| Because of the possibility of fossil resources becoming scarce in the forthcoming decades, the synthetic polymers which are produced from the fossil resources will encounter a shortage of resources, and a renewal of interest in extending the application potential of renewable resources can be seen. As one of the four industrial raw materials, natural rubber (NR) is the only renewable one. How to take full advantage of this resource has become an arduous work for rubber workers. In the last decade, many attempts were made through graft copolymerization between NR and vinyl monomer to synthesize functional polymers with special properties, to prepare thermoplastic elastomer, or to use the graft copolymer as a compatibilizer. The graft copolymerization was normally performed via free-radical polymerization method. However, because radical termination can not be controlled in free-radical polymerization, it’s impossible to obtain graft copolymer with controllable structures.Synthesis of polymers with the precisely defined molecular architecture is a prerequisite for the development of new advanced materials targeting specific applications. As the most widely used controlled "living" radical polymerization technique, atom transfer radical polymerization (ATRP) is known to be an important method for architecture design of macromolecules. In this thesis, ATRP was used to synthesize graft copolymer of NR by introduction of initiation sites onto the polymer chains in the first step, giving rise to a macroinitiator, followed by polymerization of a chosen monomer in the second step. Using this strategy, it’s possible to obtain graft copolymer of NR with controlled degree of polymerization (molecular weight) of graft chain, graft uniformity (polydispersity) and predetermined graft density (average spacing in-between the side chains).(1) Squalene containing six isoprene units was used as a model compound of NR. To obtain bromoalkyl-functionalized compound using as ATRP initiator, two-step procedure was used, epoxidation of double bond to introduce epoxy group at first, and then ring-opening reaction of the epoxide with2-bromoisobutyric acid to obtain bromoalkyl-functionalized squalene (SBr). The relationship between reaction condition and the composition of SBr, and the distribution of initiating site along the polyisoprene backbone in terms of hexamer were studied. Through this modelization, it provides a reference to predetermine the graft density and graft distribution of graft copolymer of NR. And the results found that temperature is the key factor determining the reaction rate of ring-opening, rising temperature is more effective than increasing the amount of acid, and that the distribution of the initiating site in the model compound, investigated by inverse gated13C NMR, is uniform. The influence of ATRP catalytic system on the ATRP of MMA initiated from SBr was investigated, by changing the structure of ligand, to supply a research base for controllable degree of polymerization of graft chain. It indicates that the catalytic system has important influence on the controllability of the molecular weight and the initiation efficiency of the initiator. The optimum catalyst is the one comprised of CuBr and4,4’-Dinonyl-2,2’-bipyridine (dNbpy) or1,1,4,7,10,10-hexamethyltriethylene-tetramine (HMTETA).(2) Macroinitiator (NRBr) was prepared by introduction of initiation sites onto the polymer chains with two-step method, i.e. epoxidation and then ring-opening reaction, and used to initiate the ATRP of St and MMA to synthesize graft copolymer of NR. The relationship between the structure of NRBr and reaction conditions was studied and found that the density of initiation site can be controlled by adjusting the epoxidation degree of the rubber molecule, so that three kinds of NRBr with different density of initiation site were prepared. The PS grafts were cleaved from the NR-g-PS by hydrolysis and then the structure was analyzed, it showed that the molecular weights of PS grafts increased with increasing conversion, but were much higher than predicted, indicating inefficient initiation, and that the polydispersity was relatively high. In view of the problem of low initiation efficiency of macroinitiator and poor controllability of molecular weights of graft chain, normally encountered in the ATRP "grafting from" process, the influence of catalytic system on the ATRP of MMA initiated from NRBr was researched, and the results imply that the catalyst has a great influence on the controllability of molecular weights and the initiation efficiency of macroinitiator. When CuBr/HMTETA was used as catalyst, the molecular weights increased in a linear fashion with increasing conversion and were close to the theoretical values, the initiation efficiency was high, and the Mw/Mn value was as low as1.36. So that the graft copolymer of NR with controllable structure was synthesized. Based on the research of the reaction kinetics of ATRP graft and compared with the model compound, it suggests that the activation energy for ATRP initiated from macroinitiator was much higher than that from model compound.(3) In view of the problem existing in the normal ATRP, that the transition metal catalysts are used at high concentrations and therefore needs to be removed, activators regenerated by electron transfer for ATRP (ARGET-ATRP) was used to overcome this limitation by using a tiny amount of Cu catalyst (down to single digit ppm vs. monomer). At first, research on model compound was carried out and found that [CuⅡ] can be reduced to lOppm and still control of the molecular weight was excellent when CuCl2/TPMA was used as catalyst. The Mw/Mn value was as low as1.22. Secondly, NR-g-PMMA was obtained by ARGET-ATRP of MMA initiated from NRBr, a "living" characteristics was observed, and the polydispersity of the PMMA grafts was low. Moreover, the graft polymerization can be carried out in ambient temperature (35℃) by increasing the amount of reducing agent. Finally, NR-g-PMMA with different graft density and targeted molecular weight was synthesized. |
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