201007-41-2Relevant academic research and scientific papers
Dual Polymerization Pathway for Polyolefin-Polar Block Copolymer Synthesis via MILRad: Mechanism and Scope
Dau, Huong,Keyes, Anthony,Basbug Alhan, Hatice E.,Ordonez, Estela,Tsogtgerel, Enkhjargal,Gies, Anthony P.,Auyeung, Evelyn,Zhou, Zhe,Maity, Asim,Das, Anuvab,Powers, David C.,Beezer, Dain B.,Harth, Eva
supporting information, p. 21469 - 21483 (2020/12/21)
This work explores the mechanism whereby a cationic diimine Pd(II) complex combines coordination insertion and radical polymerization to form polyolefin-polar block copolymers. The initial requirement involves the insertion of a single acrylate monomer into the Pd(II)-polyolefin intermediates, which generate a stable polymeric chelate through a chain-walking mechanism. This thermodynamically stable chelate was also found to be photochemically inactive, and a unique mechanism was discovered which allows for radical polymerization. Rate-determining opening of the chelate by an ancillary ligand followed by additional chain walking allows the metal to migrate to the α-carbon of the acrylate moiety. Ultimately, the molecular parameters necessary for blue-light-triggered Pd-C bond homolysis from this α-carbon to form a carbon-centered macroradical species were established. This intermediate is understood to initiate free radical polymerization of acrylic monomers, thereby facilitating block copolymer synthesis from a single Pd(II) complex. Key intermediates were isolated and comprehensively characterized through exhaustive analytical methods which detail the mechanism while confirming the structural integrity of the polyolefin-polar blocks. Chain walking combined with blue-light irradiation functions as the mechanistic switch from coordination insertion to radical polymerization. On the basis of these discoveries, robust di- and triblock copolymer syntheses have been demonstrated with olefins (ethylene and 1-hexene) which produce amorphous or crystalline blocks and acrylics (methyl acrylate, ethyl acrylate, n-butyl acrylate, and methyl methacrylate) in broad molecular weight ranges and compositions, yielding AB diblocks and BAB triblocks.
Mechanistic studies of the palladium-catalyzed copolymerization of ethylene and α-olefins with methyl acrylate
Mecking, Stefan
, p. 888 - 899 (2007/10/03)
Mechanistic aspects of palladium-catalyzed insertion copolymerizations of ethylene and α-olefins with methyl acrylate to give high molar mass polymers are described. Complexes [{N@?N)Pd(CH2)3C(O)-OMe]BAr'4 (2) or [(N@?N)Pd(CH3)(L)]BAr'4 (1: L = OEt2; 3: L @? NCMe; 4: L @? NCAr') (N@?N @? ArN = C(R)-C(R) = NAr, e.g., Ar @? 2,6-C6H3(i-Pr)2, R @? H (a), Me (b); Ar' @? 3,5-C6H3(CF3)2) with bulky substituted α-diimine ligands were used as catalyst precursors. The copolymers are highly branched, the acrylate comonomer being incorporated predominantly at the ends of branches as -CH2CH2C(O)OMe groups. The effects of reaction conditions and catalyst structure on the copolymerization reaction are rationalized. Low-temperature NMR studies show that migratory insertion in the η2-methyl acrylate (MA) complex [(N@?N)PdMe{H2C = CHC(O)OMe}]+ (5) occurs to give initially the 2,1-insertion product [(N@?N)PdCH(CH2CH3)C (O)OMe]+ (6), which rearranges stepwise to yield 2 as the final product upon warming to -20°C. Activation parameters (ΔH@? = 12.1 ± 1.4 kcal/mol and ΔS@? = -14.1 ± 7.0 eu) were determined for the conversion of 5a to 6a. Rates of ethylene homopolymerization observed in preparative scale polymerizations (1.2 s-1 at 25 °C, ΔG@? = 17.4 kcal/mol for 2b) correspond well with low-temperature NMR kinetic data for migratory insertion of ethylene in [(N@?N)Pd{(CH2)(2n)Me}(H2C = CH2)]+. Relative binding affinities of olefins to the metal center were also studied. For [(N@?N)PdMe(H2C = CH2)]+ + MA @? 5a + H2C = CH2, K(eq)(-95°C = (1.0 ± 0.3) x 10-6 was determined. Combination of the above studies provides a mechanistic model that agrees well with acrylate incorporations observed in copolymerization experiments. Data obtained for equilibria 2 + H2C = CHR' @? [(N@?N)Pd{(CH2)3C(O)OMe}(H2C = CHR')]+ (R' @? H, Me, (n)C4H9) shows that chelating coordination of the carbonyl group is favored over olefin coordination at room temperature. Formation of chelates analogous to 2 during the copolymerization is assumed to render the subsequent monomer insertion a turnover-limiting step.
