α-diimine Catalysts Modulated By Various Strategies And Application In Ethylene (Co)Polymerization

At the end of the last century,Professor Maurice Brookhart pioneered a class of cationic α-dⅡmine Ni(Ⅱ)/Pd(Ⅱ)catalysts,which is also a milestone in the field of olefin polymerization catalyzed by late-transition metal complexes.In the past three decades,in order to improve the catalytic performance of α-diimine system,researchers have developed many types of ligand modification methods and achieved unprecedented results,but there are still some key issues that have not been well resolved.For example:1)the contradictory relationship between polar functional groups and polymer molecular weight has not been well solved,and it is difficult to prepare high valueadded polar functionalized UHMWPE;2)chain transfer is easy to occur in polymerization at high temperature,which results in the molecular weight of the polymer decreases and the distribution becomes wider,and the catalyst even decomposes and deactivates directly.In this article,we have developed a variety of αdiimine ligand modulated strategies,which not only achieved the preparation of polar functionalized UHMWPE at ambient conditions,but also greatly improved the thermal stability of the catalyst.Moreover,remarkable progress has been made in inhibiting chain transfer at high temperature,which proves that late-transition metal catalysts can also exhibit excellent living polymerization ability at high temperature.1.We delineated an unsymmetrical modulated strategy to shed light on the effect of steric bulk in α-diimine active species via the unsymmetrically pentiptycenyl/dibenzhydryl α-diimine nickel and palladium catalysts versus symmetrical pentiptycenyl analogues and symmetrical dibenzhydryl analogues.In the nickel catalyzed ethylene polymerization,the following new polymerization features have been revealed:1)with the increase of steric bulk,in a relatively long 30 min polymer molecular weights increase,yet moderately hindered unsymmetric nickel catalyst produces the highest molecular weight(Mw=123.0 × 104 g mol-1)in a short 5 min;2)with the increase of ligand steric bulk,branching density first rises and then falls.Consequently,unsymmetric palladium catalyst gave the highest activity and the highest molecular weight(Mw=41.2 ×104 g mol-1)at challenging high temperature of 70℃.In addition,unsymmetrical nickel and palladium catalysts also enabled copolymerizations of ethylene with various polar comonomers.2.Fluorine substituents in transition metal catalysts are of significantly great importance in olefin polymerization catalysis,but the comprehensive effect of fluorine substituents is elusive in seminal late transition metal α-diimine catalytic system.Fluorine substituents at various positions(ortho-,meta-,and para-F)and with different numbers were installed into the N-terphenyl amine and thus were comprehensively studied for the first time in the α-diimine nickel promoted ethylene polymerization and copolymerization with polar monomers.As a notable result,the ortho-F substituted αdiimine nickel catalyst produced highly linear polyethylenes with an extremely high molecular weight(Mw=870.3 × 104 g mol-1)and a significantly low branching density of 1.4/1000 C;however,the meta-F and/or para-F substituted α-diimine nickel catalysts generated highly branched(up to 80.2/1000 C)polyethylenes with significantly low molecular weights(Mw=2.0～5.0 × 104 g mol-1).3.Direct copolymerization of ethylene with polar monomers to produce polar functionalized polyethylene is the most ideal route,but obtaining high molecular weights of polar copolymers represents one of the biggest challenges.we designed a series of novel α-diimine nickel catalysts,polar functionalized ultrahigh numberaverage molecular weight polyethylenes(UHMWPE,Mn=82.5～110.2 × 104 g mol-1)can directly be generated at 1 bar and 30℃.This enhances copolymer molecular weights with up to two orders of magnitude relative to previous reports.Notably,this nickel catalyst family also exceptionally produces the highest number-average molecular weight polyethylenes(Mn=603.7 ×104 g mol-1)at 1 bar using the nickel species.The Sterimol steric parameter of nickel catalyst quantitatively correlates to polymer molecular weight.Mechanistic insights from DFT calculation reveal that the low barrier(10.4 kcal mol-1)of ethylene insertion as the rate-limiting step should be responsible for the formation of UHMWPE.4.Living polymerization by suppressing chain transfer is a very useful method for achieving precise molecular weight control and microstructure control.However,the suppression of chain transfer at high temperatures is extremely challenging in any catalytic polymerization.We report the unprecedented living polymerization of ethylene at 130℃,with a narrow molecular weight distribution range of 1.04 to 1.08.Tailor-made α-diimine nickel catalysts that exhibit both the steric shielding and fluorine effects play an essential role in this breakthrough.These nickel catalysts are even active at 200℃,and enable the formation of semi-crystalline,ultrahigh molecular weight polyethylene at 150℃.Mechanistic insights into the key chain transfer reaction are elucidated by DFT calculations.