Living Ethylene Polymerization

As reported, a common Ti-FT catalyst combined with MAO possesses some characteristics of living ethylene polymerization under limited conditions (e.g., short polymerization time and/or controlled ethylene concentration in a polymerization medium). 

For example, we have described that nearly monodisperse PEs can be formed by 2/ MAO (1 min polymerization, atmospheric pressure 25 °C Mn 52,000, MJMn 1.12 50 °C Mn 65,000, MJMn 1.17) and 38 (Fig. 25)/MAO (1 min polymerization, atmospheric pressure W25n°C M 8000, M /M 1.05 50 °C M 9000, M IM 1.08) [28, 68, 69]. Additionally, Coates and coworkers subsequently reported that Ti-FI catalysts 34 (Fig. 22) and 39 (Fig. 25) can form nearly monodisperse PEs under controlled conditions [70]. With these Ti-FI catalysts, however, synthesizing high molecular weight and narrow molecular weight distribution PEs is generally difficult (e.g., 5 min polymerization, atmospheric pressure, 50 °C 2Mn 132,000, MJMn 1.83 38Mn 24,000, MJMn 1.46) [28, 68]. Moreover, normally, these catalysts cannot be applied to block copolymer formation. 

We successfully synthesized dimethyl complexes of the fluorinated Ti-FI catalysts 42 and 43 (Fig. 26), which possess an essentially octahedrally coordinated Ti center and C2 symmetry, with a cis-O, cis-N, cd-methyl arrangement (Fig. 27) [73]. 

The Ti-FI dimethyl complexes 42 and 43 can readily be activated with B(C6F5)3 or [CPh3]+[B(C6F5)4] to form a methyl cationic species. The addition of ethylene to the cationic species derived from 42 allows room temperature observation of the living propagating species, which displays XH NMR peaks centered at 1.45 and 2.88 ppm that are attributed to the diastereotopic a-methylene protons connected 

We demonstrated that a series of Ti-FI catalysts 40 (Fig. 25) and 44-47 (Fig. 29) possessing a t-Bu, cyclohexyl, i-Pr, Me, and H ortho to the phenoxy-O (thus having various steric environments in close proximity to the active site) all initiate room temperature living ethylene polymerization, though, for the non-fluorinated congeners, the steric bulk of the substituent ortho to the phenoxy-O significantly influences product molecular weight (Table 6) [28, 33]. 

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It should be pointed out that a structurally related bis(imine-phenoxy)Ti complex 54 (Fig. 32) having a C6F5 on the imine-N with MAO activation does not initiate living ethylene polymerization. Interestingly, DFT calculations suggested that there is virtually no interaction between the ortho-F and the 3-H (ortho-F/ 3-H distance 3.66 A) (Fig. 33) [21]. 

Coates and co-workers reported on bis(phenoxy-ketimine) Ti complexes, which can also be catalysts for living ethylene polymerization at temperatures between 0 and 50 These complexes do not have fluorine atoms as the... 

In a similar way, n-butyl acrylate was copolymerized by ATRP with methacrylate macromonomers containing highly branched polyethylene prepared by Pd-catalyzed living ethylene polymerization. The observed reactivity ratios depend on the molecular weight and concentration of the macromonomer. The resulting graft copolymers showed microphase separation by AFM [304]. 

Reinartz S, Mason AF, Lobkovsky EB, Coates GW. Titanium catalysts with ancillary phenoxyketimine ligands for living ethylene polymerization. OrganometaUics 2003 22 2542-2544. 

To achieve living ethylene polymerization with palladium catalysts 72a,b, Brookhart and coworkers demonstrated that specific reaction conditions, particularly quenching reactions with Et3SiH to prevent chain coupling, were crucial (Gottfried and Brookhart, 2001). At 5 °C, highly branched ( lOO branches/1000 carbons), amorphous PEs with very narrow... 

Matsugi, T., Matsui, S., Kojoh, S. et al. (2001) New titanium complexes having two indolide-imine chelate ligands for living ethylene polymerization. Chemistry Letters, 566—567. 

In 2005, Wang and Nomura reported that aryIimido(ary-loxo)vanadium dichloride complex 82 (Figure 23) activated with Et2AlCl exhibited characteristics of living ethylene polymerization. At 0 ° C, the PE produced had a narrow molecular weight... 

Finally, MacAdams et investigated chromium complexes bearing 2,4-pentane-iV,iV -bis(aryl)ketiminato ((Ar) 2nacnac) ligands for ethylene polymerization. At room temperature in the presence of ethylene, 88 (Figure 24) formed linear PE with narrow molecular weight distributions My,/Mn= 1.17-1 A], The Afn was shown to increase linearly with polymer yield. These results represented the first report of living ethylene polymerization with a chromium-based catalyst. 

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