Olefin Polymerization from the Gas Phase

大多数烯烃聚合方法是催化的， 最经常涉及多相催化剂。 的 聚合反应发生在聚合物上的一些活性位点处。 催化剂在其表面上或在其孔内。 这 术语“接触”是指单体必须接触催化剂， 聚合物生长在表面上或孔内， 催化 所涉及的运输过程包括单体 从扩散通过聚合物层的体相到 活性中心和反应热（反应是 放热的）通过该相同的聚合物层扩散出去。 因此，这些分数与温度和运输有关 有限公司 因此，产物聚合物的性质是 由催化剂的孔结构控制。 为 反应继续进行，聚合物产生于 催化剂越来越多地阻塞产生 单体浓度和温度梯度。 聚合速率降低， 产物聚合物变化很大。 催化剂最终 碎片，暴露更多的表面积以及更多的访问 到以前无法进入的毛孔。 新的地点暴露了， 传输路径长度减少，然后聚合 加快 断裂是由于受限生长而发生的， 聚合物链在狭窄、易碎的通道中膨胀， 催化剂是一种自催化现象。 这 破裂速率取决于催化剂孔结构， 因此在分子量方面具有显著的重要性 聚烯烃的分布和产率。 气相聚合具有以下优点：（1）低 需要资本投资，（2）它是无溶剂的 工艺（即蒸馏、干燥和溶剂精制不 （3）成本和能耗 消费低。 PI计划继续 实验研究了三维孔隙的作用 结构对烯烃气相聚合的影响。 孔隙 结构将通过物理吸附、X射线 层析成像和孔隙率测定。 扩展N2吸附和129 将采用XeNMR来表征微孔隙度， 催化剂/聚合物体系和单体 扩散到活动表面。 X射线成像在两个和 将开发三个维度来确定空间 催化剂在生长的聚合物中的分布。 的 结果应该产生一个更好的基本了解 动力学和输运现象的相互作用和设计 不仅是更好的催化剂，还有整个反应器。

The majority of olefin polymerization processes are cataytic, most often involving a heterogeneous catalyst. The polymerization reaction occurs at some active site on the catalyst either on its surface or within its pores. This means that monomer has to contact the catalyst and that the polymer grows either on the surface or within the pores of the catalyst. The transport processes involved include monomer from the bulk phase diffusing through a polymer layer to an acitve site and the heat of reaction(the reactions are exothermic) diffusing out through this same polymer layer. The ractions are thus temperature related and transport limited. The properties of the product polymer are therefore controlled by the pore structure of the catalyst. As the reaction proceeds, polymer created in the internal pores of the catalyst increasingly blocks transport paths generating gradients in monomer concentration and temperature. Polymerization rate decreases and the molecular weight of product polymers vary greatly. The catalyst ultimately fragments, exposing more surface area as well as more accesss to previoulsy inaccessible pores. New sites are exposed, transport path length is reduced, and polymerization is then accelerated. The facture occurs due to confined growth and expansion of polymer chains in narrrow, friable channels of the catalysts and is an autocatalytic phenomena. This fracture rate depends on the catalyst pore structure which is thus of significant importance in the molecular weight distribution and yield of polyolefins. Gas phase polymerization has the advantage that: (1) low capital investment is required, (2) it is a solvent free process (i.e. distillation, drying and solvent rcovery are not necessary), and (3) therefore operating costs and energy consumption are low. The PIs plan to continue to experimentally study the effects of thethree-dimensional pore structure on the gas-phase polymerization of olefins. Pore structure will be characterized by physical adsorption, X-ray tomography and porosimetry. Extended N2 adsorption and 129 XeNMR will be employed to charactized the microporosity of the catalyst/polymer system and the rate by which the monomer diffuses to the active surface. X-ray imaging in two and three demensions will be developed to determine the spatial distribution of the catalyst within the growing polymer. The results should yield a better fundamental understanding of the interaction of kinetics and transport phenomena and the design of not only better catalysts, but whole reactors as well.