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Syndiotactic Polymerization Of Styrene With Homogeneous Metallocene Catalysts

Posted on:2003-02-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:R FanFull Text:PDF
GTID:1101360062475896Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
Syndiotactic polystyrene (sPS) is a new type of polymer with excellent properties of heat-resistance, water-resistance and size stability, which is produced by the coordination polymerization initiated by the homogeneous metallocene catalysts. The crystallization rate of sPS is more than ten times as high as that of IPS, which provides the new product with broad field in application. Up to present, the researches about sPS mainly lie in the following three fields: designing and synthesis of highly active syndiospecific metallocene catalysts, studying on the properties of sPS and its processing, and scaling-up of the polymerization process and the reactor for sPS production. However, most domestic researchers focus their interests in the former two fields and have seldom contacted the last field which is utmost important in industrialization of sPS. In our work, the full-range polymerization kinetics, crystallization of the nascent sPS, nucleation mechanism, polymer morphology, and particle size distribution, etc., of the syndiotactic polymerization of styrene with titanocene/methylaluminoxane(MAO)/tri(iso-butyl) aluminum(TIBA) catalyst system, were studied in detail. A preliminary two-phase rate model was set up also to describe the polymerization processes. Under the basis of these studies, a pilot reactor and the relative polymerization technology were specially designed for bulk syndiotactic polymerization of styrene.The production of sPS is a very complicated process. It is easily to form gel-like products and block during the polymerization, and thus led to the difficulties to continue the process, when the conversion is below 10% and the MWD is almost uncontrollable. How to avoid the problems on gelation, flowing and heat-transfer, and serious reactor fouling, and obtain powdery sPS products, becomes a key topic in sPS development, international speaking.The experiment results reveals that the activity of half-titanocene with pentamethylcyclopentadienyl ligand is more than 1.26 g sPS/gTi hr for syndiotactic polymerization of styrene. In proper polymerization conditions, the final conversion of this polymerization is more than 80%, which takes the practical value to the industrialization of sPS. It is also indicated that with specific recipe, the agitator type and agitation speed are vitally important in avoiding gelation and producing powdery sPS.V.Kinetic study of syndiotactic polymerization of styrene in high conversion period is very difficult, and it has been seldom dealt with before. In this work, under the basis of producing powdery sPS successfully, kinetic studies on bulk polymerization in full conversion range were performed. The results indicate that a typical kinetic curve takes on "ascent and descent" type. An obvious acceleration phenomenon occurs as the continuous phase disappears during the bulk polymerization, which is resulted from the accumulated polymerization heat in particle phase and thus the temperature rises therein. Addition of small amount of inert solution, such as, pentane and isooctane, to the bulk polymerization system is helpful to remove the reaction heat. Factors such as non-cp ligand and polymerization temperature may change the activity of the active centers, and factors such as metallocene kinds, MAO concentration, TIBA concentration etc. may change the active center number formed. All these factors have big influence on kinetics of the bulk polymerization. Small amount of TIBA to the initiation system will improve the activity of the catalyst greatly, however, excess amount of TIBA may further reduce the Ti(IV) species to the inactive Ti(IL) one and decrease the number of the actual active centers of Ti(Ill) formed.The average molecular weight, molecular weight distribution of the as-polymerized sPS samples and their impact factors were studied in detail. The results show that the acceleration during the bulk polymerization seriously decreases the average molecular weight and broadens the MWD of sPS samples. Compared with Cp*TiC13,...
Keywords/Search Tags:Syndiotactic Polystyrene, Metallocene, Kinetics, Morphology of Nascent Polymer, Crystallization during Polymerization, Nucleation Mechanism, Two-Phase Rate Model, Reactor Scaling-up
PDF Full Text Request
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