高性能聚乙烯产品设计

Synthesis of high-performance polyethylene

聚烯烃是国民经济建设和日常生活必不可少的重要基础材料和战略物资.目前,我国聚烯烃行业严重依赖进口,高端聚烯烃产品占进口量的一半以上,行业发展严重受制于人.金属有机催化理论的迅猛发展极大地增强了对聚合物分子结构的调控能力,使人们有可能设计出更多性能优异的聚烯烃新产品.但是,如何实现聚烯烃新产品的工业生产,并使之转化成为商品,是制约聚烯烃产品高性能化的瓶颈,涉及化学工程和高分子科学的深度交叉.本文分别从催化剂结构设计、非均相催化剂结构解析、聚合反应工艺以及产品加工工艺的层面介绍了当前聚乙烯高性能化的重要进展;重点介绍了从活性中心聚合微环境设计出发,通过与催化剂聚合行为的匹配,开发的高性能聚乙烯产品制备新工艺.利用烃类冷凝液的喷射蒸发,在单一流化床反应器中构建温度、浓度差异化的持液颗粒聚合和气相聚合共存的复合流型,打破流化床固有的温度浓度均一性特点,相继开发了超低/极低密度聚乙烯高端产品数万吨,率先生产热收缩膜、拉伸套管膜、双向拉伸聚乙烯膜等高端产品;利用催化剂在惰性组分纳微气泡富集区域的穿梭和碰撞,在生长的聚乙烯颗粒周围营造反应组分的传质开关,使分子链生长和结晶有节奏地交替进行,降低了初生态聚乙烯的链缠结程度,开发了易加工的超高分子量聚乙烯产品.

Polyolefin is one of the important and fundamental materials in daily life and has a great influence on the national economy.With the advances in catalysis in organometallic chemistry,the molecular structure of the polyolefins can be modulated reasonably,resulting in the design of high-performance polyolefins.However,synthesizing high-performance polyethylene on the industrial scale and making them a commodity is still a big challenge,thus limiting its current applications in chemical engineering,polymer engineering,and science.In this study,progress in the high-performance polyolefin synthesis is reviewed based on the design of homogeneous catalyst,heterogeneous catalyst,polymer reaction engineering,and product processing engineering.The in situ characterization of the evolution of active species is described based on advanced techniques favorable for revealing the catalytic behavior and synthesizing the polyolefin with the desired structure.In view of chemical engineering,the gas-liquid-fluidized bed process and bubble-assisted slurry polymerization process designed by our group are introduced for the synthesis of novel polyethylene commodities based on the matched behavior of catalytic performance and polymerization engineering.For example,using the jet evaporation of hydrocarbon condensate,a composite flow pattern of liquid-holding-particle polymerization and gas-phase polymerization with different temperatures and concentrations is constructed in a single fluidized bed reactor,breaking the inherent temperature and concentration uniformity of the fluidized bed.The catalyst can shuttle back and forth in this heterogeneous environment,where numerous polymer chains with diverse microstructures can be synthesized along with the chain propagation.This method ensures the most homogeneous distribution of different components since these components are mixed as soon as the chain grows.Thousands of tons of ultralow-/very-low-density polyethylene products have been developed successfully,which can produce high-performance products,such as heat-shrinkable film,stretch-casing film,and bidirectional stretch polyethylene film.Furthermore,ultrahigh molecular weight polyethylene(UHMWPE)has emerged as a value-added engineering polymer rather than a commodity plastic owing to its unprecedented properties.The ultrahigh molecular weights(i.e.,larger than one million g/mol)with numerous entanglements serve as the physically crosslinked points,dramatically increasing the melt viscosity.This limits their processability and the resulting mechanical properties.We have proposed an inert nano-and microbubble-assisted polymerization process based on the reconstruction of the microchemical reaction environment around the active sites.The mean contact/interval time between microbubbles and particles is observed to be 10–3 s of the mean contact/interval time,several orders less than the chain propagation time.This frequent contact/separation model between microbubbles and polymer particles instantly blocks/recovers the reactant transfer on the solvent-particle interphase,causing an intermittent“dormancy effect”to retard chain propagation and facilitate chain crystallization.The processes of chain growth and chain crystallization are performed alternately,reducing the chain entanglement degree of nascent polyethylene.The nascent UHMWPE with reduced entanglements is achieved by the catalyst synthesizing the highly entangled polyethylene.The synchronous increment is found in stiffness,toughness,and strength for the synthesized polyethylene owing to reduced chain entanglements.