| The mechanism of olefin polymerization reactions catalyzed by late transition metalshas been extensively in into organic substrates and studied by experimentalists andtheoreticians during the last decades. The studies demonstrate that the initial step is thecoordination of ethylene to the empty site of the active species to form an olefin-alkylπ-complex. Following migratory insertion of the ethylene into the Pt—Cαbond takes placevia a four center transition state to form the direct product,γ-agostic complex, which caneasily rotate around Cα—Cβbond with a small barrier to rearrange into the more stablecomplex. but there is little theoretical report about the mechanism of the platinum(Ⅱ)-catalyzed ethylene dimerization and Polymerization.In this paper, we chose several typical reactions that have been carefully studiedusing quantum methods, obtained some interesting results. On the basis of the traditiontransition state theory and quantum chemistry theory, the systems chose have beeninvestigated using Density Functional Theory (DFT) and the polarized continuum model(PCM). The structures of the reagents, the reaction products and transition states along thereaction paths have been obtained, then obtained the reaction surfaces, the spectrumdatum, thermodynamic datum as well as the information of orbitals. The reactionmechanism has been argued deeply using these data.The whole paper consists of four chapters. Chapter 1 mainly reviews the evolution ofmethane catalyzed by ligated transition metal. The second chapter summarizes the theoryof quantum chemistry and calculation methods of this paper. The contents of two chapterswere the basis and background of our studies and offer us with userful and reliablequantum methods.In chapter 3, We have studied the ethylene insertion reactions catalyzed by thecationic complexes L2Pt(Ⅱ)R+( L2: none, (NH3)2, (CHNH)2; R: H, CH3, C2H5). Theresults demonstrate that the barriers of migratory insertion are lowest when L2 =(CHNH)2. In the process of ethylene dimerization, the path of the formation of ethylethylene complex from ethylene hydride through ethylene trapping of a reversibly formedβ-agostic ethyl intermediate is favored. In chapter 4, we have studied the mechanism of diimine-Pt(Ⅱ)-catalyzed ethylenepolymerization reaction has been studied in detail theoretically using the B3LYP densityfunctional method. The chain initiation reaction proceeds with the coordination ofethylene to the active catalyst Pt(CHNH)2CH3+, followed by ethylene insertion intoPt—CH3 bond with a 27.55 kcal/mol free energy barrier to form a -agostiγcintermediate, which with a small barrier rearranges to a more stableβ-agosticintermediate. The chain propagation starting with theβ-agostic complex can proceedvia pathway A through coordination, migratory insertion, and rearrangement, leading tolinear polyethylene, or pathway B, throughβ-H elimination, propylene rotation, andreinsertion to give a branched alkyl intermediate, which can then coordinate ethylene andproduce branched polyethylene. The rate-determining step is ethylene insertion among allthe reactions. Furthermore, we also have studied the mechanisms of several chaintermination and transfer reactions.
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