Synthesis Of Copolymers With Complex Structures Based On RAFT Polymerization And Their Self-assembly Behavior

In the field of polymer science, it is always attractive for polymer scientists to design and synthesize the copolymers with well-defined structures and investigate their physico-chemical properties. Nowadays, as the various living polymerization techniques are developed rapidly, polymer scientists could prepare conveniently the copolymers with well-defined and complex structures.In this thesis, on the basis of reversible addition-fragmentation chain transfer polymerization, a series of copolymers with complex structures (comb-like, linear-comb diblock, comb-linear-comb triblock and linear triblock) were synthesized by combination RAFT with other living polymerization techniques using a variety of chain transfer agents, and they are characterized by different means as UV, ¹H NMR and GPC etc in details. The aggregation behavior of these copolymers in aqueous media was also investigated.1. Random copolymer P(St-co-HEMA) with many hydroxyl groups was successfully synthesized by copolymerization of St and HEMA via RAFT polymerization in which PPDTB was used as chain transfer agent, and polymerization data demonstrated that the copolymerization was controllable. The average number of hydroxyl groups of copolymer chains was obtained by NMR analysis for copolymers composition. P(St-co-HEMA)s with controlled molecular weight, narrower molecular weight distribution and different amount and density of hydroxyl groups were obtained by changing the polymerization conditions.Comb-like copolymer P(St-co-HEMA)-g-PCL was then prepared by coordination-insertion ring-opening polymerization of ε -CL using P(St-co-HEMA) as macro-initiator and stannous octoate (Sn(Oct)₂) as catalyst, the grafting polymerization was also under control. By changing the ratio of monomer to macro-initiator, the graft copolymer P(St-co-HEMA)-g-PCLs with different length of side chains were obtained. Here, a new method of calculate the grafting efficiency was used, and the grafting efficiency was as high as 100%. It meant that nearly all the hydroxyl groups of macro-initiator took part in the initiation, and the final comb-like copolymer P(St-co-HEMA)-g-PCLs with narrow molecular weight distribution, different grafting density and different lengths of grafting side chains were successfully synthesized.It was confirmed by DSC that the melting point of all graft copolymer P(St-co-HEMA)-g-PCL was lower than their corresponding homopolymer PCL and was affected by length of graft chains, that was the shorter the length of graft chain, the lower the melting point2. A novel linear-comb amphiphilic copolymer mPEO-b-[P(St-co-HEMA)-g-PCL] was successfully synthesized by combination of RAFT, ring-opening anionic polymerization and coordination-insertion ring-opening polymerization (ROP).The α -methoxy poly(ethylene oxide) (mPEO) with ω,3-benzylsulfanylthio-carbonylsufanylpropionic acid (BSPA) end group (mPEO-BSPA) was prepared by the coupling reaction of hydroxyl end group of mPEO with 3-benzylsulfanylthiocarbonylsufanylpropionic acid chloride (BSPAC), and the reaction efficiency was close to 100% which meant nearly all the hydroxyl groups of mPEO were modified by BSPA.After that, the mPEO-BSPA was used as a macro-RAFT agent to mediate the copolymerization of St and HEMA using 2,2-azobisisobutyronitrile (AIBN) as initiator, and the diblock copolymer mPEO-b-P(St-co-HEMA) was obtained, its molecular weight was increased proportionally with the monomer conversion and the polymerization process was controllable. It was noticed that the molecular weight of macro-RAFT agent could exert effect on the polymerization procedure.Finally, the ROP of ε -caprolactone was then completed by initiation of hydroxyl groups of the mPEO-b-P(St-co-HEMA) precursors in the presence of Sn(Oct)₂. Thus, the amphiphilic copolymer of linear mPEO linked with comb-like P(St-co-HEMA)-g-PCL was obtained. The final and intermediate products were characterized in detail by ¹H NMR.3. Macro-CTA agents based on PEO with two BSPA end groups (BSPA-PEO-BSPA) were prepared by the reaction of Poly(ethylene oxide) with hydroxyl group at both ends (HO-PEO-OH) with BSPAC in the presence of pyridine. The obtained macro-CTA agents were used to mediate the copolymerization of St and HEMA. The hydroxyl groups of the HEMA units of the resulting triblock copolymers P(St-coHEMA)-b-PEO-b-P(St-co-HEMA) then initiated the ROP of ε-CL in the presence of Sn(Oct)₂. Thus, comb-linear-comb like triblock copolymer [P(St-co-HEMA)-g-PCL]-b-PEO-b-[P(St-co-HEMA)-g-PCL] with novel structureand complex composition was successfully obtained.By means of the investigation of aggregation behavior of these triblock copolymers in the aqueous solution, it was found that the self-assembled morphologies of the copolymers were dependent on the water contents, and the morphologies were varied from rods to vesicles with the increase of the water content in THF. In the transition from rods to vesicles, a novel intermediate morphology named "pearl necklace" was detected. In addition, the composition of the copolymers also exerted great effect on the morphologies.4. A macro-RAFT agent mPEO-DDAT was designed and prepared for the polymerization of alkyl methacrylate and alkyl acrylate, the well-defined hydrophiphilic triblock copolymer mPEO-b-PDMAEMA-b-PNIPAAm was synthesized by consecutive RAFT of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and N-isopropylacrylamide (NIPAAm) monomers using mPEO-DDAT as starting macro-RAFT agent. The polymerization data demonstrated that the RAFT polymerization was under control.Due to the temperature-response of PNIPAAm block and pH-sensitive property of PDMAEMA block of the triblock copolymer mPEO-b-PDMAEMA-b-PNIPAAm, the copolymer could self-assemble to micelles in the aqueous media under special conditions. The investigation indicated that the aggregation of triblock copolymers depended on the temperature, pH and composition of copolymers. When the temperature was higher than LCST, the copolymer could form the global micelles, the higher the pH, the larger the diameters of micelles. It was also found that the higher the molecular weight of PNIPAAm block, the lower the LCST and the larger the micelles formed by copolymers. When the PDMAEMA block was crosslinked with 1,4-dibromobutane, the diameters of formed shell cross-linked (SCL) micelles increased with the drop of the temperature.