空间位阻与氟效应协同增强镍系乙烯聚合

Enhancement on Nickel-Mediated Ethylene Polymerization by Concerted Steric Hindrance and Fluorine Effect

烯烃聚合在工业上是最重要的化学反应之一.过渡金属催化剂是烯烃聚合反应发展的核心,其中水杨醛亚胺后过渡金属中性镍催化剂由于杂质耐受性强与无需助催化剂的双重优势备受青睐.为增强镍催化剂的催化性能,空间位阻效应或者氟效应策略已有广泛报道;然而将两者结合形成协同策略仍鲜有研究.在本工作中,对位氢和空间位阻取代基(苯基、萘基和蒽基)与邻位氟取代基被同时引入至水杨醛亚胺中性镍催化剂中,利用空间位阻效应与氟效应来协同增强镍系乙烯聚合.系统研究空间位阻效应、氟效应、聚合反应温度、聚合时间对乙烯聚合反应的活性、分子量、支化度的影响.结果表明,邻位氟取代基显著提升活性、催化剂寿命与聚合物分子量,降低聚合物支化度;对位空间位阻取代基的体积根据催化活性或聚合物分子量的需求而定,但对聚合物支化度几乎无影响.本工作发展了一种新的调控水杨醛亚胺镍烯烃聚合催化剂手段.

Olefin polymerization is one of the most important chemical reactions in industry.Transition metal catalysts are the key to the development of olefin polymerization.Neutral salicylaldiminato nickel catalyst stands out due to the nature of both functional-group tolerance and cocatalyst-free.Either sterically hindered effect or fluorine effect has extensively been reported over the past decades to improve properties of neutral and single-component salicylaldiminato nickel catalyst;however,combination of these two effects to generate a concerted strategy is much less studied.In this work,both para-sterically hindered substituents including phenyl,1-naphthyl or 9-anthracenyl group and ortho-fluorine substituents are concurrently installed into salicylaldimine ligands,and thus five salicylaldiminato nickel catalysts have been synthesized and fully identified by~1H andC NMR spectroscopy,elemental analysis and X-ray diffraction technique if possible.Without the addition of any activator,these single-component nickel catalysts are used to ethylene polymerization.Influence of sterically hindered effect,fluorine effect,reaction temperature and reaction time on catalytic activity,polymer molecular weight,and branching density of polymer is comprehensively investigated.ortho-Fluorine substituents particularly elevate catalytic activity,lifetime of catalyst species,and polymer molecular weight,while decreases branching density of polymer.Enhancement of catalytic activity and polymer molecular weight reaches two orders of magnitude and 36 times,respectively;and linear structure(5branches/1000 carbon)of polyethylene can be accessible.This should originate from the inhibition of both chain transfer and chain walking pathways.The bulk of para-sterically hindered substituents can be designed according to the required catalytic activity and molecular weight,and notably it has a minor influence on branching density of polymer.The concerted combination of fluorine effect and steric shielding effect enables the formation of linear ultrahigh molecular weight polyethylene(UHMWPE).This work develops a new strategy for the efficient design of salicylaldiminato nickel olefin polymerization catalyst.