Study On Living Free-Radical Polymerization Mediated By TEMPO

The control of macromolecular structure has become an important facet of polymer science from both an academic and industrial viewpoint. Combining the advantages of free-radical polymerization and living polymerization, a range of new polymerization techniques based on living free radical procedures have been developed.Of the living free radical techniques developed, the procedures mediated by stable nitroxide free radicals, such as 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), have attracted considerable interest. In the nitroxide mediate radical polymerization (NMRP), the polymerization can be considered as pseudo-living in nature and the condition of NMRP is mild. Meanwhile, the procedure does not suffer from the gel effect, which may be beneficial for industrial-scale production. One of the major drawbacks of NMRP is the long reaction time and elevated temperatures that are required for these reactions to reach completion. A definite need for the development of simple rate accelerating additives therefore exits.The main purpose of this work is to optimize the condition of NMRP, study the monomer reactivity ratio in NMRP, achieve the UV-irradiated NMRP and prepare the ABA type block copolymer. The main results and progresses of this dissertation are underlined as following:1,The bulk polymerizations of styrene were completed in the presence of TEMPO using the derivates of dimethyl malonate, acetoacetic ester and malononitrile (DMM, DEMM, DEBM, DEDEM, MPD, HFA, Ac-MN, DM-MN) as new rate-accelerating additives respectively. Varying amounts of these additives were added to the polymerization. Significantly, all of these additives were found to have a great rate-accelerating effect on the polymerization. Among them, the polymerization rate of styrene is so quick that the conversion reaches 99% using Ac-MN as additive and 96% using MN as additives within 1.5 hours at 125℃. The derivates of malononitrile had more dramatic rate enhancement effect and resulted in more than 20 times higher rate of polymerization of styrene. There is a common ground that the methylene was substituted by electron-withdrawing groups in all the additives. A possible explanation for these effects is that because of the electron-withdrawing effect, the C atom of methylene had an acidic property and could aggregate around TEMPO due to the electron effect. It was found that the rate enhancement effect is greater with the stronger electron-withdrawing and greater steric hindrance groups substituted.2,Monomer reactivity ratios are important quantitative values to predict the copolymer composition for any starting feed and to understand the kinetic and mechanistic aspects of copolymerization. They are considered as a possible mechanistic probe, as they express the effects of rate constants. Usually, monomer reactivity ratios are obtained by establishing the relationship of the composition between the monomer. Therefore, this work is devoted to studying the monomer reactivity ratios of the living free-radical copolymerization of St with methacrylate (MMA,EMA, BMA), acrylate (MA, EA, BA), vinyl acetate (VAc), 2-hydroxyethyl acrylate (HEA), N,N- dimethylacrylamide (DMAA), and 2-(dimethylamino)ethyl acrylate (DMAEA). The living free-radical copolymerizations had been analyzed by GPC. The data from GPC showed that the polydispersities of the most resulted copolymers were below 1.5, and the rate of polymerization increased, while the polydispersity declined with increase in St molar fraction in the feed. Monomer reactivity ratios had been determined by expanded Kelen-Tudos method by ¹H-NMR and FT-IR. In the copolymerization of St with acrylates the monomer reactivity ratio r1 (St) decreases and in the copolymerization of St with methacrylates the monomer reactivity ratio r1 (St) increases while the length of the substituted group increases.3,Living free radical polymerizations are employed to synthesize polystyrene in the presence of TEMPO irradiated with UV light at 50℃. In the polymerizations, the conversion increased linearly with time, the molecular weight increased linearly with the conversion, and polymers exhibited narrow molecular weight distribution as the value of PDI below 1.5 characterized by GPC, which is characteristic of a controlled/living free radical polymerization. Compare to the thermal-initiated polymerization, the condition of UV-irradiated polymerization is milder and the rate is faster. The polymers were analyzed by ¹H-NMR and FT-IR.4,Atom transfer radical polymerization (ATRP) and Reverse ATRP (RATRP) are useful methods for realization of living polymerization and syntheses of block copolymers. First, polybutylacrylate caped with chlorine, PBA-Cl, was synthesized by RATRP method using AIBN as initiator and FeCl₃/PPh₃ as catalyst at 80℃. Then using the PBA-Cl as macroinitiator, ATRP of styrene was realized and the copolymer, PBA-b-PSt-Cl was obtained. Finally, using the nano Cu power as catalyst, atom transfer radical coupling (ATRC) of the copolymer PBA-b-PSt-Cl was realized and the ABA type copolymer, PBA-b-PSt-b-PBA, was prepared.