| In this dissertation, we designed and synthesized cleavable polystyrene- b-poly(ethylene oxide) (PS-b-PEO) block copolymer. First monomethylated poly (ethylene oxide) was modified byω,3-benzylsulfanythiocarbonylsufanyl propionic acid (BSPA) and the macro-RAFT agent mPEO-BSPA was prepared. The cleavable linking unit thiocarbonylthio group, which can be broken under mild conditions, was introduced between the two blocks of PS-b-PEO by reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene in the presence of mPEO-BSPA. The polymerization was initiated with azobisisobutyronitrile (AIBN) under bulk condition or in toluene solution. The RAFT polymerization was also initiated at high temperature. The influences of the way of initiation and the concentration of the chain transfer agent on molecular weight distribution were investigated. It was found that the higher concentration of the chain transfer agent led to the narrower the polydispersity. Taking advantage of the living/radical polymerization, we synthesized two series of PS-b-PEO block copolymers (PEG44PSn and PEG113PSn) with different molecular weights but narrow polydispersity. The block copolymers obtained were characterized by 1H-NMR and GPC.Two methods were applied to cleave the block copolymers: (1) chemical reduction. Potassium borohydride or n-butylamine was used as reducing agents and the trithioncarbonate in the polymers were cleaved to give the separated two blocks. (2) photoreduction. Azobisisobutyronitrile (AIBN) or benzoperoxide was used as the photo-initiator. Under UV light, the trithiocarbonate linkage can be cleaved by radicals generated by photo-initiator. The results showed that the photoreduction is a more rapid and efficient that can cleave the trithiocarbonate while avoiding the coupling of the thiol end groups.The self-assembly behaviors of PS-b-PEO in cyclohexane/dioxane and in N, N-dimethylormfamide (DMF)/ethanol and subsequent photoreduction of these two micellar solutions were studied. For PS-b-PEO in cyclohexane/dioxane, reverse micelles with the crystalline PEO core were formed. After photo-reduction, we observed that the radius of the micelles decreased, but the crystalline PEO would aggregate without the protection of the PS corona. In DMF/ethanol mixed solvent, PS-b-PEO copolymer self-assembled into micelles with glassy PS as the core surrounded by a soluble PEO corona. After photo-reduction, the micelles were cleaved and the micelles' radius decreased. As the irradiation time was prolonged, the efficiency of photo-reduction was improved. However, the PS block might precipitate from the solution without the stabilization of the PEO block..During the micellization of PS-b-PEO copolymer, we exploited the utility of the functionality sulfhydryl group generated by reduction of the cleavable group -- thiocarbonylthio group between the two blocks of PEO-b-PS with sodium borohydride (NaBH4). The thiol functionalities were used to synthesize polymer-gold composite nanoparticles. |
