新型钒/锆双中心催化剂聚乙烯釜内合金制备及其聚合机理研究

Ultra-high-molecular-weight polyethylene (UHMWPE), though respected to be “the king of engineering plastic”, is limited in promoting the application further for its poor processibility. Physical mixing UHMWPE with high density polyethylene (HDPE) is commonly used to solve this issue, but problems such as phase separation, poor mechanical properties and high energy cost are often confronted. Our preliminary work indicated that one pot synthesis of UHMWPE/HDPE in-reactor alloy with bimetallic catalysis technique was an important solution to these problems. In this work, approached through combined experimental and theoretical methods, controllable synthesis of UHMWPE/HDPE in-reactor alloy with the novel V/Zr bimetallic catalysts will be studied, and the polymerization mechanism of the catalysts will also be investigated. The experimental studies will focus on the effects of catalyst composition, as well as preparation and polymerization conditions on the polymerization behaviors. Their effects on the structure and processing and mechanical properties of the polymer will also be investigated. EXAFS and in-situ FTIR will be applied to characterize the structure of the catalyst active species as well. To investigate the polymerization mechanism of V and Zr active sites with a synergistic effect, the theoretical studies will be carried out by density functional theory (DFT) using the molecular models of catalysts, which will be constructed on the basis of the experimental results. The results of this work will provide strong instruction for the development of novel high-efficiency catalysts for high-performance polyethylene products with independent intellectual property.

超高分子量聚乙烯（UHMWPE）作为“工程塑料之王”因加工困难而使其应用难以进一步推广，目前通常将其与高密度聚乙烯（HDPE）物理共混的解决方案也面临相分离、力学性能差、能耗高等问题。本项目前期工作表明，利用双中心催化技术一釜法合成UHMWPE/HDPE釜内合金是解决以上问题的一条重要途径。本项目将采用实验与分子模拟相结合的方法，展开基于新型钒/锆双中心催化剂的UHMWPE/HDPE釜内合金可控制备及聚合机理研究。实验研究将重点考察催化剂组成、制备和聚合条件等对其聚合行为及聚合物结构、加工和力学性能的影响规律，并利用EXAFS和原位FTIR等技术表征催化剂活性中心结构。分子模拟将利用基于实验结果构建的催化剂分子模型和DFT方法研究钒、锆双中心之间协同机制下的乙烯配位聚合机理。本项目研究成果将对开发具有自主知识产权的新型高效催化剂及其高性能聚乙烯产品提供理论指导。

超高分子量聚乙烯（UHMWPE）作为“工程塑料之王”因加工困难而使其应用难以进一步推广，目前通常将其与高密度聚乙烯（HDPE）物理共混的解决方案也面临相分离、力学性能差、能耗高等问题。本项目前期工作表明，利用双中心催化技术一釜法合成UHMWPE/HDPE釜内合金是解决以上问题的一条重要途径。项目采用实验与分子模拟相结合的方法，基于钒、铬单金属中心乙烯聚合催化剂制备双中心催化剂以实现UHMWPE/HDPE釜内合金的可控合成，同时探究催化剂的聚合机理。取得了如下重要结果：（1）对于VOx/SiO2，其可能的活性价态为+3价，且通过Al、Ti和Zr改性可使V中心的缺电子性提高，空间位阻降低，同时活性中心LUMO与乙烯HOMO轨道间能隙减小，因此其乙烯插入能垒更低，聚合活性更高。另一方面，VOx/SiO2的β-H链转移能垒比插入能垒高约7kcal/mol，预示其合成UHMWPE的能力。一氯二乙基铝对VOx/SiO2较强的活化能力主要源于其与V中心形成的半稳定配位作用，且可使部分无活性V(IV)转变为活性中心；（2）VOx/SiO2相比于CrOx/SiO2，其乙烯插入时过渡态结构更稳定，但是因V中心缺电子性较强，乙烯与金属中心配位过紧密，导致总体插入能垒反而高2-3kcal/mol，这是前者聚合活性更低的根本原因。（3）活性中心铬硅氧烷环尺寸的差异以及F改性可显著改变CrOx/SiO2活性中心电子和空间结构，从而影响其催化活性以及HDPE产物分子量分布；（4）Chromocene/SiO2独特的乙烯链引发机理可能源于其活性位点双铬中心结构，同时电子自旋极化在引发过程中作用关键；（5）制备的一系列CrOx/Chromocene/SiO2和VOx/Chromocene/SiO2双金属中心催化剂可成功合成UHMWPE/HDPE釜内合金，且通过调节两种活性组分的配比或者利用二者氢调响应的显著差异，可有效调控聚合产物中UHMWPE与HDPE的相对含量，从而实现UHMWPE/HDPE釜内合金的可控合成。以上结果从分子水平一定程度揭示了硅胶负载铬、钒单中心催化剂的乙烯聚合机理，阐释了其催化和聚合产物特性，并为双金属中心催化剂的设计提供理论指导，同时实验结果也表明基于铬、钒的双金属中心催化剂具有单釜可控合成UHMWPE/HDPE釜内合金的潜力。