Study On Synthesis And Modification Of Polypropylene With Different Tacticity

Low isotactic polypropylene (LIPP), as a unique kind of polypropylene, is usually synthesized by low-Ti loading Ziegler-Natta catalyst system. However, the polymerization activity per-gram Ti of the traditional MgCl₂-supported catalyst is rather low and therefore how to improve the catalyst's polymerization activity is being paid more and more attention.A one-pot ball-milling method (Process I) is used to prepare the MgCl₂/AlCl₃ supported Ziegler-Natta catalyst. The process is available in industry due to its simplicity, easily being controllable and good stability. The effects of different AlCl₃/MgCl₂ molar ratio of the catalyst on the propylene polymerization are investigated and the polymerization activity reaches its maximum 8478gPP/(gTi*h), which is larger than that of MgCl₂-supported Ziegler-Natta catalyst (5500gPP/(gTi*h)), when the AlCl₃/MgCl₂ molar ratio is 0.12 and Ti-loading of the catalyst is 1.2wt%. LIPP can be synthesized under the optimum conditions: [Ti]=6.4*10^-6gTi/ml, Al/Ti (mol/mol) =40, T=40℃, t=30min. Based on Process I, a simplified method (Process II) is proposed. The polymerization activity of the catalyst prepared by Process II is higher than that of the catalyst prepared by Process I while both kinds of catalysts are of the same Ti-loading. LIPP is also synthesized under the optimum conditions: [Ti]=6.4*10^-6gTi/ml, Al/Ti (mol/mol) =40, T=40℃, t=30min. MgCl₂-supported Ziegler-Natta catalyst is also prepared using the chemical method. The polymerization activity of this kind of catalyst is much higher than that of the above two catalysts. LIPP can be synthesized using this kind of catalyst under the optimum conditions: [Ti]=1.1*10(-5)gTi/ml,Al/Ti (mol/mol) =80, T=20℃, t=30min.Although polypropylene has many advantages, its non-polar properties result in its low compatibility with other polar polymers and inorganic filling, and in turn confine its many uses as engineering plastics. Therefore the modification of polypropylene seems very significant. So far grafting modification and chlorination modification of polypropylene are the two common and effective modification methods.As to the chlorination modification of polypropylene, a novel aqueous chlorination method is put forward in the paper. The polypropylene particle of which the particle diameter is no more than 0.18mm can be chlorinated uniformly using this method. A several-step chlorination method is proposed in order to make polypropylene particle of larger particle diameter be chlorinated uniformly in the aqueous medium. Using three-step chlorination method, polypropylene particle of particle diameter 0.35mm can be chlorinated uniformly and the chlorinated products can be dissolved in CC1₄ at room temperature. The chlorinated products can be dissolved in xylene at 80℃ and no deposits are formed after the solution is cooled. The two-step grafting method is used in the surface modification of polypropylene in the paper. Compared with traditional graftingmethods, the two-step grafting method is more effective and easily being controllable. It was found that the most appropriate preparation conditions are: the concentration of BPO=1.25% (wt/v);the decomposition temperature of BPO=353K;the grafting temperature=353K;the monomer concentration^ .09mol/l;the time for grafting reaction=lh. Also the mechanism of the grafting reaction and the microstructure of grafted products are investigated by HPLC. The results indicate that at the above grafting conditions one active site is formed for every 914 propylene units and the average grafting length is 136 acrylic acid monomer units.