Synthesis And Self-assembly Of PNBMI-b-PS-b-PI Triblock Copolymers

A series of triblock copolymers, poly(N-butyl maleimide)-b-polystyrene-b- polyisoprene (PNBMI-b-PS-b-PI), with different block length, were synthesized via reversible addition fragmentation chain transfer (RAFT) polymerization. All triblock copolymer molecularly dissolve in toluene at room temperature. Upon addition of cyclohexane, the polymer can self-assemble into three-layer'onionlike'micelles. Micellar shell cross-linking were conducted via the reaction of side double bond groups of PI block with 1,4-butanedithiol under UV irradiation. The main contents were as follows:Preparation of triblock copolymer. A series of well-defined poly(N-butyl maleimide) (PNBMI) polymers were synthesized via RAFT polymerization of NBMI monomer at 60oC, using S-ethyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate (EMP) as chain transfer agenting (CTA) and azobisisobutyronitrile (AIBN) as initiator. Kinetic studies demonstrated the living/controlled characteristics of this polymerization process. In order to clarify the living character of this RAFT polymerization, RAFT polymerizations of styrene were conducted at 60oC using the PNBMI as a macromolecular CTA and AIBN as initiator. Moreover, a variety of well-defined PNBMI-b-PS-b-PI triblock copolymers were synthesized via RAFT polymerization of isoprene using PNBMI-b-PS as macro-CTA and DCP as initiator at 115oC.Micellization of PNBMI-b-PS-b-PI triblock copolymer. The triblock copolymer PNBMI-b-PS-b-PI was dissolved in freshly-distilled toluene at room temperature and the solution was added of freshly-distilled cyclohexane. Micellization occurred to inform micelles consisting of PNBMI core, PS shell, and PI coronas. The process of micellization was confirmed by 1H NMR. The results of 1H NMR show the copolymer chains in toluene are fully solvated, and all the signals expected for each block are visible. In contrast, the signals due to the PNBMI block atδ3.45 ppm completely disappeared in d8-toluene and d12-cyclohexane mixed solvent(v/v), while the signals for the PS and PI are still clearly evident, indicating the formation of the PNMBI-core micelles with an inner shell PS chains and an outer corona of PI chains. DLS studies revealed narrow, monomodal size dissolutions and average hydrodynamic diameter was about 30 nm. Hydrodynamic dimensions of the triblock copolymer micelles depend on both triblock copolymer concentration and the proportion of toluene and cyclohexane.Shell cross-linking of the triblock copolymer micelles (SCL). SCL micelles were synthesized via"thiol-ene"click reaction using 1,4-butanethiol as a cross-linking agent and 2,2-dimethoxy-2-phenylacetophenone (DMPA) as a photo-initiator under UV irradiation. 1H NMR results indicated that the"thiol-ene"click reaction was successfully conducted. The DLS results demonstrated that the formed SCL micelles were quit stable in toluene. From a TEM image of SCL micelles, we can see that SCL micelles are spherical micelles and the sizes range from 20 nm to 30 nm, similar to the value derived from DLS.