and graft polymers in selective solvents, and one advantage of this method is that the polymerization, micellization, and cross-linking reactions occur in one pot, forming stable, narrow micelles with functional cores.