Study On Synthesis Of Star And Branched Polymers Based On A Polymerizable RAFT Agent

This research is aimed at synthesis and properties of hyperbranched and star polymers obtained by RAFT self-condensing vinyl (co)polymerization or normal RAFT polymerization. Their relationships between structure and properties were investigated, and the potential applications were also discussed. To this end, a polymerizable RAFT agent- S-(4-vinyl)benzyl S'-propyltrithiocarbonate (VBPT) was synthesized, and a series of hyperbranched copolymers and multiarm star polymers were obtained by one or two-step reaction via RAFT process. Dual stimuli-responsive PNIPAM hydrogels were obtained by tandem aminolysis and oxidation of RAFT-synthesized star PNIPAM, and their properties were investigated. The main contents are listed as follows.In part 1 (Chapter 2), facile synthesis of hyperbranched and star polymers on the basis of S-(4-vinyl)benzyl S'-propyltrithiocarbonate (VBPT) was described. RAFT copolymerization of VBPT with vinyl monomers such as methyl methacrylates (MMA), styrene (St), methyl acrylate (MA), and tert-butyl acrylate (tBA) afforded hyperbranched copolymers with variable branch length and degree of branching. Hyperbranched copolymers obtained at a low feed ratio of vinyl monomers to VBPT usually possessed repeat units per branch higher than the expected values due to the presence of VBPT unit with pendant trithiocarbonate group and side reactions resulting in partial loss of CTA functionality. RAFT copolymerization at various feed ratios afforded poly(VBPT-co-MA) branched copolymers with weight-average CTA functionality up to hundreds or even thousands, which were further used to generate star PSt and PtBA with adjustable molecular weight and variable polydispersity (1.12 < PDI < 1.88). The approach based on two successive RAFT processes is general and versatile to synthesize multiarm star polymers with controllable arm length and arm number similar to CTA functionality of macro CTAs such as 7.26, 13.0 and 55.8. The resultant polymers were characterized by 1H NMR, GPC-MALLS, DSC, and TGA. The intrinsic viscosities of branched and star-shaped polymers were lower than those of their linear analogues with the same molecular weights; both Mark-Houwink- Sakurada exponent and contractor factor (0.19 < g'< 0.83) of branched copolymers were observed to increase with decreasing degree of branching, thus confirming a branching nature. The single glass transition temperature in DSC traces indicated branched copolymers obtained at various feed ratios had good compatibility, thus RAFT copolymerization could be used as an efficient approach to improve the compatibility between polymeric chains with different polarities.In Part 2 (Chapter 3), hyperbranched PVBPT with CTA functionality in range of 8.1-28.3 were synthesized by RAFT SCVP of VBPT, and PNIPAM stars were obtained by a subsequent RAFT polymerization of N-isopropylacrylamide (NIPAM) in dioxane at 60 oC. RAFT-synthesized star PNIPAM was subjected to aminolysis using hydrazine and simultaneous oxidation by exposing to air, and temperature and redox dual stimuli-responsive PNIPAM hydrogels were obtained. It was found that the gelation time was liable to increase with enhanced number of arms of star PNIPAM. The swelling kinetics of PNIPAM hydrogels in deionized water at different temperatures were studied, the maximum swelling ratio at 20 oC was ranged between 14 and 21, whilst the maximum swelling ratio at 40 oC was estimated to be in the range 1.5-4. In addition, loading and release of Rhodamine B from various PNIPAM hydrogels originated from various star PNIPAM samples were also investigated, and about 50-70% of absorbed dye molecule could be released in 2-5 h.In summary, we have developed and improved a general approach to the synthesis of multiarm star polymers and further understood detailed evolution to form branched polymers during RAFT self-condensing vinyl copolymerization using a styrenic-type chain transfer agent.