The Forming Process Of SPS Nascent Crystalline And Its Coupling Behavior With Syndiospecific Polymerization Of Styrene

Gelation probably occurs during the syndiospecific polymerization of styrene with homogeneous metallocene catalysts. This dissertation is mainly focused on study of the specific precipitation polymerization process of syndiotactic polystyrene(sPS), with the aim of avoiding the gelation and preparing fine sPS powdery particles. On the basis of our previous work, crystallization mechanism of nascent polymer, morphologies of sPS powder and gel and the effect of nascent crystalline on polymerization process were investigated in detail, which helps to elucidate the relationship of polymerization, crystallization mechanism and particle forming process. Based on these fundamental work, the polymerization was successfully conducted in pilot scale with 120L reactor.For general polymerization process, the reaction mixture undergoes complex phase transitions. The homogeneous system transforms into slurry system(continuous and particle phases coexist), then into powdery system(only particle phase). It is found that the phase evolution of sPS is caused by the coupling behavior of polymerization and crystallization, especially the formation of nascent 8 crystalline. The crystalline structure of the δ form is characterized by the presence of clathrate form occupied by guest molecules such as toluene and styrene. Gelation can be formed if the rate of crystallization during polymerization is slow. On the contrary, powdery sPS can be prepared if the rate of crystallization is fast. Moreover, the content of small molecules in 8 clathrate structure is range from 9wt% to 14wt%.During the course of polymerization, the power consumption of agitation goes up rapidly above the critical solid weight fraction ≈ 15% and it reaches peak value when solid weight fraction is around 40%. It has been found that the main factor of determining the power consumption of agitation is volume weight fraction of system, which is directly affected by the morphology of sPS particles formed during the polymerization process.In terms of solubility difference between sPS and solvents, two kinds of particle formation mechanism were presented: successive crystallization and precipitation, simultaneous precipitation and crystallization. The particles formed in poor solvents are agglomerates of primary particles or the secondary particles. Near-globular primary particles pack on the surface ofthese sPS particles. However, the resulting morphology of particles formed in toluene dilute or bulk(no solvent) have no obvious spherical primary particles.The probable gel appearing during the early process of polymerization takes on network structure. The thermo-reversible physical gels of sPS are formed through the cross-link domains of 8 crystalline clathrates. In order to avoid the gelation, the following effective methods can be taken: 1) Decreasing the solid fraction of system;Powdery sPS can be easily prepared in the presence of proper solvents.2) Syndiospecific polymerization of styrene up to high temperature;The thermo-reversible gel structure is destroyed at high temperature. A small amount of soluble PS(aPS) is inevitably produced by thermal free radical polymerization of styrene. But the syndiospecific insertion mechanism of styrene is not influenced up to high temperature. 3) Choosing solvents with large molecular volume;It will essentially avoid the gelation because the solvents don't match with the empty cavity of § clathre form. It is the first time that we succeed in preparing sPS with the non § crystalline form( y or P ) in the polymerization by control of temperature and solvent.A mathematical model for syndiotactic polymerization of styrene has been developed. The simulated results are in good agreement with the experiment data reported by us and other researchers. With this model, some probable results during the sPS polymerization were also predicted.The foregoing work gives a comprehensive knowledge on the heterogeneous polymerizationprocess of syndiotactic polystyrene, and lays an important foundation for industrialization of sPS with homogeneous metallocene catalyst system.