Synthesis Of Cationic Amphiphilic Triblock Copolymers And Their Self-Assembly In Solution

Micellization of block copolymers in selective solvents has attracted a great deal of interest due to the potential applications of micelles in drug delivery systems, microcapsules and catalysis. The micelles are stabilized in solution via the interactions of the soluble blocks with the solvent molecules. Depending on the composition of the block copolymers, two types of micelles can be distinguished:star and crew-cut. It has been found that amphiphilic block copolymers can form crew-cut aggregates of various morphologies, including spheres, rods, bicontinuous rods, lamellae, vesicles, large compound micelles (LCMs), large compound vesicles (LCVs), hexagonally packed hollow hoops (HHHs), and many others. There are many factors influencing the self-assembly of amphiphilic copolymers:the water content, the block length and structure of polymers, the initial copolymer concentration, nature and composition of common solvents and selective solvents and the presence of additives. Although aggregates can be prepared from a wide range of amphiphilic block copolymers, the self-assemble behaviors of cationic amphiphilic block copolymers have rarely been reported. Compared with other controlled/living free radical polymerization, reversible addition-fragmentation chain transfer (RAFT) polymerization has many advantages. For example, there is no residue in the polymers. In this research, polystyrene-block-poly(vinylbenzyl chloride)-block-polystyrene(PS-b-PVBC-b-PS) and poly(vinylbenzyl chloride)-block-polystyrene-block-poly (vinylbenzyl chloride)(PVBC-b-PS-b-PVBC) were synthesized by RAFT polymerization. Cationic amphiphilic copolymers PS-b-Q-PVBC-b-PS and Q-PVBC-b-PS-b-Q-PVBC were achieved by the quaternization of PVBC.And then self-assemble behaviors of PS-b-Q-PVBC-b-PS and Q-PVBC-b-PS-b-Q-PVBC in solution were studied.RAFT polymerization technique was applied to prepare triblock copolymers PS-b-PVBC-b-PS and PVBC-b-PS-b-PVBC.Cationic amphiphilic triblock copolymers PS-b-Q-PVBC-b-PS and Q-PVBC-b-PS-b-Q-PVBC were prepared by quaternarization of VBC with excess triethylamine. The molecular weight and molecular weight distributions of macro-CTA and corresponding triblock copolymers were obtained by GPC (gel permeation chromatography). FI-R and1H-NMR spectra were used to characterize the structure of polymers. The results showed that cationic amphiphilic triblock copolymers PS40-b-Q-PVBC26-b-PS40, Q-PVBC13-b-PS85-b-Q-PVBC13and PS54-b-Q-PVBC56-b-PS54were successfully prepared. PS4o-b-Q-PVBC26-b- PS40and Q-PVBC,3-b-PS85-b-Q-PVBC13have almost identical molecular weight and block ratio.The self-assemble aggregate morphologies of copolymers PS40-b-Q-PVBC26-b-PS40and Q-PVBC,3-b-PS85-b-Q-PVBC13in different selective solvents were observed by TEM. It’s found that, under same conditions, aggregate morphologies are spheres in amphiphilic triblock copolymers with the corona-forming segment at the end and for the amphiphilic triblock copolymers with the corona-forming segment in the middle, the aggregate morphologies are core-corona micelles (with relatively low content of selective solvent) or rods mixed with irregular spheres (with higher content of selective solvent). Selective solvents also have a great effect on the aggregate morphologies of the cationic amphiphilic triblock copolymer Q-PVBC13-b-PS85-b-Q-PVBC13. When water is used as the selective solvent, the micelles are spheres and the diameter is403nm. On the contrary, the swelling, hollow, widely distributed and irregular spheres with size about350nm are formed when using methanol as the selective solvent. In addition, the aggregate morphologies of Q-PVBC,3-b-PS85-b-Q-PVBC13were affected by the nature and composition of common solvents. When pure DMF was used as common solvent, the aggregate morphologies were sphere. As the content of dioxane increases, the aggregate morphologies changed from spheres to rods, vesicles, and large compound micelles.The critical water content (CWC) of PS54-b-Q-PVBC56-b-PS54decreases as the polymer concentration increases. A linear relationship is found between the CWC and the logarithm of the polymer initial concentration. The micelle fraction also depends on the copolymer initial concentration. At certain water content range, the micelle fraction increase as the initial concentration increases. The aggregate morphologies of PS54-b-Q-PVBC56-b-PS54are affected by the water content, composition of selective solvent, temperature and the rate of adding water. Once the water content reaches CWC, the polymer starts to aggregate to form sphere micelles. As the water content increases, the aggregate morphologies change from sphere to rod, vesicle and large compound micelles. Similarly, when pure methanol is used as selective solvent, there are sphere micelles. As water is added into the selective solvent, the aggregate morphologies change from sphere to rod, bilayer and vesicle. In addition, with the temperature increasing and the rate of adding water decreasing, the aggregate morphologies also change from sphere to rod and finally to large compound micelles.