Design Of P,O-Coordinated Nickel/Palladium Catalysts For The Carbonylative Polymerization Of Ethylene

Keto polymer features carbonyl groups in the polymer backbone,it has excellent mechanical and chemical properties,such as high strength,organic solvent and corrosion resistance.In particular,keto polymers have the unique photodegradability,a desirable feature in view of environmental plastics pollution.Ethylene is low cost bulk chemicals,and carbon monoxide（CO）is important C1 resource,the direct transition-metal-catalyzed carbonylative polymerization of ethylene is a convenient and prominent method for the synthesis of keto polymers.However,due to the strong binding affinity of CO to the metal center,most of the reported catalyst systems are easily decomposed by CO or only afford perfectly alternating polyketones,and it is very challenging to control the carbonyl distribution in keto polymers.Therefore,based on the strategies of electronic and steric regulation,we have designed and synthesized a series of diphosphazane monoxide（PNPO）-ligated nickel/palladium complexes for copolymerization of ethylene with CO,affording a series of keto polymer materials ranging from alternating polyketone,nonalternating polyketone to in-chain ketone polyethylene,as follows:（1）Based on the strategy of electronic regulation,a series of PNPO-Ni complexes were synthesized and employed for the copolymerization of ethylene with CO.By investigation on electronic effect of susbituents,C₂H₄/CO pressure and reaction temperature,the perfectly alternating polyketone materials were afforded.The installation of substituents with strong electrondonating capacity on both of the phosphine and arylamine moieties could produce alternating polyketones with an excellent productivity of 31150 g（g Ni）^-1.Notably,this unique nickel catalyst could operate at low total C₂H₄/CO pressure of 0.5 MPa,which favors the thermal stability and longevity of Ni catalyst.Density functional theory（DFT）indicates that both of the electronic effects and weak interactions of ortho-substituents on the phosphine moiety impact the catalytic performance.（2）Based on the strategy of electronic asymmentry,a series of PNPO-Pd complexes were synthesized and employed for the nonalternating copolymerization of ethylene and CO.By investigation on the effect of susbituents,C₂H₄/CO feed and solvents,nonalternating polyketones were afforded with different carbonyl contents and melting temperatures.Up to24.2%extra ethylene incorporation has lowered T_m values to 147°C and further improved thermal stability（T_d～339°C）of the resultant materials.In addition,DFT calculations have suggested that the PNPO-Pd catalyst favors the competitive coordination to metal center of ethylene and subsequent insertion to form nonalternating structure compared with CO.Our study reported here also points out that phosphine-sulfonate ligated palladium complexes are no longer the singular family of catalysts that can promote the nonalternating copolymerization of ethylene with CO,since cationic PNPO catalyst can also exhibit excellent reactivity and nonalternating degree toward the carbonylative polymerization with a broad tolerance of organic solvents.（3）PNPO-Ni/Pd catalyzed ethylene polymerization with the introduction of an extremely low ratio of CO afforded in-chain ketone high density polyethylenes（HDPE）with a carbonyl content of 1.5%by investigation on reaction temperature,C₂H₄/CO feed and polymerization time.Differential Scanning Calorimetry（DSC）and Wide Angle X-ray Diffraction（WAXD）showed that these in-chain ketone polymers do not compromise crystalline behaviors of polyethylenes,and the tensile test has revealed that it featrues a ductile behavior similar to commercial HDPE.In addition,the regulation of the polymers structure from in-chain ketone polyethylene to nonalternating polyketone could be achieved by simply changing C₂H₄/CO feed.In particular,the incorporation of carbonyl with a low denscity renders this in-chain ketone polyethylene material with unique photodegradability.